The pedigree is for a condition that shows complete penetrance. Which individual's phenotype definitively rules out X-linked recessive inheritance?
- Individual II-2
- Individual III-1
- Individual III-2
- Individual III-4 (correct answer)
Explanation: To rule out X-linked recessive (XLR) inheritance, we need to find a contradiction. Let's analyze the key individuals. For XLR, an affected female must have the genotype XaXa. To have this genotype, she must inherit one Xa from her mother and one Xa from her father. This means her father must have the genotype XaY and thus must be affected. In this pedigree, individual III-4 is an affected female. Her father is individual II-4, who is shown as unaffected. An unaffected father cannot have an affected daughter under XLR inheritance. Therefore, the existence of affected female III-4 with her unaffected father II-4 definitively rules out the X-linked recessive mode. Other individuals are consistent with XLR. For example, II-2 (affected male) could have inherited Xa from a carrier mother I-2.