In birds, males carry two identical sex chromosomes (ZZ) and females carry two different sex chromosomes (ZW).
If a harmful recessive mutation is sex-linked, which gender of bird is most likely to be affected?

The correct option is B Females
If a recessive mutation is sex-linked the heterogametic individual is prone to mutation. Sex-linked mutation will be expressed easily in females (who have only one Z chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In males (who have two Z chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that male will have two altered copies of this gene, it is very rare for males to have the diseased condition. Thus, the correct answer is option B.