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Discussion Starter #1 (Edited)
Genetic crossovers are hard to understand, so here's an illustrated attempt to explain it. The only crossovers that make any difference for color mutations are when the male has multiple sex-linked genes.

What is a crossover? Well, chromosomes normally travel in pairs. When sperm and ova are formed these pairs break apart, so that each parent gives the chick one chromosome from each pair. But it's called recombinant DNA for a reason. Before the chromosome pairs break apart some of the genes are shuffled from one chromosome to the other, and each reshuffling may be different so there's a wide range of possible combinations. A segment of one chromosome will trade places with the matching segment on the other. Because this exchange happens in segments, genes that are close together on the segment tend to not get separated and genes that are farther apart are frequently separated.

Lutino and cinnamon are apparently close together so crossovers between them are relatively rare (estimated rate = 3%). Suppose we have a male with lutino on one X and cinnamon on the other. I'm making up the locations, but suppose that both genes are carried on the top right arm of the X. The colored segments show the location of each gene on the father's chromosomes:


Now suppose that there's a crossover between these entire segments. This is what you get:


Now the cinnamon gene is on X1 and lutino is on X2, but for practical purposes there is no difference. We still have one X with lutino on it and one with cinnamon on it.

Now let's get a little more precise about the location of those genes. Here's where they are on the father's chromosomes:


This time only a little segment from that part of the X trades places. Here are the results:


Now we have a difference! Instead of one X with lutino and one with cinnamon, we have one X with lutino cinnamon and one with no sex-linked mutations. Each of these X's is going to go into a sperm, and if one of them fertilizes an ovum then the breeder will have some unexpected results in the nestbox.

But the most common crossover involves pearl and lutino and/or cinnamon, because pearl is further away from the other genes and recombinations are more common (estimated rate = 30%). Now our father bird has lutino pearl on one X and cinnamon on the other:


In our first example, nothing important happened when the whole upper-right segment of the X traded places. But look what we get when that same segment moves now:


Instead of lutino pearl on one X and cinnamon on the other, we now have cinnamon pearl on one X and lutino on the other. There are surprises in the nestbox once again.
 
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