Final answer:
The student's claim is incorrect. A person expressing a dominant trait for eye color could have a genotype of either homozygous dominant (bb) or heterozygous (Bb), assuming 'b' is the dominant allele.
Step-by-step explanation:
A student states, 'if a person expresses the dominant trait for eye color, then the person's genotype must be bb.' This statement is incorrect if the lowercase 'b' is meant to represent the dominant allele, because in genetics, uppercase letters usually represent dominant alleles, while lowercase letters represent recessive alleles. Additionally, assuming that 'b' is indeed the dominant allele for eye color, a person expressing the dominant trait could be either homozygous dominant (bb) or heterozygous (Bb). This is because the presence of at least one dominant allele ('b' in this case) is sufficient to express the dominant phenotype.
The concept of dominance indicates that the phenotype of an individual will express the dominant trait when at least one dominant allele is present. Therefore, a person with the dominant eye color could have a genotype of either 'bb' or 'Bb'. Being 'Bb' means the individual carries one dominant allele ('b') and one recessive allele ('B'), but will still express the dominant eye color phenotype due to the dominant allele overshadowing the effects of the recessive allele.