Patterns of Inheritance

1. Patterns of Inheritance

2. Gregor Mendel • Pea plants • Complete dominance • Law of Segregation • Law of Independent Assortment

3. Law of Independent Assortment

4. Practice • Two organisms, with genotypes BbDD and BBDd are crossed. Assuming independent assortment occurs for both genes, what genotype ratios do you expect?

5. Practice • Y = yellow seeds and y = green seeds / T = tall plant and t = short plant • An F1 generation yields 12 yellow, tall plants and 4 yellow, short plants. What are the genotypes of the P generation plants?

6. Law of Multiplicatiom • Used to determine whether two or more independent events will occur together in some specific combination. • e.g. What is the probability that an Rr x Rr cross will yield the rr genotype? 1/2 x 1/2 = 1/4

7. Law of Additon • Used to determine the probability that any one of two or more mutually exclusive events will occur. • e.g. In the Rr x Rr cross, the Rr genotype can result from two mutually exclusive events. The sperm can donate the R and the egg r or vice versa. So, 1/4 + 1/4 = 1/2 chance at Rr.

8. Practice • What is the probability that YyRr will result from a YyRr x YyRr cross? • What is the probability that ppYyrr will result from a PpYyRr x Ppyyrr cross?

9. Mendelian Inheritiance • Aka. complete dominance • Each character is determined by a single gene, for which there are two alleles, one completely dominant and the other recessive.

10. A note on dominance... • Dominance does not mean the allele is most common in a population. It means that in a complete dominance situation, the heterozygote expresses only the dominant trait.

12. Pedigrees

13. Practice • Draw the following pedigree. Identify the genotypes and whether the trait is dominant or recessive.

14. Types of Characters • Discrete (e.g. Tongue rolling, ear-lobe attachment) • Quantitative (e.g. Height, skin color) • Multifactorial (e.g. Height, build, skin, intelligence)

15. Non-Mendelian Patterns

16. Incomplete Dominance • Neither gene is completely dominant. • e.g. flower color in some plants,

17. Codominance • Each allele affects the phenotype in separate, distinguishable ways. • e.g. Human mn blood group, flower color in some plants

18. Multiple Alleles • More than two alleles for a character exist in a population of organisms. • e.g. ABO blood groups

20. Practice • Cross an individual heterozygous for type A blood with one who is heterozygous for type B blood. What are the expected genotype and phenotype ratios?

21. Polygenic Inheritance • When two or more genes have an additive affect on a single phenotypic character. • e.g. Skin pigmentation

22. Plieotropy • One gene has multiple phenotypic effects. • e.g. cystic fibrosis, sickle-cell disease

23. Epistasis • The phenomenon when the phenotypic expression of one gene alters the phenotypic expression of another gene. • e.g. Fur color in mice and dogs

24. Practice • B = black fur, b = brown fur and E = pigment deposition, e = no pigment deposition • These genes have an epistatic relationship. • What are the expected fur color phenotype ratios (black : brown : yellow) when two dogs heterozygous for both traits are crossed?

25. X-linked • When a gene is located on the X chromosome. • e.g. Color blindness, hemophilia

27. Practice • Neither Tim nor Rhoda have Duchenne muscular dystrophy, but their first born son has it. What is the probability that a second child of this couple will have the disease?

28. Practice • A man with hemophilia (a recessive, x-linked condition) has a daughter of normal phenotype. She marries a man normal for the trait. What is the probability that a daughter of this mating will be a hemophiliac?

29. Patterns of Inheritance Overview • Complete dominance • Incomplete dominance • Codominance • Multiple Alleles • Polygenic Inheritance • Plieotropy • Epistasis • Sex-linked, X-linked

30. The relationship between dominance and phenotype • For any character, the observed dominant/recessive relationship of alleles depends on the level (organism all, molecular, biochemical) at which we examine the phenotype. • e.g. See Tay-Sachs description on p. 272 • Remember an allele is a nucleotide sequence.