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Warning: All rights reserved. This article appeared in the issue of the National Finch and Softbill Society Bulletin Volume 22. No. 1. Jan./Feb. 2005. (p. 6-12). Anyone wishing to reproduce this article for another bulletin, newsletter, article, journal, CD, or any other public forum needs express written consent of the NFSS and of the author michael@exoticfinches.com
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by Michael Marcotrigiano, NFSS Science Editor
Higher organisms have a significant number of chromosome pairs. All but one pair is shared by both sexes. The shared pairs are known as autosomal chromosomes. The remaining pair, known as the sex chromosomes, distinguishes the sexes. In humans, males can be distinguished from females by the presence of the Y chromosome. Women have two X chromosomes (XX) and males have one X and one Y (XY). Because the Y chromosome is present only in men, it contains little genetic information that is essential for basic life function. If this were not true, women would be missing some genes necessary for essential processes. Instead, the Y chromosome functions almost exclusively to determine the sex of the human. Interestingly, in birds and some insects it is not the male that has the unique sex-determining chromosome, but the female. Using the common designations, female birds would be XY and male XX. In order to avoid confusion with mammal genetic diagrams, the standard way to express sex chromosomes in birds is ZW for hens and ZZ for cocks. I will use these symbols in this article.
Those of us that are interested in the color mutations of cage birds know that sooner or later we must come to grips with their genetics if we are to obtain the desired color combinations. Yet, to this day I encounter hobbyists that are clueless about such matters and either inbreed to maintain a certain color or just perform random matings hoping for the best. Usually, the results are disappointing since a large number of normal colored birds begin to appear as recessives genes become masked by dominant genes.
In this article, I will explain in text and graphics how to manage a trait whose genetic control resides on a sex chromosome. Table 1 below shows you some of the most important color morphs that are controlled by genes on the Z chromosome. To my knowledge all sex-linked color mutations are recessive to normal with the exception of the yellow body mutation in the Gouldian finch, which is a codominant trait (split males are dilutes, double factor males have yellow bodies).
Table 1. Some of the more common sex-linked color traits known in captive raised finches
Society Finch Lonchura striata pearl creamino Zebra Finch Poephila guttata fawn chestnut flanked white continental chestnut flanked
white lightback Gouldian Finch Chloebia gouldiae red head yellow body Blue Faced Parrot Finch Erythrura trichroa lutino Diamond Firetail Emblema guttata fawn Shafttail Poephila acuticauda fawn Parson's Finch Poephila cincta fawn cream
* names from: Clement, P., A. Harris, and J. Davis. 1993. Finches
and Sparrows - An identification guide. Princeton University Press,
Princeton, New Jersey.
Since hens only contain one Z chromosome, hens carrying a Z-linked mutation will be visuals as there is no dominant allele to mask the expression of the mutation. In other words, hens cannot be split for a sex-linked mutation. Males on the other hand, are ZZ. So a male can be split for the mutation and still look normal (i.e. wild type for the trait). The only way to prove he is a split male is to mate him and observe his female offspring or have a pedigree that shows he must be split because you know the makeup of his parents.
Pairing birds with sex-linked mutations if fun. If the pairings are planned with knowledge of sex-linked inheritance it allows you to sex the birds while they are still in the nest box, a great treat for those of us struggling with sexing monomorphic species like society finches. In the following figures I use the pearl mutation of the society finch as my example. Pearls are kept in a self background meaning they have no white feathers (i.e. are not pied). Pearls are easy to distinguish from chocolates since pearl turns chocolate to a mocha color and it imparts a variable amount silver-gray to certain body parts. Keep in mind that any sex-linked mutation on Table 1 can be substituted for pearl in the figures below with the same result, except for yellow body in Gouldians where split males look dilute, not yellow.
In all the Figures below I have reduced their complexity by showing only the sex chromosomes and if only one genetic type of sperm is possible from a give male I did not depict multiples. I used letter symbols only for the genes that I'm concerned with in the specific example. Note that you can follow the lines leading from the final offspring to the egg and sperm that combined to give rise to them.
