Some alleles act in such a way as to bring about variable effects, depending on several factors, including the rest of the animal’s genetic make-up. One recessive gene causes the condition known as Collie Eye Anomaly (CEA), seen in sheepdog breeds such as the Rough Collie, Shetland Sheepdog and, to a lesser extent, the Border Collie. Dogs carrying the combination cea cea exhibit the disease, which may vary from total blindness with detached retinas to almost unimpaired vision but with an abnormal eye picture when seen through an ophthalmoscope. The normal dog (CEA CEA) and the carrier (CEA cea) will have correct vision.
Another type of inherited disorder is caused by incomplete penetrance, in which a particular gene, usually dominant, fails to express its presence in the phenotype, as in Centralized Progressive Retinal Atrophy (CPRA) in Labradors. This eye disease is thought to be so dominant that the presence of only one allele, CP, will cause impaired vision. In most cases CP CP and CP cp animals are affected and only cp cp animals have unimpaired vision. However in some 20 per cent the CP cp heterozygote appears phenotypically normal, and the gene is therefore said to show 80 per cent penetrance.