MENDEL’S LAW OF INHERITANCE


Gregor Mendel was a monk in Austria. He developed the fundamental principles of genetics. Mendel proposed that there are “special factors” in organisms, which control the expression of traits and their transmission to next generations. These factors were eventually termed genes.
Mendel selected pea to carry out a large number of experiments. In his writing, gave reasons for this selection. He argued that an organism for genetic experiments should have the following features:
  • ·         There should be a number of different traits that can be studied.
  • ·         The organism should have contrasting traits e.g. for the trait of height there should be only two very different phenotypes i.e. tallness and dwarfness.
  • ·         The organism (if it is a plant) should be self-fertilizing but cross fertilization should also be possible.
  • ·         The organism should have a short but fast life cycle.


All these features are present in pea plant. Normally, the flowers of pea plant allow self-pollination. Cross pollination can also be done by transferring the pollen grains from the flower on one plant to the flower in another plant. Each trait studied in pea plant had right organisms for his experiments but also because he analyzed the results by using the principles of statistics (ratios).

Mendel’s Law of Segregation

Mendel studied the inheritance of shape first. For this purpose, he crossed two plants having one contrasting trait i.e. seed. A cross, in which only one trait is studied at a time, is called monohybrid cross. Mendel crossed a true breeding round-seeded plant with a true-breeding wrinkled-seeded plant. All resulting seeds of the text generation were round. Mendel declared the trait “round seeds” as dominant, while “wrinkled seeds” as recessive. The following year, Mendel planted these seeds and allowed the new plants to self-fertilize. As a result, he got 7324 seeds: 5474 round and 1850 wrinkled (3round: 1wrinkled).
The parental generation is denoted as P1 generation. The offspring of P1 generation are F1 generation (first filial). The cross in F1 generation produces F2 generation (second filial).
Similarly, when “true-breeding” tall plants were crossed with “true-breeding” short plants, all offspring of F1 were tall plants i.e. tallness was a dominant trait. When members of F1 generation were self-fertilized, Mendel got the ratio of tall to short plants in f2 as 3:1.
Mendel concluded that the traits under study study were controlled by discrete factors or genes. In each organism, the genes are present in pairs. During gamete formation, the genes of each pair segregate from each other and each gamete receives one gene from the pair. When the gametes of male and female parents unite, the resulting offspring agains gets the genes in pairs. These conclusions were called the Law of Segregation.

Mendel’s Law of independent Assortment

In the next crosses, Mendel studied two contrasting trait at a time. Such crosses are called dihybrid crosses. He performed experiments on two seed traits i.e. shape and colour. The trait of round seeds was dominant over wrinkled seeds. Similarly yellow seed colour was dominant over green. Mendel crossed a true-breeding plant that had round yellow seeds with a true-breeding plant having wrinkled green seeds. All seeds in F1 generation were round yellow.
When F1 seeds grew into plants, they were self-fertilized. This cross produced seeds with four phenotypes. There were 315 round yellow seeds, 108 round green seeds, 101 wrinkled yellow seeds and 32 wrinkled green seeds. The ratio of these phenotypes was 9:3:3:1.
Mendel explained that the two traits i.e. seed shape and seed colours are not tied with each other. The segregation of ‘R’ and ‘r’ alleles happens indepdently of the segregation of ‘Y’ and ‘y’ alleles.
From his second experiment, Mendel concluded that different traits are inherited independently of one another. This principle is known as law of independent assortment. It states as: “the alleles of a gene pair segregate independently from the alleles of other gene pairs”


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