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Wednesday, October 17, 2018

PRINCIPLES OF SEGREGATION (MENDEL’S FIRST LAW OF INHERITANCE)

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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