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, and the mutations on each X chromosome are always inherited together, so 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, according to this model, 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%.

The word “parblue” refers to the pastelface-creamface-whiteface series of mutations, which reduce the level of yellow pigment in the cockatiels’ plumage.  Many wild-type parrots have green plumage due to a combination of feather structure and pigments; melanin and structure produce blue feathers, which combine with yellow pigments to create green plumage.  A total lack of yellow pigments, then, results in a bird that is blue rather than green, and a reduction of yellow pigment creates varying shades of bluish-green, thus the term “partial blue,” or parblue.

Cockatiels’ feathers lack the structural features needed to produce blue, and yellow/red pigments tend to be confined to the face, so the mutations that would cause a blue or partial-blue appearance in other parrots instead produce a grey-bodied bird with a white or pale-yellow face.

Although “reduced-yellow” describes the actual appearance of these cockatiels better than “parblue,” it’s better to stick with a term that’s accepted by parrot breeders instead of making up a label that no has heard of.  If the experts can agree on a better label for these mutations, the Virtual Breeder’s terminology will be updated.

There are three mutations in the parblue series: pastelface, creamface, and whiteface.  All of these mutations reduce yellow and red pigments to different degrees; pastelface reduces the appearance of these pigments by about 50%, creamface by about 90%, and whiteface eliminates these pigments completely (these percentages are just estimates).  Because all of these mutations affect the same gene, which will be called the “parblue gene” in this program for the sake of clarity, they interact differently than other mutations, which can be very confusing for breeders who are just learning about genetics.

As you learned in the genetics lesson, autosomal (non-sex-linked) genes come in pairs, and the parblue genes are autosomal.  This means that a bird can carry a maximum of two mutations from the parblue series: one creamface and one pastelface mutation, or two whiteface mutations, or one normal gene and one pastelface mutation, just to name a few possibilities.  As long as a bird carries two mutated copies of the parblue gene — even if the two mutations are not the same — the bird will not be normal grey.  Instead, the bird will have the appearance, or phenotype, of the “yellowest” of the two mutations it carries.  A bird carrying one pastelface and one creamface mutation, then, will look exactly like a pastelface; a bird with one whiteface and one creamface mutation will look like a cream face.

If the male you enter is split to one or more sex-linked mutations, the breeder will let you specify which of the male’s two X chromosomes (X1 and X2, as they are named here) contain these mutations (this applies only to splits, since a visual sex-linked mutation occupies both chromosomes). For the sake of order, let’s say that X1 is the chromosome he inherited from his father, and X2 is the one he got from his mother. If he only has one sex-linked mutation, it doesn’t matter whether you put it on X1 or X2 — the results will be the same. Of course, if you know that your male is the result of a crossover, assign the mutations accordingly. In the list of results, any sex-linked split mutations will be followed by the X chromosome it occupies. At this time, the Virtual Breeder in the Cockatiel Color Palette app includes crossover possibilities, but the online Virtual Breeder does not.

There is no true albino mutation in cockatiels (yet). Whereas a true albino cockatiel would be the result of a single mutation, the all-white cockatiels that are often called albinos are actually the result of combining the whiteface and lutino mutations — the two mutations work together to remove all pigment from the bird, just as a true albino mutation would.

Sometimes you will see chicks in the nest box that you don’t see on the results page. There are three main reasons that this happens:

The parent birds carry unknown splits. If you have two greys, the Virtual Breeder will tell you that all of your pair’s chicks will be grey. If their chicks, to your surprise, are fallow, pearl, and whiteface (just as an example), you know that your tiels have been hiding something from you — their splits. If you aren’t familiar with your pair’s parentages, they may be split to any number of mutations, and you won’t have any way of knowing until they produce chicks with mutations you didn’t expect. Once you see “new” mutations in the chicks, you can deduce what the parents’ splits must be. For example, if two greys produce a pearl chick, you know that the male grey must be split to pearl (and the pearl chick must be female). If the greys then produce a whiteface chick, you know that both parents must also be split to whiteface. This kind of detective work can get complicated, but it’s fun… well, in my opinion, anyway!

A genetic crossover has occurred. We’ll use two greys as an example again. If you know for a fact that your grey male is split to {X2: lutino pearl}, because his mother was a lutino pearl, you would expect half of his daughters to be grey and half to be lutino pearl. If one of his daughters turns out to be a regular pearl, the cause is a genetic crossover. Crossovers happen relatively rarely, but if your male is split to more than one sex-linked mutation, you can probably expect to see at least one chick which doesn’t inherit those mutations in the expected way. The Virtual Breeder does not yet take crossovers into account in its predictions, but this functionality may be added in the future.]

Probability isn’t an exact science. If the Virtual Breeder is telling you that you’ll get pied chicks 50% of the time, but none of your pair’s 8 chicks are pied, that’s just one of the quirks of probability. The parent birds’ genes combine randomly to create each chick, and even though we can predict that a certain combination will occur 25% of the time, it might happen 90% of the time, or it might never happen. Theoretically, the more chicks a pair produces, the more closely the chicks’ types will reflect the percentages expressed by probability. If your pair has a million chicks, you may well see that almost exactly 50% of the chicks are pied. But then you’d have to find homes for a million chicks.