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INTRODUCTION
You will recall that Mendel succeeded where many before and after him had failed.
Mendel succeeded for two major reasons.
1. He analyzed his results both in a qualitative as well as in a
quantitative way.
2. In making his initial crosses, he chose pairs of clearly contrasting
characters for which each of the plants he started with words true/pure
breeding. The term true breeding is used to describe cases in which a
cross between two individuals possessing the same character yields
only progeny which are identical with one another and with the
parents with respect to that character. It is also applicable to cases of
self-fertilization yielding the same results.
2.0 Objectives
In this and subsequent units you are not expected to memorise specific examples,
instead, you should understand how the principles involved can be derived from the
examples used. This unit is supposed to inculcate an understanding of:
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1. Some of the terms used in Genetics. In this and other units new terms
are used. They are an essential part of the vocabulary of Genetics.
You have to learn them whenever they occur.
2. What is involved when a cross is written.
3. The steps, types of evidence and types of deductions which led to
Mendel’s formulation of the first law of inheritance.
4. To recognize the evidence which indicates monohybrid inheritance.
5. To explain the bases for the various phenotypic and genotypic ratios.
6. To make the necessary deductions of phenotype from genotype and
vice-versa, as well as derive offspring from parents and vice-versa.
7. To state and explain the first law in your own words.
3.0 MAIN CONTENT
3.1 Mendel’s first Law of Inheritance
Mendel made many crosses, and for each cross he used a pair of characters which
were such that a plant can only exhibit one but not both characters. A cross was
made by transferring pollen grains from the anthers of one plant to the stigma of
another plant or of the same plant for cross-pollination and self-pollination
respectively.
The plants used for the initial cross constitute the parental or P-generation. Their
progeny constitute the First Filial generation, abbreviated as F1
– generation. The
progeny of the F1 as a result of either crossing two F1 or of self pollinating an F1
constitute the F2 or, second filial generation.
In one experiment, Mendel crossed parents which were true breeding for yellow
seeds with parents which were true breeding for green seeds. This cross was done in
two ways:
1. Yellow (ovum) x green (pollen) Yellow F1
2. Green (ovum) x yellow (pollen) Yellow F1
In cross-1, the yellow parent was the female parent and in cross-2 the role were
reversed. Cross-2 is referred to as a reciprocal cross of cross-1 or vice-versa. In other
words the characters used in a reciprocal cross are exactly the same at the initial
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cross; the difference is merely a reversal of male and female roles. The F1 progeny of
the two crosses are indistinguishable from each other and from the yellow parent. In
both crosses also all the F1 were yellow.
The reciprocal cross provides a very important piece of information. The fact that the
progeny of the two crosses are identical indicates that the male and female
contributions to the progeny are equal. This is in spite of the fact that the pollen grain
contributes virtually no cytoplasm to the offspring. Mendel deduced that fact of
equal hereditary contribution from his results and as we saw earlier, it was only
much later that Hertwig and others provided cytological evidence that the nuclear
contributions are indeed equal. Mendel’s conclusions for reciprocal crosses are also
applicable to animals. The result obtained from the reciprocal cross is therefore,
evidence in support of the chromosomes theory of inheritance.
In the next step of the experiment, Mendel planted the yellow F1 seeds and selfpollinated
(selfed) them when they flowered. This step of the experiment is the same
as crossing two F1 yellow. The yellow F1 seeds gave different results from crosses
between two parental yellow types. While the parental yellows were pure breeding
the F1 yellow were not. Yellow F1 progeny from reciprocal crosses gave identical F2,
confirming the initial conclusion. The F2 progeny consisted of yellow and green
seeds. When Mendel pooled the results of the F1 crosses he got 6,022 yellow and
2,001 green F2. Further analysis gave a ratio of 3.01 yellow: 1 green among the F2.
Using the same scheme Mendel tested a number of characters. His results for some
crosses are shown below:
Note that it is no longer necessary to specify the sex of each parental type. You are
not expected to memorize this table. It is simply to give credence to the conclusions
drawn.
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Table 3.1: Some results of Mendel’s experiments on Sweet Pea
S/No Parental
Characters
F1 F2 Ratios
1
2
3.
4.
Yellow x Green
Seed
Round x wrinkled
Green x yellow
Pods
Axial x terminal
flowers
All yellow
All round
All green
All axial
6022 yellow; 2001
green
5474 round; 1850
wrinkled
428, green; 152
yellow
651 axial: 207
terminal
3.01;1
2.96:1
2.82:1
3.14:1
Although only four crosses are shown in the table, it is obvious that even though a
particular character is not visible in the F1 it is not lost nor is it modified i.e. it does
not blend with the other character. The fact that it remains unchanged can be shown
by comparing the F2 green with the parental green; they are indistinguishable in other
words the hereditary unit responsible for the green colour was merely latent in the F1.
Mendel called the hereditary units “factors”. Wilhelm Ludvig Johannsen
(1857-1927) called them “genes” later.
Also in the table we find that in each cross all the F1 resemble one parent and there is
a constant ratio of approximately 3:1 of the two parental characters. In order to
account for these results Mendel made assumptions and explained his results along
the following lines.
He assumed that each of the true breeding parents carries two identical hereditary
factors which are responsible for their particular character. For instance, in the first
cross the yellow parent would carry two identical factors making for yellowness, and
the same would be true for the green parent. These factors can be represented with
symbols. We can, therefore, represent the two factors in the yellow parent as YY.
The two factors in the green parent can be represented as YY. When each parent
produces gametes, the pairs of factors separate so that only one factor enters a
gamete (compare Mendel’s assumption which the separation of homologous
chromosomes in anaphase-I and also with August Weismann’s theory of reduction).
As a result of the separation, the gametes from the yellow parent contain only Y
factor and those from the green parent contains only one y factor also. Each parent
produces only one type of gamete but there is no way to distinguish between the two
Ys or the two ys in the green parent.
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When the gametes from the two parents fuse at fertilization, a zygote i.e. the F1 is
formed containing two factors, one Y and one y. Hence the F1 may be designated Yy.
From the table, the observed character exhibited by the F1 is yellow, which
corresponds to the Y-factor inherited from the yellow parent. Since a y-factor was
also inherited from the green parent but not exhibited, the y-factor is latent in the F1.
