Genetics: Part III Extending Mendel Figure 14.8 P Generation F 1 Generation Predictions Gametes EXPERIMENT RESULTS YYRR yyrr yr YR YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predicted offspring of F 2 generation Sperm or Eggs Phenotypic ratio 3:1 Phenotypic ratio 9:3:3:1 Phenotypic ratio approximately 9:3:3: /21/2 1/21/2 1/21/2 1/21/2 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 9 / 16 3 / 16 1 / 16 YR yr 1/41/4 3/43/4 Yr yR YYRR YyRr yyrr YYRRYYRrYyRR YyRr YYRrYYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr YyrryyRr yyrr Which of these genes are most likely to segregate as a unit? 3 4 Genes that are adjacent and close to each other on the same chromosome tend to move as a unit; the probability that they will segregate as a unit is a function of the distance between them. 5 In Drosophila.. 6 Traits for wings, leg length, and eye color are carried on the same chromosome. These genes are linked together on the same chromosome and will sort into the same gamete. 7 8 9 10 11 12 13 Crossover Value 14 Calculating Crossover 15 expectedunexpected 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 gene produces multiple phenotypes Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Figure P Generation Red White Gametes CWCWCWCW CRCRCRCR CRCR CWCW Figure P Generation F 1 Generation 1/21/2 1/21/2 Red White Gametes Pink Gametes CWCWCWCW CRCRCRCR CRCR CWCW CRCWCRCW CRCR CWCW Figure P Generation F 1 Generation F 2 Generation 1/21/2 1/21/2 1/21/2 1/21/2 1/21/2 1/21/2 Red White Gametes Pink Gametes Sperm Eggs CWCWCWCW CRCRCRCR CRCR CWCW CRCWCRCW CRCR CWCW CWCW CRCR CRCR CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW Multiple Alleles 21 Figure Carbohydrate Allele (a) The three alleles for the ABO blood groups and their carbohydrates (b) Blood group genotypes and phenotypes Genotype Red blood cell appearance Phenotype (blood group) A A B B AB none O IAIA IBIB i ii IAIBIAIB I A I A or I A i I B I B or I B i Key to blood type genotypes 23 Solve It Sara, who is type O, claims that Ira is the father of her baby. The baby has type O blood. Ira is blood type B. Is it possible that Ira is the father? 24 Solve It 25 The inheritance pattern of many traits cannot be explained by simple Mendelian genetics. Many traits are the product of multiple genes and/or physiological processes. Patterns of inheritance of many traits do not follow ratios predicted by Mendels laws and can be identified by quantitative analysis, where observed phenotypic ratios statistically differ from the predicted ratios. 26 Polygenic Inheritance Figure Number of dark-skin alleles: / 64 6 / / / / 64 6 / 64 1 / 64 Eggs Sperm Phenotypes: 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 AaBbCc Influence of Environment