What are multiple alleles?

Three or more alternative forms of a gene (alleles) that can occupy the same locus are called multiple alleles. Multiple alleles cannot be observed in the genotype of a diploid individual but can be observed in a population. It exists in a population when there are many variations of a gene present.

What are multiple alleles ? Describe multiple alleles with the help of ABO blood groups in man.

Three or more alternative forms of a gene (alleles) that can occupy the same locus are called multiple alleles. Multiple alleles cannot be observed in the genotype of a diploid individual but can be observed in a population. It exists in a population when there are many variations of a gene present. A gene has two alternative forms/versions called alleles. They are present at the same locus in a pair of homologous chromosomes. Two alleles of a gene can form three genotypes in a diploid organism. Sometimes a gene may have more than two alleles. When more than two allelic forms occur at the same locus on the homologous chromosomes of an organism so, they are called multiple alleles. When more than two alleles exist in a population of a specific organism, the phenomenon is called multiple allelism. A well known example of multiple allelism in man is the expression of ABO blood types by three alleles of a single gene which can produce six genotypes. Genotype Antigens present on red blood cells ABO blood group phenotype Antibodies present in blood plasma Blood types that can be tolerated Blood types that can accept blood for transfusion I A I A A Type A Anti-B A&O A&AB I A i A Type A Anti-B A&O A&AB FF B Type B Anti-A B&O B & AB Fi B Type B Anti-A B&O B & AB I A F A&B Type AB Neither anti-A nor anti-B A, B, AB &O AB only i i /FF Neither A nor B O only Anti-A & anti-B O only A, B, AB&O The A and B antigens are actually carbohydrate groups (sugar polymers) that are bound to lipid molecules (fatty acids) protruding from the membrane of the red blood cell. They are also called isoagglutinogens because they cause blood cell agglutination in the case of incompatible blood transfusions.

How is sex determined in human beings ?

Genes determining sex in human beings are located on two chromosomes, called allosomes. In mammals, sex determination is associated with chromosomal differences between the two sexes, typically XX denotes females and XY males. 23 pairs of human chromosomes include 22 pairs of autosomes and one pair of sex chromosomes. Females are homo gametic producing only one type of gamete, each containing one X chromosome while the males are heterogametic producing two types one with X and another with Y chromosome.