Quantitative Characters

Question 1

what is discontinuous variation?

Answer 1

individuals can be classified unambiguously into discrete categories which have no intermediates

Question 2

what is an example of discontinuous variation in nature? what is the nature of the genetic behind the traits?

Answer 2

shell character in land snails:

• shell background colour is either pink or yellow decided at one locus
• there can be bands - influenced by serval loci

Question 3

what is an example of discontinuous variation in traits due to environmental variation in an organism ?

Answer 3

acorn barnacle- is either free swimming in development or attached to a rock in adulthood
- also when they develop in an environment where its snail pred is present- forms a bent shell more not conic to protect itself

Question 4

what is continuous variation?

Answer 4

individuals have traits which can’t be discretely categorised, the values of the trait show a normal distribution of phenotypes0 with an average

Question 5

what are examples of continuous variation?

Answer 5

number of abdominal bristles on a fly, direction chosen my migrating black caps, gene expression levels

Question 6

what causes variation in continuous traits?

Answer 6

the environment and genes ( normally multiple loci)

Question 7

what causes variation in continuous traits?

Answer 7

the environment and genes (normally multiple loci)

Question 8

what causes variation in continuous traits?

Answer 8

the environment and genes (normally multiple loci with multiple alleles)

Question 9

how would you determine where there was a genetic bases to continuous variation in a trait? give an example of an example in the lab and discuss the issue with it

Answer 9

mate relatives and select for an extreme of the trait and see if the trait is “heritable” - ask whether the offspring resemble their parents more than they resemble the general population - covariance. look at wing length of parents (mid parent value) and offspring value- the environment will be the same of course but this is not always the case in the wild

Question 10

how would you determine where there was a genetic bases to continuous variation in a trait (in the wild not a lab)?

Answer 10

• it is important to ensure that the environment is the same for all offspring- this means ensuring that territory quality, parental size, amount of food supplied have no effect on the growth of the offspring that could cause bias (correlation doesn’t mean causation)
• this was done when looking at beak depth in song sparrows
• to rid variables eggs were swapped between nests so that the correlation seen between offspring and original parent would only be genetic.
• you then compare beak depth and they saw that there was correlation

Question 11

once you have established that genes contribute to a quantitative character, how can you determine whether multiple genes contribute to it?

Answer 11

tobacoo has a flowers which length varies

Question 12

once you have established that genes contribute to a quantitative character, how can you determine whether multiple genes contribute to it?

Answer 12

• tobacoo has a flowers which length varies
• fully inbred lines can be produce containing high homozygosity- 40mm and 90mm in length approx
• however there is variation in these pure lines which must be due to the environment as they are all genetical identical (this shows that environment effects variation)
• when you cross these all of the offspring in F1 are het so expect that they will all be idential- however this is not the case and the variance is about the same as P. the F1 phenotype was intermediate however this could still arise from one locus
• then cross F1 and look at the ratio of phenotypes- if there is 1 gene with 2 alleles then sleeved f1= 1:2:1 ratio of 3 phenotypes. BUT if many loci contribute then there will be lots of variation - the mor loci contribute the greater variation
• lots of variation was seen
• make several F3 populations by breeding from different parts of the F2 rang- variation between F3 populations. this further showed that multiple alleles contribute to phenotype

Question 13

what is additive variation?

Answer 13

genetic variation in which each allele has a + or a - effect on a trait causing a bell shaped distribution when looking at all loci involved. this can be the same for genotypes at different loci

Question 14

what is additive variation?

Answer 14

genetic variation in which each allele has a + or a - effect on a trait causing a bell shaped distribution when looking at all loci involved.

Question 15

how can the causal components of the variation of continuous traits be looked at?

Answer 15

cane broken down in different components- first environment and genetic then into Va, Vd, Vi and Ve

Question 16

how can the causal components of the phenotypic variation of continuous traits be looked at?

Answer 16

cane broken down in different components- first environment and genetic then into Va, Vd, Vi and Ve

Question 17

in the simplest case, what is assumed about genes and environment in terms of their influence on phenotypic variance?

Answer 17

they are assumed to act independently

Question 18

what is Va?

Answer 18

variance from genotypic differences caused by heritable,
additive effects of individual alleles.
If assume inheritance entirely additive then heterozygote’s phenotype will be exactly midway between the homozygotes. Va can be subdivided into average effect and breeding value

Question 19

what does average effect ad breeding value mean?

Answer 19

or one allele at one locus,
mean deviation from population mean of individuals receiving that allele from one parent, when other allele chosen at random from the population.

Question 20

what does average effect ad breeding value mean?

Answer 20

or one allele at one locus,
mean deviation from population mean of individuals receiving that allele from one parent, when other allele chosen at random from the population.
Breeding value: = average effect of alleles summed within & across loci for an individual.

Question 21

what part of phenotypic variance is heritable?

Answer 21

Va

Question 22

what is Vd and Vi?

Answer 22

dominance variance - variation caused by intra-locus
combinations of alleles (homozygotes & heterozygotes), in a generation.

epistasis/interaction variance - genotypic differences due to inter-locus interactions between alleles at different loci, in a generation.

Question 23

how do Vd and Vi act?

Answer 23

VD and VI effects of alleles depend on which other alleles are present.
Dominance and epistasis affect phenotypes in context dependent way (vary with genotype), in a given generation.
In sexually reproducing organisms, not all genetic effects are transmitted to offspring. Typically, context dependence of alleles showing dominance or epistasis dissolves every generation due to meiosis.
not the case in asexual organisms

Question 24

how can you calculate heritability?

Answer 24

• can look at the covariance in traits between relatives and find Va. the covariance in siblings will be 0.5 of Va and the covariance in half sibs will be 0.25 of Va. then Va/Vp= heritability
• can also compare the mid parent mean to the mean for the offspring- the regression line gradient of the line will give you heritability

Question 25

when can heritability vary?

Answer 25

• between populations because Va depends upon allele frequencies which will differ
• when environments differ becayse Ve contributes to Vp