Quantitative Genetics





Reference: Griffiths et al. Modern Genetic Analysis, 2nd Edition 2003

Chapter 18

Quantitative Genetics - The study of genetics of continuosly varying characteristics

Quantitave versus Qualitative

Mendelian Genetics

  • Mendelian Genetics applys qualitative measurements and is not useful when dealing with continuous phenotypic range

THEREFORE

Statistical techniques and analysis are applied to measure these variations


Multiple factor hypothesis - describes that a large number of genes, each with a small effect, are segregating to produce quantitative variation.


Example: Wilhelm Johannsen corn experiment

Johannsen used pure line ( family lines that are true breeding from generation to generation) in his quantitative genetics analysis experiment.

He produced these true lines of corn by inbreeding and selfing.

He hypothesized that each subsequent generation would exhibit the same phenotypic expression. He calculated the number of expression by the formula :


             

Heritability of a Trait             

Does the observed variation in the character influenced by genes at all???????


Two general methods of finding the heritability of a trait
  1. Depends on phenotypic similarity between relatives
  2. Uses marker-gene segregation
Determining whether a trait is heritable in a human population, relys on adoption studies to avoid enviromental similarities between relatives

Ideal subjects are identical twins



Quantifying Heritability

Phenotypic variance (Sp^2) can be broken into two parts

  1. Genetic variance (Sg^2) - variance between genotypic means
  2. Enviromental variance (Se^2)
Sp^2 = Sg^2 + Se^2

Problem: Excludes co-variance between genotype and enviroment

Example: Musical parents rearing musical children - Genetics versus Enviroment ???

Therefore, essential to include co-variance: Sp^2 = Sg^2 + Se^2 + 2 cov ge

Broad heritability (H^2): the degree of heritability of a character 


Method of estimating H^2


H^2 = 4 [(correlation of full siblings) - (correlation of half-siblings)]
Can also use parent or twin inforamtion


Meaning H^2

Two conclusions from heritability studies

  1. If H^2 is non-zero, genetic differences have influenced variation between individuals, if 0, does not neccesarily mean genes are relevant
  2. H^2 is limited in prediction of effects of enviromental modification under particular circumstances

Locating the Genes

Not possible with genetic techniques to identify all genes that influence the development of a given trait.

Genetic Analysis - detects only when there is some allelic variation

Molecular Analysis - can identify genes even when they do not vary (provided the gene products can be identified

A trait may show continuous phenotypic variation, but the genetic basis for the differences must be an allelic variation at a single locus.

It is possible to use prior knowledge of biochemistry and development to guess that variation at a known locus is responsible for at least some of the variation in phenotype.

This locus is then a candidate gene for investigation of continuous phenotypic variation

Marker Gene Segregation


Linkage Analysis


More analysis of variance


Two Types

  1. Additive Variance (Sa^2) - The genetic variance associated with the average effects of substituting one allele for another
  2. Dominance Variance (Sd^2) – The genetic variance at a single locus that is attributable to dominance of one allele over another
Sg^2 = Sa^2 + Sd^2    OR    Sp^2 = Sg^2 + Se^2 =  Sa^2 + Sd^2


h^2 – proportion of phenotypic variance accounted for by additive genetic variance only (heritability in a narrow sense). This value is useful in determining whether a program of selective breeding will succeed in changing a population.

h^2 = Sa^2 / Sp^2

The effect of selection depends on the amount of additive genetic variance in a population and not on the genetic variance in general. Therefore, the narrow heritability, h2, not the broad heritability H2, is relevant for a prediction of response to selection.


Estimating the Components of Genetic Variance

•    All non additive variance is attributed to dominance variance

•    The slope of a regression line in comparing offspring values to midparent values for a character with heritability can be used to estimate h2. This can then be used to predict the effects of artificial selection (figure 18-14)

Midparent value – average phenotype of two parents

Selection Response
– difference between the offspring of the selected parents and mean of the parental generation

Selection differential – the difference between the mean of a population and the mean of the individuals selected to be parents of the next generation.

Selection Response = h2 * Selection Differential

•    h2 estimate depends on the assumption that there is no correlation between similarity of individuals’ environments and the similarity of their phenotypes

•    h2 in one population in one set of environments will not be the same as h2 in a different population in a different set of environments (figure 18-13)

Use of h2 in Breeding

•    Higher value of h2 reflects a higher offspring-parent correlation in heritability

•    If h2 is low, then only a small fraction of parental superiority will appear in the next generation

•    If h2 and H2 are both low, then this signifies a large proportion of genetic variance

•    If h2 is low, but H2 is high, then there is not much environmental variance


Hybrid – Inbred Method – Large number of inbred lines from selfing. Lines are then crossed in many different combinations, and the cross that gives the best hybrid is chosen


The subdivision of genetic variation and environmental variation provides important information about gene action that can be used in plant and animal breeding.