Phenotype refers to the outward appearance of the structure. Phenotype may be the result of either genes or effects of the environment.
Genotype refers to the actual genes acting on that structure.
The most simple way in which genes operate is on a dominant-recessive basis. Usually the dominant gene codes for the normal condition while the recessive gene is regarded as a variant which codes for the abnormal condition. The genes are found in the cell nucleus on structures called chromosomes which occur in matching pairs. Pairs of genes have a very well defined location on each of a specific pair of chromosomes. This position on the chromosome is known as a locus. In the classic situation the locus can have either a dominant or a recessive gene. Where two of the same type of gene i.e. dominant or recessive are found on matching chromosomes the animal is said to be homozygous for that gene. Where the two different variants i.e. a dominant and a recessive gene, are found at the locus on each of the chromosome pair then the animal is said to be a heterozygote. Heterozygotes are sometimes called “carriers” i.e. they carry the faulty gene on one chromosome but because there is a dominant gene on the other chromosome the normal phenotype is expressed. The abnormal phenotype is only seen when the animal is homozygous recessive for the condition. (i.e. has two recessive genes on each of the pair of chromosomes). Occasionally the defect is suspected to be due to the effect of the dominant gene rather than the recessive gene in this case only breeding homozygous recessives together will prevent the abnormal phenotype from occurring.
There has been an explosion in our understanding of the molecular basis for many genetic diseases in man in the last 10-20 years. This explosion is now starting to occur in the canine world especially in our knowledge of genetic retinal diseases in dogs. Many retinal degenerations which only become apparent in middle age in susceptible breeds can now be identified by DNA testing at the puppy stage. In many of the diseases carriers can also be identified as well so this should make things easier in future. Unfortunately many ocular abnormalities still do not have an identifiable genetic cause. Therefore eye certification (i.e. examination by a qualified veterinary ophthalmologist) is still the only way we have at present to potentially identify breeding stock with a given abnormality and make decisions about whether they be bred or not, assuming the problem might be caused by a genetic defect. The effect of the abnormality on vision is probably the most important factor which is taken into consideration when a recommendation is made as to whether the animal should be prevented from breeding.
In the rest of this section are some of the more common abnormalities of the eyes, with some general remarks about their cause and possible heritability. These abnormalities are covered in more detail in Information on Eye Diseases.