S.6 Biology Gene Interaction Caltec S.6

1 SENIOR SIX BIOLOGY (MR . ABIB AMIR) GENE INTERACTIONS. Sometimes a single characteristic is controlled by the alleles of two or more genes interacting with one another. A characteristic which is controlled by more than one gene is known as polygenic character and its transmission is called polygenic inheritanc e. There are many situations when genes interact to control phenotypic characteristics of organisms, these include, • Incomplete dominance . • Multiple alleles. • Lethal genes. • Gene complex/simple gene interactions/ complementary genes. • Epistasis. MULTIPLE ALLELE This is where a phenotypic characteristic of an individuals is controlled by many alleles occurring at a specific gene locus, however, only two of such alleles may occupy the same gene loci of homologous chromosomes. Characteristics control led by multiple alleles include; eye color and coat color in mice and blood group in humans Multiple alleles can result in offsprings with characteristics which differ from both parents. INHERITANCE OF A, B, O BLOOD GROUPS A good example of such mult iple alleles is provided by the alleles controlling the ABO blood group system in humans. Blood group is controlled by an autosomal gene. The gene locus is represented by the symbol I, which stands isohaemoglutinogen. Blood group in individuals is determin ed by the types of mucopolysaccharides that function as antigens (agglutinogens) on the plasma membrane of blood red blood cells. There are 3 alleles represented by the symbols A, B, and 0. The alleles A and B are dominant to O which is recessive to both a lleles A and B. Alleles A (IA) and B (IB) codes for the enzymes that are involved in the formation of the antigens A and B respectively and allele O (IO) does not code for any known proteins. Individuals of blood group A have antigens …