Cockatiel Color Genetics
By David M. Slater
COCKATIEL COLOR GENETICS: THE BASICS
When someone considers the idea of breeding their Cockatiels, the subject of genetics comes up. Everyone wants to know what their pair of birds is capable of producing. If the breeder doesn't know what to pair with what, the results can be disappointing. The inexperienced breeder may ask an "expert" for help. That can be a disappointment, too.
Usually, when someone who thinks they understand the topic begins to explain it to someone who doesn't, a strange thing happens: The speaker launches into a long-winded tirade of X's and Y's, Punnett Squares, and reverse double crossover gobbledygook. All too soon, the listener begins to get very quiet, responding with less understanding in his voice. His eyes take on a glazed appearance, and his jaw goes slack. Soon, the soft, steady sound of snoring is heard...
When the speaker realizes his student is asleep, he gets upset and storms off, yelling about never, EVER again wasting his time explaining something SO VERY SIMPLE to this ungrateful dunce!!! (Sound familiar?)
Unfortunately, this scenario happens all too frequently, and it is a shame. How many people would be more confident about genetics if they could only "catch on" to this elusive topic. It isn't HARD, but it is COMPLEX. It takes clear thought, and the ability to think in terms of "how" and "whys" not just "what." If you start at the beginning making sure you're clear on the principles being discussed, and progress slowly, there is no reason why anyone, even YOU, can't get a grip on it. This lesson will explain how genetic principles work, and I promise I'll try not to put you to sleep. Honest. At any point in this discussion, if you get confused by the subject matter, stop right there. Back up, and read it over again. If you read it enough, it'll make more sense. I'll make a point of doing a review after each principle is explained, to give you a chance to be sure you understand that point. Ready? Here goes:
WHY BABIES ARE MALE AND FEMALE?
The first thing you need to understand is why and how baby birds become male or female. This is the hinge upon which most genetic principles are based, and its importance cannot be overemphasized.
When you stop and think about it, there must be SOMETHING that decides the sex of the baby. It isn't by magic. (Well, maybe just a LITTLE bit...) It's in the combination of genes given to the embryo by the parents. If there was nothing saying that this chick should be a male, or that one a female, there would only be one sex, right? Since we all know that's crazy, what's the scoop here?
Well, it goes like this: Genes work in pairs. As living things, we each have many gene pairs, inherited from our parents. Half came from our fathers, the other half from our mothers. When a hen builds the "ovum," (the egg before the shell goes on,) she gives it a copy of one gene from each pair she has. When the male fertilizes the ovum, he does the same. These genes each have a purpose, and will decide EVERYTHING about the new baby, from its color, to its personality, and yes, even to its sex.
Here's where the little bit of magic comes in: Each gene knows he can't do his assigned job alone. During the instant of fertilization, he runs around to find his corresponding partner from the other parent. They link up, forming the gene pairs that make everything happen. They march off to work, doing whatever job they're supposed to do. (That's pretty magical, don't you think?)
For the sake of clarity, let's pretend that these genes have a number on their backs, kind of like runners in a marathon. This marathon, though, is a "Couples Only" marathon. Each gene is holding hands with his partner, and each pair has a unique number. (Are you with me? If not, go back and study this until you are clear about it. I'll wait here...)
In this particular marathon, let's pretend the job that gene pair number 1 has to do is name the sex of the chick. (It could be any pair of genes, but let's say it's pair number one.) Gene number 1 who was sent by the father is always named "X," (I think maybe it's because he can't spell his name...) The number 1 gene the mother sent THIS time also happens to be named "X." When they found each other, during the big link-up, they discovered they had the same name, and it made their job real easy: They gave the baby their two names, and called him "XX." Babies named "XX" are always boys. Sometimes, the X gene that dad sends discovers his partner is a gene named "Y." They link-up, spelling "XY." They know "XX" is always a boy's name, so "XY" must be a girl's name, right? (I don't know of any other choices, do you?)
To review what I just said: During fertilization, single genes sent by each parent pair up with their same-numbered partner from the other parent, because all genes work in pairs. Each pair decides something about the new baby. The task of one pair of genes is to determine the baby's sex. If this gene pair is made up of two X's, the chick is a male. If it's an XY pair, the chick will be female.
Now you know how and why babies are born male and female, right? It's important to understand that this XX and XY linkup is totally random during fertilization. The parents have no control over which gene is being given, nor of what the outcome will be.
If you have a pair of birds who produce more female than male chicks, it’s only because the combined genes are more often XY than XX. It’s all up to chance. Nobody can predict which it will be.
