genetics 23: genetic basis of complex traits

What best defines a multifactorial trait?
A. A trait controlled by a single gene
B. A trait influenced by multiple genes and environmental factors
C. A trait that does not vary among individuals
D. A trait determined only by the environment

A trait influenced by multiple genes and environmental factors

Which of the following is an example of a complex trait?
A. Blood type determined by one gene
B. Height, influenced by many genes and environment
C. Eye color determined by a single gene
D. Huntington’s disease caused by a single mutation

Height, influenced by many genes and environment

Genetic architecture of a complex trait includes all EXCEPT:
A. Specific genetic effects of individual genes
B. Gene-gene interactions
C. Gene-environment interactions
D. Only environmental factors without genetic contribution

Only environmental factors without genetic contribution

Gene-gene interactions refer to:
A. How environmental factors affect gene expression
B. How genes influence each other’s effects on a trait
C. How mutations cause new alleles to arise
D. How alleles segregate independently during meiosis

How genes influence each other’s effects on a trait

True or False: Complex traits typically show a wide range of variation in the population.

true

Which of the following is an example of a continuous quantitative trait?
A. Number of eggs laid by a hen
B. Height in humans
C. Presence or absence of diabetes
D. Blood type

Height in humans

Threshold traits differ from continuous traits because:
A. They show a smooth range of phenotypes
B. They have discrete categories but are influenced by continuous underlying factors
C. They are determined by a single gene
D. They are unaffected by environmental factors

They have discrete categories but are influenced by continuous underlying factors

Which type of quantitative trait can be counted in whole numbers but still shows genetic and environmental variation?
A. Continuous traits
B. Categorical traits
C. Threshold traits
D. Qualitative traits

Categorical traits

Traits that show a smooth range of phenotypes with no distinct categories are called __________ traits.

continuous

The number of offspring produced by an organism is an example of a __________ quantitative trait.

categorical

True or False: Both genetic and environmental factors contribute to variation in quantitative traits.

true

Explain why adult-onset diabetes is considered a threshold trait.

Adult-onset diabetes is considered a threshold trait because it shows only two phenotypes—affected or unaffected—but underlying this is a continuous range of genetic and environmental risk factors. The disease appears only when a person’s combined genetic susceptibility and environmental exposures pass a certain threshold.

What does the mean of a quantitative trait represent?
A) The range of phenotypic values
B) The most common phenotype
C) The average phenotypic value across the population
D) The spread of phenotypic values

The average phenotypic value across the population

The variance of a quantitative trait measures:
A) The average phenotype
B) The spread or variability of phenotypes around the mean
C) The difference between the highest and lowest phenotype
D) The shape of the phenotypic distribution

The spread or variability of phenotypes around the mean

The standard deviation is:
A) The variance squared
B) The square root of the variance
C) The mean phenotype value
D) The frequency of each phenotype

The square root of the variance

A small standard deviation in a population's trait distribution means:
A) Most individuals have very different phenotypes
B) The phenotypes are spread widely across the population
C) Most individuals have phenotypes close to the mean
D) The mean phenotype is very low

Most individuals have phenotypes close to the mean

According to the empirical rule, approximately what percentage of individuals fall within two standard deviations of the mean?
A) 50%
B) 68%
C) 95%
D) 99.7%

95%

f a trait follows a normal distribution, what percentage of individuals fall outside three standard deviations from the mean?
A) 0.3%
B) 1.5%
C) 5%
D) 32%

0.3%

Within one standard deviation of the mean, about __________ percent of individuals in the population are found.

68%

A broad curve in a trait distribution indicates a __________ standard deviation and __________ variability.

large; high

The empirical rule helps us understand how __________ or __________ a certain phenotype is in a population.

typical; rare

Explain what it means for a phenotype to lie more than two standard deviations away from the mean in a population.

A phenotype more than two standard deviations away from the mean is considered rare because it falls outside the range that contains approximately 95% of the population. This suggests that only about 5% of individuals have such extreme trait values.

