Diploid Mapping

Question 1

Unlinked genes

Answer 1

50% recombinants

Question 2

Linked genes

Answer 2

< 50% recombinants

Question 3

What percentage of recombinants would occur if there were never a crossover?

Answer 3

0%

Question 4

What percentage of recombinants would occur if there were always a crossover?

Answer 4

50%

Question 5

Thomas Hunt Morgan’s Theory of Crossovers

Answer 5

If there exists a small but equal probability for a crossover event at any given unit of chromosome distance, then crossover frequency between genes should be a function of their distance

• i.e, the relative distance between genes could be determined by crossover frequency and all the genes on a chromosome could be mapped
• 1% recombinants = 1 map unit (mu)

Question 6

Genes A/a, B/b and D/d are linked

P AABB x aabb
F1 AaBb testcrossed to aabb

Answer 6

AaBb 47%
aabb 47%
Aabb 3%
aaBb 3%

6% recombinants (6 map units)

Question 7

P AADD x aadd

F1 AaDd testcrossed to aadd

Answer 7

AaDd 40%
aadd 40%
Aadd 10%
aaDd 10%

20% recombinants

Question 8

Three-point testcross

Answer 8

Testcross a trihybrid and produce enough progeny to map all three genes in one cross

Question 9

Double recombinants

Answer 9

-Always the lowest frequencies

Question 10

Parentals

Answer 10

-Always the highest frequencies

Question 11

Determining gene order

Answer 11

• Look at double recombinants and parentals genotypes only
• gene that switches position between d.r. and parentals (not the two that always stay together) is the one in the middle

Question 12

Determining distance

Answer 12

(look at notes. but basically, take middle gene and keep one, move other gene downwards. The one you move downwards is the one you are determining distance from middle gene. Then find out the frequency for the middle and upper, add lower and then add the double recombinant frequencies, divide all over total amount) for determining distance between two ends, add middle + 1 plus muddle + other end

Question 13

Coefficient of coincidence

Answer 13

Multiply middle + left and middle + right, then multiply that by total amount of progeny to determine expected amount of double recombinants

Then divide observed (double recombinants added up together) by expected amount. Multiply by 100 to determine percentage

Question 14

Interference

Answer 14

1 - coefficient of coincidence

Question 15

Unlinked

Answer 15

50% recombinants on separate chromosomes

-more than 50 map units apart on same chromosome

Question 16

Linked

Answer 16

Less than 50% recombinants

-within 50 map units on same chromosome