Question

The allele for the black coat colour (B) is dominant over the allele for white coat colour (b) in horses.
A cross between two black horses produces a foal with white coat colour. The probability of the second foal having black coat would be:
(a) 0
(b) ¼
(c) ½
(d) ¾
(e) 1

Answer 1

Hint: If black coat colour gene B is dominant over white coat colour gene
b and if two black horses produce a white foal, then the parent horses’ genotypes must be Bb and Bb.

Complete answer:Allele for black coat colour- B
Allele for white coat colour-b
Allele B is dominant over allele b i.e. B>b.
If a cross between two black coloured horses produces one white coloured foal, its genotype must be bb (because allele b is recessive) and both the parent horses’ genotypes must be heterozygous i.e. Bb (expressing black coat colour phenotype).
If we want to know about the genotypes of the other offsprings (foals), we have to look at the complete cross.

Gametes	B	b
B	BB(Black coat colour)	Bb(Black coat colour)
b	Bb(Black coat colour)	bb(White coat colour)

From the above cross (put into a Punnet’s square), we can see that the phenotypic ratio of black coat colour: white coat colour = 3:1
So, the probability of producing a black coloured foal is 3 out of 4 i.e. ¾.

Hence, the correct option is ‘ ¾ ’

Note:Here, coat colour means simply the colour of the horse or the colour of
the fur/hair.
The genotypic ratio obtained from the cross is- BB: Bb: bb = 1:2:1.