USA Biology Olympiad (USABO) Practice Exam

Which type of inheritance pattern is exhibited by color-blindness?

1. Autosomal dominant

2. Autosomal recessive

3. X-linked recessive

4. X-linked dominant

The correct answer is: X-linked recessive

Color-blindness is primarily caused by mutations in genes located on the X chromosome, which is why it is classified as an X-linked trait. In terms of inheritance pattern, this means that the gene responsible for color-blindness is found on the X chromosome, and because males have only one X chromosome (XY) while females have two (XX), the expression of this trait differs between genders. In males, if the single X chromosome carries the mutation for color-blindness, they will express the trait since they do not have a second X chromosome to potentially carry a normal version of the gene. In contrast, females would need to have mutations in both of their X chromosomes to be color-blind; if only one of their X chromosomes carries the mutation, they would typically be carriers without expressing the condition. This pattern of inheritance also explains why color-blindness is more prevalent in males than in females. Females can be carriers without showing symptoms because their second X chromosome can often have a normal allele. The trait is recessive because the normal allele can mask the expression of the mutated allele when present. Thus, the mode of inheritance for color-blindness is correctly identified as X-linked recessive.