Variation

Variation XII Std Zoology Chapter XII Std Zoology Chapter Principles of Inheritance and Variation determines B antigen and I O allele specifies no antigen. Individuals who possess these antigens in their fluids such as the saliva are called secretors. I A allele produces N-acetyl galactose transferase and can add N-acetyl galactosamine (NAG) and I B allele encodes for the enzyme galactose transferase that adds galactose to the precursor (i.e, H substances) In the case of I O /I O allele no terminal transferase enzyme is produced and therefore called “null” allele and hence cannot add NAG or galactose to the precursor. From the phenotypic combinations it is evident that the alleles I A and I B are dominant to I O , but co-dominant to each other (I A =I B ). Their dominance hierarchy can be given as (I A =I B > I O ). A child receives one of three alleles from each parent, giving rise to six possible genotypes and four possible blood types (phenotypes). The genotypes are I A I A , I A I O , I B I B , I B I O , I A I B and I O I O . • Antigens similar to those found among human beings have been recognized in the blood of other organisms. A-type antigens have been found in Chimpanzees and in Gibbons, A, B and AB antigen in Orangutans. • New world monkeys (Platyrrhina) and Lemurs have a substance similar but not identical with B antigen in humans. • Three blood groups have been distinguished in cats with a genetic system similar to those in humans. • The secretors (antigens found in the body fluids) can be detected in tears, saliva, urine, semen, gastric juice and in the milk of animals.