MULTIPLE ALLELES Definition Genes which have more than

MULTIPLE ALLELES

Definition • Genes which have more than two alleles. • Have 3 or more alleles for the same gene (or at a single locus). • In Other Words – there are 3 or more alleles that create the different traits. • At most one individual will have 2 alleles or 1 pair (unit factors exist in pairs).

Example – ABO Blood Group • The A, B, AB & O are the blood types of human. • Each individual has 1 of 4 phenotypes: – Type A – Type B – Type AB – Type O

• The ABO locus has three common alleles: 1. IA 2. IB 3. IO IA and IB are codominant. IO is recessive.

Why is this called a phenotype and not a genotype? A A A B Both alleles equally expressed (example: blood types— genotype and surface antigens A B B

Donor-recipient compatibility Recipient Type A B AB O A Donor B AB O = Agglutination = Safe transfusion Note: • Type O blood may be transfused into all the other types = the universal donor. • Type AB blood can receive blood from all the other blood types = the universal recipient.

• The ABO locus controls the type of glycolipids found on the surface of erythrocytes apparently by specifying the type of glycosyltransferases synthesized in rbc’s. • The specific types of glycolipids on the rbc provide the antigenic determinants that react with specific antibodies present in blood serum.

• When type A blood and type B blood are mixed, the anti-A antibodies in the type B blood react with the antigens of type A, which produces agglutination or clumping of cells. • Cross-matching to determine compatibility is essential in transfusion.