Question

In guinea pig, black short hair(BBSS) is dominant over white long hair(bbss). In the dihybrid cross, F2 genotypes BBSS, BbSS, BBSs and BbSs appear in the ratio of
A)9:3:3:1
B)4:2:1:2
C)1:2:1:4
D)1:2:2:4

Answer 1

Hint: Guinea pig is a rodent that belongs to the family Caviidae and unlike their name are not native to Guinea. It not only serves as an important food source but also plays an important role serving as model organism in biological experiments.

Complete answer:
The dihybrid crossing system was introduced by Mendel by his experiments. It is a cross between two different genes that differ in their observed traits. The rules of Mendel’s that apply to the dihybrid cross are Law of Segregation and Law of Independent Assortment.

For genes which are located on different chromosomes the allele pairs showed independent assortment. In the first filial generation( F1) four identical offspring were produced which in this case would be BbSs then the offspring produced in the first generation are crossed leading to more varied offspring in the second filial generation(F2). The offspring produced in the F2 generation can be easily observed by means of a Punnett square given below:

	BS	Bs	bS	bs
BS	BBSS	BBSs	BbSS	BbSs
Bs	BBSs	BBss	BbSs	Bbss
bS	BbSS	BbSs	bbSS	bbSs
bs	BbSs	Bbss	bbSs	bbss

So the genotypic ratio of BBSS:BbSS:BBSs:BbSs is 1:2:2:4.

Hence, the correct answer is option (D)

Note: Dihybrid crosses are important not only to demonstrate the dominance of different alleles by giving different phenotypic observations but also to get an idea about the relationship between different allelic pairs.