The yellow character is said to be dominant over the green character because when
the two types of factors responsible for both characters are present in the same
individual only the yellow character is exhibited. In the same way the Y-factor is
said to be dominant over the y-factor. The green character is said to be recessive to
the yellow character. The same terminology is used to describe the relationship of
the y-factor to the Y-factor.
The factor for the yellow trait is designated Y because yellow is dominant and the
factor for green is designated y because green is recessive. The same letters used as
the symbol for both the yellow and green characters because they are alternate forms
of the same character. In other words a seed is either yellow or green but not both.
Although we have been using gene (hereditary factor) and character interchangeably,
the character is the effect produced by the gene. The symbols Y and y are therefore
alternate forms of the same gene. They are called alleles. Alleles are modifications of
the same gene, hence variations of the same symbol are used to designate them.
We assumed earlier that each parent carried a pair of alleles for the characters in
question, hence we would use symbols to represent the genetic constitution of each
parent and also of the offspring. The term for the genetic constitution is genotype.
For example the genotype of the yellow parent is YY. The effect produced by the
genotype (which we had called character) is called the phenotype. Before continuing
with our discussion of Mendel’s experiment, it is important to draw your attention to
the fact that identical phenotypes do not necessarily indicate identical genotypes. In
the example under consideration the phenotype of the F1 are indistinguishable from
that of the yellow parent yet according to our explanation so far the yellow parent is
YY while the yellow F1 are Yy.
According to Mendel’s assumption, given the parental genotype and the types of
gametes produced, the F1 are Yy. What type of gametes would the F1 produce? We
had concluded that because the F2 green was not different from the green in the Pgeneration,
the contribution of the green parent to the F1 must have retained its
integrity and merely remained latent. In effect therefore, we also have to assume that
the y allele remained unchanged in the F1. In spite of the difference in genotype
there is no reason to assume that the processes leading to gamete formation in the F1
would be different. Again the two alleles must separate so that only one, Y or y,
enters each gamete. It is most important that you recognize the fact that only one
allele would be in any given gamete. When both alleles were identical as in the
parental generation, each parent produced only one type of gamete. But you will
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recall that at the end of meiosis-I each daughter cell contains one member of a
homologous pair of chromosomes. Genes are on chromosomes so the same situation
applies. More specifically then, 50% of the gamete formed by each F1 would contain
the Y-allele and the other 50% would contain the y-allele.
Fertilization i.e. gametic fusion according to Mendel is a random process, i.e. the Ybearing
pollen does not preferentially fertilized either the Y-bearing or y-bearing
ovule. Both types of fusion are equally frequent because there are equal amounts of
the two types of gametes. We can easily represent random fertilization by using the
Punnett squares (designed by Reginald C. Punnett, 1875-1967). All the four boxes
are equally possible in this case, and together constitute a unit.
Fig. 3.2 The Punnett Square
POLLEN
Y
Y
EGG Y YY Yy
y Yy
Yellow
Yy
Green
The genotype in each box is produced by the fusion of the corresponding gametes.
The contents of the boxes represent the F2 and they are equally visible. Mendel’s
actual results given earlier in Table 3.1 show that the ration of yellow: green in the F2
was 3:1. The Punnett squares show the same type of ratio, and in addition, how the
ratio was arrived at. It shows the genotypes contained in the two phenotypic groups.
The results produced by Mendel’s assumptions and shown in the Punnett square
allow the following predictions to be made:
1. The green F2 will be pure breeding if they are either self fed or crossed
to the pure-breeding green of the P-generation because they have the
same genotype. (yy).
2. One-third of the yellow F2 i.e. ¼ of all the F2 will also be pure
breeding for the yellow phenotype since they are YY in genotype.
3. Two-third of the yellow F2 i.e. 2/4 of all the F2 will yield the same
results as the F1 if they are self fed. They will give yellow and green
F3 in a ratio of 3:1.
You can convince yourself with respect to the fractions which are expressed in
quarters by indicating the fractions of the gametic types i.e. ½ Y and ½ y. A fusion
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event in the Punnett square is “like” an algebraic multiplication such that Y x Y
YY (NOT Y2, that is why an arrow is used instead of =). If therefore you
now include the fraction of the gametic type we shall have ½ Y x ½ Y ¼
YY.
Mendel tested these predictions and obtained the expected results, thus confirming
the correctness of these assumptions – there are a pair of factors (alleles); there is
segregation and there are dominant and recessive alleles.
We can re-summarise these and other facts as follows:
A diploid organism contains pairs of homologous chromosomes such that the
numbers of each homologous pair separate into two cells during meiosis. A gene
may occur as different forms of the same functional unit; the different forms are
called alleles. A diploid organism contains only two alleles for any give phenotype,
and the alleles may be identical as in YY or different as in Yy. Because there are
only two of any alleles and because there is only a pair of any given chromosome
type, we can say that one allele is on one chromosome and the other alleles is on its
homologous partner. Recall the parallel behaviour of the genes and the
chromosomes.
We can summarise Mendel’s experiments with seed colour as shown below:
P Yellow x green
YY (cross) yy
Gametes Y x y
F1 Yy
x x = selfing
Gametes Y , y Y , y
F2 ¼ Y Y; 2/4 : ¼ yy
¾ yellow ¼ green
3 yellow : 1 green
Mendel derived the First law of inheritance, also called the Principle of segregation
from these results. Mendel’s First law of inheritance states that:
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“In the formation of gametes, the members of a gene pair i.e. a pair of
alleles, segregate from each other so that only one or the other
member is contained in each gamete.”
Although the law has been formally stated, it is not intended that you should
memorise it. Rather, you should understand it and be able to apply it. As you can see,
it deals only with gamete formation. If you cannot correctly derive the gametes then
the offspring you derive would not be viable!
3.2 Some Definitions
3.2.1 Locus
This is the specific point on a chromosome, occupied by a gene. Thus
alleles occupy the same locus on homologous chromosomes. We had
said earlier, that genes do not normally move from chromosome to
chromosome. The locus of a gene is constant. The only aspect that
varies is the allele that may be at that locus on a particular
chromosome.
3.2.2 Homozygous/Heterozygous
A genotype is said to be homozygous when both alleles are identical
e.g. YY or yy, and it is heterozygous when the alleles are different
e.g. Yy. Homozygous organisms are called homozygotes. By the
same token heterozygotes are heterozygous individuals. From the
definitions and the discussions above homozygotes are pure breeding
types if self fed or crossed to similar homozygotes.
