Slide 1 Chapter 11 Complex Inheritance Patterns Slide 2 Concept 11.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics Not all heritable characters are determined as simply as the traits Mendel studied. However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance. Slide 3 Complete Dominance: Homozygous and heterozygous phenotypes are indistinguishable. These look the same Slide 4 Dominant alleles are not necessarily more common in populations than recessive alleles. Six finger trait is dominant to the five finger trait Slide 5 Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations. When alleles are not completely dominant or recessive. When a gene has more than two alleles. When a single gene influences multiple phenotypes. Slide 6 In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties. Slide 7 In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways x Homozygous White Homozygous Black Heterozygous Black & White Slide 8 Carbohydrate (b) Blood group genotypes and phenotypes Allele Red blood cell appearance Genotype none B A IBIB Phenotype (blood group) i IAIA IAIBIAIB ii I A I A or I A i I B I B or I B i B A O AB (a) The three alleles for the ABO blood groups and their carbohydrates Recessive Slide 9 Pleiotropy: The property of a gene that causes it to have multiple phenotypic effects. Example: Sickle cell anemia is caused by one gene but has the following phenotypic effects: Anemia Episodes of pain Swollen hands and feet Frequent infections Delayed growth Slide 10 BE Be BE be BBEE bbee BbEE BbEe bEbe bE Be BBEe BbEEbbEE bbEe BbEe BBEeBbEeBbeeBBee BbEebbEe Bbee 9 : 4 : 3 Eggs Sperm BbEe Epistasis: a gene at one locus alters the phenotypic expression of a gene at a second locus. Slide 11 Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum. Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype. Skin color in humans is an example of polygenic inheritance. Slide 12 Figure 11.13 Eggs Sperm AaBbCc Phenotypes: 0 Number of dark-skin alleles: 12 3 4 5 6 1 64 6 64 1 64 15 64 20 64 1 8 Slide 13 Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype. Slide 14 Integrating a Mendelian View of Heredity and Variation An organisms phenotype includes its physical appearance, internal anatomy, physiology, and behavior. An organisms phenotype reflects its overall genotype and unique environmental history.