Question

In humans, brown eye color (B) is dominant over blue eye color (b). A brown-eyed woman and a brown-eyed man mate and have a blue-eyed son as their first child. What are the most likely genotypes of the parents for the eye color gene in question

Answer 1

Answer:

the genotype for the man is Bb and that of the woman is Bb

Explanation:

Blue eye colour (b) in human is recessive to brown eye colour (B), thus blue eyes(b) will express itself in homozygous condition only (bb). The dominant "B" allele can express itself in both homozygous(BB) and heterozygous conditions(Bb). Since the man is browned eyed and the woman is browned eyed and they have a child which is blue eyed.

For the child to be blue eyed (that is bb), the genotypes of the parent which are both brown eyes must be Bb. That is the genotype for the man is Bb and that of the woman is Bb

Answer 2

Answer:

In the case of these two parents, a quarter of their children would have blue eyes.

Explanation:

It is not only possible, it is expected that this would happen an appreciable fraction of the time.  Brown eyes are dominant.  Suppose eye color were dependent on a single Mendelian gene with two alleles, Br for Brown eyes and Bl for Blue eyes.  (As Tracey Bryan has explained, this is not true -- I want to explain what the answer would be even in the simple case.)  This means the three possible combinations of two alleles give the following eye colors:

BrBr => Brown

BrBl => Brown

BlBl => Blue

Now suppose two brown-eyed people have a child.  We can't tell from the fact that they have brown eyes which pairs of alleles they have.  Each of them could have either BrBr or BrBl.  Suppose they each have BrBl.  When they are crossed (i.e., they have a child), the child gets one allele from its mother -- it has a 50-50 chance of getting either one -- and one allele from its father -- again it has a 50-50 chance of getting either one.  So, the child has:

25% chance of getting Br from mother and Br from father => brown eyes;

25% chance of getting Br from mother and Bl from father => brown eyes;

25% chance of getting Bl from mother and Br from father => brown eyes;

25% chance of getting Bl from mother and Bl from father => blue eyes.

So in the case of these two parents, a quarter of their children would have blue eyes.