lecture 16- quantitative genetics

Question 1

polygenic inheritance

Answer 1

Polygenic inheritance refers to the inheritance of a trait governed by more than one genes. Generally, three or more genes govern the inheritance of polygenic traits. Multiple independent genes have an additive or similar effect on a single quantitative trait.

Question 2

continuous traits

Answer 2

influenced by

1) alleles at many loci (polygenic) and/or

2) environmental effects

many genes contribute to single phenotype

Question 3

quantitative trait loci

Answer 3

Question 4

heritability

Answer 4

the proportion of phenotypic variance that can be attributed to genetics

*if same genetics but different environment, lower heritability (increase in environmental variance)

***specific to particular population in particular environment

Question 5

backcross

Answer 5

Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, to achieve offspring with a genetic identity closer to that of the parent. It is used in horticulture, animal breeding, and production of gene knockout organisms

Question 6

intercross

Answer 6

A mating between two members of the F1 generation or between two animals that are heterozygous at the same locus

A cross between two identically hybrid individuals (A/a X A/a).

Question 7

heritability

Answer 7

the proportion of total phenotypic variation that is due to genetic differences

Question 8

mean (x with bar on top)

Answer 8

average measure which provides information about the center of the distribution

Question 9

variance (s^2)

Answer 9

indicates the variability of a group of measurements, or how spread out the distribution is

(the great the variance, the wider the spread in the distribution)

Question 10

phenotypic variance (v with subscript p)

Answer 10

can be measured in a population and then partitioned into two components: genetic and environmental

Question 10

phenotypic variance (v with subscript p)

Answer 10

can be measured in a population and then partitioned into two components: genetic and environmental

Question 11

broad sense heritability (h^2)

Answer 11

proportion of phenotypic variance that is due to genetic variance

(i.e.) how much of the variation is genetic vs how much is environmental?

Question 12

broad sense heritability

Answer 12

Broad-sense heritability, defined as H2 = VG/VP, captures the proportion of phenotypic variation due to genetic values that may include effects due to dominance and epistasis.

Question 13

narrow sense heritability

Answer 13

Narrow-sense heritability is defined as the fraction of phenotypic variance that can be attributed to variation in the additive effects of genes ( V A ) : h 2 = V A / V P ⋅ . Narrow-sense heritability is always less than or equal to broad-sense heritability.

Question 14

monozygotic twins

Answer 14

single egg breaks in two
100% same genes, so same phenotype

Question 15

dizygotic twins

Answer 15

two eggs fertilized at same times
50% same genes

Question 16

correlation coefficient (r)

Answer 16

measures the strength of association between two variables, as in the degree of scatter in the points from the line (NOT slope)

higher r, stronger correlation

Question 17

how do we calculate heritability in twin studies

Answer 17

broad sense heritability can be estimated by taking twice the difference of the correlation coefficients for a quantitative traits in monozygotic vs dizygotic twins

H^2= 2(r for monozygotic- r for dizygotic)

Question 18

interaction variance

Answer 18

arises when the effect of a genotype depends on the specific environment it was found

(such as man and woman having same genotype for height but man is taller because of testosterone and hormones affecting gene expression)

Question 19

norm of reaction

Answer 19

the pattern of phenotypic expression of a single genotype across multiple environments

Question 20

components of phenotypic variance

Answer 20

genetic variance (vg)
environmental variance (ve)
genetic environment interaction variance (vge)

**a GE interaction doesnt require the lines to cross, they just have to have different slopes

Question 21

dominance effects

Answer 21

when interactions between alleles complicate things

Question 22

another way to find little h^2 besides v additive/v phenotype?

Answer 22

R/S

Selection differential/response to selection

Question 23

limitations of heritability

Answer 23

Question 24

There are many aspects of experimental design that can help to increase power (i.e., chances to find QTL). what are the steps?

Answer 24

1) Ideally, when you set up a cross between two “parental” lines, you would like those parents to be as different as possible in the trait of interest to increase power 2) one should consider the genetic makeup of the parents of the cross. If the parents of the cross are inbred strains (i.e., they are homozygous at all sites), then any differences between the strains will be a maximally informative marker locus, and useful for QTL mapping 3) Another factor to consider (once you have decided on the parents of the cross) is the cross design itself; that is, if you choose to perform an F2-intercross or a backcross 4) the final step is to ask which marker loci show an association with the phenotype of interest in the second generation hybrids. The closer the marker is to the causal genetic variant, the stronger the association

Question 25

how do you determine if you should perform an intercross vs a backcross?

Answer 25

This decision can be based on the phenotype of the first generation (F1) hybrid phenotype. If the F1 (let’s call it “Aa” genotype) phenotype strongly resembles parent 1 (AA), this suggests that the alleles in parent 1 (A alleles) are dominant. Thus, it makes sense to “backcross” the F1 hybrids to parent 2 with the recessive aa alleles; this way, the F2 offspring will be either Aa or aa in equal frequency. (Note: if you did an intercross Aa x Aa, then you would have 1:AA, 2:Aa 1:aa, but since AA and Aa are indistinguishable, the phenotypic ratio is 3:1, which is less diverse than 1:1 of the backcross). If the F1 hybrid resembles parent 2 in phenotype, then you should backcross it to parent 1, for the same reasons stated above. In either case, we want the individuals in the next generation to be as genotypically and phenotypically diverse as possible. Finally, if the F1 phenotype is intermediate in phenotype between the two parents, then the F2- intercross design provides maximal power and will likely produce a range of phenotypes and genotypes between the two extreme phenotypes of the parents.

Question 26

total phenotypic variance can be apportioned into three components

Answer 26

VP = VG + VE + VGxE

Question 27

Additive genetic variance (VA)

Answer 27

comprises the additive effects of genes on phenotype, which can then be summed to determine the overall effect on phenotype. This can also be thought of as incomplete dominance, whereby the heterozygote has an intermediate phenotype to the two homozygotes

Question 28

dominance genetic variance (VD)

Answer 28

In the case of VD, the heterozygote has the same phenotype as the dominant homozygote, which means that one cannot predict the offspring’s phenotypes based solely on the parent’s phenotypes. VD is therefore a complicating factor that can make investigating the genetic basis of traits harder. Importantly, it’s the additive variance that determines the resemblance between parents and offspring. For these reasons, scientists are often more interested in the proportion of VP that is due to VA, which is referred to as the narrow sense heritability: h2 = VA/VP

Question 29

response to selection (R)

Answer 29

is dependent on the narrow sense heritability. R is defined as the differences in means between the trait in the new generation and the original population. When h2 is high, offspring resemble their parents, making the selection process more efficient. Conversely, when h2 is low, offspring will bear less resemblance to their parents, impeding selection efforts. The response to selection is therefore dependent on h2