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.
Important info about sex-linked inheritance I mentioned above that a male cockatiel inherits one X chromosome from his father (let's call this chromosome "X1") and the other from his mother ("X2"). Each of these chromosomes can carry sex-linked mutations. The sex-linked mutations that a male inherits from his father will occupy X1, and the ones he gets from his mother will occupy X2. The important thing to remember here is that the mutations on each X chromosome are always inherited together. The mutations on X1 will travel together to the next generation (likewise for the ones on X2). Let's say that a lutino male and a cinnamon pearl female have one male chick. One of the chick's X chromosomes (call it "X1") will carry lutino (from his father), and the other ("X2") will carry cinnamon and pearl (from his mother): X1: lutino X2: cinnamon pearl If this male is mated to a grey female, these are the possible results: Male chicks: 50% grey split to cinnamon pearl (if they inherit X2) 50% grey split to lutino (if they inherit X1) Female chicks: 50% cinnamon pearl (if they inherit X2) 50% lutino (if they inherit X1) Note that it is not possible for any of the female chicks to be just pearl, just cinnamon, or lutino pearl. She is limited to the combination of mutations on the single X chromosome she inherits. In some cases, genetic crossovers can occur which switch the placement of sex-linked mutations. Crossovers occur in individual gametes when the male produces sperm. In the example above, it would be possible for the two X chromosomes to switch some genetic material and thus swap mutations, leaving, for example, pearl on X2 and transferring cinnamon to X1 (meaning that X1 would then carry both cinnamon and lutino). The affected gamete would then produce either a cinnamon lutino hen or a pearl hen (if the resulting chick were male, it would be split to pearl or cinnamon lutino). If you know a male's parentage and find that his chicks carry unexpected combinations of sex-linked mutations, a crossover is the cause. The rate of crossovers seems to vary from mutation to mutation, but it can be as high as 30%. If this doesn't quite make sense, you'll find a more in-depth explanation on the North American Cockatiel Society site (with a section on crossovers here). A note on pastelface: Pastelface is sometimes called "dominant," but it is recessive to normal grey. It does, however, have a relationship with the whiteface mutation that makes it dominant to whiteface. The two mutations affect the same gene, and a bird with one copy of the whiteface mutation and one copy of pastelface will look exactly like a bird with two copies of pastelface (although, in genetic terms, the bird is still split to both whiteface and pastelface). There are a few things to keep in mind regarding whiteface and pastelface. Because the two mutations affect the same gene, a bird can only have one of the following combinations of the two mutations:
Pure pastelface (two copies of the pastelface mutation) Pure whiteface (two copies of the whiteface mutation) Split to whiteface and pastelface (one copy each of the pastelface and whiteface mutations) Split to whiteface (one whiteface mutation and one normal gene) Split to pastelface (one pastelface mutation and one normal gene) ...and, of course, both copies of the gene may be normal.
Note that there is no way a bird can be a pure pastelface AND a pure whiteface. Remember, too, that a visual whiteface can not be split to pastelface, because pastelface always "shows through" whiteface. So, why does pastelface become visual when combined with whiteface? I think the best way to describe the relationship between whiteface and pastelface is to say that they are two forms of the same mutation. Both affect the same gene -- the one that controls lipochrome, or yellow/red pigment, production -- but they do it with different "intensities". Whiteface has the greatest intensity -- it reduces lipochrome production 100%. Pastelface is weaker -- it reduces lipochrome production by about 50%. When a tiel has one copy of the pf mutation and one copy of the wf mutation, both of its lipochrome-production genes are affected -- one of the genes is switched off completely by the whiteface mutation, and the other gene is turned down halfway, so to speak, by the pastelface mutation. Since the tiel has no normally functioning lipochrome-production genes, it will not look like a normal grey; it's still got one of the genes working at 50%, though, so it will produce reduced-intensity yellows and reds -- and it will look like a pure pastelface tiel (one that has two copies of the pf mutation).