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Warning: All rights reserved. This article appeared in the Zebra Finch Society Bulletin and the NFSS Bulletin. Volume 18. No. 6 Nov/Dec 2001 (p. 15-19). 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
Michael Marcotrigiano
Montague, Massachusetts
All Rights Reserved
As most finch breeders know, new mutations or mutations that are newly imported can cost as much as 10 times more than a "normal" bird. In addition, it is often difficult to find someone that is willing to sell you a new mutation. Yet, on occasion, you may be able to obtain or afford a "split". How can you recover fully mutant types from the split without resorting to aggressive inbreeding?
Before I get to my recommendations, a few definitions and a quick genetic lesson are in order. For those of you familiar with the Punnett square diagrams I have made charts for all the crosses I'll mention, using the Black Cheek Zebra as an example.
By now you most likely know that mutations can be dominant or recessive. Dominant mutations would show up in the first generation of mutant x "normal" crosses. This simplifies things so much that there is no lesson needed for dominant mutations. Examples of dominant mutations are few but include Crested in Society finches and Black Face in Zebra finches. Examples of recessive mutations are many and include Blue Body in Gouldian finches, Fawn in Society finches, and Black Cheek in Zebra finches. Most of the finch mutations that I'm aware of are recessive so I will restrict my comments to recessive single gene mutations. I will not tackle the sex-linked mutations in this article but expect that discussion in a future article.
What exactly is a "split" bird ? First of all, animals have chromosomes that are in pairs. The only exception is in sperm and egg where there is only one of each chromosome. This means that in all but sex cells each gene is represented twice, one on each "homologous" chromosome. A long time ago, a now famous monk, Gregor Mendel, studied the inheritance of many traits in garden peas. He figured out that traits could be recessive or dominant and that it is possible for a pea plant to be carrying a recessive trait but not show it. In later generations, he showed that this trait could pop up in offspring even if parents did not show the trait. Today, we know that the same system is present in animals like birds.
Bird breeders call birds carrying one copy of the mutant gene "splits" while serious geneticists would call them "heterozygous". If both copies of the gene are mutant the bird will appear mutant and it is said to be "homozygous". Sticking with the bird terminology, I've decided to call the condition when both chromosomes have a mutant gene "whole", since "split" means only one chromosome has the defective gene. Please don't attribute this new term to me. My genetics colleagues would kill me for introducing more slang into the literature.
Now let's get back to the situation where all you own is a split and you want to establish the mutation in your flock. For the purpose of this article I will use a "split" for Black Cheek in the Zebra finch as my example. There is no way to recover "whole" mutants from a ""split"" in one generation if you start out owning a single "split" bird. But don't lose hope, it is possible to obtain "wholes" with patience. Assuming you have just the one "split" and all "normals", you have no choice but to mate your "split" to a "normal". For single gene recessive traits like Black Cheek, a mating like this results in all "normal" looking birds, but on average half of the offspring will be "splits" (Figure 1). So, now you seem to have gotten no where. Yet, you have increased the number of birds in your flock that carry the mutation. You should sleep better at night, knowing that if your original "split" should die, you've probably secured the mutant gene.
Making sure each bird is banded with a unique number or colored band is important since you don't know which of the offspring are "split" and once you figure this out you don't want to misidentify the bird in the future. Let's assume for argument sake that the first mating between the "split" and a "normal" results in two hens and two cocks, none of which you can be sure are "split". Your temptation is to take one of the offspring and mate it back to the original "split" in a father/daughter or mother/son mating. If you get lucky enough to choose a "split" to mate back to the parent, this would result on average a 1:2:1 "normal" : "split": "whole" ratio (Figure 2). The problem here is that you've also done some serious inbreeding. It is my personal opinion that at least some of the mutations that we say are weak, small, and inferior are this way, not because the mutation affects growth or vigor, but because most breeders are clueless when it comes to managing recessive "splits" and they inbreed to make sure they maximize their production of "wholes". Then, they compound the problem by taking this narrow genetic base and mating "whole" to "whole" to get all "whole" (e.g. Black Cheek to Black Cheek but related - Figure 3). But often, since they start with only one mutant bird or one "split", they inbreed and inbreed bringing out many recessive deleterious traits. If you feel it is important to avoid inbreeding you can use my strategy to obtain a "whole". Start out with the same "split" to "normal" mating (Figure 1). Foster them out if you want to get to the "whole" more quickly and if you want to relieve the stress from your only "split". Now take the original "split" and mate it to another "normal" that is NOT related to the first "normal". You now have two clutches of babies. Each of these clutches should possess half "splits" on average (Figure 1) but have one parent that is not related. Now, instead of mating back to the parent, you can mate members of the two clutches. They only shared one parent and this is the least inbreeding that is possible considering you had to use two "splits to make a "whole". Your expectations are again 1:2:1 "normal" : "whole": "split" (Figure 2).
Once you have gotten this far you probably do not want to continue matings where you are unsure if a baby is "normal"or "split". At this point take any recovered "wholes" and mate them to "normals" (again unrelated birds) to obtain 100% "splits". That's right! A mating of a "whole" with a "normal" yields ALL "splits" (Figure 4). Once you have some "splits" to work with, mate them to the least related "whole". When you mate a "split" to a "whole" you get 1:1 "split": "whole" (Figure 5). This pairing means that you can tell them apart and know exactly what you have.
Your future strategy should be to mate "splits" to "wholes." Take some of the new "wholes" and make new "splits" by using unrelated "normals". By continuing this strategy you will know exactly what you have and you will avoid inbreeding. Remember, once you have the mutation established avoid "split" x "normal" matings because you will not know if the offspring are "split" or "normal" without doing test matings to "wholes."
In a future article I will discuss how to obtain ""wholes" when the mutation is sex-linked. In the meantime look for a good deal on a "split" and make it "whole" again!
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Sample Crosses - Black Cheek Zebras as an example. Lower case "b" indicates the recessive mutant gene for Black Cheek.
Key:
BB = "normal" bird with wild-type color
Bb = "normal" appearing but "split" for Black Cheek
bb = Black Cheek bird
B = "normal" sperm or egg
b = mutant sperm or egg
FIG. 1
Above: Result of crossing a "normal" hen (BB) with a "split" male
(Bb). Offspring are two "normals" (BB) and two "splits" (italics).
Since Black Cheek is not sex-linked expect on average an equal number
of males and females to be "split".
FIG. 2
Above: When two "splits" are mated expect a 1:2:1 ratio with 1
"normal" : 2 "splits" (in italics): and 1 "whole" visual Black Cheek
(bb)
FIG. 3
Above: When two Black Cheek birds are mated all offspring are Black
Cheek. This seems like an easy tempting solution but you would want
to be sure the two Black Cheek birds were only remotely related since
inbreeding problems (e.g. loss of size and vigor, birth defects
leading to 'dead in shell') can occur if they are closely related
birds.
FIG. 4
Above: When a "normal" bird is mated to a "whole" (Black
Cheek) bird all offspring will appear "normal" but all will be
"split" for the mutation.
FIG. 5
Above: An ideal pairing is a cross of a "split" bird (Bb) to a
mutant bird (bb). The end result is a 50:50 mix of "splits" and
mutants. You know that no bird will be a "normal"!
the end
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