Question

Below is a pedigree of a fairly common human hereditary trait in which the boxes represent males and the circles represent females. Shading symbolizes the abnormal phenotype. Given that one gene pair is involved, what is the inheritance pattern?

1) Autosomal dominant
2) Autosomal recessive
3) Recessive lethality
4) X-linked dominant
5) X-linked recessive

Answer 1

The correct option is 5. To identify a human hereditary trait's inheritance pattern from a pedigree, one must distinguish if more males than females are affected indicating an X-linked recessive pattern. Affected males will have unaffected sons and possibly carrier or affected daughters. The correct inheritance pattern based on this description is X-linked recessive.

Step-by-step explanation:

To determine the inheritance pattern of a human hereditary trait using a pedigree, several key observations are necessary. Autosomal dominant traits will be present in every generation and affected individuals may have unaffected offspring. Autosomal recessive traits will sometimes skip generations, and affected individuals are often born to unaffected parents. For X-linked dominant traits, affected males pass the trait to all their daughters and none of their sons, whereas X-linked recessive traits can be more common in males with mothers who are carriers.

X-linked recessive inheritance is identifiable in pedigrees when more males than females are affected, and it does not skip generations. Affected males have unaffected sons and carrier or affected daughters. Females can be carriers and typically unaffected, unless they receive two recessive alleles. Considering the example of a male with a normal phenotype and a female who is a carrier for an X-linked recessive disorder, the male offspring have a 50% chance of being affected, while the female offspring will be carriers or unaffected usually.

Given a pedigree where squares represent males, circles represent females, and shading symbolizes the abnormal phenotype, you would look for a pattern where affected males have unaffected sons and carrier or affected daughters to suggest X-linked recessive inheritance. If the pedigree shows that affected individuals appear only in the male members and are absent in the females, it would strongly indicate an X-linked recessive pattern. Therefore, based on the information provided, the correct option would be 5) X-linked recessive.