For example, a dog's size as an adult is determined partly by his or her genes, but is also influenced by environmental factors such as its health as a puppy and the food it eats. Each member of a gene pair is called an allele. A gene can have many alleles within a population but an individual animal will have only 2 alleles which influence a particular trait. Upper case letters are traditionally used to represent dominant traits, lower case letters for recessive traits. Thus for a dominant trait, either AA or Aa will express the particular characteristic, while for a recessive trait only aa will express the characteristic.
P represents the dominant allele, and p the recessive allele.
Since PRA is a recessive trait, p is the affected allele, and P the normal allele. The genotypes PP and pp are homozygous. Dogs with the genotype PP have normal sight and those with pp are affected. Pp is heterozygous. These animal have normal sight but are carriers.