Many human and horse genes are inherited in a Mendelian manner. We are usually unaware of their existence unless a variant form is present which causes an abnormal or different phenotype. We can follow the inheritance of the abnormal phenotype and deduce whether an allele is dominant or recessive.

·         Autosomal dominant

A dominant condition is transmitted in unbroken descent from each generation to the next. Most matings will be heterozygote (Aa) to homozygous (aa) recessive. We would therefore expect every child of such a mating to have a 50% chance of receiving the mutant gene and thus of being affected. Huntington’s disease is an example, progressive mental deterioration beginning in the 40’s. The age at which this appears is imprinted and depends on whether it is the maternal or paternal copy of the allele that is dominant. Using Mendel's peas Yellow is dominant to green. If a Y allele is inherited the offspring will always be yellow.

·         Autosomal recessive

A recessive trait will only manifest itself when homozygous. If it is a severe condition it will be unlikely that homozygotes will live to reproduce and thus most occurrences of the condition will be in matings between two heterozygotes (or carriers). An autosomal recessive condition may be transmitted through a long line of carriers before by ill chance two carriers mate. Then there will be a ¼ chance that any child will be affected. Cystic fibrosis is an example. A green pea allele is recessive. It will only be expressed if the offspring inherits an allele from both parents so the offspring is homozygous for this factor.

·         Incomplete dominance

A gene can also show incomplete dominance. When a true breeding black animal (AA) is crossed to a true breeding white (aa) and the first-generation offspring are grey-blue (Aa). Then when the offspring are mated to each other the second generation’s offspring consist of 1 black, 1 white and 2 grey-blue. Since the black and white reappear undiluted in the second generation the genes have not merged but have remained independent, despite the fact that in a heterozygote (Aa) they affect each other.

·         Co dominance

Both alleles are manifested in a heterozygote (Aa). An example of this is found in some human blood groups where both forms of the allele are found in the red blood cells of MN individuals. Both forms of the genes are expressed. Neither gene is dominant or recessive they exist side by side.

For autosomal traits generally it makes no difference how these will be expressed in offspring regardless of the sex of the parent and the sex of the offspring. This will be mentioned again under sex balancing. Only if the trait is imprinted will sex balancing make sense for an autosomal factor. To date there are no know horse traits known to be imprinted.

Three things to keep in mind when looking at pedigrees:

1. If a condition is common, this does not mean the allele is dominant. Extra digits (polydactyl, 6 fingers or toes) are rare but caused by a dominant allele. Just one copy inherited by an individual will mean the individual will display the trait.

2. If offspring differs from both parents the offspring is displaying a recessive condition since both the parents must have been carriers but it was not expressed. This is where for example where both parents are green but the offspring yellow.

3. If the parents are related to each other, perhaps by being cousins, there is an increased chance that any gene present in a child may have two alleles identical by descent. The degree of chance that both alleles of a pair in a person are descended from the same recent common ancestor is the degree of inbreeding of the person.

The above is not a complete list of the various forms of dominance and recessivity but they are the main ones that you need to understand before looking at the other forms.

It is important to understand that a gene is not termed “dominant” because it somehow subdues a recessive allele. When a dominant allele (A) coexists with a recessive allele (a) in a heterozygous genotype (Aa), they do not interact at all. It simply reflects the mechanisms by which specific alleles are expressed in the phenotype. It has nothing whatsoever to do with superiority and the frequency with which they may appear in a population.

As breeders you will often see comments like “he stamps his stock” as if this is an expression of superiority of a stallion’s genes. All it reflects is that the stallion may be homozygous (AA) or (aa) for that trait not that he has superior genes that somehow subdues less worthy genes. Whether this “stamping” is good or bad will also depend on the mare and whether the trait is important for the sort of horse being bred. The expression stamping of stock as such gives the wrong impression as it only reflects that an individual maybe homozygous for a particular trait. It is important only for giving you an increased probability of the trait being inherited by the offspring. It does not mean the stallion’s genes subdue some other less desirable gene carried by the mare.

The same idea applies if you want to avoid an undesirable characteristic in a mating.