Question

Cross between homozygous black rough (BBRR) guinea pig and homozygous white smooth guinea pig (bbrr) produced black and rough animals in the F1 generation. Presuming the genes to be present on different chromosomes, the percentage of F2 individuals which are heterozygous for both the gene pairs would be
A. $25\%$
B. $35\%$
C. $50\%$
D. $75\%$

Answer 1

The first generation resulting after a cross is known as the first generation (F1 generation) and F2 generation is the generation resulting from selfing or intercrossing of first-generation. P1 generation is the parental generation used in the cross. Since the genes are present on both chromosomes, the genes follow independent assortment and don’t show any linkage.

Complete answer: Crossing between the parent organisms BBRR and bbrr:

Gametes: BBRR bbrr	BR	BR
br	BbRr	BbRr
br	BbRr	BbRr

So the F1 generation formed is of genotype BbRr and the organism formed is a black rough guinea pig.
When the guinea big obtained in F1 generation is crossed :

Gametes: BbRr BbRr	BR	Br	bR	br
BR	BBRR	BBRr	BbRR	BbRr
Br	BBRr	BBrr	BBRr	Bbrr
bR	BbRr	BbRr	bbRR	bbRr
br	BbRr	Bbrr	bbRr	bbrr

In the F2 generation, 9 black rough guinea pigs,3 smooth black guinea pigs, 3 smooth white guinea pigs, and one white smooth guinea pig are obtained.
So the number of guinea pigs that are heterozygous for both gene pairs (BbRr) is $4/16$ which is equal to$\;1/4$which is $25\%$.
So the correct option is A.

Note: The cross carried out here is a dihybrid cross. A dihybrid cross allowing us to look at the pattern of inheritance of two different traits at the same given point in time. The $9:3:3:1$phenotypic ratio is the classic ratio for the dihybrid cross.