Exploring Beyond Mendel: Dominance, Gene Interactions, and Conditional Phenotypes

1. Going Beyond Mendel. Chapter 11

2. Introduction • Sometimes one gene simply controls a one trait (another reason why Mendel was lucky—genotypes matched phenotypes) • However, sometimes a genetic trait can be controlled by: • a number of genes that interact with each other with some being dominant over other and… • the “conditions” the gene is under to finally produce the phenotype.

3. Dominance Relationships and Gene Interactions • There are various ways in which two alleles of a gene can affect the phenotype. • Often they are simply dominant or recessive • Other times, there are situations where the recessive allele may function a little or interact with the dominant allele.

4. 1. Incomplete dominance (partial dominance) • phenotype of a heterozygote is between the two phenotypes of the two homozygotes • Example • cR cR = red flowers • cW cW = white flowers • cR cW = pink flowers

6. 2. Codominance • the equal expression of both alleles resulting in a mixed phenotype. • examples • Cattle – • Blood type/group –

8. 3. Multiple alleles • Himalayan Rabbit example

9. Blood type/group example

11. 4. Lethals • alleles that cause the death of the organism • recessive lethal alleles • Corn: • Huntington’s disease

12. 5. Epistasis • one gene interferes with the expression of another gene. Fig 11.8 in text • Example B = black mouse b = brown mouse C = makes the pigment melanin necessary for either black or brown c = can’t make pigment necessary for either color

14. Conditional Phenotypes • How genes are expressed depends on the conditions under which they exist • The same genotype can result in different phenotypes in: • separate individuals • different tissue of the same individual

15. Environment

16. Development • As an organism grows from fertilized egg to a mature organism, certain nerves and tissues may or may not be “completed” • incomplete penetrance - • variable expressivity – • Ex.) Polydactyly: tendency to have extra fingers or toes

18. Sex • a sex-limited or linked trait shows up in only one sex or the other • Ex.) uteran cancer. • a sex-influenced trait can affect both sexes but the effect is different. • Ex.) middle-aged male go bald but women’s hair only thins.

19. Age • Some genetic traits can have variable ages of onset • Ex.) muscular dystrophy, Huntington’s disease, and baldness.

20. Pleiotropy • Example • Phenylketonuria (PKU) • Victims can’t metabolize a.a. phenylalanine which then builds up and leads to brain damage. • Victims also have light hair and skin color.

21. Polygenetic inheritance • Examples • skin color: • height: • intelligence • birth weight