Ch 21 Quantitative Genetics and Multifactorial Traits

Question 1

Intro

Answer 1

1. Many traits not distinct and clear cut—show a continuous range of phenotypes
   - Height in humans, milk and meat production in cattle, yield in various crops
2. Continuous variation
   - Measured and described in quantitative terms known as quantitative inheritance
3. Polygenetic
   - Varying phenotypes result from input of genes at more than one loci.
4. Continuous quantitative trait
   - Often the result of polygenic inheritance and frequently multifactorial
   - Result of interaction between genes and environment

Question 2

Quantitative Traits Can Be Explained in Mendelian Terms

- Quantitative Patterns

Answer 2

Quantitative patterns
– Mendelian factors could not account for range of phenotypes seen in quantitative patterns of inheritance.
Multiple-gene hypothesis
–Many genes, individually behaving in Mendelian fashion, contribute to phenotype in a cumulative/quantitative way.
- ex: grain color in wheat

Question 3

Additive and Nonadditive Alleles

Answer 3

1. Multiple-gene hypothesis: additive & nonadditive alleles
   –Various grain color phenotypes due to additive and nonadditive alleles
   –Additive allele: contributes equally to red grain color
   –Nonadditive allele: fails to produce red pigment
2. Greater number of additive alleles in genotype=more intense red color expressed in phenotype

Question 4

Additive Alleles: The Basis of Continuous Variation

Answer 4

Multiple-gene hypothesis—major points

1. Phenotypic traits show continuous variation and can be quantified by measuring, weighing, and so on.
2. Two or more loci show an additive effect on the expression of phenotype (polygenic inheritance).
3. Each additive allele contributes an equal amount.
4. Additive alleles contribute to a single quantitative trait to produce substantial phenotypic variation

Question 5

The Study of Polygenic Traits Relies on Statistical Analysis: Polygenic Traits

Answer 5

• Measured in large sample and representative individuals of population
• Data form normal distribution: characteristic bell-shaped curve when plotted as frequency histogram

Question 6

The mean

Answer 6

• Provides information on where central point lies along range of measurements
• Gives quantitative trait
• arithmetic average of set of measurements
• Variance—average squared distance of all measurements from mean; provides information about spread of data around mean

Question 7

Standard deviation

Answer 7

Standard deviation (s)
–Square root of variance
–More than 95% of all values are found within two standard deviations to either side of mean.
–Standard deviation can be interpreted in form of probability.

Question 8

Heritability Values Estimate the Genetic Contribution to Phenotypic Variability
- Heritability

Answer 8

• Describes proportion of total phenotypic variation in population due to genetic factors
• Does not indicate how much of the trait is genetically determined or the extent to which an individual’s phenotype is due to genotype
• Example: heritability mean (h2)of 0.65 human height
• ——65% of overall variation in height due to genotypic difference in individuals
• ——Not 65% due to genes

Question 9

Phenotypic Variance

Answer 9

Phenotypic variance (Vp) components

1. Genotypic variance
2. Environmental variance
3. Genotype-by-environment interaction variance(Figure21-5)
4. Heritability estimates obtained using experimental and statistical techniques
   - Partitions Vp into genotypic variance (Vg) and environmental variance (Ve)

Question 10

Artificial Selection

Answer 10

• Choosing specific individuals with preferred phenotypes from initially heterogeneous population for future breeding
• Purpose: to develop population containing high frequency of individuals with desired traits

Question 11

Twin Studies Allow an Estimation of Heritability in Humans

- twins

Answer 11

Human twins
– Useful subjects for examining phenotypic variance for multifactorial traits due to genotype as opposed to environment
– Underlying principle
Monozygotic (MZ)—derived from single zygote
Dizygotic (DZ)—derived from two separate fertilization events

Question 12

Identical and Fraternal Twins

Answer 12

Identical twins (monozygotic) 
–Phenotypic variance equals environmental variance and no genotypic variance.
Fraternal twins (dizygotic)
–Phenotypic differences represent environmental variance and approximately half genotypic variance.

Question 13

Concordance

Answer 13

• Twins are concordant for a given trait if either both or neither express trait.
• Discordant: One expresses while the other does not.
• Comparison of concordant values (MZ vs. DZ) gives potential value for heritability assessment.

Question 14

Monozygotic twins

Answer 14

Assumption that MZ twins share the same genome has been valuable for estimating heritability for many multifactorial diseases such as
–Cardiovascular disease
–Diabetes
–Mental illness
The results from genomic research have challenged whether MZ twins are truly identical.

Question 15

Most relevant genomic discoveries about twins

Answer 15

– By the time they are born, MZ twins do not necessarily have identical genomes.
–Gene-expression patterns in MZ twins change with age; this leads to phenotypic differences.

Question 16

Quantitative Trait Loci Are Useful in Studying Multifactorial Phenotype
- Quantitation Trait Loci

Answer 16

Quantitative trait loci (QTLs)
– Multiple genes contributing to quantitative trait
– Involve finding associations between DNA markers and phenotypes
QTL mapping
– Artificial selection over many generations
— highly divergent lines created

Question 17

QTL Mapping

Answer 17

Genetic mapping
– When many QTLs for a given trait are located, the genetic map gives the positions of genes involved on different chromosomes.
QTL mapping
– Extensively used in agriculture
– Corn, rice, wheat, and tomatoes (Table21.5)
– Livestock: cattle, pigs, sheep, and chickens