What is a mutation?
A mutation is a change in a piece of genetic material. Some mutations have little or no effect on the animal (or other living thing) whose genes they alter, while others can cause dramatic change or even be fatal. The mutations that cockatiel breeders are most concerned with are harmless, affecting only the colors in the birds’ plumage. These color variations occur because the mutations change the levels of melanin (which produces browns, blues, and greys) and lipochromes (which produce yellows and reds) in the birds’ feathers. The lutino mutation, for example, removes all melanin from a cockatiel’s plumage, and the whiteface mutation removes the lipochromes. A combination of these mutations would result in a pure-white bird.
How are the mutations inherited?
Genes come in pairs; a pair is composed of a gene from each parent. A mutation can affect both copies of the gene, only one copy, or neither. The different combinations of gene pairs alter the way in which the mutation affects the bird; the effects of these combinations are determined by whether the mutation is dominant, recessive, or sex-linked. A dominant mutation needs only to be present in one copy of a gene to change a bird’s appearance.
Normal grey (which is not considered to be a mutation, but the wild coloration of a cockatiel) is dominant to all recessive and sex-linked colors.
The two true dominant mutations are dominant silver and dominant yellowcheek. A bird with one dominant silver gene will appear silver, and is called a single-factor bird. A cockatiel with two copies of the gene is called a double-factor. The two forms can be told apart visually; a double-factor is much paler than a single-factor (think of the double-factor as having inherited a double dose of melanin reduction). Dominant yellowcheek is inherited in the same way as dominant silver, but single-factor and double-factor birds cannot be visually told apart.
A recessive mutation must affect both copies of a gene in order to change the bird’s appearance; a visually whiteface cockatiel must have inherited a copy of the “whiteface” gene from each of its parents. A bird that possesses only one copy of a recessive gene is called a split, and will pass that mutation on to half of its offspring. Most splits will not show any sign of the hidden mutation, although cockatiels that are split to pied will often have a patch of yellow feathers on the backs of their necks.
A sex-linked mutation is one that is carried on one of the sex chromosomes. When dealing with sex-linked mutations in birds, it is important to note that humans’ and birds’ sex chromosomes do not work in the same way. While a human female is homozygous (which means that she has two copies of the same sex chromosome — “XX”) and a male human is heterozygous (“XY”), it is the other way around in birds; female birds are heterozygous, and males are homozygous. This means that females can have only one copy of a sex-linked mutation (the mutation is carried on the X chromosome), and it follows that females cannot be split to a sex-linked mutation. If a female does not visually possess the sex-linked trait, she does not carry it at all.