In the gene locus for eye colour in humans, the allele for brown eye is dominant...

This question is asking about the inheritance of eye color in humans. The gene locus for eye color has two alleles: brown and blue. Brown eye color is dominant over blue eye color. The question tells us that a girl with brown eyes has a brother with blue eyes. We are then asked to determine the likely phenotypes of their parents.

To begin, we need to understand some basic genetics. Each person has two copies of each gene, one from their mother and one from their father. These two copies can be the same or different, and they are called alleles. In this case, the gene locus for eye color has two alleles: brown and blue.

If a person has two copies of the same allele, they are homozygous for that allele. If they have one copy of each allele, they are heterozygous. In the case of eye color, a person with two brown alleles will have brown eyes, while a person with two blue alleles will have blue eyes. A person with one brown and one blue allele will also have brown eyes, since brown is dominant over blue.

Now let's apply this knowledge to the question. We know that the girl is homozygous for brown eyes, meaning she has two brown alleles. Her brother has blue eyes, meaning he must have two blue alleles. Since they have different phenotypes, we know that their parents must be heterozygous for eye color, meaning they each have one brown allele and one blue allele.

Using Punnett squares, we can predict the possible genotypes and phenotypes of their parents. The father could contribute either a brown or blue allele, while the mother could also contribute either a brown or blue allele. The possible combinations are: BB (brown eyes), Bb (brown eyes), bB (brown eyes), and bb (blue eyes). The only combination that would result in a homozygous brown-eyed child and a blue-eyed child is if both parents are heterozygous (Bb) for eye color. Therefore, the likely phenotypes of their parents' eye color are both brown.

In summary, if a homozygous brown-eyed girl has a brother with blue eyes, it is likely that both of their parents have brown eyes, with each parent carrying one brown allele and one blue allele.