Qualitative genetics

Question 1

Complex Genetic Trait

Answer 1

A trait that results from interplay of multiple factors, each with a relatively small effect.

Question 2

Define qualitative complex trait and give examples.

Answer 2

Discontinuous and categorial heritable trait

e.g most diseases (diabetes and asthma)

Question 3

Do qualitative traits follow Mendel’s laws?

Answer 3

genetic transmission preserves categories, genes that contribute to qualitative traits do not segregate according to typical Mendelian ratios

gene = yes (one from mum one from dad)

trait = no

Question 4

Discontinuous Phenotypic variation - meaning

Answer 4

Resulting phenotypes from qualitative traits fall into distinct categories that can be easily classified.

Question 5

Disconcordance vs concordance

Answer 5

Dis-concordance = trait that is only present in 1 of the 2 individuals

Concordance = probability (0-1) that trait will be shared between 2 individuals.

Question 6

Susceptibility and liability definition

Answer 6

Susceptibility

Innate (genetic) tendency to develop a trait

Liability

Total risk of developing a trait (incorporates all sources of variation)

Question 7

Where does the threshold of liability exist?

Answer 7

Above which the trait appears.

Question 8

Threshold trait

Answer 8

A trait that manifests when the liability exceeds the threshold

Question 9

Describe Falconer’s threshold Model

Answer 9

Hypothetical scale of liability

Person who exceeds a certain level of risk/ liability/ threshold they have a disease.

Question 10

For Falconers Threshold model what does:

q

x

mean?

Answer 10

q

incidence

x

How far threshold is from mean (in SDs)

Question 11

For Falconers Threshold model what is:

Xg-xr?

Answer 11

Xg-xr

How far the curves have moved against each other

-how far the relatives’ curve has been pushed to the right

Question 12

For Falconers Threshold model what is:

ag

Answer 12

Difference in mean liability of affected individuals in the general population from mean liability in total population

Question 13

For Falconers Threshold model what is:

b

Answer 13

Regression coefficient (slope is not heritability, have to correct for relatedness)

Question 14

For Falconers Threshold model what is:

Answer 14

Mean liability of general population

Question 15

For Falconers Threshold model what is:

A

Answer 15

Mean liability of affected individuals in the general population

Question 16

For Falconers Threshold model what is:

R

Answer 16

Mean liability of relatives of affected individuals

Question 17

Selection differential

Answer 17

A-G

Affected individuals are ‘selected’ out of the general population

(same as Xg-xr)

Question 18

Response differential

Answer 18

R-G

Difference between mean of relatives and mean of the general population

(same as ag )

Question 19

Steps for calculating heritability for Complex Qualitative traits.

Answer 19

1- find incidence of general population
2- use this to find x (xg)
3- find the incidence of affected (qr)
4- find the relevant x (Xr = relatives)
5- calculate the difference (Xg-Xr)
6-determine a
7- calculate b (xg-xr/a)

Question 20

what does the coefficient of relatedness indicate?

Answer 20

the proportion of genes that types of relatives share

Question 21

Qualitative vs Quantitative

Answer 21

Qualitative = categorical (discontinuous) heritable character (sick, color blind)

Quantitative = metric (continuous and measureable) heritable trait due to a interplay of multiple factors with relatively small effects.

Question 22

what are the r values for:

a- identical twins
b- siblings/ progeny
c- grandparents and aunts
d- first cousins

Answer 22

a- identical twins, degree, r= 1
b- 1st degree, r=0.5
c- 2nd degree, r= 0.25
d- 3rd degree, r= 0.125