Lecture 4

Question 1

With three linked loci it is possible to determine: (2)

Answer 1

• The recombination rates/distances between loci

- the proper order of the loci on the chromosomes

Question 2

Without crossover interference, what happens to the number of crossovers?

Answer 2

You get the maximum number of crossovers

Question 3

What is involved with linkage genetic maps?

Answer 3

Ordering the loci along the chromosomes

Measuring the distance between them

Question 4

How are map distances expressed?

Answer 4

Morgans: 1M equals to one expected crossover per meiosis between two loci
Centimorgans: 1cM equals to one hundredth of a Morgan, that is one crossover per 100 meiosis

Question 5

What is the simplest measure of distance?

Answer 5

The rate of recombination (r) between two loci

Question 6

What assumptions are made with basic linkage maps where m=r?

Answer 6

Recombination rates are non additive

Assumes interference is less than 1

Question 7

What do mapping functions account for that recombination rates cannot?

Answer 7

The fact that double crossover and other even number of crossovers cannot be observed

Question 8

What do mapping functions do/how do they work?

Answer 8

Mapping functions attempt to predict the number of crossovers from the observed recombination frequency

Question 9

What are the two most popular mapping functions?

Answer 9

Haldane and Kosambi mapping functions

Question 10

What does haldanes mapping function assume?

Answer 10

Assumes that the crossover occurred randomly and independently on the entire chromosome with no interference.

Question 11

What type of distribution doe Haldane’s mapping function follow? And what year was it discovered?

Answer 11

Poisson distribution and 1919

Question 12

What is the problem with haldanes mapping function?

Answer 12

The presence of a crossover in one region may affect the frequency of crossovers in other regions lowering frequency of double recombinants.

Question 13

What does kosambi’s mapping function allow for?

Answer 13

Positive interference

Question 14

What is the disadvantage of kosambis map distances?

Answer 14

They are not additive

Question 15

At a small observed recombination fraction, what is the difference between haldane and kosambis map distances?

Answer 15

The Haldane mapping function and the Kosambi mapping function will predict very similar map distances at small observed recombination rates.

Question 16

With a large recombination fraction, what is the difference between the two mapping functions? Why?

Answer 16

With large recombination rates the difference between the predicted distances by the two mapping functions will increase as double recombinants are increasingly more likely.

Question 17

What are some problems when mapping many loci?

Answer 17

• loci closely located will be difficult to unequivocally determine their order
• Some loci will not be informatively the genotypes will be missing

Question 18

What method was developed to help determine order and map distances of multiple loci?

Answer 18

Maximum likelihood