MULTIPLE ALLELES

=Multiple Alleles= So far we have always looked at genes for which there are only two possible versions (alleles). But what happens if there are more than two possible versions of a gene? This is the case for blood groups. Blood cells express particular carbohydrate groups on their surface. There are three different versions of the gene that code for this surface carbohydrate. These three alleles cannot be represented by a single letter and are thus denoted A, B and O. The alleles A and B code for different carbohydrate groups, while the allele O codes for no carbohydrate group at all! Because the O allele does not code for a surface carbohydrate, it is considered recessive to the other two alleles. If the alleles A and B are both present, they are both expressed simultaneously and are thus considered co-dominant to one another. As a person can only carry two versions of a gene (alleles), the following genotypes and phenotypes are possible: [image:http://i.imgur.com/XDRjes8.png] Extra for Experts: If a person is given blood, their immune system will recognise the blood as foreign only if it has any surface carbohydrates that it has not encountered before. Thus a person with the blood group O will recognise all other blood groups as foreign (and reject them), whereas a person who is blood group AB can receive blood of any type.