Quantitative Genetics, Heretability and Twins

Question 1

Be able to identify some characteristics that are under polygenic (as opposed to single gene) control.

Answer 1

Intelligence
Height
Weight
Hair Color
Eye Color
Skin Color

Question 2

What are some characteristics of multifactorial/polygenic traits?

Answer 2

o Many genes control the trait,
effect of each gene is small,
effects of genes are additive (no dominance),
environment usually influences phenotype,
large number of genotypes resulting in large number of phenotypes,
underlying discrete genotypic distribution, but appears continuous due to environmental effects,
often assume normal distribution

Question 3

Describe the difference between monozygotic and dizygotic twins.

Answer 3

Monozygotic: Identical
- Same phenotype for a large number of traits

Dizygotic: Fraternal
- Separate placentas and amnions

Question 4

What are “Super Twins”?

Answer 4

Combinations of identical and fraternal twinning events that sometimes occurs with multiples

Question 5

Which type of twinning is more likely to run in families? Why?

Answer 5

Fraternal (DZ) is inherited and increases with age of mother

Question 6

What is meant by term “conjoined” twins? Are they identical or fraternal?

Answer 6

Identical twins joined in utero that did not separate fully from one another and are still partially united.

Question 7

Who were Chang and Eng Bunker? Who are Abby and Brittany Hensel?

Answer 7

Chang and Eng Bunker: Born in 1811, conjoined twins in Siam Thailand.
Abby and Brittany Hensel: Dicephalus Conjoined Twins born 1990.

Question 8

Define heritability

Answer 8

The proportion of phenotypic variance that is due to genetic effects, not environmental.

Question 9

Define concordance

Answer 9

The probability that a pair of individuals will both have a certain characteristic, given that one of the pair has the characteristic.
Compare pairs of monozygotic and dizygotic twins for a trait.

Question 10

Define concordant

Answer 10

Twins same for trait

Question 11

Define discordant

Answer 11

Twins differ for trait

Question 12

Define correlation

Answer 12

The proportion of variance that two traits share due to genetic causes.

Question 13

Be able to compare values of MZ:DZ concordance ratios to determine which trait has a higher heritability.

Answer 13

• High heritability: High MZ: Low DZ
• Environmental Factors: Similar MZ and DZ
• Genetic predisposition, but still some environmental factors: Low MZ but still much higher than DZ

Question 14

Use the idea of the additive effects of alleles to determine whether an individual of a particular genotype has a genetic disorder

Answer 14

(see practice problems)

Question 15

Which trait is expected to have a higher heritability based on the MZ:DZ ratios given? Trait A 95:85 or Trait B 95:4

Answer 15

Trait B

Question 16

T or F: The expression of quantitative characteristics depends on the genotype and the environment

Answer 16

True

Question 17

How are twin studies used to help us understand the heritability of quantitative characteristics?
o In order to evaluate the genetic contribution of a characteristic it is best to compare which of the following 2 types of twins? MZ reared together, MZ reared apart, DZ reared together, DZ reared apart
o Which 2 should be compared to evaluate the environmental contribution to a trait?

Answer 17

Compare data from MZ and DZ twins to determine the relative contribution of genotype and environment.

• Genetic effects: MZ reared together and DZ reared together.
• Environmental effects: MZ reared together and apart.