Chapter 19(3)

Question 1

What commonly displays discontinuous variation?

Answer 1

the phenotypes of single-gene traits

Question 2

What commonly displays continuous variation?

Answer 2

polygenic and multifactorial traits are controlled by many genes and show continuous variation, with a range of values in an uninterrupted continuum

Question 3

What is an example of a multifactorial trait with continuous variation? Why?

Answer 3

human height
-influenced by multiple genes
-phenotypes are influenced by environmental factors, too, such as nutrition
-parents transmit a “genetic potential” to offspring that may or not be met, depending on the environment

Question 4

What does continuous variation of polygenic traits result from?

Answer 4

the effects of multiple gene that may exert different amounts of influence

Question 5

What is a major gene?

Answer 5

gene(s) that strongly influence a trait

Question 6

What are modifier genes?

Answer 6

genes that influence a trait to a lesser degree

Question 7

What are additive genes?

Answer 7

the continuous distribution of phenotype in some polygenic traits that results from incremental contributions by multiple genes

Question 8

What is an additive trait?

Answer 8

a value assigned to each additive gene that indicates its contribution to the polygenic trait

Question 9

Do all additive genes have largely different values?

Answer 9

No, for certain traits, each of the additive genes has an approximately equal effect, so that no single gene has a major influence on the phenotype

Question 10

What is the multiple gene hypothesis?

Answer 10

in early 1900s, the idea that segregation of alleles of multiple genes played a role in phenotypic variation

Question 11

What is the relationship between gene-environment interactions and range of phenotypic values?

Answer 11

The more gene-environment
interaction occurs, the wider the
potential range of phenotypic
values that may occur

Question 12

What happens when no gene-environment interaction occurs?

Answer 12

genotype corresponds to a distinct phenotype

Question 13

What are threshold traits?

Answer 13

some traits may have continuous variation of phenotype but the phenotypes can still be divided into distinct categories

Question 14

What two categories can individuals with threshold traits be?

Answer 14

-“affected” (having the condition)
-“unaffected” (normal)

Question 15

What is the variance?

Answer 15

s^2
-is a measure of the spread of distribution around the mean; it interprets how much variation exists within the sample

Question 16

What is genetic liability? What kind of distribution?

Answer 16

the organism’s risk of having the affected phenotype
-continuous distribution

Question 17

What is the standard deviation?

Answer 17

s
-expresses deviation from the mean in the same units as the scale of measure for the sample
-found by taking square root of variance

Question 18

What is Vp?

Answer 18

key part of analyzing quantitative trait variation is to analyze the factors thought to contribute to
phenotypic variance

Question 19

What can phenotypic variance be divided into?

Answer 19

-genetic variance (VG)
-environmental variance (VE)
found by adding VG and VE

Question 20

What is genetic variance?

Answer 20

the proportion of phenotypic variance due to genotype differences

Question 21

What three different kinds of allelic effects can genetic variance be divided into?

Answer 21

1. Additive Variance (VA): derives from the added effects of all alleles contributing to the trait
2. Dominance variance (VD): results from dominance relationships in which heterozygous individuals are not intermediate between the two homozygous states
3. Interactive variance( VI): derives from epistatic interactions between alleles contributing to a trait

Question 22

What does heritability help measure?

Answer 22

the proportion of phenotypic variation that is due to genetic variation

Question 23

Does heritability differ from trait to trait? Why or why not?

Answer 23

Yes, heritability differs from trait to trait; a trait with high heritability is largely the result of genetic influences, while one with low heritability is mainly attributable to
environmental variation
* Heritability is an important measure of the potential
responsiveness of a trait to selection

Question 24

What are the two measures of heritability?

Answer 24

1. Broad sense heritability (H^2): estimates the proportion of phenotypic variation that is due to genetic variation
2. Narrow sense heritability (h^2): estimates the proportion of phenotypic variation that is due to additive genetic heritability

Question 25

How are both broad sense heritability and narrow sense heritability expressed?

Answer 25

as proportions ranging in magnitude from 0.0 to 1.0

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 and population can change
4. high heritability does not preclude environmental factors

Question 27

How can broad sense heritability be used?

Answer 27

as a general measure of the magnitude of genetic influence over
phenotypic variation in a trait, when VG cannot be partitioned

Question 28

How can narrow sense heritability be used?

Answer 28

Estimates of phenotypic variation that is due to the effects of alleles of additive genes is particularly
useful in agriculture

Question 29

What do high narrow sense heritability values correlate with?

Answer 29

a greater degree of response to selection

Question 30

What is the selection differential (S) and how is it used?

Answer 30

-finding this at start in order to estimate the potential response to selection for a trait
-it gives the difference between the means of the whole population and the breeding population

Question 31

What does the response to selection (R) depend on?

Answer 31

the extent to which the difference between the population mean of the mating individuals can be passed on to progeny
-R=S*h^2