“Sex Linkage” – What does it mean?
Tom Barnhart, Lima, OH
The term “sex linkage” by itself simply means that a particular characteristic is linked to the sex chromosome, or more specifically, the gene for that particular characteristic lies on the sex chromosome. A “sex-linked mating”, on the other hand, is a mating in which sex linkage of a particular gene is used to determine the sex of an offspring based on plumage color.
As is the case with all living organisms, chromosomes occur in pairs, where one member of each pair comes from each parent. Pigeons have 31 pairs of chromosomes, but for our discussion we only need concern ourselves with one such pair, the pair that determine the bird’s sex. What makes things a little less complicated when discussing sex linkage – and what makes sex-linked matings possible at all – is the fact that in pigeons (and birds in general), the female has only one active sex chromosome while the cock has two. (The female actually has two sex chromosomes, but the one is so small and insignificant as to be virtually negligible for our purposes.)
In pigeons, the following characteristics are due to genes that are on the sex chromosome and therefore such characteristics can be used to determine the sex of youngsters as soon as they feather out: Ash red (symbol BA), brown (symbol b), almond (symbol St), faded (symbol StF), qualmond (symbol StQ), reduced (symbol r), dilute (symbol d), and pale (symbol dP). There are also a few others, but they are rather obscure and the average fancier need not concern himself with them. Capital letters indicate that a particular gene is dominant to normal (which we consider to be a common blue bar, and sometimes refer to as “wild type”), and the characteristic will be visible any time the bird possesses the gene for that characteristic on either or both sex chromosomes. Lower case letters mean the gene is recessive to normal and the corresponding characteristic will only be visible when that gene is present twice, that is, on both chromosomes.
For the genes mentioned above, each of these occurs at a single spot (“locus”) on the sex chromosome. Some are alternate genes for the same locus, and they are called “alleles”. Consider the following schematic diagram of a pigeon’s sex chromosome, where four specific spots on the chromosome have been labeled from1 to 4.
1 2 3 4
The relative proximity of the numbers indicates the relative spacing of these positions on the chromosome itself. All of the genes mentioned occur at one of these four locations. Locus 1 is called the ash-red locus; because it is here that the ash red and brown genes occur. Locus 2 is the almond locus, and here the almond, faded, and qualmond genes occur. Locus 3 is the reduced locus and locus 4 is the dilute locus, where dilute and pale genes occur. On a normal blue pigeon (any pattern – bar, check, or barless), there will be a “normal” or “wild-type” gene (symbol “+”) at each of these locations, and none of the other color genes will be present. The presence of any of the other color genes may or may not alter the appearance of the pigeon, depending on whether the gene is recessive or dominant to normal and whether the bird in question is a cock or a hen.
Now, let’s look at those four loci individually and consider what genes might be present at each one.
At locus 1, there will be one – and only one - of the following genes: normal (+), ash red (BA), or brown (b).
At locus 2, there will be one – and only one – of the following: normal (+), almond (St), faded (StF), or qualmond (StQ).
At locus 3 there will be either normal (+) or reduced (r).
At locus 4 we will find exactly one of normal (+), dilute (d), or pale (dP).
As noted earlier, a normal blue pigeon will be have “+” at each locus.
A blue cock with no other mutant color genes will have a pair of chromosomes that can be diagrammed like this: + + + +
+ + + +
A blue hen will look like the above, but with only one chromosome: + + + +
Her second chromosome is usually represented by a dot or nothing at all.
An ash red bar (“silver” in the parlance of Racing Homer breeders in the U. S.) or red check cock will have a chromosome diagram like this: BA + + +
+ + + +
or like this: BA + + +
BA + + +
Both will appear as ash red pigeons, but the first is “heterozygous” (one copy of the ash red gene) and the second is “homozygous” (2 copies). The first pigeon is ash red on one chromosome and blue on the other, but the pigeon appears ash red because the ash red gene is dominant to the blue. The only indication that the first pigeon is not homozygous is that he may show some small dark spots on the tail or primaries.
So you see here that a cock can “be” ash red, but also “carry” the gene for blue. Similarly, a cock can “be” ash red but carry the gene for brown. That is not the case for a hen, because she has just one active sex chromosome. Therefore, as far as sex-linked genes go, in the case of a hen, “What you see is what you get.” It is this difference that we exploit in choosing certain combinations for sex-linked matings, which is a topic we will save for the next article.
A brown cock, on the other hand, must be homozygous for the brown gene if he is to appear brown. This is because the brown gene is recessive to the normal gene. The diagram for a brown cock is
b + + +_
b + + +
An almond, faded or qualmond cock, like an ash red, need have just one copy of the respective gene in order to show the effects, since those genes are dominant to normal.
A reduced cock will be + + r + because reduced is a recessive gene.
+ + r +
Likewise, a dilute or pale cock must be homozygous because those genes are also recessive.
A dilute hen, of course, will be + + + d and a reduced hen will be + + r +
Again, I emphasize that a hen cannot carry any hidden sex-linked factors, and this is because she has just the one active sex chromosome.
In a future article, we will look at some specific matings, what color of offspring can be expected, and how the breeder can judiciously choose his matings so that the youngsters can be sexed based on their color.