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.

Simply-Inherited Traits
A simply-inherited trait is one affected by one or a very small number of genes, such as polling (without horns) in cattle or coat colour in Labradors. Genetic diseases, eg Huntington’s Disease in humans, are invariably simply-inherited as well.

Phenotypes of these traits tend to be ‘either/or’ and can be placed in categories. Cattle either have horns or they don’t. Labs are either black, chocolate or yellow. People either carry the gene for Huntington’s or they don’t. Simply-inherited traits can also be called qualitative or categorical traits for this reason.

These traits are also not influenced much by environment.

Polygenic Traits
A polygenic trait is one affected by many genes, with no one gene overly influential. Rather, all genes involved contribute in some way to a final phenotypic expression. Examples of such phenotypes include growth rate, milk production, and meat yield. Unlike the ‘either/or’ of the phenotypes of simply-inherited traits,  the phenotypes of polygenic traits are often expressed numerically: 0.3 kg/day, 6,000 L/year, 71.6%. Thus these traits are usually quantitative or continuous.

And unlike simply-inherited traits, polygenic ones are influenced by environment. Lack of feed will result in lower growth rates and milk production for example.

Exceptions
While simply-inherited traits are usually qualitative (categorical), and polygenic traits are usually quantitative (continuous), there are exceptions.

Though rare, there are simply-inherited traits which are continuous. One example is human skin pigmentation, of which there is a continuum from very light-skinned to very dark-skinned.

And there are polygenic traits which are categorical. One example is dystocia, or difficulty birthing. Birthing is either assisted or unassisted, and this ‘either/or’ nature implies a simply-inherited trait. Yet dystocia is influenced by many genes including those determining the size of the foetus, the size of the pelvic opening, and the mother’s stamina.

Another example is mastitis in cows, where many genes influencing udder size, milk yield and milk flow play a part. But a cow either has mastitis or she doesn’t — there is no spectrum of affectedness.

Threshold Traits
Dystocia and mastitis are examples of threshold traits, polygenic traits that exhibit categorical phenotypes. Fertility is another example: ability to conceive is either successful or it isn’t.

Having covered the differences between simply-inherited and polygenic traits here, next week we’ll go over the properties they share in common.