Lecture 10

Question 1

Classic Mendelian Traits:

Answer 1

• Single locus
• Two alleles
• Generally two phenotypic classes (or three classes in the case of partial dominance)

Question 2

Quantitative/complex traits:

Answer 2

• Polygenic or many ‘quantitative trait loci: QTLs’
• Trait value often partially influence by environment
• Continuum of phenotypic classes (‘continuous variation’)
• Often exhibit a normal distribution

Question 3

Quantitative traits are important in:

Answer 3

• Complex disease (risk factors, genetics, environment, genetics)
• Agricultural traits (production and yield characters and their trade-offs)
• Evolution (what maintains variation?)

Question 4

QTL mapping experiments utilise:

Answer 4

• Genetic/linkage mapping and molecular markers

- Without recombination, relying on independent assortment

Question 5

In an experiment using drosophila bristles on the sternopleural and abdominal body segment pre-molecular markers were used because..

Answer 5

• limited to model organisms/phenotypic markers
• Model traits
• Balancer chromosomes/No recombination in Drosophila males

Question 6

In order to map polygenes to a chromosome two fly strains were used. They were

Answer 6

• Low score (with a phenotypic marker, unrelated to the trait but still attached)
• High score (selected for)
• Perform a backcross with F1 to the selected stain (assuming bristle related genes are recessive).
• Take sons of F2 and determine which chromosomes contain genes contributing to this trait.

Question 7

Phenotype score:

Answer 7

• A tally of the number of offspring that contain a particular genotype..
• ABC, AB+, A+C, A++, +BC, +B+, ++C, +++

Question 8

How do you tell which flies have bristle loci?

Answer 8

• Look at which genotype shows no difference when graphed on phenotype score.
• There is no bristle loci on the chromosome that has no effect on the bristle number.

Question 9

Can also calculate the effect (in addition to the position):

Answer 9

• The effect of substituting c with + is __
• The effect of substituting b with + is __
• The effect of substituting a with + is __
• We must also make sure this is statistically significant

Question 10

Students t-test

Answer 10

• Compares two populations with
• Assumes variation in both populations are normally distributed
• Ho: the means of the two populations are the same
• P=0.05, means that the probability that the null hypothesis is true and you get this result is 1/20. This is sufficiently rare that we reject the null hypothesis.

Question 11

To map within a chromosome we need:

Answer 11

• Recombination
• Markers
• Maps

Question 12

Physical map:

Answer 12

• Composed of nucleotide sequences (eg. restriction sites, contigs of clones, nucleotide sequences) where distance is measured in base pairs

Question 13

Cytological map:

Answer 13

• Composed of chromosomal features (eg. puffs, bands), an example if the polytene chromosomes of Drospohila which are divided into ‘divisions’ and ‘bands’.

Question 14

Linkage/genetic map:

Answer 14

• Composed of polymorphic markers where distance is measured from recombination fractions (cM)

Question 15

Recombination frequencies are not additive:

Answer 15

• We only ‘see’ recombination if there has been an odd number of recombination events between the markers we are looking at.
  A—B—C
• To see recombination between A and C you can have either recombination between A and B, or B and C.
• rAC = rAB(1-rBC)+(1-rAB)rBC

Question 16

Haldanes mapping function:

Answer 16

• Accounts for double crossovers
• Let c be the actually recombination distance between markers
• We estimate c from the observed recombination fraction (r)
• c+-1/2In(1-2r)