Short essay on Rules for the Inheritance of Traits: Mendel’s Contribution

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Inheritance is the transmission of genetically controlled characteristics (or traits) from one generation to the next. We will now describe how Mendel studied the inheritance of characters or traits in various generations of pea plants cultivated by him.

First we will discuss ‘monohybrid inheritance’ which concerns the inheritance of a single characteristic (or single trait) such as plant height. After that we will describe the dihybrid inheritance which involves the inheritance of two characteristics (or two traits) such as seed shape and seed colour.

1. Monohybrid Inheritance and the Law of Segregation :

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In order to trace the inheritance of a single pair of contrasting characteristics among the pea plants (like tall stem and short stem), Mendel crossed (cross-bred) the pure-bred pea plants differing in these traits and noted their occurrence in the progeny of succeeding generations.

(i) Mendel first crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall pea plants were produced in the first generation or F1 generation. No dwarf pea plants (or short pea plants) were obtained in the first generation of progeny. From this Mendel concluded that the first generation (or F1 cross) showed the traits of only one of the parent plants: tallness. The trait of other parent plant, dwarfness, did not show up in the progeny of first generation.

(ii) Mendel then crossed the tall pea plants of the first generation (F1 generation) and found that tall plants and dwarf plants were obtained in the second generation (or F2 generation) in the ratio of 3 : 1. In ‘ other: words in the F2 generation, three-fourth plants were tall and one-fourth was dwarf.

Mendel noted that the dwarf trait of the parent pea plant which had seemingly disappeared in the first generation progeny reappeared in the second generation. Mendel said that the trait of dwarfness of one of the parent pea plant had not been lost, it was merely concealed or suppressed in the first generation to re- emerge in the second generation. Mendel called the repressed trait of ‘dwarfness’ as ‘recessive trait’ and the expressed trait of ‘tallness’ as the ‘dominant trait’. In this way, Mendel’s experiments with tall and dwarf pea plants showed that the traits may be dominant or recessive.

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Mendel also noted that all the pea plants produced from the hybrid tall parents of F1 generation, were either tall or dwarf. There were no plants with intermediate height (or medium height) in-between the tall and dwarf plants.

In this way, Mendel’s experiment showed that the traits (like tallness and dwarfness) are inherited independently. This is because if the traits of tallness or dwarfness had blended (or mixed up), then medium sized pea plants would have been produced.

Out of total 1064 pea plants of F2 generation, Mendel found that there were 787 tall pea plants and 277 dwarf pea plants. The ratio of tall plants to dwarf plants comes to be 787 : 277 = 2.84 : 1, which is approximately equal to 3 : 1. Thus, a yet another result obtained from Mendel’s monohybrid inheritance experiment is that the ratio of tall plants to dwarf plants in the F2 generation is 3 : 1. Since tallness is a dominant trait and dwarfness is a recessive trait, so we can also say that the contrasting progeny in the F2 generation occur in the ratio of 3 dominant to 1 recessive. The ratio 3 : 1 is known as the monohybrid ratio.

The results of monohybrid cross enabled Mendel to formulate his first law of inheritance which is called the law of segregation. According to Mendel’s first law of inheritance: The characteristics (or traits) of an organism are determined by internal ‘factors’ which occur in pairs. Only one of a pair of such factors can be present in a single gamete. We will now explain the results of monohybrid cross of tall and dwarf pea plants theoretically by using Mendel’s first law of inheritance.

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Explanation of Results of Monohybrid Inheritance :

(i) Mendel said that each trait is determined by a pair of ‘factors’. This means that the pure-bred tall pea plant has two factors TT for the trait of tallness, and the pure-bred dwarf pea plant also has two factors tt for the trait of dwarfness.

(ii) The factors of inheritance of tallness TT separate into two gametes T and T, and the factors for inheritance of dwarfness tt separate into two other gametes t and t (The traits are transmitted to progeny through these gametes).

(iii) The gametes of tall pea plant then cross with the gametes of the dwarf pea plant by the process of fertilization to form zygotes which then produce various progeny in the F1 generation (or first generation) which consists of all tall plants. Thus the F1 generation possesses one factor of inheritance each parent plant which were carried in gametes. The parental cross is shown clearly in the following chart:

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In the F1 generation shown above, all the progeny plants have factors Tt in which T is the factor for tallness which is a dominant trait. Since all the plants in the F1 generation have the factors Tt, so all of them are tall. The small letter t represents recessive trait of dwarfness, which does not show up in first generation in the presence of dominant trait T.

(iv) When two hybrid, tall pea plants (Tt) produced in the first generation (F1) are now cross-bred with each other, then they will produce second generation (F2) pea plants. This again happens by the separation of factors of inheritance of these tall plants into individual gametes and then crossing of the gametes during fertilisation as shown below:

We can see from the above chart that in the F2 generation (or second generation), the pea plants produced have genotype or inheritance factors TT, Tt, Tt and tt. Now, the plants having genotype TT, Tt and Tt all contain the factor T for dominant trait ‘tallness’, so all the three plants (TT, Tt and Tt) are tall. The plant having the genotype tt has both factors t for the recessive trait ‘dwarfness’, so it is a dwarf plant. Please note that though a single copy of factor T is enough to make a plant tall but both copies of factor t (that is tt) are necessary to make a plant dwarf (or short).

In the F2 generation, we get 1 plant having genotype TT, 2 plants having genotype Tt and 1 plant having genotype tt. So, the genotypic ratio in monohydrid cross will be:

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Again, in the F2 generation, we get 3 tall plants and 1 dwarf plant, so the phenotypic ratio in monohybrid cross will be:

Tall plants: Dwarf plants = 3:1

This result is the same as that obtained by Mendel through experiments.

2. Dihybrid Inheritance and the Law of Independent Assortment :

Dihybrid inheritance involves the inheritance of two pairs of contrasting characteristics (or contrasting traits) at the same time. The two pairs of contrasting characteristics chosen by Mendel were shape and colour of seeds: round-yellow seeds, and wrinkled-green seeds. In order to trace the inheritance of two pairs of contrasting traits, Mendel crossed pea plants having round-yellow seeds with pea plants having wrinkled-green seeds and noted their occurrence in the succeeding generations of pea plants. Mendel made the following observations:

(i) Mendel first crossed pure-bred pea plants having round-yellow seeds with pure-bred pea plants having wrinkled-green seeds and found that only round-yellow seeds were produced in the first generation. No wrinkled-green seeds were obtained in the Fj generation. From this it was concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green colour of the seeds.

(ii) When the F1 generation pea plants having round-yellow seeds were cross-bred by self pollination, then four types of seeds having different combinations of shape and colour were obtained in second generation or F2 generation. These were round-yellow, round-green, wrinkled-yellow and wrinkled-green seeds. Mendel collected a total of 556 F2 seeds and counted them shape wise and colour wise. He got the following result:

Round-yellow seeds 315

Round-green seeds 108

Wrinkled-yellow seeds 101

Wrinkled-green seeds 32

Thus, the ratio of each phenotype (or appearance) of the seeds in the F2 generation is 9 : 3 : 3 :1. This is known as the dihybrid ratio.

Mendel observed that he had started with two combinations of characteristics in seeds: round-yellow and wrinkled-green, and two new combinations of characteristics had appeared in the F2 generation : round- green and wrinkled-yellow. On the basis of this observation, Mendel concluded that though the two pairs of original characteristics (seed shape and colour) combine in the F1 generation but they separate and behave independently in subsequent generations.

The results of dihybrid cross enabled Mendel to formulate his second law of inheritance which is called the law of independent assortment. According to Mendel’s second law of inheritance: In the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.

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