Advanced Placement (AP) Biology Practice Exam

What is a key distinguishing feature of sex chromosomes?

1. They are always diploid

2. They determine genetic traits

3. They include X and Y chromosomes

4. They consist only of autosomes

The correct answer is: They include X and Y chromosomes

A key distinguishing feature of sex chromosomes is that they include X and Y chromosomes. In many organisms, particularly in humans, these chromosomes play a crucial role in determining the sex of an individual. The presence of two X chromosomes typically indicates a female, while the presence of one X and one Y chromosome indicates a male. This chromosomal pair is responsible for the inheritance of sex-related traits and can determine various biological characteristics associated with gender. The option stating that sex chromosomes are always diploid is misleading; while they are part of a diploid organism, sex chromosomes themselves consist of one X and one Y in males (XY) or two Xs in females (XX), thus making the distinction in their role unique to sex determination rather than just being categorized as diploid. While sex chromosomes can affect genetic traits, asserting that they "determine genetic traits" is too broad. Traits can be influenced by many other chromosomes and genetic mechanisms beyond just the sex chromosomes. Therefore, while they are involved, this option does not precisely pinpoint their unique distinguishing feature. Lastly, stating that they consist only of autosomes is incorrect since autosomes are non-sex chromosomes, and sex chromosomes are a separate category altogether. Therefore, the definition and role of sex chromosomes are uniquely characterized