Lecture 12: Quantitive Genetics

Question 1

Name 3 types of phenotypic variation

Answer 1

discontinuous
continuous (quantitive)
Threshold

Question 2

Discontinuous variation:

Answer 2

-Discrete classes of phenotype
- NO intermediates
- Follows Mendelian ratios
- Simple genetic basis, limited effect of the environment
e.g. red/white pea flowers
widow peak, dimple and blood groups

Question 3

Continous variation:

Answer 3

Also known as: quantitive, complex or 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, blood pressure, Beak size

Question 4

Graph used to show continuous variation data:

Answer 4

‘Normal’ (Gaussian) Graph. Upside bell shape.

Question 5

What does the graph used for continuous variation data show us?

Answer 5

Peak = MEAN.

68% oh width = standard deviation (measure of spread)

Question 6

Variance =

Answer 6

sd^2

Question 7

Threshold variation:

Answer 7

• appears like a discontinuous trait BUT is caused by a continuous distribution.
• distribution caused liability
• phenotype depends on critical threshold
• no simple segregation in families

Question 8

E.g of threshold variation:

Answer 8

Type 2 diabetes.
As number of predisposing alleles in genotype increase past threshold zone then you’re considered to have DIABETES. Measure of liability.

Question 9

How do genes cause continuous variation?

Answer 9

Think of coin tossing.
1 toss = 1H 1T
2 toss = 2H 1H1T 2T
3 toss = Even more inbetween values (More possible ways for middle values to be made) =NORMAL GRAPH SHAPE.
same thing if many genes contribute to variation in a trait

Question 10

Lots of genes contributing to one trait that are close together ==

Answer 10

Quantitive Trait Locus (QTL)

Question 11

Complications to continuous variation?

Answer 11

-Genes vary in the size of their effect
-Allele frequencies vary
-Some alleles are dominant/ co-dominant
AND ENVIRONMENT has a huge effect

Question 12

You’re more similar to your siblings than a stranger? WHY?

Answer 12

you inherited some of the same alleles from your parents (NATURE) you grew up in the same environment (NURTURE)

Question 13

Monozygotic twins:

Answer 13

IDENTICAL

Question 14

Dizygotic twins:

Answer 14

NOT IDENTICAL

Question 15

do genes or environment contribute to quantitive traits?

Answer 15

BOTH

Question 16

The total variation within a population for a phenotype is measured by the..

Answer 16

PHENOTYPIC VARIANCE (Vp)

Question 17

Phenotypic variance 2 components:

Answer 17

• Genetic variance (Vg)

- environmental variance (Ve)

Question 18

Genetic variance (Vg)…

Answer 18

Variation due to differences in genotype

Question 19

Environmental variance (Ve)…

Answer 19

Variation due to environmental effects

Question 20

If genetic & environmental effects are independent you can write…

Answer 20

Vp = Vg + Ve

Question 21

Heritability =

Answer 21

H^2 = Vg/Vp

Question 22

H^2 is called the..

Answer 22

‘broad sense heritability’ of the trait

Question 23

Broad sense heritability…

Answer 23

‘the proportion of the phenotypic variation in a population that is due to genetic differences among individuals’

Question 24

Broad sense heritability is often expressed as a

Answer 24

%

Question 25

H^2 = Vg/Vp =

Answer 25

Vg/(Vg+Ve)

Question 26

What happens to heritability when we DECREASE genetic variance?

Answer 26

Heritability decreases

Question 27

What happens to heritability when we INCREASE genetic variance?

Answer 27

Heritability increases

Question 28

Do estimates of heritability for a particular trait change among populations?

Answer 28

Populations living in different environments can show different degrees of heritability for a particular trait.

Question 29

Heritability estimates for a trait are…

Answer 29

relative to the genetic & environmental factors in the population

Question 30

The effect of inbred individuals on heritability..??

Answer 30

= 0 heritability after long time. 
As Vg (genetic variance) = 0 over time

Question 31

What does 0 heritability mean?

Answer 31

A zero heritability DOES NOT mean that a trait is not genetically determined.

Question 32

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

Answer 32

Allele frequencies change over time due to selection and/or genetic drift. This change affects Vg & in turn, heritability.

Question 33

Genotype-environment interaction is known as…

Answer 33

Nature vs nurture

Question 34

Genotype-environment interaction is represented by th equation:

Answer 34

Vp = Vg + Ve + VgXe

Question 35

One way to handle GxE is to consider….

And whats it called

Answer 35

the response to environmental variation as the quantitive trait
called ‘norm of reaction’

Question 36

The norm of reaction:

Answer 36

a curve that relates, for a given genotype, the contribution of environmental variation to observed phenotypic variation.

Question 37

Heritability is specific to…

Answer 37

the population and environment in which it is measured.