3.2.3 Backcross
This is a cross between an offspring and one of its parents an
individual that is genotypically identical with one parental type.
3.2.4 Testcross
This is a cross between an individual whose genotype is not known
and another individual who is known to be homozygous recessive for
the trait in question. The testcross by its design makes it possible to
determine the unknown genotype. For example we know that in the
garden pea, axial flowers are dominant over terminal flowers.
Suppose a plant had axial flowers and we had to determine the
genotype of the plant. We would make a testcross.
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i.e. Axial Flowers x terminal
7 x aa
The genotype which we give the plant with axial flowers will be
determined by the types of progeny we get. The critical aspect of the
test-cross however lies in the fact that the homozygous recessive
parent (terminal flowers in our example) produces only one type of
gamete and the gamete contains only recessive alleles. Because the
allele is recessive, any allele from the other parent which it fuses with
can be easily determined. Suppose our test-cross yielded two types of
offspring as shown below:
Test cross: Anxial flowers x terminal
Genotype: 7 x aa
Gamete: (a)
Test cross: Anxial flowers x terminal
Genotype: 7 x aa
Gamete: (a)
F1 Phenotype: Axial : terminal
Ratio: 1 : 1
Partial genotype -a -a
Since the recessive parent produces only one type of gamete half of
the F1 genotype is known, as indicated. In order to have a terminal
phenotype, a recessive trait, there must be homozygosity for the
recessive allele. Hence, that genotype is aa, and the axial parent must
have contained “a” as part of its genotype. The axial F1 also has “a” as
part of its genotype but the phenotype is a dominant one, thus
requiring that at least a dominant “A” allele be present. Such an allele
could only have been contributed by the parent with unknown
genotype which also has an axial phenotype.
Therefore, the genotype of the axial parent is Aa and the cross is
Axial x terminal
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Aa x aa
Gametes: ½ A : ½ a (a)
Axial : Terminal
1 : 1
½ Aa : ½ aa
3.2.5 Phenotypic Ratio
This is the ratio of the different phenotypes in the progeny of a cross,
based on the fraction of the different phenotypes. For instance in the
testcross above, the phenotypic ratio is 1 : 1, but among the F2 in
Mendel’s experiment the ratio was 3 yellow: 1 green.
3.2.6 Genotypic Ratio
This is the ratio of the different genotypes among the progeny of a
cross. The genotypic ratio may or may not be identical with the
phenotypic ratio. It depends on the parental genotypes.
3.2.7 Monohybrid Cross
This is a cross in which the parents differ with respect to only one
trait which is controlled by only one gene (and its alleles). The
example of Mendel’s cross is a monohybrid cross. One pure breeding
parent was yellow and the other green, but the trait was seed colour
controlled by the one gene with the alleles Y and y. The F1 combining
the traits and alleles from both parents is a monohybrid. It is a hybrid
with respect to one locus.
3.2.8 Genetic Symbols
As we found earlier symbols are used to designate the game
responsible of a given trait. The same basic symbol may be modified
to designate the alleles of that gene. We therefore use symbols to
represent the genotypes of an individual.
The choice of symbols is somewhat arbitrary so you will sometimes
find different symbols for the same gene in different books. There are
however some common patterns which we shall adopt, except when
convention demands something different.
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Usually a single letter chosen from one of the phenotypes is used and
the capital form represents the dominant allele while the lower case
represents the recessive. It is often best to state which phenotype
corresponds to a symbol, e.g. yellow = Y and green = y. Equally
important is the need to ensure that the same letter is used for alleles
since that is the only way of making it unambiguous that the
phenotypes belong to the same gene. You would be correct if you use
yellow = G and green = g, but I would mark you wrong if for the
alternate phenotypes of yellow and green you wrote the allelic
symbols as “Y” and “g” respectively. I would take it that these are
alleles to two different genes occupying two different loci, so that a
genotype such as Yg would not be taken as heterogenous. It would be
taken as incomplete, since as we shall see later it represents a gamete
carrying alleles from two loci.
One deviation from the above pattern is found in Drosohila genetics.
By convention the wild type alleles (i.e. the most common type found
in the wild) are written with a “+” as superscript e.g. “w+.” The less
common allele is written as “w”. The symbol implies neither
dominance nor recessiveness. This aspect has to be stated.
4.0 CONCLUSION
We have covered very specific information as well as principles which apply
equally to plants and animals. You are not expected to commit to memory
whether a particular trait is dominant or recessive. On the basis of the facts
you can easily determine that if you know the principles. In the example,
yellow is said to be dominant because in the F1 from a cross between pure
breeding yellow and green was also passed on to the F1.
I expect you to be able to give the genotypic and phenotypic ratio from a
cross and also to be able to derive the types of offspring a cross between two
parental types will produce as well as the converse i.e. to be able to derive the
probable parental genotype given sufficient information about the offspring.
You would almost certainly have a lot of difficulty if you did not try to
understand how results are obtained, you will never be able to memorise all
the different situations. Yet you can quite easily master the principles for
deriving gametes, hence offspring and parental genotypes. “F1” or “F2” do
not designate any specific genotypes or phenotypes, nor does “backcross”
imply a specific genotype. Yet a testcross must definitely include a
homogenous recessive parent. You should memorise definitions but you
should equally know how and when to apply them.
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PRINCIPLES OF SEGREGATION (MENDEL’S FIRST LAW
OF INHERITANCE
items
1.0 Introduction
2.0 Objectives
3.0 Main content
3.1 Mendel’s First Law of Inheritance
3.2 Some Definitions
3.2.1 Locus
3.2.2 Homozygous/Heterozygous
3.2.3 Backcross
3.2.4 Testcross
3.2.5 Phenotypic Ratio
3.2.6 Genotypic Ratio
3.2.7 Monohybrid Cross
3.2.8 Genetic Symbols
4.0 Conclusion
5.0 Summary
6.0 Tutor-marked Assignment
7.0 References

Deduction of Chromosome Theory of Inheritance
Hertwiig working with sea urchins and some other investigators working
with other organisms, discovered that two equal-sized nuclei, one from the
sperm and the other from the egg fuse at fertilization. This is in spite of the
fact that the egg is much larger than the sperm. In other, words the difference
is in the amount of cytoplasm not the nuclear content. Based partly on this
fact and the results of crossing (mating) different types, Hertvig, and
Strasburger also in 1885 advanced the theory that the cell nucleus must
contain the hereditary materials.
