Guide on Sex-Linked Mutations

[Russ Lawson]

I just finished writing the fifth and final (at least for now) genetics guide installment for my website on sex-linked mutations that has been in planning since last year, but I never got around to. Last I checked, there was a lot of confusion about sex linkage in reptiles with them having Z and W chromosomes instead of X and Y chromosomes, so I am optimistic that this should help clear up some of the confusion and misconceptions about them. I've cut/pasted directly from my website, followed by a link to the actual page. The forum doesn't appear to have a function to allow superscripts in text, so just assume any + signs or lowercase letters following a Z or W are in superscript if you read it here.

Z-Linkage

Up until recently, all of the ball python mutations that have been discovered have been autosomal, or located on chromosomes that are not sex chromosomes. However, as more mutations are discovered and bred in any species, it is inevitable that one will eventually arise that is located on one of the sex chromosomes. Most people will remember learning in high school biology that humans have X and Y sex chromosomes, and that females have a genotype of XX, whereas males have a genotype of XY. This is not true for reptiles, however, which have Z and W sex chromosomes. In reptiles (and birds) females have a genotype of ZW and males have a genotype of ZZ. Even without a knowledge of genetics, most people know that colour-blindness is more common in males than females. This is because the mutation for colour-blindness is located on the X-chromosome, and females require two copies of the mutant allele in order to express the mutation, whereas males only require one. In reptiles mutations associated with the Z-chromosome will work oppositely, with females that express a mutant phenotype being more common than males for the same reason. One mutation which appears to be associated with the Z-chromosome in ball pythons is coral glow. For purposes of this guide, we will assume a static 1:1 ratio in offspring of male to female offspring from all pairings.

The first pairing I will demonstrate is of a female coral glow to a normal male. Because we are working with sex chromosomes, it is important to mark the chromosomes for each parent in the Punnet Square. Remember, females will have a ZW genotype, whereas males will have a ZZ genotype, and that each parent passes only one of its chromosomes to any individual offspring. To denote the wild-type a + is used in superscript, and the mutation for coral glow will be denoted by a c in superscript. Thus, Z+Z+ will be our wild-type male, and ZcW will be our coral glow female.

From this breeding, we get a 1:1 ratio of animals with a ZcZ+ genotype to animals with a Z+W genotype. All of these animals express the wild-type phenotype. Because all of the females get their W-chromosome from the mother and their Z-chromosome from the father, none of them carry the coral glow mutation. However, any males in the clutch will be heterozygous for coral glow because all of them will get one Z-chromosome containing the mutation for coral glow from the mother. Note that it is IMPOSSIBLE to obtain a female expressing the wild-type phenotype to carry a mutant Z-linked allele - therefore, there is no such thing as a female het coral glow.

The above pairing obviously isn't satisfactory; we want to make more coral glows! So this time, let's go ahead and take one of our male heterozygous coral glows (ZcZ+) and pair him back to his coral glow mother (ZcW).

Paydirt! The above pairing gives us 1:4 ZcZc or MALE coral glows! We also get 1:4 ZcW (female coral glows), 1:4 ZcZ+ (heterozygous coral glow male), and 1:4 Z+W (wild-type female). Note that again, all of the males with the wild-type phenotype will be heterozygous coral glow, while the females will not. So now that we have our male coral glow we decide to pair him with a normal female to see what happens...

This time we get a phenotypic ratio of 1:1 wild-type to coral glow. However, this time we don't even have to sex the animals to know what we have. Every animal expressing the wild-type phenotype will be a male het coral glow (ZcZ+) and every animal expressing the coral glow phenotype will be a female. I think this pairing alone shows just how valuable a male coral glow is. All female offspring he produces will be coral glows. This type of pairing was the reason NERD originally thought they had a dominant mutation on their hands with coral glow. Their original whitesmoke albino male sired about 50% coral glow offspring in every clutch. After a while though, they realized that all of these were females. Only more recently have they begun to produce more male coral glows.

W-Linkage

Although none have been recognized in ball pythons yet, for the sake of completion I feel it is necessary to include W-linked mutations in this guide. For this purpose, I propose the fictional spurless mutation on the W-chromosome, which seems reasonable considering the lack of spurs is a reliable way to sex some species of boas. In this instance, a female with the Z+W+ genotype will possess anal spurs at the cloaca (wild-type), whereas a female with the Z+Ws genotype will lack spurs (spurless). It is only necessary to show one pairing for this particular type of mutation. Here we will breed a spurless female (Z+Ws) to a wild-type male (Z+Z+) to see what we might get out of such a pairing...

From this pairing we would get the following ratio 1:1 wild-type (Z+Z+) to spurless (Z+Ws). Note that all wild-type offspring in this pairing will be males, and all mutant offspring will be females. This demonstrates that it is impossible to transfer a W-linked mutation to a male due to their lack of a W-chromosome. Additionally it shows that in the case of a W-linked mutation, all female offspring of a mutant mother will possess the same mutation. For the fictional mutation above, this would be somewhat convenient, as it would offer a way to sex offspring visually, but at least for now we don't have such a mutation in ball pythons.

Sex-linked mutations work in a very unique manner that may be a bit more difficult to wrap your head around than the autosomal mutations that were described in the previous guides. The two most important things to remember when using these Punnett Squares are that anything with a W-chromosome is a female, and that you should always assume that you will get half males and half females in any pairing. The above should provide a useful guide to any individuals who choose to breed animals with sex-linked mutations. As always best of luck with your future breeding projects!

Source: http://www.russreptiles.com/resourcegenetics5.htm