In chickens, the dominant Cr allele produces chickens with extremely short legs (called creepers). When a male creeper is mated with a female creeper, 2/3 of the progeny are creepers and 1/3 of the progeny are wild-type (with long legs). Explain how this is possible.

The male creeper and the female creeper are both heterozygous for the creeping trait.

Explanation:

The creeping allele Cr is dominant. For any chicken to have the wild-type long leg attribute, the dominant allele must be absent in its genotype. This means that such an individual would have two recessive alleles, each from either parent.

Hence, in order for two parents to produce wild-type offspring at all, both of them would have to be heterozygous for the creeping trait.

Assuming that the alternate trait is represented by cr allele:

           Crcr    x    Crcr

    CrCr   Crcr   Crcr   crcr

The two parents in the above cross are creepers but were able to produce wild-type offspring as a result of both parents being heterozygous.