Genetic susceptibility

A certain number of inherited disorders are limited to specific breeds. Rebecca Hamilton-Fletcher MRCVS explains the what and why of such cases

- Rebecca Hamilton-Fletcher

IT is well-recognised that some breeds are more predisposed to certain conditions than others. Such conditions may have an inherited genetic basis, or be the result of selective breeding for highly desirable qualities, such as appearance or speed, at the cost perhaps of long-term soundness, for example.

A horse’s individual characteristics – such as size, colour or temperament – are dictated by his or her genetic constitution, or “genotype”.

In every cell of the horse’s body, there are about 22,000 protein-coding genes, consisting of specific segments of DNA. These are arranged on 32 pairs of chromosomes, one inherited from each parent.

“Alleles” are pairs of genes (again, one from each parent) which control the same trait, the final expression of which will depend on whether the allele is dominant or recessive.

Human blue eye-colour, for example, is a recessive trait and occurs only if both parents contribute a blue-eye allele. If one parent is brown-eyed however, the offspring is likely to have brown eyes, since the brown-eye allele is dominant.

TRIPLE TROUBLE

EQUINE genetic disorders encompass those health problems which occur when something goes wrong with the horse’s genotype – usually as a result of a mutation.

There are three types of genetic disorder. The most common are classic Mendelian or simple genetic disorders. There are two types:

• Autosomal recessive, such as cystic fibrosis in humans – both inherited alleles need to be mutant to be clinically affected.

• Autosomal dominant, for example Huntington’s disease in humans – only one of the inherited alleles needs to be mutant to be clinically affected.