Sex linkage refers to the inheritance pattern that results from a mutation occurring on a gene located on a sex chromosome. A classic example of a sex-linked trait in humans is red-green color blindness; females can only be red-green colorblind if they have two copies of the mutation (one on each X chromosome), while a single copy of the mutation is sufficient to confer colorblindness in males (since males only have one X chromosome).

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In birds, females are the heterogametic sex (with sex chromosomes ZW) and males are the homogametic sex (with sex chromosomes ZZ). A commonly known sex-linked trait in domestic chickens is the rapid feath- ering trait, which is carried on the Z chromosome. Chickens with the rapid feathering trait grow feathers at a faster rate; this difference is especially pronounced within the first few days from hatching. The wild-type allele K is dominant to the mutant alelle K+; presence of the K- allele produces slow feathering. Females can be either genotype ZK* W or ZK W. Males can be either heterozygous (ZK+ZK), homozygous for slow feathering (ZKZK), or homozygous for rapid feathering (ZK+Zk+). In a population of chickens, 9% of males are rapid feathering and 16% of females are rapid feathering. Suppose that slow feathering chickens are mated. What is the probability that out of their 12 offspring, at least two are rapid feathering?