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flatheadfisher wrote:

Here is a link to an animation. of what happens in meiosis (the kind of cell division that produces sperm & egg). It shows how the homologous chromosomes line up during one part of the process. The homologous chromosomes are identical to each other except they may contain different alleles. In other words, they are the same size and shape and they contain the same sequence of genes. But, they may have the same alleles or different alleles. If they have the same alleles then they are homozygous at that locus (AA or aa). If they have different alleles, they are heterozygous at that locus (Aa). When you watch the animation, pay attention to the colors on the chromosomes. You will see that the colors get mixed up on the chromosomes in two of the cells. The chromosomes that have two colors are called "recombinate" chromosomes. This is one of the things that results in so much genetic diversity in sexually reproducing organisms - along with random fertilization and something called independent orientation. The point here is that even though you get different colors on two of the chromosomes, the chromosomes are still precisely the same length. This is why crossing over can't produce a chromosome with two of the same gene loci on one chromosomes. The only thing that comes close is called "translocation."

Translocation happens when one of the chromosomes breaks and attaches to another chromosome. This results in one of the cells having a chromosome that is longer and one that is shorter. This usually results in an embryo that doesn't survive. However, this kind of error sometimes results in viable offspring with a chromosomal abnormality. For example, in humans a form of Down's syndrome is the result of this sort of error of meiosis.

Here is the link to the animation.

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