Quantitative genetics Flashcards
Types of variation – discrete
AKA - discontinuous, categorical, mendelian
Discrete classes of phenotype
No intermediates
Follows mendelian ratios
Simple genetic basis, limited effect of the environment
E.g. blood groups, red/white pea flowers
Types of variation – continuous
AKA - quantitative, complex, multifactorial
Phenotypes do not fall into classes
Offspring of intermediate phenotype
Cannot observe mendelian ratios
Complex genetic basis plus effect of the environment
E.g. height, beak size
The combined effects of several polymorphic loci result in a nearly continuous distribution
Quantitative traits are heritable
Types of variation – threshold
Appears discontinuous but caused by continuous distribution
Distribution is called liability
Phenotype depends on critical threshold
No simple segregation in families
How can genes cause continuous variation?
Polygenic traits, such as height or weight, are determined by the combined effects of many genes that contribute to the trait
Each of these genes may have a small effect on the trait, but together they can add up to produce a wide range of phenotypic variation
In addition, environmental factors, such as nutrition, exercise, and exposure to toxins, can also affect the expression of these genes and contribute to the overall phenotype.
Continuous variation can also arise through interactions between genes and the environment.
E.g. some genes may only be expressed in response to specific environmental cues or conditions, leading to different phenotypes under different environmental conditions.
Continuous traits heritable?
Looking at twin studies
-If trait similarity is due to genes, not environment, correlation should be stronger in monozygotic twins rather that dizygotic twins
-Should still be present even if twins reared apart
Continuous traits influenced by genetics and environment
Environmental variance complicates the inference of the effects of genotypes at individual loci
