GENETIC PRINCIPLES

This chart, which depicts the inheritance of recessive alleles, is very useful if you want to get a concise idea of why matings with known carriers of genetic diseases are best avoided.

A short list of genetic diseases of horses

(across all breeds) that are inherited as recessives and expressed only when the foal inherits two copies, “aa,” of the recessive alleles: HYPP (hyperkalemic periodic paralysis), HERDA (hereditary equine regional dermal asthenia), LFS (lavendar foal syndrome), CA (cerebellar abiotrophy), GBAD (glycogen branching enzyme deficiency),

SCID (severe combined immunodeficiency) and

JEB (junctional epidermolysis bullosa).

However, any deleterious phenotypic trait—such as thin shelly hoof walls, weak ankles, or retained placenta—is best regarded as charting exactly the same as recessive genetic diseases. Avoidance should be the rule even when the deleterious trait may be linked to (i.e. inherited together with) a desirable trait such as racing speed.

This chart brings out the fact that carriers of deleterious recessive alleles (colored red) can never be completely eliminated from a population, even by outcrossing with non-carriers (blue). Only when homozygous non-carriers are mated, as on the left- hand side of the chart, will the resulting gene pool be free of the deleterious allele.

Anytime when studying genetic formulae, such as represented on this chart, it is crucial to remember that only one pair of alleles— and thus only one set of phenotypic traits—is being considered. Pleiotropic effects and heterotic interactions between genes are not represented, while the number of allelic variants not pictured staggers the imagination. Charts such as this one are simplified models that help us to understand natural systems and to derive the principles by which they work, but as Sewall Wright himself said, “Organisms must be aviewed as vast networks of interacting systems.”