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alleles

I thought there is only 2 possible allele on a chromosome. so why does the book say two or more

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Original post by WWEKANE
I thought there is only 2 possible allele on a chromosome. so why does the book say two or more

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You're right in the sense that a person (usually) inherits two alleles for a given gene - one from the mother and one from the father. However, this isn't what the textbook is saying. Consider the alleles for ABO blood groups. There is an A allele, B allele and an O allele, so there are three different versions of the same gene (i.e. three alleles). Or think about HLA alleles - there are hundreds of different alleles in the population, hence why finding a HLA matched transplant can be difficult - however, an individual will only have two alleles for a given gene, one maternal and one paternal.

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