Question

Describe the dihybrid cross upto F₂ generation as conducted by Gregor Mendel using pure lines of Garden Pea for characters-seed shape and seed colour.

Answer 1

Yellow colour was dominant over green and round shape was dominant over wrinkled. These results were identical to those that he got when he made separate monohybrid crosses between yellow and green-seeded plants and between round and wrinkled-seeded plants. Let us use the genotypic symbols Y for the dominant yellow seed colour and y for the recessive green seed colour, R for round-shaped seeds and r for wrinkled seed shape. The genotype of the parents can then be written as RRYY and rryy. The cross between the two plants can be written down showing the genotypes of the parent plants. The gametes RY and ry unite on fertilisation to produce the F₁ hybrid RrYy. When Mendel self-hybridised the F₁ plants, he found that 3/4^th of F₂ plants had yellow seeds and 1/4^th had green. The yellow and green colours are segregated in a 3 : 1 ratio. Round and wrinkled seed shapes are also segregated in a 3 : 1 ratio; just like in a monohybrid cross. In the dihybrid cross Figure, the phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9 : 3 : 3 : 1. Such a ratio was observed for several pairs of characters that Mendel studied. The ratio of 9 : 3 : 3 : 1 can be derived as a combination series of 3 yellow : 1 green, with 3 round : 1 wrinkled. This derivation can be written as follows: (3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green Based upon such observations on dihybrid crosses (crosses between plants differing in two traits) Mendel proposed a second set of generalisations that we call Mendel’s Law of Independent Assortment. The law states that ‘when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters’.

Phenotypic ratio:

round yellow : round green : wrinkled yellow : wrinkled green

9                     3                      3                         1