RECESSIVE GENES: SEX-LINKED OR SIMPLE?
Sometimes, when certain genes are getting ready to go do their task in life, they take some tools along with them. (When you have a job to do, you use tools, don't you? Well, genes do too.) In this case, though, the tools aren't hammers and wrenches and such. Instead, the tools are certain genetic traits, (carefully packed in little genetic toolboxes, right beside little genetic sandwiches and milk.)
These traits are for all sorts of things. They're the plans for how large the bird will be, or how long the crest will be, or what color the eyes will be, or whether the baby will be a sweetheart-mushy- face-lover kind of bird, or a nasty, mean-old-stick-in-the-mud bird. EVERYTHING that has to do with this bird is in there, in somebody's toolbox. Each gene that controls a given trait will carry the needed tool, and these traits are brought out and used by those genes. One such trait is for what color the baby will be.
Some of these color traits are very pretty, but they aren't very strong. They're called "recessive" colors because they’re, genetically weak, when compared to the very strong, "dominant," Normal grey color trait. In order for these recessive colors to show themselves, they have to "double up" on the dominant Normal to overpower it. It takes two of any "simple recessive" gene to do the baby any VISUAL good. If there is only one present, it won't have enough strength to out-do Normal. It loses out and Normal takes charge. The recessive gene is still there, but it’s hidden in the background, and you won't be able to see it. (Are you with me?)
Certain other recessive colors are "Sex-Linked". These also require two genes to out-do the Normal color gene, but ONLY FOR MALE BABIES. For some reason, (probably due to a soft spot Mother Nature has for little girl birds,) female chicks only need one gene for a sex-linked color for it to appear. (Got that? I know its heady stuff, but if you think about it, it’s not so bad. Ponder it awhile, then continue reading.)
To review this part: Two simple recessive genes must be present to overcome dominant Normal. If only one gene for a Recessive color trait is present, the recessive is hidden by the stronger Normal grey color. This applies to both sexes. In sex-linked recessives, males require two genes of the same trait for it to appear, and females only need one. (Simple, right?)
Ready for two major points? (Here's the big breakthrough:)
The toolboxes carried by those number 1 genes named "X" are the ONLY ones big enough to hold sex-linked color traits. The genes named "Y" can't carry as heavy a toolbox, so Y's don't deal with sex-linked colors, and neither does any other gene. X is the only one who can.
Simple recessive color traits are carried by other genes and NOT the X and Y genes that determine sex, the gene pair we call number 1. (Think about these a minute. They're VERY important points.)
When the X's are pairing up, and are deciding if they're an XX or an XY combination, they also bring out any sex-linked recessive traits they might have brought along. (Not all the X's bring along this trait. It’s only carried by those who were assigned the task by the little gene foreman who is in charge of this sort of thing.) A bird will be Normal grey in its coloration unless a color-altering recessive trait is present.
If the gene pair happens to be XY, any sex-linked trait the X has with him will decide the color of the chick, (who is a female, as you'll recall.) If the pair happens to be XX, (a male chick,) the two X's compare any sex-linked trait they may have. If they are two of the SAME trait, the male baby will BE that color, whatever it is. But if they are for two DIFFERENT sex-linked colors, neither has the power to out-do the other, and Normal will dominate. The sex-linked colors don't go away, though. You won't be able to see them, but they're still there. We say that male chick is "SPLIT" for those colors. (This means he carries the genes, but it isn't visible. The only time they do him any good is when he fathers chicks of his own. We'll get to that later on.)
The same thing applies if only ONE of the male's two X's is for a sex-linked color, and the other one isn't. Normal is still stronger. The baby will be a Normal grey, but it is split for the one sex-linked color. (Remember: In this example, this is gene pair number 1, ONLY. No other pair of genes has this same job to do. All the other guys are busy doing something else.)
A quick review: Sex-linked colors are carried with the X gene. In a male chick, since they have two X's, both of the X's must be for the SAME sex-linked color in order for the color to be visible. If they're for DIFFERENT sex-linked colors, or only one of the X’s is for a sex-linked color, the chick will be Normal grey, but he'll be split for the sex-linked color or colors.
In a female chick, (an XY,) only one sex-linked color gene is needed, since a female chick only has one X. But since she only has one X, HENS CANNOT BE SPLIT FOR A SEX-LINKED COLOR. She will BE the color she appears to be. Nothing more, nothing less. (Remember that!)
Make sure you're clear on these three things:
How and why chicks are born male and female.
Sex-linked colors are carried by the X gene that determines sex.
Simple recessive colors are carried by another gene.