The Laws of Inheritance Formulated by Mendel on the basis of his experiments are:
Law I: Law of Dominance:
Mendel concluded that there are two alternate forms of alleles for each hereditary trait. Mendel postulated that when two alleles of contrasting character of a hereditary trait are brought together by fertilization, only one is expressed, while the other is suppressed.
The characteristic which is expressed is called dominant or expressive characteristic, and the characteristic which is not expressed, is called recessive or suppressive characteristic. In his experiment, the tallness is described as dominant, while dwarfness is described as recessive. This is Mendel’s first law of heredity—law of dominance.
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Law II: Law of Segregation:
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According to Mendel, each organism that reproduces sexually by producing gametes has 2 factors (genes) for a characteristic. Of these, one is inherited from the male parent and other from the female parent. These are alleles for a characteristic. If the alleles are similar, they are in homozygous condition.
However, if the alleles are dissimilar, they are in heterozygous condition. All gametes produced by a homozygous individual will have similar alleles. Gametes of a heterozygous individual will be dissimilar. No gamete receives both the alleles of a characteristic.
On the basis of above shown two crosses, it may be concluded that even though the recessive characteristic of dwarfness was not seen in F1 hybrid, it was present and reappeared in the F2 progeny. Mendel concluded from this experiment that alleles come together in the hybrid but do not mix with each other. They simply remain together in F2 generation and maintain their purity. They recombine in the F2 generation.
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On the basis of results obtained from monohybrid crosses, Mendel formulated the second law of inheritance law of segregation.
Law III: Law of Independent Assortment:
The second law of inheritance was formulated on the basis of monohybrid crosses that involved only a single pair of alleles at a time. However, at any given instance, two organisms may differ in many characteristics.
Each of these characteristics is governed by their own pair of alleles in an organism. Mendel had questions in his mind such as, would one pair of alleles influence the inheritance pattern of other pairs of alleles? To find answers to these questions, he performed dihybrid crosses.
Dihybrid Cross:
A dihybrid cross is between two organisms that differ in two characteristics, i.e. heterozygous for two pairs of alleles. For example, two plants differing in seed shape (round/wrinkled) and cotyledon colour (yellow/green).
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Mendel selected a pure line variety of peas for yellow round seeds and another for green wrinkled seeds. He crossed these plants and all the F1 generation plants had yellow round seeds. This showed that:
i. Yellow was dominant over green which was a recessive characteristic.
ii. Round was dominant over wrinkled shape. So, the cross would be represented as follows:
In the next step, Mendel self-pollinated these dihybrids obtained in the F1 generation. The F2 progeny showed four different kinds of phenotypes. These were yellow round, yellow wrinkled, green round, and green wrinkled in the ratio of 9:3:3:1, respectively. Of these, two are of the parental P, types and two are new combinations or recombinants. The dihybrid ratio is therefore, 9:3:3:1.
One the basis of these observations, Mendel formulated his third law of heredity—law of independent assortment of characters. It states that, when a dihybrid organism forms gametes iii. Each gamete receives one allele from each allelic pair (or each characteristic), and
iv. The assortment of alleles of different characteristics during gamete formation is independent of their parental combinations.