What does genotypic variance ( sg2sg2​ ) represent?
A) Variation caused by environmental differences
B) Variation caused by genetic differences among individuals
C) Total variation in phenotype
D) Variation caused by random chance

Variation caused by genetic differences among individuals

Environmental variance ( se2se2​ ) refers to:
A) Variation caused by genetic mutations
B) Variation caused by differences in genotype
C) Variation caused by environmental factors
D) Variation caused by alleles being lost

Variation caused by environmental factors

The total phenotypic variance ( sp2sp2​ ) in a population is:
A) The difference between genotypic and environmental variance
B) The sum of genotypic and environmental variance
C) Equal to genotypic variance only
D) Equal to environmental variance only

The sum of genotypic and environmental variance

Explain why both genetic and environmental variance together often result in a normal distribution of phenotypes for a quantitative trait in a population.

Because multiple genetic factors and environmental influences act additively and independently on a trait, the combined effect produces a range of phenotypes that cluster around an average value. This additive variation tends to follow the normal distribution due to the central limit theorem.

Why do inbred lines show minimal genetic variation within the line?
A) They are hybrids of different species
B) They have a high mutation rate
C) They are genetically identical due to extensive homozygosity
D) They experience intense natural selection

They are genetically identical due to extensive homozygosity

What does phenotypic variance in an inbred line primarily represent?
A) Genetic variance only
B) Environmental variance only
C) Both genetic and environmental variance
D) Neither genetic nor environmental variance

Environmental variance only

Crossing two genetically distinct inbred lines produces F1 offspring that are:
A) Genetically uniform with high heterozygosity
B) Genetically variable due to segregation
C) Genetically identical to the parents
D) Genetically uniform but homozygous

Genetically uniform with high heterozygosity

In an inbred line, individuals are ___________ (genetically identical / genetically diverse), so all phenotypic variation observed is due to ___________.

genetically identical; environmental factors

The F1 offspring of two distinct inbred lines are genetically ____________, and phenotypic variation among F1 individuals is primarily due to ___________.

uniform; environmental variance

Phenotypic variation in the F2 population results from both __________ variance and __________ variance.

genetic; environmental

When measuring phenotypic variance in an inbred line, the expected value of genetic variance is __________.

zero (or near zero)

T/F: The F2 population is genetically uniform because it is derived from two inbred parents.

false

T/F: The phenotypic variance in an F1 population is usually greater than that in the F2 population.

false

What best describes a genotype-environment (G-E) interaction?
A) All genotypes respond identically to environmental changes
B) Different genotypes respond differently to the same environment
C) The environment has no effect on phenotype
D) Genetic effects are independent of the environment

Different genotypes respond differently to the same environment

f Genotype A grows better at low temperature, and Genotype B grows better at high temperature, this is an example of:
A) Genetic drift
B) Gene flow
C) Genotype-by-environment interaction
D) Epistasis

Genotype-by-environment interaction

Which of the following scenarios best illustrates genotype-by-sex interaction?
A) A genotype has the same effect on height in males and females
B) A genotype causes disease only in one sex due to hormonal differences
C) Environmental factors affect males and females differently
D) Females have more genetic diversity than males

A genotype causes disease only in one sex due to hormonal differences

How can genotype-by-sex interactions affect population-level phenotype distributions?
A) They create identical distributions for both sexes
B) They result in males and females having different average trait values or variances
C) They eliminate phenotypic variation
D) They only affect environmental variance

They result in males and females having different average trait values or variances

hy are genotype-environment interactions important in evolutionary biology?
A) They show that phenotypic plasticity is impossible
B) They can maintain genetic variation in populations by favoring different genotypes in different environments
C) They always reduce fitness
D) They prevent natural selection from acting on traits

They can maintain genetic variation in populations by favoring different genotypes in different environments

If males and females with the same genotype have different average heights due to hormonal differences, this is called __________.

genotype-by-sex interaction

In a study where Genotype A performs better in environment 1 and Genotype B performs better in environment 2, natural selection may maintain both genotypes due to __________.

genotype-environment interaction

When a genotype shows stronger expression in males than females, it demonstrates __________ effects on phenotype.

sex-specific or genotype-by-sex interaction

Broad-sense heritability (H²) includes which of the following genetic effects?
A) Only additive genetic effects
B) Additive and dominance effects, but not epistasis
C) Additive, dominance, and epistatic effects
D) Only environmental effects

Additive, dominance, and epistatic effects

Narrow-sense heritability (h²) specifically estimates the proportion of phenotypic variance due to:
A) All genetic effects combined
B) Environmental variance
C) Additive genetic variance only
D) Dominance variance only

Additive genetic variance only

Which of the following is true about narrow-sense heritability?
A) It predicts the response to natural selection
B) It ignores additive genetic effects
C) It includes epistasis and dominance
D) It is always greater than broad-sense heritability

