1. The Genetic Basis of Complex Inheritance
BIO 304
Genetics
Lecture Outline
Chapter 18
The Genetic Basis of Complex Inheritance

2. The distribution of heights among 4,995 British women is very close to a normal distribution

3. Properties of 3 normal distributions having the same mean but different variances

4. A normal distribution can be described by two variables, Mean and Standard deviation

5. F2 progeny from a Trihybrid (3 gene) cross
In this example, all three genes quantitatively affect the same trait. The F2 phenotypic
ratio is 1:6:15:20:15:6:1

6. The distribution of phenotypes with 3 or 30 genes affecting the same phenotype
The colored bars show the result with 3 genes. The hatched bars show the result for 30 genes. Either result closely matches a normal distribution shown by the black line

7. The distribution of a trait
approaches a normal distribution as the number of genes increases

8. In a homozygous population, variation is caused only by the environment

9. Distribution of a quantitative trait
UConn students
From Heredity (1914)
by Albert Blakeslee

10. Students and faculty from UConn Remake of the famous photo

11. Distribution of a Quantitative Trait
Phenotypic
Distribution of a Quantitative Trait

12. Statistics in Quantitative Genetics
Population samples (random samples)
Mean
Phenotypic variance
Standard deviation

13. Statistics in Quantitative Genetics
Correlation
Correlation coefficient
Covariance of x and y
Regression analysis

14. The combined effect of genotypic and environmental variation
Genotypic variance alone
Environmental variance for the genotype aa
Environmental variance for the genotype Aa
Environmental variance for the genotype AA
Combined total variance in the population

15. Components of Variance
Phenotypic variance can be divided into genetic and environmental components.
Vp=Vg + Ve + V i

16. Interaction between the genotype and the environment

17. Components of Variance
Environmental variance can be estimated from variances of inbred parents and their F1 hybrid:
Ve=(VP1 + VP2 + VF1)/3

18. Components of Variance
Genetic variance is made up of several other variances:
Vg=Va + Vd + Vi
Vg=(VF2 - Ve )

19. Components of Variance
Additive variance can be calculated:
Va= 2[VF2 – (VB1 + VB2)/2]

20. Heritability Estimates
Useful in predicting phenotypes of offspring from crosses.
Heritability is a statistical measure of how strongly the offspring resembles the parents.

21. Hypothetical Parent-Offspring Regressions

22. Parent-Offspring Regressions
Offspring vs. midparent
A. Migratory activity in a bird (h2 = 0.45  0.08)
B. Shell length in a snail
(h2 = 0.36  0.17)

23. Note the full-sibs nested within half-sibs
Half-sib Design
Note the full-sibs nested within half-sibs

24. Means and variances in F1 and F2 progeny from a cross with unlinked genes and complete additivity

25. Effect of selection on the distribution of phenotypes in the progeny relative to the parents

26. Depression of yield in corn due to inbreeding

27. The fraction of affected individuals in the parents and the progeny of affected parents for a threshold trait

28. Actual risks for many congenital abnormalities and the theoretical estimates of risks for each type of inheritance
Narrow sense heritability (h2) = fraction of genotypic variance due to additive genes

29. Map locations for many quantitative trait loci (QTLs) for 3 phenotypes in the tomato genome