Quantitative genetics Flashcards
What is quantitative genetics?
A field where you try to predict how characters will evolve when you don’t know anything about their genetic architecture. A theory evolved for crop and livestock breeding.
What is a breeding value?
A phenotypic trait is made from various components (genes and environment). The genetic component assumes that there are very many loci each of small, independent effect (i.e. genotypes at loci don’t interact). These genetic contributions add together to give a “breeding value”, giving it the name of an “additive genetic” map.
Why was the method developed?
To try and understand the evolution of phenotypic traits when we don’t know anything about the underlying genetics. Assume that each genetic contribution has a small, independent effect.
Can you assume that the genes controlling a phenotypic trait are Mendelian?
When you have multiple genes contributing to a phenotype, even just with 3 genes you can get a continuous range of variation. For milk, many more genes involved. Many traits are continuous. Provides support for taking a continuous trait and assuming that the underlying genes are Mendelian. Wheat is a hybrid of 3 different species and therefore similar genomes. Each genome has a single locus affecting kernel colour. Each locus has a red (A, B, D) and white (a, b, d) allele. Inheritance is additive: amount of red in a kernel is proportional to the number of red alleles (0 to 6). Breeding heterozygotes leads to 7 different traits. Combined with environmental variation leads to a continuous character from white to dark red.
What is the breeder’s equation?
Evolution = heritability x selection. Evolution also called response of selection. H^2 is the heritability, ranges from 0-1. S is selection.
How do you estimate additive genetic variance when you don’t know the genes?
Relatedness tells you how many gene you are expected to share with someone. See how the phenotype varies with relatedness. The slope of the line is heritability.
Can take the average of both sexes traits.
How is selection measured?
Difference between trait mean across parents and trait mean of parental cohort
An example of quantitative genetics success?
Broiler chickens - bred to be eaten. In 1920s, market chickens were 2lbs, now 5lbs. Also halved the age at which they go to market.
Most of this difference is due to genetics.
An example of quantitative genetics failure?
Fails to explain evolution in the wild. Predict evolution but often doesn’t happen. Reasons include - can target selection in the individuals you want and targeted at one trait, environment is easy so no other selection pressures. In the wild, there are many more ways to die. Selection is targeting many traits, and the ability to live in a harsh environment. The environment is also very variable in the wild. If you can’t statistically correct for environmental variation, predictions are likely to fail. Complications of density dependence.
