If the allele for brown eyes (B) is dominant to the allele for blue (b), could two homozygous brown‑eyed parents have blue‑eyed children? 

Quick answer:

Two homozygous brown-eyed parents could not have blue-eyed children due to the fact that their brown-eye genes are more dominant than blue-eye genes. Homozygous indicates that there are two dominant brown-eye genes in each parent, therefore making them BB. Without the gene for blue eyes there would not be an option for blue-eyed children.

Expert Answers

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The answer is no, two homozygous, brown-eyed parents could not have a blue-eyed child. The reason is very simple. If the brown-eyed trait is dominant, that means that you only need to have one of the alleles to code for a brown-eye phenotype. Thus, a person with either the BB or Bb genotype will have the brown-eyed phenotype. The only way for a person to have the blue-eyed phenotype is to be homozygous recessive with a genotype of bb.

Now the question states that both parents are homozygous (meaning they have two of the same alleles for the trait), and that they are homozygous for the brown-eyed trait. This means that both the mother and the father’s genotype is BB. You may already be able to see why the answer is no, even without a Punnett square. If both parents have the BB genotype, that means that there is no little “b” allelewhich codes for blue eyesto pass on to the next generation. However, we can chart this by using a Punnett square.

BB x BB = ¼ probability of getting either BB, BB, BB, or BB.

Thus, you can see that the only allele being passed down is the big “B” allele, which codes for brown eyes. There is thus a 100% chance that the child will have at lease one big “B” allele (and, in fact, a 100% chance that they will actually have two, and thus have the BB genotype). Because brown eyes is a dominant trait, you only need one big “B” allele to display it. It is therefore impossible for the child to have blue eyes.

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Could a homozygous brown‑eyed man and a blue‑eyed woman have a blue‑eyed daughter? 

The answer to your question is no. A homozygous, brown-eyed man and a blue-eyed woman can't produce a blue-eyed child of either sex. The reason that these parents can't produce a blue-eyed child is due to the fact that the father is homozygous for the brown-eyed trait. This means that he is carrying two dominant genes for brown eyes. Another way of saying this is that the man is a purebred brown-eyed person. He does not carry the gene for blue eyes. The only eye color gene that he can donate is the brown eyed gene. Genotypically, we would write that the man is "BB." A capital "B" stands for the brown-eye gene, and a lower case "b" stands for the blue eye gene. The woman's phenotype is blue eyes. Her genotype is "bb." No matter what, her egg will contain the blue eyes gene. The man's phenotype is brown eyes. Genotypically, that could be written as "BB" or "Bb," however, the stated cross indicated that the man is homozygous. This means that he carries identical versions of the gene. His sperm will always contain a "B" gene for brown eyes. What is guaranteed in this cross is that every child possible will be heterozygous (Bb). The children will be brown eyed, but they will all carry the recessive blue eye gene. It simply won't show in the phenotype.

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