Earlier in 1883, Eduoard van Beneden (1846-1910) had discovered in
Parascaris equorum (formerly Ascaris megalocephala – these names seems to
be still preferred) that the fertilized egg of this nematode contains only four
chromosomes. Furthermore, at the time of fertilization, the sperm and the egg
nuclei contain two chromosomes each. In the light of this fact one could be
more specific about the equal nuclear contribution by both the male and
female parent to the zygote. The components of the nucleus that are visibly
distributed during cell division are the chromosomes. It is therefore, quite
logical to conclude that because the parents contribute equal numbers of
chromosomes, the chromosomes must be the carriers of hereditary material.
Reasoning without the benefit of knowledge of van Beneden’s discovery,
Wilhelm Roux (1850-1924), also in 1883, in a purely hypothetical discussion
of the significance of the mitotic process strongly implied (did not say so
categorically) that the chromosomes are the bearers of hereditary materials.
Roux’s approach was teleological i.e. he started from the standpoint that there
must be a reason for the elaborate mitotic process. (For example, it is
teleological to say that we developed eyes because we needed to see). The
question in essence was “why should the division of a simple structure like
nucleus be so complicated?’
According to Roux, if one assumed that there are in the nucleus, very many
submicroscopic units which control the life processes of cell, then it would be
understandable that great care should be taken in dividing the nuclear
content.
On the other hand, mere constriction of the cell would be sufficient for
dividing the cytoplasm. Roux reasoned that a suitable method for ensuring an
identical distribution of the very many submicroscopic units into each
daughter cell would be for each unit to be divided first, and then the sister
units would be separated. The tasks of division and separation would
however be greatly facilitated if the units were arranged like beads on a
string. There would be several such assemblies, carrying different units, in
the cell. During cell division each “string of beads” would then split
longitudinally, and the halves would move into separate daughter cells. Roux
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then went on to say that because the mitotic process is so elaborate it must
serve a purpose in the organism. The purpose is the equal distribution of the
nuclear material important for the physiological and developmental processes
of the cell. We know today that Roux’s “units” are the genes, the hereditary
material, and they are carried on the chromosomes.
In formulating his theory of the Germplasm in 1885, Weismann specifically
said that the chromosomes function as the carriers of hereditary units, but the
chromosome theory was still to be clearly stated.
After the rediscovery of the Mendelian Laws in 1900, it did not take long
before the genes and the chromosomes were identified. The fact that the
observable type of transmission of chromosomes (i.e. the cytological
evidence) corresponds to the deduced type of transmission of genes (the
Mendelian Laws of inheritance) was pointed out independently by Sutton and
by Boveri in 1903. Their conclusions constitute the Chromosomes Theory of
Inheritance. The main points of the theory are:
1. That genes are located on chromosomes such that one member of a
pair of genes is on one chromosome and the other member is on a
partner chromosome, i.e. the homologous chromosome with which it
synapses in meiosis.
2. Different pairs of genes are located on different chromosomes. This is
not to say that there is only one gene on each chromosomes. Rather,
the point is that non-homologous chromosomes carry different genes.
There is more than one gene on each chromosome.
The parallels between the genetic and cytological facts which form the basis
for the theory are:
i) In diploid organisms, genes occur in pairs and so do chromosomes.
ii) Members of a gene pair separate at the time of gamete formation so
that each gamete receives only one member of the pair. The same is
true for chromosomes (cf. Anaphase-I).
iii) The members of different gene pairs recombine at random at the time
of segregation during gamete formation.
Sutton and Boveri did not have corresponding evidence for chromosomes but
they also did not have evidence to the contrary. Recall the fact that the
metaphase-I orientation of one bivalent did not influence the orientation of
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another vivalent. This piece of evidence was provided later and it confirmed
the assumption that No. (iii) was also applicable to chromosomes.
The most convincing proof of the theory that genes are on chromosomes was
provided by Theodor Boveri in his experiments with the sea urchin. Boveri
worked with a species in which 2n = 36. In other words at fertilization each
gamete contributes a haploid number of chromosomes of n = 18. Normally,
only one sperm fertilizes an egg but there are rare exceptions in which more
than one sperm fertilizes the egg. This condition is called polyspermy. It is
called dispermy when only two sperm are involved. Polyspermic embryos die
early in development. We shall consider the simplest case, i.e. the dispermic
embryos. Boveri found that there was great viariability in the time of death
and also in the type of organ whose abnormal development led to death.
The sea urchin embryo can be divided into four quadrants, each of which
arose from one of the first four cleavage blastomeres cells. Boveri observed
that the four quadrants often develop differently, thus one quadrant may be
normal and the other three abnormal but in different ways and to different
degrees. This variability in development of different parts of the same
embryo was a very important observation by Boveri. How does one acccount
for it?
At fertilization in the sea urchin the sperm contributes a centriole which
divides to form the two poles i.e. the asters of the mitotic spindle which is
formed as the asters move apart. Each of the 18 chromosomes contributed by
each gamete in normal fertilization becomes duplicated and comes to lie at
the metaphase plate (equatorial plate). This is normal mitosis. The zygote
contains 36 chromosomes and two blastomeres are formed as a result of the
first cleavage. Following the second cleavage a total of four blastomeres
gives rise to cells which will form one quadrant of the embryo.
When there is dispermy, two centrioles are introduced into the egg. Each
divides giving rise to two asters. The effect of dispermy is the production of
four asters in the zygote. The four asters are arranged like the corners of a
square. When such a zygote divides, four blastomeres are formed at once in
the first division. As mentioned earlier, each blastomere gives rise to a
quadrant in the embryo.
In order to answer the question we posed earlier, we have to try to answer
another question, namely, “How do the chromosomes behave in a quadripolar
division?’ The zygote in question is made up of contributions from two
sperm and the egg. The nucleus of each of these gametes contains 18
chromosomes, therefore, there will be 54 chromosomes. This is a 3n number
of chromosomes and it is said to be a triploid number. The chromosomes are
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duplicated as in normal mitosis. However, when they move on to the
equatorial plates of the spindle, they are distributed at random on the
spindles. The consequence of this random distribution is that each of the four
resulting blastomeres may contain different types of numbers of
chromosomes.