In Figure 1 we see just how useful a visual male for a sex-linked trait can be. When a pearl male is mated to a chocolate hen, all of his daughters will be pearls and all of his sons will be chocolates split for pearl. In this case the young can be sexed in the nest box and you can be positive that the chocolate males are definitively split for pearl. Keep in mind that pearl is recessive.
So, what about pairings that use a split male? Here's a real example of the result. I once got an email from a woman who bought two chocolate society finches from a pet shop in New Jersey (very close to where Shimpei Tanaguchi, one of my collaborators in the pearl importation, used to live). Her chocolates produced a few offspring that were "silvery hens". I immediately suspected that they might be pearls. Figure 2 shows how this can happen. If the male happened to be split for pearl he would produce on average one pearl for every four babies and it would always be a hen. It is the pearl sperm that joined with the W chromosome that made the pearl hen. This pearl hen is depicted in the third circle on the bottom line of circles on Figure 2. On average a pairing between a split for pearl male and a chocolate hen produces a ratio of 1:1:1:1 chocolate male : split male : chocolate hen : pearl hen. It would be impossible to know which male is split without performing test matings with him.
If you acquired a pearl hen and mated it to one of your chocolate males, all the babies would appear chocolate (Figure 3). Once you could sex them by song, you could be assured that all the males were splits and all the hens were chocolates. So unlike the pairing in Figure 2 at least you know the genetics of all of the offspring. In Figure 3, we see that a chocolate male mated to a pearl hen yields a 1:1 ratio of split males: chocolate hens.
In Figure 4 below we see the expectation if a split male is paired with a pearl hen. On average one can expect a ratio of 1:1:1:1 pearl male: split male: pearl hen: chocolate hen from this pairing. This is another pairing where you can eventually figure out the makeup of all the offspring because the males are either pearl or split and no chocolate males could ever result from the pairing.
Figures 1 to 4 are informative because they visually depict the results of all possible pairings with a single sex-linked trait. Note that only in Figure 2, where a split male is mated to a chocolate hen, is their any confusion about the genetic makeup of the male offspring. In all other pairings you know the genotype of the offspring.
Another item worth mentioning is the linkage issues with sex-linked genes. In my article in the last issue of the NFSS Journal, which was on linkage, you learned that two mutations on the same chromosome cannot be easily combined if they start out being on separate birds. The same goes for sex-linked genes. For example, if you wanted to make pearl creaminos to see what they would look like, it would not be easy. The resulting offspring would be all pearl hens and all chocolate males split for pearl and creamino (see Figure 5 below).
Even if you paired the resulting brothers and sisters you would not recover the double mutant, unless crossing over occurred.Finally, I need to mention what an allelic series is, because in zebra finches there appears to be three alleles of one sex-linked gene that result in similar but distinct phenotypes. What this means is that at the same point on the chromosome different forms of a similar mutation have occurred on separate birds. Chestnut flanked white (CFW), Continental CFW, lightback, and perhaps a few others form such a series. Because these are on the same location on the chromosome, they cannot be combined on one chromosome - there is no opportunity for crossing over to occur, as has happened with fawn and lightback, both sex-linked genes but far enough apart from each other on the Z chromosome to allow crossing over to occur.
The allelic series reveals allele relationships in split males. Males that are split for two forms of the gene (e.g. CFW and lightback) are not normal grays that are double splits. Rather, the two alleles interact, sort of acting as one in the sense that you do get mutant expression (i.e.visuals), but with one having a dominant effect the other. To read more about such interesting phenomenon go to www.efinch.com and read the sections on CFW, fawn, and lightback in the zebra finch variety section.
As you can see, the more we know the less we know. In a species such as the zebra finch, where the number of mutations keep piling up, we are bound to come up with new puzzles. Perhaps sex-linked genes can interact in strange ways with genes on autosomal chromosomes? By understanding more about genetics we will able to control the outcome of our pairings and hopefully create new and exciting color combinations in the future.
In the next issue of the journal we move away from single gene traits and begin to discuss the breeding strategies used for quantitatively inherited traits such as bird shape and size.
Acknowledgement
Special thanks to Roy Beckham, Garrie Landry, Larry Baum, Russ Beasty, and Danny Maldonado for helping with the compilation of Table 1.
