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Warning: All rights reserved. This article appeared in the issue of the National Finch and Softbill Society Bulletin. Volume 21. No. 6. Nov/Dec. 2004. (p. 9-13). 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

You may have acquired some birds of a given species carrying different mutations. You've probably tried to get combinations of your favorite single gene mutations into one bird? You may want to have a black cheek yellow beak zebra finch or perhaps something even more complex. Or perhaps someone gives you a bird that already has two mutations and you want to breed one of them out of your line. You may obtain a fawn penguin zebra but you are really only interested in the gray penguin. You buy books or read web pages and they tell you how to combine or separate the mutations. They tell you what ratios to expect. They tell you things like "one out of every sixteen babies will have both mutations". Unfortunately they almost always disregard one important biological phenomenon, genetic linkage. Genetic linkage occurs, as will be explained in detail below, when the two genes of interest reside on the same chromosome. Genetic linkage can alter expected ratios, sometimes to the point that it makes it impractical to attempt your ultimate goal.

To understand linkage we need to go back to the famous monk, Mendel, and his pea breeding experiments. But before doing so, I need to make sure you understand a few facts. One is that within a given species the chromosome number is, barring rare events, the same for all individuals. Each individual has two complete sets of chromosomes. When the individual makes sperm or eggs the number is halved so that when sperm and egg combine in fertilization the species number is restored. If this reduction did not happen the number of chromosomes would double with each generation.

After performing the experiment described below Mendel concluded that units of inheritance (now termed genes and known to be located on chromosomes) sort themselves independently in offspring during the creation of sex cells. This makes it possible to predict what ratios one should recover when studying the inheritance of two or more genes.

It is now known that Mendel, as brilliant as he was, was one of the luckiest investigators of inheritance. He happened to choose pea traits, most of which were later discovered to be on different chromosomes, so his ratios were nearly textbook perfect. Mendel proposed that traits sort independently and proved it by crossing peas with two different single gene characteristics.

For the sake of this article and to make things clearer, we will pretend that Mendel used zebra finches rather than peas. Remember that normal (wild type) zebra finches have orange cheek patches and body color extending through the belly. Let's say Mendel obtained two new but uninvestigated zebra finch mutations, which he called penguin and black cheek. The penguin mutation suppresses all gray and black pigments on the lower portion of the bird. The black cheek mutation turns the orange cheek patches to black. When he made hybrids between, let's say, a black cheek normal gray and a normal cheek (i.e. orange cheek) penguin all the offspring were normal because black cheek and penguin are recessive to the normal colors. The offspring were double split (i.e. heterozygous) for penguin and black cheek. Upon crossing two of the offspring to obtain multiple clutches, the expectation for the offspring were as follows - 9 normal zebras, 3 black cheeks, 3 penguins, and only 1 bird that was a penguin with black cheeks. Below you see why this might be expected, the assumption being that the sex cells have, on average, the same possibility for possessing any one of the possible genetic makeups. This principle is call independent assortment (note: for simplicity sake only the chromosomes with the relevant mutation are depicted). 

 The figure above shows a penguin zebra and black cheek zebra cross. The long rectangles are the chromosomes, two pairs in all cells but the sex cells which have only one of each type. There is only one type of genetic makeup for the sex cells of each type of bird. When you pair penguin and black cheek you get the double split in the first generation - a bird that looks normal. The double splits make 4 possible genetic types of sex cells (sperm or eggs). These are depicted as the four smaller circles on the right. If the sibs are mated you expect to get four types of offspring. This is made clear on the table below (known as a Punnett square). The genotype of the possible sperm cells is along the top row and the genotype of the eggs is down the first column. All possible combinations of sperm and egg are given; O is normal orange cheek, o is black cheek; G is normal gray body; g is penguin body. In parentheses ( ) below each bird's genetic makeup is what one might expect for the appearance of each bird. Later in this article I will discuss Mendel's disappointment on the double mutation bird.

 The above discussion, figure, and table show you what one can expect when crossing birds carrying different recessive mutations that happen to be on different chromosomes. Given that there are dozens of mutations in zebra finches it is simplistic to think that each one is on a separate chromosome. While I could not find out if the number of chromosomes for a zebra finch has yet to be determined, it is unlikely that all the mutations could reside on different chromosomes. That brings us to linkage. Let's suppose that black cheek and penguin were both on the same chromosome (they are not but we will pretend they are for this lesson). If you obtain a bird with the penguin mutation and another bird with the black cheek mutation you will have a situation like that in the diagram below (note: for simplicity sake only the chromosomes with the relevant mutation are depicted). Note that the penguin bird has a normal dominant copy of the black cheek gene, while the black cheek bird has a normal dominant copy of the penguin gene. You goal is to get them in one bird.

 

 From the figure above you see that the linked mutations do not sort independently and essentially the sex cells of the double split offspring are the same as the sex cells of the parents when it comes to these two genes. Is it ever possible to combine these two recessive traits in one bird? Well, the answer is yes. One of the nature's marvels is that in order to create diversity a system has evolved that causes the occasional breakage and "healing" of the chromosomes, and this can result in sections being switched. This happens prior to the separation of chromosomes during the formation of sex cells. It is called "crossing over" and the end result is that it unlinks the mutations. Below we see how a crossing over event results in the production of a sex cell that has both mutations in the recessive form ("Mutations now linked" in the figure below).

 

How often does crossing over help out? Well to make a long story short, genes become unlinked more frequently if they are very far apart on the same chromosome than if they are very close together. In addition, it happens more often if they are located near the tip of the chromosome instead of near the center. Since we don't know the position of finch genes on the chromosomes (it would be expensive to find out so no one has bothered) I cannot tell you how often it will happen. What I can tell you is that you may need to get hundreds of offspring from the double splits before you get the double recessive bird. The good news is that once this is accomplished and you have the mutations linked it is just as hard to get them unlinked as it was to put them together. The message is that once the linkage has been done by someone else, buy a bird from them and forget about going through the entire procedure yourself.

One very important side topic worth mentioning here is this. Just because a bird finally has the genetic combination you want, with both mutations in one individual, it does not mean that the appearance of the bird must be the combination of traits. In my example, Mendel was disappointed in his black cheek penguin. It had white cheeks! Why? Normal zebras have orange cheek patches so the penguin mutation does not affect the cheek patch and you might not anticipate that the penguin mutation affected anything above the neck. But the black cheek mutation causes the orange to become black. Penguin wipes out black so you lose the black cheek patch even though the bird is genetically a black cheek penguin.

While much is already known about the expected inheritance in zebra finches, I used them as an example because they are familiar to many of the readers. As we discover new mutations in Gouldians, Owl finches, Parrot finches, etc. we will eventually come up with linked mutations and unexpected and disappointing results. It is my hope that his article helps with the "why".

If you had trouble with any of the terms in this article check out my prior articles on Breeding for Quality. They are posted at:

http://www.exoticfinches.com/mypubs/pubHOME.htm

In my next installment of the series I will talk about sex-linkage and how to manage mutations on the sex chromosome.

Note: Special thanks to Garrie Landry, Roy Beckham, and Bob Merritt for discussing and/or reviewing the contents of this article.

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