Boveri was able to show that the abnormal development of a dispermic
embryo was the result of the erratic chromosome distribution rather than
dispermy per se. In other words, dispermy does not invariably lead to
abnormal development. Bovery analyzed his results as follows: He found that
the size of a nucleus is dependent on the number of chromosomes present in
it. Therefore, he compared the sizes of the nuclei with the degree of
developmental success (i.e. the degree of normal development) in each
quadrant of an embryo as well as with degree of developmental success in
quadrants having similar-sized nuclei in other embryo.
Table 2.1 Comparison of Development in Two Dispermic Embryos
EMBRYO A EMBRYO B
Nuclear QUADRANTS QUADRANTS
Size I II III IV I II III IV
1 + 111
2 1111 + 11
3 +
4 11 +
1111 = Highest degree of developmental success.
From Table 2.1, one can see that similar-sized nuclei may result in different
abnormalities, hence the different degrees of developmental success. Boveri
therefore concluded that the variability in development is a reflection of
qualitative rather than quantitative differences between nuclei in different
quadrants. For instance if development were dependent on nuclear size only,
quadrants I and III having similar-sized nuclei should have had similar
degrees of developmental success.
Let us now look at a hypothetical example using only four instead 18 types of
chromosomes. In this example we shall also assume that in order to have
normal development, each type of chromosome must be represented at least
once. Since n = 4, the dispermic zygote would contain 12 chromosomes.
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Recall that the distribution of the chromosomes on the spindles is a random
process. The diagram below is therefore only one of many possible ways in
which the 12 chromosomes might be distributed on the four spindles. In this
arrangement, only one quadrant develops normally.
Note: 1 - 4 = Chromosome types
1 - IV = Blastomeres that will form quadrants
I & IV = Have equal-sized nuclei. Some for II and III.
Only IV is normal since all 4 types of chromosomes are present.
Since Boveri was aware that the chromosomes vary in shape and size he
concluded that there are qualitative differences between chromosomes.
Specific abnormalities would therefore, arise when particular chromosomes
were missing. This would be the case only if different chromosomes carried
different genes.
As a further test of his hypothesis about qualitative differences between
chromosomes, Boveri found the expected frequency with which any quadrant
might lack all three of any one of the 18 types of chromosomes. He found
that the expected frequency compared favourably with the observed
frequency of abnormally developing quadrants.
One of the main points of the chromosome theory is that different
chromosomes carry different genes. It is pertinent under the circumstances to
ask whether the chromosomes are stable structures or whether they
disintegrate during interphase and are reassembled during prophase. If that
were so it would also be probable that genes would “move” from one
chromosome to possibly a non-homologous chromosome. There would also
be the possibility that the genes are not normally carried on chromosomes.
The fact that chromosomes are stable structures which maintain their
integrity even during interphase, was established by Boveri using the
18
fertilized eggs of Parascaris equorum. In this nematode the arms of the
chromosomes are not completely retracted at the end of telophase to give a
spherical nucleus. Boveri found that at the end of telophase, the two daughter
nuclei are mirror images of each other as shown in Fig. 2.2.
Fig. 2.1: Mitotic Daughter Nuclei of P. Equorum (2n = 2)
These nuclei retain their shape until the next prophase when the
chromosomes reappear. The chromosomes reappear with their tips still in the
projectins from the nucleus. It is therefore, reasonable to conclude that the
chromosomes did not lose their identity from generation to generation.
3.2 Other Evidence in Support of the chromosome Theory
In our consideration of cell division, we found that the chromosomes in a cell
could be considered as sets, such that a diploid cell would have two sets of
chromosomes. The general terms used to describe the number of whole sets
of chromosomes is "“ploidy”. Continuing on the same theme, there are
euploid aneuploidy conditions. The term euploidy is used to describe
variations in the numbers of whole sets of chromosomes haploid = n; diploid
= 2n; traploid = 3n. These variations which involve whole sets of
chromosomes generally result in normal development. Aneuploidy on the
other hand refers to variations in the numbers of individual chromosomes.
Such variations give unbalanced sets of chromosomes.
From the discussion of Boveri’s sea urchin experiments above, it is obvious
that aneuploidy provides a lot of information in support of the theory that
genes are located on chromosomes. The same is true for the assertion that
different chromosomes carry different genes. In this section then we shall be
considering mainly evidence from aneuploid conditions.
In discussions of chromosomes one often talks of karyotype and idiogram. A
karyotype is an individual’s chromosomes complement in terms of number
and size of chromosomes as well as the location of the centromere in the
different chromosomes. The idiogram on the other hand is a diagrammatic
representation of an individual’s karyotype with the different chromosomes
arranged in order of decreasing size.
In the plant, Datura, the haploid number is 12. Occasionally unusual plants
may arise. These unusual plants contain 25 instead of the normal 24
chromosomes. These plants look different from the normal diploid plant.
Twelve different types each having 25 chromosomes can be identified in
terms of the seed capsule. It was found that each of the twelve variants
possessed a different one of the twelve types of chromosomes. In other
words, in each variant, a given chromosome was present in triplicate. This
aneuploid condition in which three instead of two of a given chromosome are
present is described as a trisomy. Thus, if the different chromosomes are
numbered 1 – 12, an individual with Trisomy – 1 (or Triplo – 1) has three of
chromosomes – 1 present. Note that as we said earlier, these trisomic plants
have only one chromosomes extra, hence the total number is 25 or 24 + 1
which can be stated as 2n + 1; with the exception of the particular
chromosome under consideration all the other chrommosomes are in pairs.
With respect to the example of Datura under consideration, the aneuplooid
effect due to Trisomy – 2 and so on. Because the effect of each trisomy is
distinguishable from all the others, it is logical to conclude that different
chromosomes carry different genes.
Normally in mitosis, the two daughter chromosomes move to opposite poles
during anaphase. Very rarely, however, mistakes do occur and both daughter
chromosomes migrate to the same pole. This situation is described as nondisjunction.
