Question

In Mendelian dihybrid cross, out of 240

individuals obtained in F2 generation, how many

of the offsprings would be pure homozygous?

(1) 60 (2) 30

(3) 120 (4) 40​

Answer 1

Answer:

(3) 120 is the ans answer of this question

Answer 2

2 out of 16 have the pure homozygous condition. Therefore 1/8th of 240, i.e., 30, is the correct answer.

Dihybrid cross is simply the cross between two pure species involving two pairs of genes.

In the Dihybrid cross, they were using pea color and shape as examples. The first step is to find all possible allele combinations.

From the monohybrid cross, we know that genotypically, one or two Y alleles will turn the pea yellow phenotypically while genotypically (yy) will lead to the green color of pea phenotypically. However, two alleles determine the shape of the pea. They can be round or wrinkled. The genotypically dominant allele R can turn the pea into a round shape. The recessive gene r will cause the pea to be wrinkled phenotypically.

So if you cross a homozygous dominant round yellow plant, genotypically (RRYY), with a homozygous recessive, wrinkled green plant, genotypically (rryy), the gametes will be RY and ry. All the plants phenotypically will be round and yellow, but they will all be heterozygous for both the characteristics genotypically (RrYy).

So if then we cross two F1 plants, the possible gametes will be RY, Ry, Ry, and very genotypically. F2 generation will be more mixed; there will be a 9:3:3:1 ratio of round yellow to wrinkled yellow to round green to wrinkled green, respectively. Dihybrid crosses are used to predict two traits at the same time.

After observing this, we obtain the following ratio

1:2:1:2:4:2:1:2:1 (1 RRYY : 2 RRYy : 1 RRyy : 2RrYY : 4 RrYy : 2Rryy : 1rrYY : 2 rrYy : 1 rryy respectively)

Hence 2 out of 16 have the pure homozygous condition. Therefore 1/8th of 240, i.e., 30, is the correct answer