Mendel’s laws of inheritance

Gregor Mendel, through his work on pea plants, discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance from one generation to the next. Mendel’s Laws of Heredity are usually stated as-

The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated into the sex cells so that sex cells contain only one gene of the pair. Offspring, therefore, inherit one genetic allele from each parent when sex cells unite in fertilization. In the case of flower colour, the Mendel Pea Experiment showed that a cross between a purple flower plant and a white flower plant produced only purple flower plants for the F1 generation. It appeared that the white flower characteristic had disappeared. However, the F2 generation threw up a surprising result; the white flower variant appeared in a quarter of this generation.

The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another. Mendel examined two different traits, seed colour and seed shape. Two alleles at the locus controlling seed shape were studied, the dominant round (R) and recessive wrinkled (r) alleles. Mendel crossed one parent that was homozygous for the dominant alleles of these two different genes (round yellow RRYY) with another parent that was homozygous for the recessive alleles of those two genes (wrinkled green rryy). Re-stating the basic question, do the alleles at each locus always stay together (i.e. round with yellow, wrinkled with green) or do they appear in new combinations in the progeny? As expected from the 1st law, the F1 generation shows a uniform round yellow phenotype, since one dominant and one recessive allele was inherited from the parents. When the F2 progeny are obtained by crossing the F1 generation, the parental phenotypes reappear (as expected from the first law), but two nonparental phenotypes also appear that differ from the parents: wrinkled yellow and round green! The results can be explained by the alleles of each different gene assorting into gametes independently. For example, in the gametes from the F1 generation, R can assort with Y or y, and r can assort with Y or y, so that four types of gametes form: RY, Ry, rY, and ry. These can rejoin randomly with other gametes from the F1 generation.

The Law of Dominance: An organism with alternate forms of a gene will express the dominant form. The genetic experiments Mendel did with pea plants took him eight years (1856-1863) and he published his results in 1865. During this time, Mendel grew over 10,000 pea plants, keeping track of progeny number and type. Mendel’s work and his Laws of Inheritance were not appreciated in his time. It wasn’t until 1900, after the rediscovery of his Laws, that his experimental results were understood.

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