Day 5

Lecture 5:
Chapter 3.3 Quantitative genetics


Many traits take quantitative instead of qualitative values, and are determined by many genes

Genetics and inheritance of quantitative traits is described in statistical terms.



Quantitative traits:

- Traits affected by many genes and the environment
- Also known as “complex traits”
- Typical examples:
o Fitness
o Body weight
o Growth rate
o Egg number
o Seed yield
o Asthma
o Schizophrenia
o Resistance to infectious diseases
o Intelligence
o Human height



Quantitative genetics:

- The genetics of quantitative traits
o Is the foundation of animal breeding
o Was developed by agriculturist
 Together with statistics
o Very much an applied science
 Livestock and plant breeding
 Powerful tools : “it works”
 You can change populations
o No knowledge of the genes required
 Statistical genetics
 In the past: Little insight in genetic background/ genes
 Recent: GWAS  more knowledge of genes



Quantitative trait vs qualitative traits:

- Qualitative traits:
o Individuals cannot be ordered from low to high values of the traits

,  E.g. coat colour types
- Quantitative traits have a value
o Trait values can be expressed on a scale from low to high
o For example
 Body weigh in rabbits
 Fitness
 Lifetime
 Litter size
 Teat number in pigs.



Continuous quantitative trait:

- Continuous: all trait values are possible
- We have a continuous distribution (often normal)




-



Categorical quantitative traits:

- Categorical traits (meristic traits)
o Trait values fall into a limited number of categories
 Countable traits
 Number of offspring, number of ears, number of seeds
 But not body weight.
o Histograms of frequency
 Bristle number in drosophila.

, 

The basic of quantitative genetic model:

- Phenotype = genotype + environment.
-




Genotypic value: the average value of the genotype.

The environmental effect is a deviation from the average.