Non-disjunction can also occur in both meiosis –I and meiosis-
II. In the former case, homologous chromosomes would be involved while
the latter would be similar to mitotic non-disjunction. Non-disjunction will
give rise to aneuploid conditions.
Trisomic conditions also occur in man. One example is Trisomy – 21. This
chromosome imbalance produces a condition known as Down’s syndrome.
The term syndrome is used when a number of symptoms characterise an
ailment. This particular case was first described by Down. In man, the diploid
number is 46 but those affected with Down’s syndrome have 47
chromosomes, the extra being chromosome – 21. Amongst other symptoms,
affected individuals are mentally retarded.
Where it has been studied (e.g. U.S.A.) the occurrence of Trisomy – 21
(production of an egg with 24 chromosomes) has been found to be associated
with the age of the mother. The proof of the effect of maternal age is that in
general population, the occurrence of Trisomy – 21 is one in 600 live births.
However, when different age groups are considered separately, the frequency
for mothers about 20 years old is one in 3,000, but for mothers around 45
years, the frequency of occurrence rises to one in 40 live births. The rise in
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frequency starts when the woman is about 35 years. A corresponding study
keeping the female age fairly constant but varying the father’s age does not
show any difference between age groups. The reason for the association with
the age of the mother is not known.
Non-disjunction is not the only cause of Trisomy – 21. Although it was said
earlier that every chromosome maintain its integrity (with the exception of
reciprocal exchange between homologues during crossing-over) it sometimes
happens that a portion of one chromosome is transferred to another
chromosome, usually or non-homologue. This phenomenon is known as
trans-location. Chromosome – 21 is a very small chromosome while 14 is
fairly large. In some very rare individuals the bulk of 21 has been
translocated to 14 to give a chromosome designated 14.21.
Figure 2.2: Translocations Involving Human Chromosomes 14 and 21
The translocation occurs as shown above in a diploid individual. The small
chromosome 21.14 is lost without any adverse effect and so the person has 45
chromosomes, but is normal because virtually all of 14 and 21 are combined in the
14.21 chromosome. If the egg produced carries both the 14.21 and the free 21, it
would have two doses instead of one of 21. Fertilisation by a normal sperm would
therefore, produce an individual with 46 chromosomes but with three effective doses
of chromosome – 21. Notice that the fact that particular effects are associated with
specific trisomic conditions and also, the fact that the translocated 14.21 can be
transmitted unchanged are proof that chromosomes retain their integrity.
If non-disjunction can produce a gamete containing two of one type of chromosome,
the reverse situation is also possible. There are cases known in which as organism
carries only one instead of two of a given chromosome; such individuals are said to
be monosomic for that chromosome. Monosomy – 21 is not known in man, so the
condition is assumed to be non-viable. The same is true for monosomy – 14. These
cases illustrate the point that in some organisms, unlike the sea urchin studied by
Boveri, the mere presence of some genes is not a sufficient condition for normal
development, rather the genes must be present in a balanced dose. In Drosophila
melanogaster, a fruit fly, the haplo – IV (monosomy – IV) condition survives
although the fines have reduced viability and fertility. Some other aneuploid
conditions are:
21
Tetrasomy = 2n + 2 i..e. two extra of a given chromosome.
Double Trisomy = 2n + 1 + 1 i.e. one extra of each of two different chromosomes.
Nullisomy = 2n – 2 i.e. a given chromosome has both members absent.
The significance of chromosomes as well as dosage of chromosomes with respect to
characteristics exhibited by organisms extends to sex determination as we shall see
later. It is sufficient to mention one extreme example here, namely, the honey –bee
in which male are haploid while females are diploid.
When an organism has more than two whole sets of chromosomes i.e. 3n or more
such as individual is described as being polyploid. The 3n individual is a triploid
individual; tetraploid = 4n and pentaploid = 5n. Polyploidy is rather common in
plants but it is rare and often easily recognizable because with certain limits they are
larger than their diploid counterparts.
4.0 CONCLUSION
Rather than try to summarize the examples considered, it is sufficient to say that the
chromosome theory of inheritance states that the genes are an integral part of the
chromosomes. The basis for this generalization is the fact that particular deviations
from say the normal diploid chromosome number, whether euploid or aneuploid
have specific detectable effects. These specific effects are an indication that
chromosomes carry genes and more specifically that different chromosomes carry
different groups of genes.
5.0 SUMMARY
We have learnt that genes are borne on chromosomes, and occur in pairs in diploid
organisms. The gene pairs separate at the time of gamete formation so that each
gamete receives only one member of the pair. Pairing is restored when members of
different gene pairs recombine at random. The randomness of recombination is the
basis of genetics.

History of Genetics
Genetics is primarily and originally a science dealing with heredity i.e. the
transmission of characteristics from parents to offspring. From such
considerations, laws are derived concerning the relationships. In addition,
genetics also involves a study of the factors, which show the relationship
between parents and offspring and which also account for the many
characteristics which organisms possess. You are familiar with the
observations that “Like begets like”, that children tend to resemble their
parents as well as their siblings (or sibs i.e. their brothers and sisters), but
they also tend to vary or look different from one another in many ways.
Genetics is the science, which tries to account for similarities and variations
between related individuals. The science studies the transmission of
hereditary factors from parents to offspring. Put differently, it is a study of
biological “communication” between generations using the hereditary
factors. Another facet of the science is the study of the expression or effect
of the factors during development.
If one were to put the above “descriptive definition” of Genetics in a capsule
form, Bateson, who coined the term Genetics in 1906 aptly defines it as
follows:
Genetics is the science dealing with heredity and variation, seeking to
discover laws governing similarities and differences in individuals related to
descent. The factors which are transmitted were called “Genes” by
Johannsen in 1909.
As mentioned above, Genetics provide explanations to the phenomenon of
heredity and variation. It is therefore, not surprising that the beginning of
genetics dated back to the centuries before Christ. Around 400 BC
Hippocrates theorized that small representative elements of all parts of the
parental body are concentrated in the semen. It is these elements, which
provide the building blocks for the corresponding parts of the embryo.
According to this theory characteristics acquired by parents can be
transmitted to offspring.
Aristotle (384-322 BCE), one century later disproved the theory postulated
by Hippocrates (about 470 BC-about 410 BC), pointing out the facts that
crippled and mutilated parents do not always produce abnormal offspring.
