Final answer:
The absence of Creeper homozygotes in a flock is likely due to the Creeper allele being recessive lethal. This means that Creeper homozygotes do not survive, leading to a genotypic ratio among survivors of 2:1 (heterozygotes to normal), rather than the expected Mendelian 3:1 ratio.
Step-by-step explanation:
The lack of Creeper homozygotes in the flock can be explained by the fact that the Creeper allele may be recessive lethal. In this case, the presence of two Creeper alleles (homozygous) would result in death before birth, and thus these individuals would not be present in the flock. Therefore, only those with one Creeper allele (heterozygous) and those with no Creeper alleles (normal) survive.
When heterozygous individuals are crossed and one of the alleles is recessive lethal, we typically see a phenotypic and genotypic ratio that diverges from the expected Mendelian 3:1 ratio, leading instead to a 2:1 ratio among surviving offspring. This inheritance pattern, known as recessive lethal, means that while the dominant wild-type allele sustains life, the presence of two copies of the lethal allele does not allow the organism to survive, even in utero.
Because 66 chicks are heterozygous and 32 are normal, and no homozygous lethal chicks are present, it is evident that the recessive lethal pattern is the reason behind the observed genotypic ratio. Hence, option A, where the Creeper allele is recessive lethal, would be the correct explanation for the lack of Creeper homozygotes.