Many beginners shy away from the term sex-linked inheritance because they believe that it must be complicated. However, sex-linked mutations occur in many varieties of birds, and some mammals. In the case of birds, cinnamon canaries, opaline, cinnamon, and ino budgerigars are all examples of sex-linked mutations. For zebra finches, there are three such mutations commonly available; namely, fawn, lightback, and chestnut flanked white.
|Pairings to Produce Sex-linked Mutation Zebra Finches|
|SXL cock x SXL hen||= 50% SXL cocks
& 50% SXL hens
|SXL cock x normal grey hen||= 50% normal grey/SXL
& 50% SXL hens
|normal grey cock x SXL hen||= 50% normal grey/SXL cocks
& 50% grey hens
|normal grey/SXL cock x SXL hen||= 25% SXL cocks
& 25% normal grey/SXL cocks
& 25% SXL hens
& 25% normal grey hens
|normal grey/SXL cock x normal grey hen||= 25% normal grey/SXL cocks
& 25% normal cocks
& 25% SXL hens
& 25% normal grey hens
There are some simple ground rules to understanding how to reproduce these mutations. In an earlier article on breeding dominant mutations, it was pointed out that if the mutation was present, both cock and hen would visually show the characteristics of that mutation so (for example) a dominant dilute cock in the fawn series would be visually cream. Therefore it follows that cream cannot be carried in hidden form (normally referred to as split) in either sex. With sex-linked forms, if the mutation is present in hens, then it must appear visually; a hen cannot be split for a sex-linked mutation. The situation with cocks is slightly different, in that the sex-linked mutation can appear in both visual form and in split form. Why is this so? The sex of birds is determined by the presence of X- and Y-chromosomes, so that the cock bird always has two X chromosomes, and the hen has one X- and one Y- chromosome. (Interestingly, in the case of humans it is the female who has two X-chromosomes, and the male has an XY pair!) In sex-linked forms, the mutation gene is directly linked to the X-chromosome, hence the name. As a result, the expectations of pairings other than pure fawn x fawn will depend on which bird is the cock, and which the hen, as will be seen below.
In the formation of the embryo, the XX chromosome pair from the cock and the XY pair from the hen break and recombine to form the new chicks:
Xc Xh Xc Xh Xc —– Xh (male chick)
| + | → → +
Xc Y Xc Y Xc—– Y (female chick)
Therefore, young males receive one X-chromosome each from the parent cock and hen, but young hens form through receiving an X- from the cock and a Y- from the hen. It follows that the sex of the young is effectively determined through the hens contribution of the Y-chromosome.
It has been stated above that hens cannot be split for a dominant or sex-linked mutation. This is correct, but it is a common misconception that sex-linked hens cannot be split for any other mutation, and this is not the case. Such hens can carry the factor for the many recessive mutations, such as pied, penguin, blackcheek, etc. In addition, it is possible to combine the sex-linked mutation with either a dominant or a recessive mutation. So, combination of the fawn with the dominant dilute factor yields creams, and combination of fawn with recessive mutations like pied, penguin, or blackcheek, gives pied fawns, fawn penguins, and fawn blackcheeks respectively. Breeding characteristics of recessive mutations will be dealt with in a future article.
If we pair two visual examples of the same sex-linked mutation together taking for example fawns we can expect the young to be 100% fawns. However, if both the parent fawns are split for the same recessive mutation pied, for example then a proportion of the young chicks will be pied fawns, showing the characteristic white markings of the pied on the base colour of the fawn. This explains why little strangers sometimes appear from parents originally thought to be pure fawns. In the case where a fawn cock is paired to a non-fawn hen (an example might be a normal grey hen) all of the hen chicks will be fawn. On the other hand, the cocks will be normal grey in appearance, but will be split (ie carriers) for fawn. The reverse pairing of normal grey cock to fawn hen gives all normal grey chicks in appearance, but the cocks will again be normal grey/fawn. These normal grey/fawn cocks can be useful to produce the next generation of fawn cocks. So, pairing such a normal grey/fawn cock to a fawn hen should yield 25% each of normal grey/fawn cocks, fawn cocks, normal grey hens, and fawn hens. Note that only the normal grey cocks will be carriers of fawn, whereas the hens will not be. Finally, mating the normal grey/fawn cock to a pure normal grey hen will produce 25% each of pure normal grey and normal grey/fawn cocks, and pure normal grey and fawn hens. This latter pairing is not to be generally recommended for breeding fawns (although it may result in quality normal greys), since there is no way of telling which of the resulting cocks are fawn carriers i.e. normal grey/fawn – and which not. In Table 1 below, I have designated the sex-linked mutation as SXL to enable the pairing expectations to be relevant to lightbacks and chestnut flanked whites also.
Two special cases need to be considered to round off a discussion about sex-linked reproduction. The first is the special relationship between the lightback and chestnut flanked white. This arises because these two mutations occupy the same position on the X chromosome. As a result, the cock birds can genetically either be described as pure lightbacks, or as lightback:chestnut flanked whites. These latter birds are, in effect, a combination of lightback and chestnut flanked white in the same bird. Usually the pure lightback cocks are of a darker shade than their lightback:chestnut flanked white counterparts, but this is only a guide as the difference in some cases is quite small. In common parlance, both types are referred to as lightbacks, because the lightback colouration predominates. The hens, however, having only one X chromosome, must either be pure lightback or pure chestnut flanked white. In the breeding cage, the difference between the two types of lightback cock is very important. Pairing a pure lightback cock to a lightback hen will give all lightback youngsters. If the cock is lightback:chestnut flanked white, then pairing with a lightback hen will give both pure lightback and lightback:chestnut flank white cocks, but lightback and chestnut flanked white hens. Table 2 gives the expectations of a number of matings, pairing lightback to lightback, and to chestnut flanked white.
The second special case is when the sex-linked mutation is paired with one of the many recessive mutations. As a brief example, let us take the pairing:- normal grey penguin cock x fawn hen. The resulting young will all resemble normal greys in appearance, but all the young cocks will be split for fawn and penguin, and all the hens will be split for penguin. This apparently puzzling result can be understood better if broken down into its component parts. Firstly, it is important to know that normal grey penguin is in the normal grey series, and we know that pairing a normal grey cock to a fawn hen will give normal grey/fawn cocks and normal grey hens (see above). The penguin is itself a recessive mutation, so if paired to a bird (either sex) which does not carry the penguin factor, ALL the youngsters will be split penguin (written normal grey/penguin). Combining the two pieces of information will show that the expectations from the normal grey penguin cock x fawn hen to be normal grey/penguin & fawn cocks, and normal grey/penguin hens.
So, although understanding sex-linked mutation reproduction is slightly more complicated than that of dominant mutations, with a little effort it can be understood sufficiently for practical purposes.