6
Aristotle, in turn advanced the theory that the father’s semen provides the
plans according to which the amorphous blood of the mother is to be shaped
into the offspring. Put differently the semen supplied the FORM while the
mother’s blood supplied the SUBSTANCES. It is important at this point to
note that Aristotle recognized that biological inheritance consists of a
transmission of information for embryonic development, and not simply a
transmission of samples of body parts. The fact that the information in the
seminal fluid could not be seen, it was regarded as a mystical influence.
Early in the 17th century, Harvey called this influence the AURA
SEMINALIS.
In the 17th and 18th centuries, new theories of inheritance were propounded,
following the discoveries of the egg and the sperm. One theory was the
PREFORMATION THEORY, which depending on the school of thought,
stated that either the egg or the sperm contains the entire organism in a
miniaturized but perfect form. In the case of men, the theory postulated a
miniature human being, called a homunculus, present in the sperm. This
theory was postulated by Jan Swammerdam (1637-1680). Not too
surprisingly there were scientists who claimed that they saw homunculi in
spermatozoa. They even drew diagrams to illustrate what they saw. One
person who made an elaborate drawing of homunculus was Nicolass
Hartsoeker (1656-1725). The major drawback with the pre-formation theory
is the fact that is implies that one homunculus contained another, which in
turn contained yet another ad infinitum.
Another theory of development was the THEORY OF EPIGENESIS. In the
18th century, Christian Wolff (1679-1754) discovers that adult structures in
plants and animals arise from embryonic tissues, which do not resemble the
corresponding adult structures. In other words, there is no pre-formation. But
Wolff thought that mysterious vital forces were responsible for what he
thought was a de novo origin of adult parts. Wolff’s view modified in the 19th
century by Karl Ernst Von Baer (1792-1876) who stated that adult parts arise
as a result of a gradual transformation or differentiation of embryonic tissues
into increasingly specialized tissues. Although the modified epigenetic theory
is correct. It did not account for the form in which the materials to be
transformed existed in the original embryonic cell, zygote.
Early in the 19th century, Pierre-Louis Maupertuis (1698-1759) postulated
that minute particles from each part of the body of the parents are united in
sexual reproduction such that during development particles from the male
dominate in some cases; in other cases those form the female parent
dominate. In one important aspect, this theory recognized the fact that an
offspring receives two of each type of particle, one from each parent, but
exhibits only one. However, by suggesting that the body parts contribute
7
particles, this theory leads to the theory of evolution advanced by Jean-
Baptiste Lamarck (1744–1829). According to Lamarck’s interpretation
characteristics such as well-developed muscles acquired by parents in the
course of their life can be transmitted to their offspring. This idea was
formalized by Charles Darwin (1809-1822) as the “Provisional Hypothesis of
Pangenesis.” According to Darwin, exact miniature replicas, called
gemmules, of the body parts and organs are carried in the blood stream, to be
assembled in the gametes. In the zygote, the gemmules from both sexes
come together and are parceled out to form the appropriate structures during
development. Since a gemmule is an exact replica of a parental part it means
that acquired characteristics should be inherited by the offspring. If that were
so it would be easy to understand evolution. Recall that the theory of
pangenesis is essentially the same theory advanced by Hippocrates in the 5th
century B. C. and disproved by Aristotle.
The theory of pangenesis lends itself readily to testing, and it was tested by
August Weismann (1834–1914), toward the end of the 19th century. He cut
off the tails of mice for 22 generations, yet the offspring of such mince
continued to show tails of normal length in every generation. The experiment
can be represented schematically as follows:
Generation I: Cut off tails of the mice and mate them.
Generation II: Offspring with tails; repeat operation
Generation III:Offspring with tails; repeat operation
Generation IV: Offspring with tails; repeat operation
: :
: :
Generation XXI: Offspring with tails; repeat operation
Generation XXII: Offspring with tails.
The result therefore showed that it cannot be true that acquired characteristics
can be inherited.
In spite of this proof there are people who still accept the inheritance of
acquired characteristics. Perhaps the most prominent adherent in recent times
was the Russian, Trofim Lysenko (1898–1976). He coerced many Russian
geneticists to accept the theory, because he wielded political power.
8
To replace the theory of pangenesis August Weismann (1834-1914) proposed
the GERMPLASM THEORY in 1885. According to this theory, multicellular
organisms are made up of two types of tissues, viz the somatoplasm and the
germplasm. The somatoplasm is made up of tissues which are essential for
the functioning of the organism, but they do not determine what is
transmitted to the offspring. In other words, changes in the somatic tissues
are not transmitted. The tail of a mouse is a type of somatic tissue. On the
other hand the germplasm is a tissue whose sole function is the formation of
gametes. Since the gametes give rise to the offspring, changes in the
germplasm may lead to changes in the offspring. Notice, however, that the
theory does not indicate what the germplasm transmits.
Many biologists including Josef Gottlieb Kolreuter (1733-1806) compared
the similarities and differences between plant hybrids and their parents. A
hybrid is an offspring from two different parental types. Kolreuter found that
although hybrids from two parental stocks are usually similar, such hybrids if
fertile usually produce offspring which show considerable diversity. The
results of such hybridization studies were recorded simply as qualitative
observations.
Kolreuter and many others after him did not record the ratios in which the
original parental characters occurred among the progeny. As we shall see
later, it is therefore not surprising that the early hybridizers did not discover
any underlying principles of inheritance. Thus, even though they made many
important observations, the hybridizers pre-date the origin of genetics.
In many ways Genetics is a precise and somewhat mathematical science
dealing with specific offspring ratios which are predictable on the basis of the
known genetic constitutions of the parents. In the reverse process, the genetic
constitution of the different types of offspring they produced.
Gregor Johann Mendel (1822-1884), an Austrian monk, is regarded as the
father of Genetics. It is generally agreed that Mendel’s success can be
attributed to the fact that he was lucky in choosing the garden pea, Pisum
sativum, for his studies. This plant, although, normally self-pollinating can be
easily cross-pollinated. Mendel was also successful because he studied the
inheritance of single contrasting characters (i.e. smooth versus wrinked),
unlike his predecessors who studied several characters simultaneously.
Equally important was the fact that Mendel counted and carefully recorded
the numbers of each type of offspring from each of his crosses.
