Chapter 2 Extensions to Mendel s laws Complete dominance 3 1 Incomplete dominance 1 2 1 Co dominance 1 2 1 Recessive lethal alleles 2 1 Sickle cell disease as a comprehensive example Chapter outline Section 2 1 1 Extensions to Mendel s Laws for Single Gene Inheritance Interaction between alleles Section 2 2 2 Extensions to Mendel s Laws for Two Gene Inheritance Interaction between genes 3 Complementation test Section 2 3 4 Penetrance and Expressivity Additive 9 3 3 4 Recessive epistasis 9 3 4 Reciprocal recessive epistasis 9 7 Dominant epistasis I 12 3 1 Dominant epistasis II 13 3 Redundancy 15 1 Interaction between alleles A1A1 A2A2 A1A2 hybrids Type of Dominance Complete Complete Incomplete Codominant A1 is dominant to A2 A2 is recessive to A1 A2 is dominant to A1 A1 is recessive to A2 A1 and A2 are incompletely dominant relative to each other A1 and A2 are codominant relative to each other Incomplete dominance a Antirrhinum majus snapdragons Co dominance Co dominance ABO blood types are determined by three alleles of one gene Relationship between multiple alleles Wild type alleles and monomorphic genes The most common alleles in a population are usually called the wild type alleles An allele is considered wild type if it is present in the population at a frequency greater than 1 A rare allele in the same population is considered a mutant allele A gene with only one common wild type allele is monomorphic In mice one of the main genes determining coat color is the agouti gene The wild type allele A produces fur that appears dark gray or agouti Common variants and polymorphic genes Genes that have more than one common allele are polymorphic The ABO blood type system has three alleles IA IB and i All these alleles have appreciable frequencies in most human populations and can be considered to be wild type however high frequency alleles of a polymorphic gene are referred to as common variants Coat color in mice example of recessive lethal alleles and pleiotropism Sickle cell disease illustrates many extensions to Mendel s analysis Hb A is the wild type allele of the globin gene that codes for a 146 amino acid long polypeptide chain A large number of mutant alleles have been identified at this locus The most common mutant allele identified at this locus is the Hb S allele which codes for a polypeptide that has Valine in place of Glutamic acid at position 6 in this polypeptide chain Glu6 Val6 a Complete Incomplete and Co dominance in sickle cell anemia Interaction between genes Additive interactions between two genes controlling a single trait result in a 9 3 3 1 F2 phenotypic ratio Two genes controlling the same trait seed color probably function additively in independent biochemical pathways Recessive epistasis results in a 9 3 4 F2 phenotypic ratio A biochemical explanation for coat color in Labrador retrievers Recessive epistasis gives rise to the Bombay phenotype Reciprocal recessive epistasis results in a 9 7 F2 phenotypic ratio A biochemical explanation for flower color in sweet pea Dominant epistasis results in a 12 3 1 F2 phenotypic ratio Dominant epistasis results in a 13 3 F2 phenotypic ratio Redundant genes result in a 15 1 F2 phenotypic ratio A biochemical explanation for redundant gene action Incomplete dominance or codominance can expand phenotypic variation With incomplete dominance two genes can produce nine 3 x 3 different phenotypes of a single trait Locus Heterogeneity Mutations in any one of several genes may cause the same phenotype Based on the results of complementation test shown here mutants x1 to x6 can be placed in how many complementation groups The same genotype does not always produce the same phenotype Penetrance the percentage of individuals having a particular genotype that express the expected phenotype Expressivity The degree to which a character is expressed Factors that alter the phenotypic expression of a genotype include modifier genes genetic background environment chance A specific example of incomplete penetrance is the human bone disease osteogenesis imperfecta OI The majority of people with this disease have a dominant mutation in one of the two genes that produce type 1 collagen COL1A1 or COL1A2 Collagen is a tissue that strengthens bones and muscles and multiple body tissues People with OI have weak bones bluish color in the whites of their eyes and a variety of afflictions that cause weakness in their joints and teeth However this disease doesn t affect everyone who has COLIA1 and COLIA2 mutations in the same way In fact some people can carry the mutation but have no symptoms Thus families can unknowingly transmit the mutation from one generation to the next through someone who carries the mutation but does not express the OI phenotype Environmental effects on phenotype In Siamese cats temperature affects coat color