BIOLOGY Volume 3 - A Guide to General Biology - 2004

24. VARIATION AND GENETICS

24.1. Mendel's Researches

24.1.3. Dihybrid Inheritance and the Law of Independent Assortment

Having established The ability to predict the outcomes of crosses involving a single pair of alternative traits, Mendel moved on to study the inheritance of two pairs of such traits. Crosses between individuals differing in two traits are called dihybrid crosses.

In one of his experiments, Mendel used pea plants differing in seed shape and colour. Applying the method described in section 24.1.1, he crossed pure-breeding (homozygous) plants with smooth yellow seeds and pure-breeding plants with wrinkled green seeds. All F₁ plants had smooth yellow seeds. Based on the results of previously conducted Monohybrid Crosses, Mendel already knew that these traits are dominant; now, however, he was interested in The Nature and ratio of Different types of seeds in the F₂ generation obtained from the F₁ plants via self-pollination. In total, he collected 556 seeds from the F₂ plants, among which there were:

315 smooth yellow

101 wrinkled yellow

108 smooth green

32 wrinkled green

The ratio of different phenotypes was approximately 9:3:3:1 (dihybrid ratio).

Based on these results, Mendel drew two Conclusions.

1) The F₂ generation exhibited two new combinations of traits: wrinkled and yellow; smooth and green.

2) The ratio of each pair of allelomorphic traits (phenotypes determined by different alleles) was 3:1, which is characteristic of a monohybrid cross: 423 smooth to 133 wrinkled, 416 yellow to 140 green.

These results led Mendel to argue that two pairs of traits (seed shape and colour), whose hereditary determinants united in the F₁ generation, segregate in subsequent generations and behave independently of one another. This is The basis of Mendel's second law — THE PRINCIPLE OF independent assortment, according to which each trait from one pair of traits can combine with any trait from the other pair.

The experiment described above can be represented using the genetic symbols familiar to us, as shown in Fig. 24.4, A. As a result of the Separation (segregation) of alleles (R, r, Y, and y) and their independent assortment (recombination), one of four allele combinations is possible in each of the male and female Gametes. To show all possible combinations of gametes arising from random fertilisation, a Punnett square is used, named after the Cambridge geneticist R. Punnett. It helps minimise errors that could occur when listing all possible gamete combinations. When filling out a Punnett square, it is recommended to first place all "male" gametes in the Cells along the vertical columns, and then all "female" ones in the Cells of the horizontal rows. Furthermore, when determining the phenotypes of F₂ individuals, it is useful to denote identical phenotypes with some easily distinguishable symbols (as done in Fig. 24.4, B). As shown in Fig. 24.4, A and B, in accordance with Mendel's First and Second laws, for each male and female F₁ genotype, The formation of gametes with the following allele combinations is possible:

R can only occur in combination with Y or y (not with r), i.e., as RY or Ry;

r can only occur in combination with Y or y (not with R), i.e., as rY or ry.

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Fig. 24.4. A. Formation of F₁ phenotypes from a cross between homozygous parental individuals. This is an example of a dihybrid cross — two pairs of contrasting traits are considered. B. Using a Punnett square to show all gamete combinations possible during the formation of F₂ genotypes.

Thus, for any gamete, the chance of receiving any one of the four allele combinations specified here is 1 in 4.

Since in a monohybrid cross 3/4 of the F₂ offspring express the dominant allele and 1/4 express the recessive one, the probabilities of expressing the four alleles under consideration in F₂ are:

smooth (dominant) 3/4

yellow (dominant) 3/4

wrinkled (recessive) 1/4

green (recessive) 1/4.

Hence, the probabilities of occurrence of the following allele combinations in the F₂ phenotypes are:

smooth and yellow = 3/4 x 3/4 = 9/16

smooth and green = 3/4 x 1/4 = 3/16

wrinkled and yellow = 1/4 x 3/4 = 3/16

wrinkled and green = 1/4 x 1/4 = 1/16.

The results of Mendel's experiments on crossing varieties differing in two pairs of alternative traits closely approximate these theoretical calculations.

24.3. In the guinea pig (Cavia), There are two pairs of alleles determining black or white coat color and short or long Hair. When crossing long-haired white homozygotes with short-haired black homozygotes, all F₁ offspring had short, black hair.

Explain:

a) which alleles are dominant,

b) what the phenotypic ratio will be in F₂?

24.4. Flower color in the sweet pea is determined by two pairs of alleles (Rr and Ss). Flowers are purple when at least one dominant Gene from each pair of alleles is present. For all other genotypes, the flowers are white.

What will be the ratio of different phenotypes in the offspring resulting from a cross between two purple-flowered RrSs plants?