Mendel published his results in 1866 after he had reported them at a Natural
Science meeting in 1865. He clearly stated the laws of inheritance which can
be derived from his results. The law constitute the foundation stones of
9
Genetics. In spite of the fundamental nature of Mendel’s discoveries and the
clarity with which he stated his results and conclusions, his papers had no
immediate impact on the scientific world. However, one Russian botanist,
Ivan Ivanovich Schmalhausen (1884-1963) stressed the importance of
Mendel’s findings soon after they were published. Mendel’s discovery did
not have an immediate effect because the related information required for
understanding his deductions were not available at the time. Thus it may be
said that Mendel was “ahead of his time”.
After publication of Mendel’s results other relevant information about
development were provided by various workers. In 1875, Oscar Hertwig
(1849-1922) and later, Hermann Fol, and Eduard Strasburger described the
process of fertiliztion including the fusion of the egg and the sperm nuclei.
Between 1880 and 1885, Fleming, van Beneden and Strasburger described
chromosomes and their division in mitosis as well as their constancy in
number. Later Hertwig and Strasburger developed the theory that the nucleus
contains hereditary materials. These discoveries were reflected in
Weismann’s theory of the Germplasm. Weismann postulated that in the
process of gametogenesis, i.e. the formation of gametes there must be a
reduction in half of the number of chromosomes. If that were not so, there
would be a doubling of the chromosome number at each fertilization.
However, as mentioned earlier the chromosome number is constant from
generation to generation. The postulate by Weismann of reduction in
chromosome number was later observed by Boveri and other investigators.
The process involved is meiosis.
Three investigators unaware of Mendel’s work and results independently
carried out similar plant breeding experiments. During the process of writing
their findings for publication, they each came across Mendel’s paper and they
referred to it in their rediscovery of the Mendelian laws of inheritance.
Although the three people, Correns, Hugo de Vries and Tschermak are
generally regarded as the rediscoverers, some scientists (Stern & Sherwood,
1966) do not think that Tschermak’s work on its own could have yielded the
laws of inheritance. Hence, there should be only two rediscoverers.
Although the laws of inheritance were first demonstrated with plants, Bateson
in 1902 showed that the laws apply equally to animals.
From this brief history of Genetics one would hope that you would derive and
appreciate the tortuous steps leading to the establishment of various laws in
science

INTRODUCTION TO GENETICS biology
A feature of students’ attitude to Genetics is that it is a very difficult course, which
one takes only because it is compulsory. Some students reinforce this negative
attitude by the rationalization that they do not intend to do post-graduate work in
Genetics. These attitudes are not deceptively correct because they make you neglect
some of the more important aims of education. If you avoided the challenge posed by
some courses, you would be denying your intellect the stimulation it needs to spur it
to greater heights.
Besides, for a long time, Nigerian secondary school students have been denied
adequate exposure to Genetics because some people avoided the challenge. Genetics
is a vital aspect of everyday life and of Biology, and no biologist, regardless of his
level or interest, should avoid a meaningful exposure to it.
Dobzhansky, aptly summarizes the need for a broad exposure as follows:
“The advancement of science is, in the main, the business of
specialists. And as science expands, the specialists tend to
become narrow specialists. Some specialists have become
disgustingly narrow. Narrow specialists are ENDANGERED
and DANGEROUS (emphais mine) – endangered because
3
their own inner lives are impoverished; dangerous because
they are liable to be easy prey for exploitation by those with
power or with money, for purposes inimical to both science
and to the interests of mankind as a whole … There should
exist, however, scientists able and willing now and then to
abandon the protective shells of their specialties, and to
engage in surveying broad vistas … people at large will have
their inner life enriched if they gain an appreciation of what
science and scientific attitude really are. Some aspects and
achievements of science are everyone’s
business” (Dobzhansky, 1964).
It is hoped that at the end of the course, you would have gained an understanding of
the principles governing the transmission of hereditary traits. All societies are
interested in understanding how certain traits are inherited in living things, including
man.
The puzzle about genetic inheritance in man is perhaps most succinctly expressed in
this portion of a poem by Aldous Huxley’s “Fifth philosopher”:
A million million spermatozoa
All of them alive;
Out of their cataclysm but one poor Noah
Dare hope to survive
And among that billion minus one
Might have chanced to be
Shakespeare, another Newton, a new Donne
But the One was Me
Why was that one me? Why do normal parents produce an albino and short parents
a tall child, or tall parents a short child? It is important for our well-being that we
should be able to answer simple questions about heredity without resorting to “old
wives tales”. But Genetics is not solely concerned with man, it is of great importance
in agriculture.
It is further hoped that at the end of this course, you will be able to appreciate the
fact that:
“Increased knowledge of heredity means increased power of control over the
living things, and as we come to understand more and more the architecture
of the plant or animal we realize what can and what cannot be done towards
modification or improvement …
4
It is not, however, in the economic field, important as this may be, that Mendel’s
discovery is likely to have most meaning for us: rather it is in the new light in which
man will come to view himself and his fellow creatures, if it is shown that the
qualities of man, his body and his intellect, his immunities and his disease, even his
very virtues and vices, are dependent upon the ascertainable presence or absence of
definite unit-characters (genes) whose mode of transmission follows fixed laws, and
if also man decides that his life shall be ordered in the light of this knowledge, it is
obvious that the social system will have to undergo considerable changes” (Punnett,
1910).
This course deals with the basic principles governing heredity. Examples are chosen
merely to illustrate these principles. To that extent therefore, you will not be
expected to memorize examples, which may be new to you. This approach is dictated
not only by the fact that the basic laws of heredity are applicable to most
organizations, but also by the belief that with a good understanding of the principles
one can make extrapolations to explain particular situations.
Much of the difficulty, which students have with Genetics stems from the fact that
they had been used to purely descriptive aspects of biology. Genetics on the other
hand largely entails logical reasoning based on a number of interdependent principles
often involving some calculations. These calculations are within the scope of anyone
who has studied elementary mathematics.
Genetics is a course which demands alertness and consistent work in the forms of
reading and practice.
A note of warning should be sounded here: You would be deceiving yourself and
also doing yourself a disservice, if you merely read genetics as literature. It indeed
entails practicing on questions that boarder on the principles and laws of genetics.
You will have to work examples typifying these principles and laws to have the
concepts of genetics running in your blood.
2.0