- To select for a simply-inherited trait requires knowing just three things: the number of loci involved (often just one), the number of alleles at each locus (usually a small number), and the genotypes or possible genotypes of the parents-to-be (again typically a small number). In the case of a simply-inherited trait that is partially dominant, such as Andalusian chicken colour, all three pieces of information are known. There is just one locus (B), two alleles (’B’ and ‘b’), and three genotypes easily identifiable by eye (’BB’, black; ‘Bb’, slate blue; and ‘bb’, white).
- As mentioned, simply-inherited and polygenic traits are equally subject to the same Mendelian and non-Mendelian inheritance forces. And both can have gene and genotypic frequencies shifted by selection and mating systems. But while it is often straightforward to observe the effect of a simply-inherited trait owing to the small number of genes involved, this isn’t the case with polygenic traits. It is often not even known how many genes are involved in a particular polygenic trait, nor what the effect of each may be. It is because of this complexity that breeders must take very different approaches when working with simply-inherited and polygenic traits.
- 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.
- The word trait, you may recall, is often used interchangeably with phenotype, but they are not the same thing at all. A trait is something that can be measured or observed, for example temperament, colour or wool staple length/year. A phenotype is the value of the trait: ‘aggressive’, ‘brindle’ or ‘120mm’. Traits fall into two categories: simply-inherited and polygenic.