I struggled with this topic until I read Frank’s page on it. This article helped me understand how it works for those things that are sex-linked. This topic probably should have been at the start of the document, however once you grasp the concepts it probably won’t be read again.
Sex-Linkage by Frank Mosca
Sex!! Neat stuff, huh? Especially if we talk about pigeons, because pigeons are different in many ways from mammals. First, pigeons have no external sex organs so it’s often difficult to tell male from female — but you knew that already. Even more interesting, at least from the bird’s point of view, is that male pigeons have two XX chromosomes and females only one. Females also carry a little slug of material in addition to their X chromosome. That little slug of material can be passed on to an egg, but for most purposes we can ignore it because it seems to carry very little information of interest to us.
All chromosomes, including these X chromosomes, carry information on them which the bird’s body uses like the commands of a computer program. This information tells the pigeon’s body to produce feet, feathers, eyes, pigment — in short, everything that goes into making a bird the animal it is. Some of this information is stuff that we as breeders want, and some is stuff which we don’t. The stuff we want might include instructions to grow feathers which curl on the wing shield, or to produce pearl eyes instead of orange ones. Some stuff we don’t want might include instructions to grow extra toes or crest or webs on the feet.
If all chromosomes carry information, what’s so special about the X chromosomes? They’re unique because they’re the ones which determine sex — gender, if you prefer. Do you see where this might lead? If we could somehow peer into a fertilized egg and see whether it had two XX chromosomes or just one X chromosome and that little slug of material, we could instantly tell whether the young to be hatched from that egg would be a male or a female (cock or hen). Well, we can’t quite do that yet – at least, not without destroying the egg — but we can do some quite amazing things with this particular piece of information. There are certain mutations, i.e., changes from the normal information associated with the wild pigeon, which have been found to sit on these X chromosomes.
Let’s suppose for the moment that one of these mutations can only show it exists (perhaps by changing the color of the feathers of the bird) when it is the only information in the egg. If the normally seen wild-type pigeon information is also present, it will override any instructions from the mutation. Remember, the mutation we’re discussing is on the X chromosome.
If the mutation happens to be on an X chromosome in an egg which has only one X chromosome, then it must obviously change the resulting bird’s color. I say obviously because the mutation is the only information available for the bird’s body to work with. There’s not another X chromosome there to carry any competing information. Just as obviously, the bird hatched from this egg will be a hen because it has only one X chromosome.
On the other hand, if this mutation is in an egg with two X chromosomes and only one of them has the mutation while the other has the normal (wild-type) information, then the mutation will remain hidden to us. The resulting pigeon, necessarily a cock because it has two X chromosomes, will show what we consider to be the normal, wild pigeon-type color. However, the mutation is in no way destroyed! It is merely hidden.
Now suppose, there is a second mutation – also on an X chromosome. This mutation has no relation to the first mutation, and, in fact, may not even be in the same bird as the first one. This mutation is different than the first we discussed because it codes information which can override that provided by the wild-type pigeon. That means, no matter what egg it shows up in, whether an egg with one X chromosome, or one with two, we will be able to see its effect. You’ve just learned sex-linkage! See, that wasn’t so hard. Let’s try a few real-life examples to explain it further.
Let’s take a brown cock, any pattern, mated to a blue hen. Brown is a sex-linked recessive mutation. Don’t panic at the terminology. We’ve already described it. Recessive simply means that in the presence of the normal information provided by the wild-type, the mutation will not be able to show its action. We’ll use the genetic language because it’s a shorthand way of saying all of that.
So we have a brown cock which can only produce sperm carrying the mutation for brown on the X chromosome. (Because he’s carrying two X chromosomes, each of which carries the mutation for brown on it — otherwise he wouldn’t be the color he is.) We have a hen which can only produce the wild-type information for color — that’s why she’s blue — in her eggs. This wild-type information is designated in genetic shorthand by the symbol +. The genetic shorthand symbol for brown is b.
An X chromosome from the cock’s sperm and the X from the hen may meet in any particular egg. In that case, we have two X chromosomes in an egg, a cock produced and a youngster hatched which must grow and feather to be a blue. Remember, the mother’s wild-type information will, in this case, override the father’s recessive information. Please understand!!!! This doesn’t mean the hen is somehow stronger or more important than the cock, nor vice versa. It simply means that, in this case, the hen happens to be carrying the information which is acted upon by the youngster’s body. Sometimes, the father may carry the information acted upon. That doesn’t make him more virile or somehow more studly. It just means he got the information from one of his parents.
Now let’s see what happens when one of the cock’s sperm which carry that information for brown on the X chromosome meets an egg which contains no X chromosome from the hen but only that little slug of information from her instead. We’ll have a hen hatched from the egg. Remember? A hen has only one X chromosome and that little slug of material. She must also be brown since that’s the only information her body has available to work with. Do you see what we must have then? All XX eggs are male and all from this mating must produce blue youngsters. All X (plus material slug) eggs must be female and all from this mating must be brown. There you have it! Sex-linkage. You can tell the sex of the young in the nest from about day six or so when their feathers begin to open.
Another example: Suppose the cock is dilute. Since dilution has been found to be a sex-linked recessive mutation, we know both his X chromosomes must have the mutation for dilution (d) on them. If he had the information for wild-type coloration on either of the X chromosomes, it would override the information for dilution. Because he does carry the information for dilution on each of his X chromosomes, it means every one of his sperm must also carry the information for dilution. The wild-type hen, however, can only produce eggs with one X chromosome carrying the information for wild-type or eggs carrying that little slug of material. If the cock’s sperm thus the X chromosome with dilution on it hits an egg carrying that little slug of material, it will be the only nformation in the egg and a short-downed, dilute hen will result. Conversely, any sperm hitting an egg with the wild-type information in it from the hen, will result in a two X chromosome embryo – a male, long-downed, wild-type in color, carrying dilution hidden. This mating is even easier to sex than the first one mentioned above since any youngster hatched with long down can be determined instantly at time of hatching, so can any short downed youngster.
I’m going to stop here, but consider for yourself — what would you get if you mated a blue cock with an ash-red hen? Ash-red is a sex-linked mutation on the X chromosome. Blue is recessive to ash-red. (Actually, it’s more correct to say ash-red is dominant to wild-type (blue), since wild-type is our standard, but I’m trying to give you a hint here.) Extra hint: Look back at the brown cock/blue hen mating. Try using this as an example.
If you get that and feel ready for more advanced testing, consider this: (reduced is a recessive mutation located on the X chromosome.) What will you get if you mate a reduced cock (r) and a non-reduced (wild-type +) hen? (I’m not even going to suggest that you check out the dilution example above.) How about the reverse of that mating?
(By the way — I know some people prefer to list avian chromosomes using a different letter arrangement including Z. I’ve purposely chosen to ignore that symbology for three reasons. 1.) This is not a college level genetics course. 2) Most of what’s known about pigeon genetics has been discovered by fanciers using X and XX as symbols. Any follow up work from here will direct you to such usage. 3) Any non-pigeon geneticist who wants to know about pigeons can come to us and use the symbols as we do. So n’yah!)
Copyright 1997 by Frank Mosca. This work may be downloaded or copied for non-commercial individual use only. All other rights under copyright are retained by the author.