Dominant and Recessive Traits

Genetic variations in characteristics, for example coat length, are called dominant if one copy of them is needed to show its effect, and recessive if two copies, one on each chromosome in a pair, are needed.

Original traits tend to be dominant, and new mutations recessive: cats originally had short coats, and the gene for it is notated L, but a mutation occurred long ago, producing a recessive gene for long hair, which is notated l.

A cat showing a dominant trait may be heterozygous, carrying the recessive alternative "masked" beneath the dominant one: a cat with a recessive trait must be homozygous for it, carrying no alternatives.

Two heterozygous shorthaired cats - both LI, carrying the recessive gene for long hair – produce, if mated, an average of two one LL kitten, and one longhaired ll kitten. Appearance gives no clue as to which of the three shorthaired kittens carry the l gene that enables them to produce longhaired kittens.

Unfortunately, not all genetics is this simple. Many traits are polygenetic, governed by an unknown combination of genes. Also, not all genes are simply dominant or recessive; some mutations show partial dominanance over others, or even over the original form.