BIOLOGY Volume 3 - A Textbook of General Biology - 2004

24. VARIATION AND GENETICS

24.7. Gene interaction

24.7.2. Multiple alleles

In all cases considered so far, each characteristic is controlled by a single Gene represented by one of two alleles. However, There are many known Examples where a single trait manifests in several different forms controlled by three or more alleles, any two of which may occupy the same loci on homologous Chromosomes. Such instances are referred to as multiple alleles (or multiple allelomorphs). For example, coat color in mice, eye color in Drosophila, and human Blood Groups are all controlled by multiple alleles.

Inheritance of ABO blood groups

Blood groups are controlled by an autosomal gene. The locus of this gene is designated by the letter I (derived from 'isohaemagglutinogen'), and its three alleles by the letters A, B, and O. Alleles A and B are codominant, while allele O is recessive to both. The genotypes presented in Table 24.5 determine the phenotypic expression of blood groups. When a dominant allele is present, the blood produces a substance called an agglutinogen, which acts as an antigen. For example, with the genotype I^AI^O, agglutinogen A is produced on The erythrocyte membrane, and the plasma contains anti-B agglutinin (corresponding to blood group A). For the method of blood typing, see section 14.9.7.

Class="center">Table 24.5. Human blood group genotypes

24.13. a) Using appropriate genetic symbols, state the possible blood groups of children born to parents who are both heterozygous: the father has blood group A and the mother has blood group B.

б) Если эти родители произведут на свет разнояйцовых близнецов, то какова вероятность, что у обоих детей Кровь будет относиться к группе А?