The presence or absence of horns in ruminant livestock species is determined by the polled gene. The polled allele (P) is dominant to the horned allele (p). What genotype (s) would result in an animal being polled (having no horns) ?

PP and Pp

Explanation:

The dominant allele "overpowers" the recessive allele, therefore that trait shows. In this case, it is the polled allele. Therefore, the heterozygous pair works. Also, the homozygous pair with only the dominant allele (polled) will of course work since that is the only gene.