CHAPTER 19

Question 1

Single Gene traits

Answer 1

• Discontinuous
• Discrete, sharply distinguishable categories

Question 2

Polygenic and multifactorial traits

Answer 2

Show continuous variation

Question 3

Genetic potential of Human height

Answer 3

• A multifactorial trait with continuous variation
• Effected by developmental and environmental factors - nutrition, exercise, rest
• Parents transmit a genetic potential to offspring that may or may not be met, depending on various influence
• Adult height >700 genetic variants

Question 4

Polygenes and differential gene effects

Answer 4

• Null model: each gene contributes the same
• Different genes contribute differently: major and modifier genes (less influence)
• Additive genes: each gene contributes an incremental amount to phenotypic value - alleles of each additive gene can be assigned a value of contribution

Question 5

Multiple-Gene Hypothesis

Answer 5

Alleles of each of the contributing genes obeyed the principles of segregation and independent assortment and had an additive effect in the production of phenotypic variation
- Two genes, two alleles each
- Alleles A1 and B1 each add one unit to phenotype
- Alleles A2 and B2 add nothing

Question 6

Number of phenotypes from additive genes

Answer 6

= 2n+1, where n is the number of genes
- more genes, more phenotypes, less demarcation between categories

Question 7

Frequency of most extreme phenotypes based on number of genes contribution to trait

Answer 7

4 ^ number of genes

Question 8

Allele segregation in quantitative trait production

Answer 8

Variation in pure-breed strands indicates;
1. Allelic segregation at multiple genes
2. Environmental effects

Question 9

Effect of the environment on phenotypes

Answer 9

1. No gene-environment interaction
   - A discrete phenotype for each genotype
2. Moderate gene-environment interaction
   - Minor overlaps between phenotypes in the F2
3. Substantial gene-environment interaction
   - Wide phenotypic ranges and significant overlaps

Question 10

GEI

Answer 10

Genotype-environmental interactions
- Estimated only when several genotypes are assayed in several defined environments

Question 11

Determining phenotype when affected by specific environmental factors

Answer 11

Phenotype = Genotype + Environment + Genotype-environment interactions

Question 12

Threshold Trait

Answer 12

dichotomous phenotypic trait controlled by multiple genes
- Continuous distribution of genetic liability in general population
- Threshold of genetic liability determines affected vs unaffected

Question 13

Threshold of genetic liability

Answer 13

The point beyond which an individual will show the affect phenotype
- Environmental factors can contribute to reaching threshold

Question 14

Goals of Quantitative Genetics

Answer 14

1. How much is the phenotypic variation contributed by genetic factors
2. How many genes influence the specific phenotypic trait
3. How much does each of the genes contribute to the phenotypic variation
4. How do genes interact with each other to influence phenotypic variation
5. How do genes interact with environmental factors to influence phenotypic variation

Question 15

Descriptive statistics of quantitative trait analysis

Answer 15

• Mean
• Mode
• Median
• SD
• Variance

Question 16

Variance

Answer 16

s^2 is a measure of the spread of distribution around the mean
= sum of (xi - x)^2/df

Question 17

Standard deviation

Answer 17

s = sqrt s^2

Question 18

Phenotypic variance

Answer 18

Vp= Vg + Ve
- Genetic variance is the genotypic contribution
- Environmental variance is the environmental contribution

Question 19

What are the 3 allelic effects on genetic variance

Answer 19

1. Additive variance: added effects of all alleles contributing to the trait
2. Dominance variance: contributions due to heterozygous individuals not having intermediate phenotype between the two homozygous states
3. Interactive variance: epistatic interactions between alleles of different genes

Question 20

Types of quantitative variation in a population

Answer 20

• Phenotypic variation
• Genetic variation
• Environmental variation
• Genotype-environment interaction variations

Question 21

Heritability

Answer 21

The proportion of phenotypic variation that is due to genetic variation

Question 22

What are the two measures of heritability

Answer 22

• Broad-sense heritability (H^2): H^2 = VG/VP
• Narrow-sense heritability (h^2): h^2 = Va/Vp

Question 23

Analyzing phenotypes of monozygotic twins

Answer 23

Share all their alleles
Vp=VE

Question 24

Analyzing phenotypes of dizygotic twins

Answer 24

About 50% their alleles are shared
VP=Ve +Vg

Question 25

Narrow sense heritability

Answer 25

The proportion of phenotypic variation due to additive genetic variation (h^2 = Va/Vp) - High NSH correlates with a greater degree of response to selection

Question 26

What are four attributes of heritability that are central to its meaning

Answer 26

1. It is a measure of the degree to which genetic differences contribute to phenotypic variation of a trait 2. Heritability values are accurate only for the environment and population in which they are measured 3. Heritability for a given trait in a population can change 4. High heritability does not preclude environmental factors

Question 27

Common sources of error in twin studies

Answer 27

1. Stronger shared maternal effects in identical twins than in fraternal twins 2. Greater similarity of treatment of identical twins than of fraternal twins 3. Greater similarity of interactions between genes and environmental factors twins than in fraternal twins

Question 28

Selection differential

Answer 28

The difference between the population mean value and the mean trait value for the mating population

Question 29

Response to selection

Answer 29

The extent to which difference between the mating trait value and the population mean value can be passed on to progeny

Question 30

Heritability and responses to natural selection

Answer 30

Higher heritability, closer in value R and S making mean of offspring resemble mean of mating population

Question 31

Quantitative Trait loci

Answer 31

Genes that contribute to phenotypic variation in quantitative traits

Question 32

GTL mapping

Answer 32

Mapping QTLs to chromosome regions/ linkage groups

Question 33

GTL mapping strategies

Answer 33

1. Analyzing genetic crosses 2. Genome-wide association studies

Question 34

Analyzing Genetic Crosses

Answer 34

- Construct genetics crosses between parental strains with different phenotypes - Develop DNA markers that differ between parental strains - Obtain phenotype and genotype of all progeny - Identify associations between phenotype and genotype at individual loci

Question 35

Genome-wide association studies for QTL Mapping Strategy

Answer 35

GWAS try to identify whether a particular sequence variant is associated with a specific phenotype in a natural population 1. Select a natural population and obtain their phenotype data 2. Obtain their whole-genome sequences 3. Compare "case" and "controls" to assess their genotypic difference at each sequence variant 4. Nucleotide frequencies showing significant differences between "cases" and "controls" are candidate QTL 5. Need to correct for "false discovery rate" to reduce false positives