Final answer:
A recessive allele expresses its phenotype only when the individual has two copies of that allele, meaning it is only visible in the homozygous state. Heterozygous individuals, with one dominant and one recessive allele, will not show the recessive trait. Recessive alleles can be located on any chromosome, not just the X chromosome.
Step-by-step explanation:
The correct answer to the question is a) It expresses its phenotype only in the homozygous state. This means that a recessive allele shows its specific physical or observable trait when an individual has two identical recessive alleles (homozygous), such as 'aa'. If the individual has one dominant and one recessive allele (heterozygous), like 'Aa', the dominant allele will mask the expression of the recessive allele. Therefore, a recessive trait will not be seen in the heterozygous state.
An example of this can be seen with the ABO blood group system. For a person to have blood type O, both alleles must be 'O' (recessive). If they carry one allele for blood type A and one for type O, the dominant 'A' allele will be expressed, resulting in blood type A.
Another important aspect:
While recessive alleles are not always located on the X chromosome, it is important to understand that X-linked recessive traits are indeed expressed differently in males and females due to the presence of only one X chromosome in males. However, the statement that recessive alleles are found on the X chromosome is not universally true, as recessive alleles can be found on any chromosome.