1. Spring 2009: Section 4 – Lecture 4 Reading: Chapter 4 Chapter 7

2. Variation in Mendelian ratios - incomplete dominance - over-dominance - co-dominance - multiple alleles - environment - epigenetics - epistasis (gene interactions) - lethal genes - gene linkage

3. Multiple alleles - when there are more than two alleles for a given gene. Can result in combinations of complete dominance and co-dominance expression.

4. examples: - eye color in Drosophila - self-incompatibility in plants - blood type in humans - coat color in animals

5. White eye alleles in Drosophila

6. Self-incompatibility in plants example: Brassica - four allele system S 1, S 2, S 3, and S 4 If the same allele is present in the male and female gamete the pollen tube will stop growing before it reaches the ovary.

7. Cross S 1 S 2 x S 1 S 2  sterile

8. Cross S 1 S 2 x S 2 S 3  S 2 S 3 S 1 S 3

9. Cross S 1 S 2 x S 3 S 4  S 1 S 3 S 1 S 4 S 2 S 3 S 2 S 4

10. Blood type in humans - Three alleles, A, B, and O - A and B are co-dominant - Both A and B are dominant to O

11. blood type genotype AIAIA or IAIO BIBIB or IBIO AB IAIB O IOIO AB - universal recipient O - universal donor

13. Coat color in rabbits c+ - wild type ch - himalayan cch - chinchilla c – albino There is a gradation in dominance for coat color c+  cch  ch  c

14. With multiple alleles the number of possible genotypes increases greatly. You can calculate the number of different genotypes with the following formula: n(n+1)/2 Where n is the number of alleles

15. examples: rabbit coat color: n = 4 number of genotypes = 4(4+1)/2 = 20/2 = 10 blood type: n = 3 number of genotypes = 3(3+1)/2 = 12/2 = 6

16. With the presence of multiple alleles it can be difficult at times to determine if the observed variation for a trait is due to two genes or allelic variation at one gene locus. The way to determine if the variation you are observing is allelic is to do a complementation test.

17. example: You have two individuals that are both white variants from the normal red flower color. If you cross them and the progeny are red then the trait is controlled by more than one gene and the two white variants have a mutation in different genes.

18. But if you cross them and the progeny are all white then the two variants have a mutation in the same gene and the trait may be controlled by only one gene.

19. Biochemically it would work like this: substrate  intermediate  product (white) A (white) B (red) ‘A’ is an enzyme that converts the substrate to an intermediate and is controlled by gene A. ‘B’ is an enzyme that converts the intermediate to the final product and is controlled by gene B.

20. 2 gene model plant 1 white aaBB x plant 2 white AAbb  F 1 AaBb all red The flowers are red because the F 1 individuals have one functional gene/allele at each gene loci. Hence the genes compliment each other.

21. 1 gene model plant 1 white a 1 a 1 x plant 2 white a 2 a 2  F 1 a 1 a 2 all white The flowers in the F 1 individuals are white because they do not have a dominant allele at the A locus. The shades of white may vary depending on the mutation in each parent.

22. So it is possible to have multiple alleles (a 1, a 2, a 3, etc.) at a single gene locus that give various shades of white depending on the location of the mutation in the gene.

23. Environment - the environment can influence the expression and level of expression of a gene. Factors such as temperature, light, and nutrition can reduce or prevent expression of a gene or genes.

24. Example: temperature response in fur color. Rabbits – Himalayan As temperature decreases the fur on the extremities darkens.

25. How to measure environmental effects: plants - grow large populations of a single genotype in several environments then compare the differences in expression of a trait. Any differences observed have to be due to the different environments.

26. animals - multiple matings to produce individuals with similar genotypes and study the F 1 progeny in several environments.

27. humans - work with twins 1. Separate identical twins at birth and place them in separate environments. 2. Study difference between identical and fraternal twins for expression of various traits.

28. If the similarity (concordance) within the sets of twins for a trait is the same between identical and fraternal twins than the expression of that trait is under more environmental than genetic control.

29. If the level of concordance differs significantly between identical and fraternal twins with a higher level of concordance in the sets of identical twins then the trait is under more genetic control than environmental control.

30. Level of concordance among identical and fraternal twins. percent concordance traitidentical fraternal hair color 89 22 diabetes mellitus 65 18 measles 95 87 schizophrenia 80 13 manic-depressive 77 19 coffee drinking 94 79

31. You can also see the effect of environment by looking at just the concordance of identical twins. example: diabetes mellitus

32. A concordance of 65% in identical twins means that out of 100 sets of identical twins 65 sets were the same for the trait while 35 sets had only one of the two showing the genetic disorder.

33. The difference observed is due to the environment. Differences in the environment (diet?) must be causing the differences because with identical twins there are no genetic differences.