It predicts the response to natural selection

If the total phenotypic variance (sp²) is 100, the genetic variance (sg²) is 40, and the environmental variance (se²) is 60, what is the broad-sense heritability (H²)?
A) 0.4
B) 0.6
C) 0.67
D) 1.5

0.4 (H² = sg² / sp² = 40/100 = 0.4)

If the additive genetic variance (sa²) is 25 and total phenotypic variance (sp²) is 100, what is the narrow-sense heritability (h²)?
A) 0.25
B) 0.75
C) 0.4
D) 1.0

0.25 (h² = sa² / sp² = 25/100 = 0.25)

Narrow-sense heritability is important for predicting __________ to selection in populations.

response

If the heritability is about 0.64, what does this imply?
A) The trait is mostly influenced by the environment
B) The trait will respond poorly to future selection
C) The trait has a high additive genetic component and responds well to selection
D) The trait is controlled only by dominance effects

The trait has a high additive genetic component and responds well to selection

What is one reason artificial selection may reach a limit in improving a trait?
A) Mutation rates increase dramatically
B) All favorable alleles become fixed or lost
C) Environmental variation increases
D) Selection differential becomes negative

All favorable alleles become fixed or lost

Natural selection can limit artificial selection because:
A) It increases genetic variation continuously
B) It always supports artificial selection goals
C) It counteracts artificial selection if selected traits reduce overall fitness
D) It only acts on non-genetic factors

It counteracts artificial selection if selected traits reduce overall fitness

A correlated response to selection occurs because:
A) The environment changes unexpectedly
B) Selection is applied to multiple traits simultaneously
C) Genes controlling different traits are linked or pleiotropic effects occur
D) Only the selected trait changes

Genes controlling different traits are linked or pleiotropic effects occur

If selecting for hens that lay more eggs causes a decrease in egg size, this is an example of:
A) Genetic drift
B) Mutation
C) Correlated response to selection
D) Genetic bottleneck

Correlated response to selection

Natural selection pushes back against artificially selected traits if they reduce overall _______.

fitness

what is an example natural selection counteracting artificial selection?

higher birth weight might decrease survival rate of offspring

T/F: Artificial selection involves choosing parents with desired traits to breed 

true

a side effect on a trait you’re not selecting for  

correlated response

What is a Quantitative Trait Locus (QTL)?
A) A single gene responsible for a Mendelian trait
B) A DNA region containing genes that influence a quantitative trait
C) A mutation causing a disease
D) A protein involved in metabolism

A DNA region containing genes that influence a quantitative trait

QTLs are usually identified by looking for associations between phenotypes and:
A) Environmental factors
B) Polymorphic DNA markers such as SNPs
C) Protein sequences
D) Chromosome number

Polymorphic DNA markers such as SNPs

Genome-wide association studies (GWAS) identify QTLs by:
A) Breeding individuals with extreme traits
B) Scanning large populations for statistical correlations between SNP alleles and phenotypes
C) Sequencing only affected individuals
D) Measuring gene expression levels

Scanning large populations for statistical correlations between SNP alleles and phenotypes

Why is there often “missing heritability” in complex traits despite identifying many QTLs?
A) All heritability is due to environmental factors
B) Gene-gene interactions and rare variants are not fully captured
C) Genetic drift removes heritability
D) SNPs are only found in coding regions

Gene-gene interactions and rare variants are not fully captured

Most QTLs identified for complex traits:
A) Have large individual effects on the trait
B) Are located on sex chromosomes only
C) Have small individual effects and collectively explain only a fraction of heritability
D) Are single mutations that cause the trait

Have small individual effects and collectively explain only a fraction of heritability

QTLs are identified by finding associations between a trait and _______ markers, usually _______.

polymorphic DNA; SNPs

GWAS uses data from large _______ consisting of affected and unaffected individuals to find QTLs.

populations

Missing heritability may be due to undetected _______ effects and rare genetic _______.

epistatic; variants

T/F: GWAS can detect all genetic factors contributing to a complex trait.

false

T/F: Each gene identified by QTL mapping usually has a large effect on quantitative traits.

false

T/F: QTL mapping helps to narrow down genetic regions that influence complex traits or diseases.

true

T/F: If a particular allele at a marker is found more often in individuals with a certain trait value, that maker is linked to a QTL 

true