Using Sex-Linked Matings to
Tell Sex of Offspring by Plumage Color
Tom Barnhart, Lima, OH
As noted in an earlier article, 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.
Here’s how it works. 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.
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 hen with none of the other sex-linked characteristics, however, will have a sex chromosome that looks like this:
BA + + +
Keep in mind that this ash red hen might be a “silver” (ash red bar), a red check, a “brick” red (T-pattern or “velvet”), or a spread ash (sometimes called lavendar or “silver barless”), or even a true barless ash red.
Now consider what happens when that blue cock is mated to the ash red hen. The cock’s two sex chromosomes are shown on the left and the hen’s single active sex chromosome is on the right. (The ■ represents the hen’s inactive chromosome.)
+ + + + BA + + +
_______ __ __ ■
+ + + +
All offspring will get one of the cock’s chromosomes, which must be + + + + , and one of the hen’s,
which can be either BA + + + (which will result in a son) or the inactive one ■ , (which yields a daughter).
All male offspring will be BA + + + on one chromosome and + + + + on the other. The result is that all males will be ash red, since the ash red gene on the one chromosome is dominant to the non-ash red gene on the other chromosome.
All female offspring will have received the inactive chromosome from the mother, along with one of the non-ash red chromosomes from the sire, resulting in a blue hen. Those two possibilities can be diagrammed as follows, ash red cocks and blue hens.
BA + + + + + + +
______ _ ____ ■
+ + + +
These ash red cocks, however, now carry the gene for blue and if one of them is mated to a blue hen he can produce daughters that are either blue or ash red, since his daughters can receive either of the cocks two sex chromosomes along with their mother’s inactive one.
The same effect can be obtained when a dilute blue cock (“true” silver, or “silver dun bar”) is mated to a normal blue hen. In this case the daughters will be dilute and the sons will be blue. Like the case noted above, the offspring are the same sex as the parent of the other color. Here are the diagrams. Note that the cock must have the gene for dilution on both chromosomes, since it is a recessive characteristic.
True silver cock Blue hen
+ + + d + + + +
______ ____ ■
+ + + d
Sons receive one of the cocks chromosomes + + + d and the active sex chromosome of the mother + + + + The result is a blue cock, since the dilute gene must be present on both chromosomes to be visible.
+ + + d
+ + + +
Daughters receive one of the sire’s chromosomes + + + d and the inactive one from the dam. The result then is a true silver hen. As noted above, the offspring are the same sex as the parent of the opposite color.
+ + + d
Here is an example of another such sex-linked mating using the almond gene. Since almond is dominant to normal, we must use an almond hen and a non-almond cock. (Cock may be of any color – blue, ash red, brown – with or without dilute or reduced).
Cock (non-almond) Hen (almond)
+ + + + + St + +
___ _______ ■
+ + + +
Sons of this mating will receive the hen’s active chromosome containing the almond gene and one of the cock’s chromosomes, which do not contain the almond gene. The result is an almond cock, since the almond gene on the mother’s chromosome is dominant to the non-almond gene on the sire’s chromosome. Daughters will receive one of the cock’s chromosomes (non-almond) and the hen’s inactive chromosome, resulting in a non-almond hen.
Sons (almonds) Daughters (non-almond)
+ + + + + + + +
+ St + +
Note that in all the above examples, the key to any sex-linked mating is using a hen that possesses a sex-linked gene that is dominant to the corresponding gene of the cock. Some other possible sex-linked matings are noted below. The list is certainly not all-inclusive, and other combinations involving more than one sex-linked gene are possible.
Ash red hen x brown cock
Blue hen x brown cock
Any intense (non-dilute) hen x any dilute cock
Qualmond hen x non-qualmond cock
Non-reduced hen x reduced cock