4: Quantitative genetics

Question 1

What are the 3 types of phenotypic variation?

Answer 1

Discontinuous (categorical/mendelian)

Continuous (quantitative/complex/multifactoral)

Threshold variation

Question 2

Describe discontinuous variation

Answer 2

• No intermediates
• Follows mendelian ratios
• Simple genetic basis, limited environ effect
  E.g height, blood groups, red/white pea flowers

Question 3

Describe continuous variation

Answer 3

• Phenotypes don’t fall into classes
• Offspring of intermediate phenotype
• Cannot observe mendelian ratios
• Complex genetic basis + environ
• Normal curve
  E.g IQ, Dermal ridges (fingerprints), crop yield, weight gain in animals)

Question 4

Describe threshold variation

Answer 4

• Appears like a discontinuous trait but is caused by a continuous distribution
• Distribution is called liability
• Phenotype depends on critical threshold
• No simple segregation in families

Question 5

Define phenotypic variance

Answer 5

total variation within a pop. for a phenotype is measured by the phenotypic variance (Vp)

Question 6

What are the 2 components of phenotypic variance?

Answer 6

Variation due to diffs in genotype → genetic variance VG

Variation due to environ effects → environmental variance VE

Question 7

Define broad sense heritability

Answer 7

the proportion of the phenotypic variation in a pop. that is due to genetic differences among individuals
→ often expressed as a %

Question 8

Why is heritability an excellent predictor for short-term but NOT long-term response?

(think about the influence of selection)

Answer 8

Allele frequencies change over time due to selection and/or genetic drift
- This change affects genetic variance (VG) and in turn heritability
- Also the effects of environ change over time so environmental variance (VE) can change too

Question 9

The more heritable a trait is, the more likely it will respond to ________

Answer 9

selection

Question 10

define norm of reaction

Answer 10

A way to view GxE as the response to environ variation as the quantitative trait