Lecture 11: Quantitative Genetics

Question 1

define a discontinuous trait and give an example

Answer 1

clear-cut, ‘either-or’ phenotypes between alternative alleles
eg. all of the traits Mendel studied in peas

Question 2

how does the phenotype vary in incomplete dominance?

Answer 2

the phenotype varies continuously with levels of protein function/the amount of functional gene product

Question 3

molecular mechanisms of incomplete dominance in snapdragon colour

Answer 3

A1: functional enzyme
A2: nonfunctional enzyme

A1A1: red (100% pigment production)
A1A2: pink (50% pigment production)
A2:A2: white (0% pigment production)

Question 4

how is familial hypercholesteremia an example of incomplete dominance?

Answer 4

the heterozygous phenotype (250-500mg/dl) is distinct from and intermediate compared to either homozygous phenotype (<250mg/dl and >500mg/dl)

Question 5

qualitative trait:
- examples
- mode of inheritance
- environmental effects

Answer 5

• blood and serum types, enzyme defects
• simple Mendelian (monogene)
• little environmental effect

Question 6

quantitative trait
- examples
- mode of inheritance
- environmental effects

Answer 6

• stature height, intelligence, milk yield
• complex (polygene)
• moderate to great environmental effect

Question 7

threshold trait
- examples
- mode of inheritance
- environmental effects

Answer 7

• hare lib, many diseases
• complex (polygene)
• moderate to great environmental effect

Question 8

what kind of distribution does continuous variation typically have?

Answer 8

a normal distribution, which is bell-shaped

Question 9

what is the Mendelian explanation of continuous variation?

Answer 9

alleles are incompletely dominant and have additive effects. the more genes or alleles, the more possible phenotypic classes, the greater the similarity to continuous variation

Question 10

what interaction results in the normal distribution of a quantitative trait?

Answer 10

genetic and environmental influences

Question 11

draw 3 graphs for no of individuals vs trait value:
- two genes, two incompletely dominant alleles
- environmental influences
- normal distribution

Question 12

what could be the result of more than two alleles of the same gene?

Answer 12

additional variation might arise

Question 13

additive alleles

Answer 13

contribute to traits in a cumulative manner. the more of a particular allele an individual has, the stronger the expression of that trait

Question 14

give an example of the impact of additive alleles on human traits

Answer 14

• a handful of ultra rare inherited mutations likely shave years off a person’s life
• each of these DNA variants can reduce lifespan by as much as 6 months
• and different combinations dictate how early age-related diseases such as cancer, diabetes, and dementia will develop
• thus, each variant is an additive allele

Question 15

define continuous traits and give examples in humans

Answer 15

are determined by segregating alleles of many genes that interact together and with the environment
eg height, weight, skin colour

Question 16

what is another name for continuous traits?

Answer 16

quantitative traits

Question 17

continuous traits are usually

Answer 17

polygenic

Question 18

three categories of quantitative traits

Answer 18

meristic (counting traits): takes on a range of discrete values. these traits are quantitative, but restricted to certain discrete values (eg salamander spots)

continuous traits: takes on a potentially infinite number of states or values over a continuous range (eg height)

threshold trait: individuals who have a certain number of risk factors will exceed a threshold and develop the disease.

Question 19

do threshold traits follow Mendelian rules?

Answer 19

no, they do not follow simple Mendelian rules or produce Mendelian ratios in pedigrees

Question 20

give an example of environmental modification of certain mutations

Answer 20

siamese cats

Question 21

quantitative trait locus (QTL)

Answer 21

region of the genome that correlates with the quantitative trait and may contain the genes affecting the quantitative trait

Question 22

how can quantitative traits be described by a frequency distribution?

Answer 22

frequency distributions associated with each genotype at the QTL overlap. we cannot determine genotype by simply looking at an individual’s phenotype as we can with genes that segregate in Mendelian ratios

Question 23

how does our predictive power of genotype from phenotype vary with trait value? draw a graph

Answer 23

at either end (low or high trait value) we can guess the genotype from the phenotype. however, at intermediate trait values, this is very hard

Question 24

phenotype =

Answer 24

genotype + environment