|MadSci Network: Genetics|
Hi Damien, You're asking a fairly straightforward answer, so I will try to be succinct in my answer. Humans and all other mammals are diploid: we carry two copies of each chromosme, one from each of our parents. Thus, there are only two copies (alleles) at every position in the genome, so pairing is not much of a problem. The exceptions to this rule pretty much prove my point. Exception #1: Down Syndrome. In DS individuals there are three copies of chromosome 21. As a result, they are usually mentally retarded, frequently have a specific form of heart disease, and are prone to early onset Alzheimer's like dementia. All of these phenotypes are consequences of trisomy 21. Thus, humans are designed for two, only two, and no more than two alleles at each locus. Exception #2: microdeletion syndromes. There are quite a few syndromes which are the result of deletions of small portions of chromosomes. Frequently these deletions are flanked by direct repeats thousands of base pairs long. When an error occurs in meiosis and unequal crossing over occurs between these repeats, one of the chromatids will now carry no copy of the region between the repeats while the other chromatid will carry two. This type of event accounts for many diseases, including some color blindness, Williams syndrome, and many others. Fortunately, such deletion syndromes are rare, demonstrating the fidelity with which the chromosomes usually pair during meiosis. Chris
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