Question

Blue eye color in humans is recessive to brown eye color. The expected children of a marriage between blue-eyed woman and brown-eyed male who had a blue-eyed mother are likely to be
A. All blue-eyed
B. Three blue-eyed and one brown-eyed
C. All brown-eyed
D. One blue-eyed and one brown-eyed

Answer 1

Hint: An allele is one of two or many forms of a given gene. A recessive allele is the one that can express itself when present only in homozygous condition while the dominant allele is the one that can express itself when present in either homozygous or heterozygous condition.

Complete answer:
Let us suppose that ‘B’ represents the brown color and ‘b’ represents the blue color. In the question, it is written that blue-eye color in human is recessive to brown eye color which means the blue color will only express itself when its alleles are in homozygous condition i.e., “bb” and brown color will express itself when its alleles are in either homozygous or heterozygous condition i.e., “BB” or “Bb”, respectively.
Also, it is written in the question that the brown-eyed male had a blue-eyed mother which means that the brown-eyed male has a “Bb” genotype, as one of its allelic pairs comes from its mother who has blue eyes i.e., “bb” genotype.
So, we have to show a cross between blue-eyed woman i.e., “bb” genotype and brown-eyed male i.e., “Bb” genotype i.e.,$Bb \times bb$

♀(female)↓ ♂(male)→	B	b
b	Bb (brown eye color)	bb (blue eye color)
b	Bb (brown eye color)	bb (blue eye color)

Thus, the genotypic as well as the phenotypic ratio of the offsprings is $1:1$ which implies that out of the two offsprings one will be blue-eyed and another one will be brown-eyed.
Option (A) is incorrect. After crossing a blue-eyed woman with a brown-eyed male whose mother was
blue-eyed, the phenotype of offsprings came out to be$1:1$ . Thus, the children were not likely to be all blue-eyed.
Option (B) is incorrect. The ratio of offsprings came out to be$1:1$and not$3:1$.
Option (C) is incorrect. After the cross, it was observed that$50\%$offsprings were blue-eyed and other$50\% $offsprings were brown-eyed.
Option (D) is correct. The cross between a blue-eyed woman and brown-eyed male whose mother was blue-eyed produced offspring in the ratio$1:1$i.e., one offspring was blue-eyed and the one was brown-eyed.
Hence, the correct answer is option (D).

Note: The eye color gene consists of two allelic forms i.e., blue-color (recessive) and brown-color (dominant). The offspring produced after a cross between a brown-eyed male and blue-eyed female were blue-eyed and brown-eyed in the ratio$1:1$.