Genetics chapter 28

Quantitative genetics

Quantitative genetics

the study of traits that can be described numerically

what is true of quantitative genetic traits

they are usually controlled by more than one gene and are called complex traits

why are quantitative genetics important

most of the key characteristics considered by plant and animal breeders are quantitative traits 

quantitative trait 

any trait that varies measurably in a given species 

what are examples of anatomical traits

height, weight, number of bristles, ear length, and the degree of pigmentation

what are examples of physiological traits

metabolic traits, speed of running and flight, ability to withstand harsh temperatures and milk production

examples of behavioral traits

mating calls, courtship rituals, ability to learn a maze, and the ability to grow or move toward light 

what are examples of disease

atherosclerosis, hypertension, cancer, diabetes, and arthritis

In this branch of genetics, the measurement of a quantitative trait is how we describe a ______

phenotype

discontinuous traits

traits that fall into discrete categories (alleles affect the phenotype in a qualitative way)

How are quantitative traits different from discontinuous traits

quantitative traits show a continuum of variation within a group of individuals 

frequency distribution

the trait is divided arbitrarily into a number of discrete phenotypic categories

mean

the sum of all values in the group/ the number of individuals in the group 

N

the number of individuals in the group 

variance

the amount of phenotypic variation that exists in a group

How can you predict the total variance for a specific trait

the variance of factors that contribute to quantitiative are added

what can you tell about a population if it follows a normal distribution

it is easier to appreciate the amount of variation by considering the standard deviation 

what percentage is accounted for by one standard deviation in a normal distribution

68%

What percentage is accounted for by two standard deviations in a normal distribution

95%

what percentage is accounted for by three standard deviations in a normal distribution

99.7%

why might you compare two variables to each other

To compare the occurrence of two phenotypic traits, compare between traits and environmental factors, or compare between traits and genetic relationships

covariance

describes the degree of variation between two variables within a group 

correlation coefficient

The strength of the association between two variables

what is the null hypothesis to test if there is no real correlation

the r value differs from zero only as a matter of chance

what is the significance of the correlation coefficient related to

sample size and degrees of freedom

when is the approach to testing the r value valid

The values of X and Y are obtained by an unbiased sampling of the entire population, and the scores of X and Y follow a normal distribution, and the relationship between X and Y is linear

can a r value that is statistically significant only be caused by genetic factors 

no they can come from environmental factors 

when is it valid to use a regression analysis to predict how much one variable will change in response to the other 

when it has been established that the variables are related due to cause and effect 

polygenic inheritance

the transmission of traits that are governed by two or more genes

quantitative trait loci (QTLs)

genomic regions identified by molecular markers that are often associated with a particular phenotype 

how many genes do QTLs contain

one or more genes

when is it most difficult to categorize traits into discrete genotypic categories

when the number of genes controlling the trait increases and the influence of the environment increases 

nearly all individuals fall into a ______ phenotypic category that corrsponds to their genotype 

single 

What do geneticists use to determine the regions of the chromosome that are associated with a particular trait

molecular marker analysis

what kind of molecular markers are being used as reference points along chromosomes 

RFLPs and microsatellites 

where have detailed genomic maps been obtained from

model organisms and organisms of agricultural importance

how are genetic maps useful

They make it easier to determine the number of loci that affect a quantitative trait

what is the basis of QTL mapping

The association between genetically derived phenotypes and molecular markers

how must strains be different to map QTLs

with regard to a quantitative trait of interest and with regard to many molecular markers (markers should correspond to the same chromosomal location)

what are the two factors that controll heritability

genetics and environment

what are all traits of biological organisms influenced by

they are influenced by genetics and environment

heritability

the amount of phenotypic variation within a group of individuals that is due to genetic variation for a particular population raised in a particular environment

What would be the heritability value if all phenotypic variation is due to genetic variation 

it would have a value of 1

What would be the heritability value if all phenotypic variation is due to environmental factors

it would have a value of 0

what assumptions have to be made to study the phenotypic variance of a quantitative trait variation 

genetic and environmental, factors are the only two components that determine a trait and these factors are independent of one another 

If Vg is very high and Ve is very low

genetics is more important in promoting variation

how to calculate total variance (Vt)

Vg + Ve

what is the total variance

the amount of variation at the phenotypic level

monomorphic

all members of a population are homozygous for the same allele of a given gene

what is the amount of variation due to genetics in a monomorphic species of mouse

Vg=0 therefore all phenotypic variation is due to environment

what is an example of genetics and environment interacting 

minerals in soil 

genotype-environment association

when certain genotypes are preferentially found in a particular environment

broad sense heritability

all genetic variation that may affect the phenotype

narrow sense heritability

the heritability of a trait due to the additive effects of alleles

equation for narrow sense of heritability

Vg= Va +Vd+Vi

Va

the variance due to the additive effects of alleles

Vd

variance due to alleles that follow of dominant/recessive pattern of inheritance 

Vi

the variance due to the effects of alleles that interact in an epistatic manner 

what are differrent ways of estimating narrow sense heritability

measure a quantitative trait among groups of genetically related individuals, use this data to compute a correlation between the individuals, and calculate narrow sense heritability

What is the expected correlation for siblings

0.5

what is the expected correlation for identical twins

1.0

what is the expected correlation for uncle-niece relationships 

0.25

who developed the method for counting the number of ridges within a human fingerprint

Kristine Bonnevie

what are the three different types of fingerprints

arch, loop, and whorl

what is the primary difference among the different types of fingerprints 

the presence of the number of triple junctions each known as a triradius 

what did Bonnevie find in her experiment

she found that ridge count correlations were relatively high in genetically related individuals

what is the hypothesis for sarah holt’s experiment

dermal ridge count has a genetic component and aims to determine the contribution of genetics in the variation of dermal ridge counts

what does a heritability value of 1 mean

that the amount of variation within this group is due to genetics 

what does a value 0.6 mean

it means that 60% of the variation in IQ testing ability is due to genetic variation in a particular population in a particular environment

is heritability a individual or population concept

a population-wide value that pertains to variation

selective breeding (artifical selection)

the modification of phenotypes in plant and animal species of economic importance 

what is the primary difference between artifical and natural selection

how the parents are chose which is either based on reproductive success and a breeder choosing individuals with traits that are desireable to humans

inbreeding

the selective breeder will often choose genetically related individuals as the parental stock

In the corn experiment what two groups were the plants divided into

members with the highest oil content and members with the lowest oil content 

In the bristle number experiment what two groups were the flies sorted into

members with the highest bristle number and members with the lowest bristle number

In unselected populations what values are quantitative traits often at

intermediate value

selection limit

After many generations, the population will eventually become monomorphic for all or most of the desirable alleles in question  and additional selective breeding will have no effect 

what does R represent

response in the offspring to selection

what does S represent

selection differential in the parents