Biological Systems MCAT Practice Exam

What is a characteristic feature of sex-linked traits?

1. They are only inherited from the father

2. They can be carried by one sex without being expressed

3. They exhibit complete dominance in all cases

4. They are always expressed in both sexes equally

The correct answer is: They can be carried by one sex without being expressed

Sex-linked traits are indeed characterized by the ability to be carried by one sex without being expressed. This is particularly true for traits that are located on the X chromosome. For example, if a male has a recessive trait on his single X chromosome, he will express that trait because he does not have a second X chromosome to mask it. However, females have two X chromosomes, so they can be carriers of a recessive trait on one X chromosome while still being phenotypically normal if the other X chromosome has a dominant allele. This characteristic of being a carrier without expressing the trait is crucial to understanding the inheritance patterns of sex-linked traits. The other options do not accurately describe the inheritance of sex-linked traits. Traits are not exclusively inherited from the father; they can be transmitted from either parent depending on the sex chromosomes involved. The claim of exhibiting complete dominance is also misleading, as many sex-linked traits follow a pattern of incomplete dominance or codominance rather than complete dominance. Lastly, sex-linked traits are not always expressed equally in both sexes; instead, they often have different manifestations based on the sex of the individual due to the presence of one or two X chromosomes.