BIOC16 - Lecture 7!
Quantitative genetics II!
We want to understand phenotypic variation in outbreeding species!
To do this we estimate variances and heritability's using phenotypic
measurements of individuals with known genetic relationships!
We do this by relating parents to offspring ( for heritability or sexual trait) or
sibling to sibling (if you care about the particular moment) and sometimes
even half siblings (eg looking at maternal effects by looking at siblings with
same mother but different fathers).
Need an adequate number of monogamously mated pairs for the
Always go for more families than more siblings !
-measure the trait in one or both parents!
- rear offspring to some developmental stage at which parents were
- measure offspring !
! If slope = 1 then a parental increase of 1leads to an offspring increase
If slope = 0.5 then a parental increase of 1 leads to an offspring
increase of 0.5!
Therefore only half of the phenotypic variance is additive genetic
What about the other half?!
- non additive!
- therefore it doesn't contribute to resemblance of parents and
Does this imply that the total phenotypic variance of the offspring is
half that of the parents?!
NO! Because of the environment and non additive genetic variance, it
is expressed in the variance among full siblings within each family.!
-you remove this variance by using a single mean of the offspring
rather than individual offspring values
This point represents the
parents and the offspring
Maternal and Paternal Effects!
-from previous example, we assumes all resemblance between parents
and offspring was due to genetics. !
- what about parental care, provisioning of the young, teaching, etc.!
Eg: the amount and quality of parental care depends on the value of
trait of interest in parents and the amount or quality of care partly
determines the value of that same trait in the offspring.!
- this gives rise to non genetic resemblance between parents and
- the most common of these are maternal effects because usually
females put more effort in caring for young. - maternal effects - gestation, lactation, direct care of young!
- paternal effects - eg care of egg etc!
So how can we obtain estimates of variance and heritability that
aren't contaminated by these effects?
-common in birds!
-phenotype of young are regressed on genetic parents ... Leads to a
good estimate of heritability since it controls for parental effects.!
- regression of offspring on foster parents to get an estimate of
-traits are expressed differently in different environments ( real
- if different genotypes differ in the level or direction of plasticity they
express then we will see GxE (gene by environment interaction) !
-we visualize these by reaction norms.!
Each line represents a different father!
-We see an effect of the father (sire) but no effect
of the environment and no effect of GxE in
-In example 2 we see an effect of sire and of the
environment but no GxE interaction.!
-In example 3, we see no effect of sire, no
predictable environment effect but there is a gene
The main representation of GxE interactions is the crossing over of the lines.!
In example 4, there is sire and GxE but no environment. The environment here is a "macro environment"!
-forest vs ﬁeld!
-lake with low pH vs lake with high pH!
-steepness of slope indicates the amount of plasticity in the family!
In example 1, we see no plasticity, the same phenotype in each
environment, families themselves differ, VA is approximately the same
in each environment!
In example 2 we see high plasticit