Mendelian genetics

Image: Mendelian genetics

Gregor Mendel is known as the father of genetics since he first worked on the inheritance studies in the pea plant. He is also the founder of modern genetics. He deeply got involved in studying the hereditary patterns in the Pisum sativum plant and discovered various traits. We need to understand certain basic concepts before proceeding with Mendel's scientific work. The characteristics of an individual get transmitted from generation to generation. These characteristics are known as hereditary traits. These traits come under the influence of genes. Before knowing the word gene, Mendel used the word factors. Hence the genes or the factors determine the expression of the trait. The observable characteristic or the trait is known as the phenotype. The genetic constitution is also known as the genotype. The expression of a phenotypic trait not only depends on the genotype of the individual but also depends on the actions of the other genes or their products accompanied by the influence of the environmental factors.

The crossing of pea plants involves a standard procedure. The pea plant reproduces using self-fertilization, meaning, the fertilization process occurs in the same flower of the same plant. Mendel successfully prevented the self-fertilization process for progressing the experiments. He instead used the technique of cross-fertilization and obtained 34 strains of the pea plant differing in the number of traits. Next, he allowed them to self-fertilize for many generations (true breeding). The selection of seven different traits further made the studies interesting. The seven traits included the flower and the seed coat color, the seed color, the seed shape, the pod color, the pod shape, the stem height, and the flower position. He checked for purple versus white flowers, and grey versus white seed coats. Similarly, he checked for other mentioned traits such as yellow versus green seed color, smooth versus wrinkled seed shape, green versus yellow pod color, inflated versus pinched pod shape, tall versus short stem, and axial versus terminal flower position.

Cross breeding leads to the offspring known as the hybrid. There are two main types of crosses such as monohybrid cross and dihybrid cross. A monohybrid cross involves a cross between the organisms differing in the single pair of the contrasting characters. A dihybrid cross involves a cross between the organisms differing in the two pairs of the contrasting characters. Example of a monohybrid cross includes a cross between the tall and the dwarf plants. Example of a dihybrid cross includes a cross between the yellow and round seeded plants with green and wrinkled seeded plants.

Monohybrid cross:

It involved a pure variety of pea plant for a single character such as the height. The pea plant either gives rise to the progeny having short or tall height. The cross-pollination between the plants involves the pollination of one flower with the pollen of the other. Hence, it leads to the first filial generation known as an F1 generation. All the offspring of the first generation had a tall phenotype indicating it to be the dominant one. Next step involved self-pollination leading to the transfer of the pollen to the stigma of the same plant. The conduction of this process involved the plants of the F1 generation. The F2 generation involved plants with two main phenotypes such as short and tall heights respectively.

The 25 percent of offspring expressed a phenotype of short height. The plants having short height heights are known as dwarf plants. The 75 percent of the offspring had tall height. Hence, the dwarf plants got produced again in the F2 generation thereby expressing the respective trait. The self-pollination of the F2 plants revealed striking features. All the offsprings of the dwarf plants were dwarfs. Only one-third of the tall plants out of all the plants yielded tall plants. However, the remaining two-thirds of the tall plants yielded 3:1 phenotypic ratio of tall and dwarf plants. Hence, Mendel conducted a series of monohybrid crosses for each of the pea plant characters.

Dihybrid cross:

This type of cross involves two characters. Hence, he selected two varieties of pea plants. For example, consider a cross between the plants with yellow and round seeds and the plants with green and wrinkled seeds. The F1 generation involved all plants having yellow round seeds. Hence, the yellow color and the round shape were dominant over the green and the wrinkled ones. Self-pollination of the F1 generation produced four types of seeds such as nine yellow round, three yellow wrinkled, three green round, and one green wrinkled leading to a phenotypic ratio of 9:3:3:1. Similarly, the F2 generation showed a different variety of plants.

Mendel's laws:

Mendel's experiments revealed that the characters or the genes do not mix or contaminate each other. There are three main laws of Mendel's inheritance studies. A branching diagram helps In calculating the phenotypic ratio of the monohybrid, dihybrid, and other crosses.

The law of uniformity:

Mating between the plants with two contrasting characters does not lead to the blending of any characters. Some of the characters that do not get expressed in the F1 generation reappear in the F2 generation.

The law of segregation:

Every individual possesses two factors for a trait. At the time of gamete formation, each gene in a pair separate from one another. Hence, each gamete carries one gene or a factor. Upon separation of the genes in the gametes, they remain unaltered. Half the gametes carry one allele and the remaining half carry another allele.

The law of independent assortment:

The members of each gene pair show assortment which is independent of the other at the time of gametogenesis. The independent assortment leads to the creation of new characters in the organism.

References:
[1] Medical genetics, G.P. Pal
[2] Principles of Genetics, Tamarin, seventh edition
[3] Genetics, Karvita B. Ahluwalia
[4] Experiments in Plant Hybridisation, Gregor Mendel

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