Chapter 20

The why is heritability of having two eyes very low?

Most of the variation in eye number is due to accidents and injuries

Variance

Average squared distance of all measurements from mean; provides information about spread of data around mean

The image below shows wild and domesticated peaches. What is this an example of?

artificial selection

Nonadditive allele

fails to produce red pigment

Expression quantitative trait loci (eQTL)

- are genetic loci that explain variation in expression levels of mRNAs. -Important tool in identifying genes that cause complex diseases: Type 2 diabetes, Asthma, Coronary artery disease

Genome-wide association studies (GWAS)

- involve scanning markers (ex. SNPs, Microsatellites) across the genomes of many individuals to find genetic variations associated with a particular trait or disease. -Two or more groups are contrasted, ex. people with and without a disease

Multifactorial (complex traits)

- quantitative traits - phenotypes results from both gene action and environmental influences

Polygenic (many genes)

- varying phenotypes result from input of genes at more than one loci - genes involved referred to as polygenes

Quantitative trait loci (QTL)

-A locus that contains a gene or genes contributing to quantitative trait -QTLs are mapped by finding associations between DNA markers and phenotypes.

Artificial Trait Selection

-Can be polygenic and multifactorial (slower and more complex). -Traits of economic importance: ▪Crops and livestock ▪Grain yield in plants ▪Weight gain or milk yield in cattle ▪Speed or stamina in horses

Correlation coefficient

-Can standardize covariance. -Indicates if two traits both increase or decrease together (r is positive), or if one increases as the other decreases (r is negative).

Standard error of the mean

-Describes errors between means determined for multiple samples of the same group. -Value always smaller than standard deviation -Accounts for sample size

Heritability

-Describes proportion of total phenotypic variation in population due to genetic factors. -Multifactorial traits: ▪High heritability attributed to genetic factors ▪Low heritability attributed to environmental 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.

Phenotypic Variance Components

-Divided between genotypic and environmental variance -Genotype-by-environment interaction variance -Heritability estimates obtained using experimental and statistical techniques ▪Partitions V p into genotypic variance (V G) and environmental variance (V E)

Polygenes

-Genes contributing to quantitative trait -Number of polygenes (n) contributing to quantitative trait—estimated based on ratio of F2 individuals resembling either of two extreme P1 phenotypes - 1/4 = ratio of F2 individuals expressing either extreme phenotype

Heritability Estimate

-Gives proportion of phenotypic variation attributed to genetic variation within a certain population in a particular environment. -Example: heritability mean of 0.65 human height: ▪65% of overall variation in height due to genotypic difference in individuals ▪Not 65% due to genes

Phenotype of polygenically controlled complex traits make it difficult to

-Identify all genes involved. -Determine effect of each gene on phenotype.

Heritability Values

-Provide no information on genes involved in traits. -Measured in populations—limited application to individuals

Mean

-Provides information on where central point lies along range of measurements for quantitative trait. - arithmetic average of set of measurements

Realized Heritability

-Selective breeding estimates potential success for artificial selection. -Example: artificial selection for increased oil content in corn

Standard Deviation

-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.

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 gene loci is occupied by either additive or nonadditive allele. 4.Additive alleles contribute to a single quantitative trait to produce substantial phenotypic variation.

Calculate the standard deviation of the following values: 8, 12, 8 and 4.

3.26

Eye color in humans is a polygenic trait influenced by 16 genes. How many phenotypic classes of eye color would you expect?

33

How many gametes are involved in the formation of dizygotic twins?

4

Interactive variance (V1)

Deviation from additive components that occurs when two or more loci behave epistatically.

Dominance variance (V D)

Deviation from additive components that results when phenotypic expression in heterozygotes is not precise

Additive variance (V A)

Genotypic variance due to additive action of alleles at quantitative trait loci

artifical selection

Humans choosing specific individuals with preferred phenotypes from initially heterogeneous population for future breeding - purpose is to develop a population with a high frequency of individuals with desired traits.

Which of the following describes an expression quantitative trait loci (eQTL)?

Lactose tolerance level is determined by lactase enzyme alleles with different mutations in the promoter that influence the gene's mRNA levels

Which of the following is an example of G x E interaction?

Lobsters with allele A grow larger than lobsters with allele a in cold water environments. In warm tropical waters lobsters with allele a grow larger.

Multiple-gene hypothesis (multiple-factor)

Many genes, individually behaving in Mendelian fashion, contribute to phenotype in a cumulative/quantitative way.

You find the heritability of height of in a population water oak trees in upstate South Carolina to be 0.67. Your colleague finds the heritability of height of in a population water oak trees in south Alabama to be 0.44. How is this possible?

Measured heritability depends on environmental variation present in population being studied

Quantitative patterns of inheritance

Mendelian factors could not account for range of phenotypes seen in quantitative patterns of inheritance.

Which of the following is NOT a major point of the multiple-gene hypothesis?

Phenotypic traits show discontinuous variation and can be cannot be quantified, only qualified

What is the difference between standard deviation and standard error?

Standard error takes sample size into account

Concordant

Twins are concordant for a given trait if either both or neither express trait.

Height in a species of plant is controlled by 4 genes with additive alleles. Which of the following is a genotype you would expect for a plant of average height?

aaBBCcDd

Which of the following is used to calculate Narrow-sense heritability?

additive variance

Genotypic variance

component includes all types of genetic variation in population

Quantitative Inheritance

continuous variation can be measured in quantitative terms

Additive allele

contributes equally to red grain color

Broad-sense heritability

estimates assume that the genotype by environment variance component is negligible

Dizygotic

fraternal twins derived from two separate fertilization events

Many traits are not distinct or clear-cut and shoe a continuous range of phenotypes:

height in humans, milk, and meat production in cattle, and yield in various crops

Monozygotic (MZ)

identical twins derived from a single zygote

Broad sense of heritability

measures contribution of genotypic variance to total phenotypic variance

Covariance statistic

measures how much variation is common to two quantitative traits

Greater number of additive alleles in genotype

more intense red color expressed in phenotype.

You are studying the Virginia opossum, Didelphis virginiana. You find 11 genes that influence the weight of newborn joeys, and that the mother's diet also influences birth weight. What term best describes this trait?

multifactorial

Discordant

one expresses trait while the other does not

Narrow Sense Heritability

proportion of phenotypic variance due to additive genotypic variance alone

You are studying corn and find that higher stalk height is correlated with larger cob size. Which of the following r values would be expected for this relationship?

r = +0.78

The data below is from a twin study. Which of these traits are monozygotic twins most likely to be discordant for?

uterine cancer

Measured Heritability

•Depends on environmental variation present in population being studied: ▪Cannot be used to estimate differences between populations.

Genotypic Variance

•can be divided into subcomponents—three kinds of gene action: -Additive variance -Dominance variance Interactive variance