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

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

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

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

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

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

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

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

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

Students and faculty from UConn Remake of the famous photo

Distribution of a Quantitative Trait Phenotypic Distribution of a Quantitative Trait

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

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

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

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

Interaction between the genotype and the environment

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

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

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

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

Hypothetical Parent-Offspring Regressions

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)

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

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

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

Depression of yield in corn due to inbreeding

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

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

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