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  Principles of segregation, Biology tutorial

Mendel's first Law of Inheritance:

Mendel made several crosses, and for each cross he utilized pair of characters that were such that plant can only show one but not both characters. The cross was prepared by transferring pollen grains from anthers of one plant to stigma of another plant or of same plant for cross-pollination and self-pollination respectively.

Plants utilized for initial cross constitute parental or P-generation. Their progeny comprise First Filial generation, abbreviated as F1-generation. Progeny of F1 consequently of either crossing two F1 or of self pollinating F1constitute the F2 or, second filial generation. In one experiment, Mendel crossed parents that were true breeding for yellow seeds with parents that were true breeding for green seeds. This cross was prepared in two methods:

1) Yellow (ovum) x green (pollen) →Yellow F1

2) Green (ovum) x yellow (pollen) →Yellow F1

In cross-1, yellow parent was female parent and in cross-2 role were reversed. Cross-2 is referred to as the reciprocal cross of cross-1 or vice versa. In other words characters utilized in reciprocal cross are exactly same at initial cross; difference is simply reversal of male and female roles. F1 progeny of two crosses are indistinguishable from each other and from yellow parent. In both crosses also all F1 were yellow.

Reciprocal cross gives a very significant piece of information. Fact that progeny of two crosses are the same indicates that male and female contributions to progeny are equal. This is in spite of the fact that pollen grain contributes virtually no cytoplasm to offspring. Mendel realized that fact of equal hereditary contribution from 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.

In the next step of experiment, Mendel planted yellow F1 seeds and self-pollinated (selfed) them when they flowered. This step of experiment is same as crossing two F1 yellow. Yellow F1 seeds gave different results from crosses between 2 parental yellow types. While parental yellow were pure breeding F1 yellow weren't. Yellow F1 progeny from reciprocal crosses gave identical F2, confirming early conclusion. F2 progeny comprised of yellow and green seeds. When Mendel pooled results of F1 crosses he got 6,022 yellow and 2,001 green F2. Further analysis gave ratio of 3.01 yellow: 1 green among the F2.

He assumed that each of true breeding parents holds two identical hereditary factors that are liable for their particular character. For example, in first cross yellow parent would hold two identical factors making for yellowness, and same would be true for green parent. These factors can be signified with symbols. We can hence represent two factors in yellow parent as YY. Two factors in green parent can be signified as YY. When each parent produces gametes, pairs of factors divide so that only one factor enters the gamete. As a result of separation, gametes from yellow parent have only Y factor and those from green parent has only one y factor also. Each parent creates only one kind of gamete but there is no way to differentiate between two Ys or two ys in green parent. When gametes from two parents fuse at fertilization, a zygote that is F1 is formed having two factors, one Y and one y. Therefore F1 may be designated Yy.

The yellow character is said to be dominant over green character as when two kinds of factors liable for both characters are present in same individual only yellow character is shown. In the same way Y-factor is said to be dominant over y-factor. Green character is said to be recessive to yellow character.

The factor for yellow feature is designated Y as yellow is dominant and factor for green is designated y as green is recessive. Same letters used as symbol for both yellow and green characters as they are alternate forms of same character. In other words a seed is either yellow or green but not both. Though we have been using gene (hereditary factor) and character interchangeably, character is the effect generated by gene. Symbols Y and y are hence alternate forms of same gene. They are known as alleles. Alleles are modifications of same gene, therefore variations of same symbol are utilized to designate them. The term for genetic constitution is genotype. For instance genotype of yellow parent is YY. Effect produced by genotype is known as phenotype. The identical phenotypes don't essentially indicate identical genotypes.

According to Mendel's supposition, given parental genotype and types of gametes produced, F1 are Yy. As F2 green was not different from green in P-generation, contribution of green parent to F1 should have retained its integrity and just remained latent. In effect thus, we also have to suppose that y allele remained unchanged in F1. Despite difference in genotype there is no reason to suppose that processes leading to gamete formation in F1 would be different. Again two alleles should separate so that just one, Y or y, enters each gamete.

Fertilization that is gametic fusion according to Mendel is random procedure i.e. Y-bearing pollen doesn't preferentially fertilized either Y-bearing or y-bearing ovule. Both kinds of fusion are evenly frequent as there are equal amounts of two kinds of gametes.

Results generated by Mendel's assumptions and shown in Punnett square allow the given predictions to be made:

i) Green F2 will be pure breeding if they are either self fed or crossed to pure-breeding green of P-generation as they have same genotype. (yy).

ii) One-third of yellow F2 i.e ¼ of all F2 will also be pure breeding for yellow phenotype as they are YY in genotype.

iii) Two-third of yellow F2 i.e. 2/4 of all F2 will yield same results as F1 if they are self fed. They will provide yellow and green F3 in ratio of 3:1.

Some Definitions:

Locus: This is specific point on chromosomes, occupied by gene. Therefore alleles occupy same locus on homologous chromosomes. Locus of gene is constant. Only aspect that differs is allele that may be at that locus on particular chromosomes.

Homozygous/Heterozygous: The genotype is said to be homozygous when both alleles are the same like YY or yy, and it is heterozygous when alleles are different like Yy. Homozygous organisms are known as homozygotes. By same token heterozygotes are heterozygous individuals.

Backcross: This is cross between offspring and one of its parents an individual which is genotypically identical with one parental type.

Testcross: This is cross between individual whose genotype is not known and another individual who is known to be homozygous recessive for trait in question. Testcross by design makes it possible to find out unknown genotype.

Phenotypic Ratio: This is ratio of different phenotypes in progeny of cross, based on fraction of different phenotypes. For example in testcross above, phenotypic ratio is 1 : 1, but among F2 in Mendel's experiment ratio was 3 yellow: 1 green.

Genotypic Ratio: This is ratio of different genotypes among progeny of the cross. Genotypic ratio may or may not be the same with phenotypic ratio. It depends on parental genotypes.

Monohybrid Cross: This is cross in which parents vary with respect to only one feature that is handled by only one gene (and its alleles). Example of Mendel's cross is monohybrid cross. One pure breeding parent was yellow and other green, but the feature was seed color controlled by one gene with alleles Y and y. F1 having traits and alleles from both parents is monohybrid. It is a hybrid with respect to one locus.

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