While — for the most part — there are differences between simply-inherited and polygenic traits, they also share much in common. Both types of trait are still determined by genes and inheritance.

Our discussion on Mendel and his three laws focussed on simply-inherited traits, but that was because these are easier to follow and understand, and confirm in real life with visual observation. Determining the genes behind polygenic traits, much less unravelling their influence in a final phenotype, is much harder to understand, and especially so if the effect of each gene is small. But they are still genes which occupy chromosomes, and are thus as subject to Mendel’s laws of segregation and independent assortment as the simply-inherited ones.

Of course, with so many genes involved in a polygenic trait, it is possible that several of these will share the same chromosomes, and that several of these again will be closely linked. But this is an exception to the law of independent assortment, and the chromosome homologues will still follow the law of segregation.

Mendel’s law of dominance will also apply to both groups, as will epistasis and other types of non-Mendelian inheritance, but in more complex ways for the polygenic traits.

Selection systems will also alter gene frequencies in the same way for both simply-inherited and polygenic traits. Selection for the simply-inherited polled gene in cattle will increase the frequency of the desired ‘poll’ allele in a herd, just as selection for speed (a polygenic trait) in racehorses will favourably shift the frequencies of many genes over many loci that positively affect speed.

It follows that mating systems will also affect the genes of both trait types similarly. Heterozygosity at the (simply-inherited) B locus will increase should a poultry farmer cross black Andalusian chickens with white ones, just as heterozygosity will increase across many loci should two breeds be crossed to produce hybrid vigour.

Next week: an overview on the different approaches when breeding for simply-inherited and polygenic traits.