MadSci Network: Genetics

Re: can one single individual have more than two alleles at the one time?

Date: Thu Jan 11 23:28:01 2001
Posted By: Christopher Carlson, Senior Fellow, Dept. of Molecular Biotechnology
Area of science: Genetics
ID: 978883233.Ge

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.


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