Understanding Quantitative Genetics: The Influence of Genes and Environment on Traits

1. Chapter 21- Quantitative Genetics

2. Strange fact for Monday • If you miss 20 days of class as a 6th grader, what is the probability that you will graduate from high school? • Assume average is 80% • 20

3. Where we’re going • Define the term • Be able to determine the # of genes involved • Phenotype-Genotype relationships- nature vs nurture, particularly as it deals with: • Heritability- and as an aside, variance. • There might be some controversy

4. Key points: • We can estimate the effects of multiple genes by making some assumptions: • 1) there are multiple genes affecting the trait. • 2) some alleles at these sites are additive, while others are non-additive. Additive is just as in normal usage- e.g., additive alleles would add to your height. Non-additive not add to your height. Other genes would produce non-additive effects- all or none effects, as seen in tall and dwarf pea plants. • 3) the genes contribute approximately equally

8. The number of gene pairs contributing is seen by counting the extremes- (¼)n ¼2= 1/16 ¼3= 1/64 ¼4= 1/256 ¼5= 1/1024

9. So, let’s do a few problems.. • 5,6, 7

10. Heritability: • This is a measure of the relative influence of genotype and environment on variance. • Variance- study on your own- it’s basically a statistical measure of, well, variance. We take the average, the difference between the average and each value, and we square it; we then divide by the # of samples -1. • What heritability means: high value- lots of genetic variation in population compared to phenotypic variation  selection will work. • Low heritability: little genetic variation; selection won’t help.

11. Broad and narrow-sense heritability- p. 478; H2= Vg/Vp; if the # is close to 1, then the effect of environment is negligible; if the # is close to 0, the effect of the environment is large. You book has a situation that allows this to be calculated. (Q 4, p. 485) • For most cases, getting broad-sense heritability is impossible- you’d need to know too much

12. Narrow sense: h2= VA/VP; here we only look at variation in traits are additive in nature- we exclude dominant variance, since these traits are less affected by selection- if the dominant trait is there, it, well, dominates. The h2 is useful in determining how easily selection can work. • experimental determination: h2= R/S

15. Concordance: if both of a pair- brothers, or twins, have a trait, it is concordant. Useful in the nature/nurture controversy. Problem, p. 488 • One of the more interesting statistics- Concordance homosexuality for identical twins is 38%, at most- many studies in the 7-15% range. • QTL’s: Through complicated patterns of breeding, and knowing where certain regions are on the chromosome, we can locate genes that affect Q.T.’s.

16. http://www.narth.com/docs/whitehead2.html • J. Michael Bailey, Michael P. Dunne, and Nicholas G. Martin, "Genetic and Environmental Influences on Sexual Orientation and Its Correlates in an Australian Twin Sample," J. Personality & Social Psychology 78 (2000): 524-536.

17. Quiz for today- work in groups! • You have a breed of dogs with long ears (20 cm) and a breed with short ears (4 cm). You mate them, and the offspring have ears that are 12 cm in length. You mate the F1’s with themselves, and 2 of 500 have 4 cm ears, and 2 of 500 have 20 cm ears. • a) how many genes are involved? • b. What contribution does each additive allele make to ear length? • c. What is the probability that a mating between an AaBbCcDd dog will produce an AAbbCCdd dog? • 1 pt extra credit: could a type A blood type and type B blood type produce a child that is type O? Give the genotypes of the parents.