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1 CHAPTER NINE Patterns of Inheritance I. Mendelian Genetics ♦ 1866 Gregor Mendel published paper on “Discrete heritable units” - work on peas rejected two common theories of Inheritance: 1. “Pangenesis” (Hippocrates theory) (all acquired traits of adult migrate to gametes) 2. Blending Hypothesis (early 19th century) (heritable traits from each parent blend in young) - Choose peas: trace 7 traits w/ 2 distinct forms of each trait (2 alleles) Self-fertilize OR Cross-fertilize True-breeding Varieties 2 II. Monohybrid Crosses ♦ track inheritance of a single trait ♦ 4 Mendelian Hypotheses: 1. Different forms of a gene exist (allele) 2. Ea. Organism carries 2 copies (diploid) 3. Gametes only carry 1 copy (haploid) due to Law of Segregation Homozygous Heterozygous 4. Forms may differ in expression: Dominant vs Recessive - Solving Mendelian Inheritance: P generation x P generation F1 generation x F1 F2 generation Organisms described in two ways: 1. Phenotype 2. Genotype 3 PUNNETT SQUARE: - visually shows principle of Independent Assortment - 4 Steps to Solving: 1. Determine P generation genotypes (diploid) 2. Determine Gametes (haploid) 3. Cross gametes in square to yield offspring (diploid) 4. Summarize P Generation __________ __________ Gametes ___ ___ ___ ___ Solve: RULES OF PROBABILITY: - Rule of Multiplication Product of each event ½ * ½ = ¼ 4 III. Dihybrid Cross - track inheritance of two traits - 4 Steps to solving: 1. Determine P generation genotypes (diploid) 2. Determine Gametes (haploid) FOIL method 3. Cross gametes in square to yield offspring (diploid) 4. Summarize P Generation __________ __________ Gametes ___ ___ ___ ___ ___ ___ ___ ___ Solve: 5 IV. Pedigrees - family genetic history illustrated, allows one to deduce pattern of inheritance and Carriers Ο = Female = Male Patterns of Inheritance: 1. Simple Dominant/Recessive: AUTOSOMAL RECESSIVE: Albinism, deafness, unattached ear lobes Cystic fibrosis (inflicts 1/1800 caucasian, 1/25 carriers) AUTOSOMAL DOMINANT: Extra digits, webbed digits, tongue rolling, dimples, achondroplasia (dwarfism), Alzheimer’s, Huntington’s 6 2. Incomplete Dominance: (intermediate phenotypes) Red x White = pink Coat color (cats, cows), hypercholesterolemia, flower colors 3. Multiple Alleles (more than 2 alleles in population) ABO Blood of humans (+ codominance) Phenotype A B AB O Genotype AA, AO BB, BO AB OO 7 4. Pleiotropy (one trait affects more than one character) Albinism and eye orientation in Siberian tigers, Sickle cell anemia 5. Polygenic (additive effect of 2 or more genes on a single trait) Skin/Hair/Eye colors & height in humans SEX-CHROMOSOMES 6. Sex-Linked X-Linked = colorblindness, hemophilia, heart-valve defect, MS, Fragile X syndrome Y-Linked = SRY gene (gonads) Hypertrichosis