Lecture 9/23 - E2

Question 1

A dihybrid cross is performed between AaBb x AaBb. Given this pathway: Colorless precursor —–(A blocks)—> Yellow Pigment ——– (B)—-> Green Pigment
Which of the phenotypic ratios do you expect in the offspring?

Answer 1

9: A-B- = Colorless
3: A-bb = Colorless
3: aaB- = green pigment
1: aabb = yellow pigment

Answer: 12 Colorless: 3 Green: 1 Yellow

Question 2

Genes on _______ chromosomes always assort independently and produce _______ numbers of all possible gamete combinations

Answer 2

1) Different

2) Equal

Question 3

_____ _____ do not sort independently.

Answer 3

Linked genes

Question 4

What are linked genes?

Answer 4

Syntenic genes (nearby genes on the same chromosome) which are bassed down together in gametes more frequently than ex[ected from independent assortment

Question 5

If there is a greater than 50% frequency of parental genotypes/phenotype gametes produced then…

Answer 5

The genes are closer together.

- If less than 50%, genes are farther apart.

Question 6

If there is less than 50% frequency of non-parental genotypes/phenotype gametes produced then….

Answer 6

The genes are closer together

- If greater than 50%, genes are farther apart

Question 7

If 100% of the gametes are parental then…

Answer 7

There is complete linkage where no recombination occured between the two genes in meiosis.

Question 8

Parental frequency + nonparental always =

Answer 8

100% because both parental will be produced at the same frequency while both nonparentla will be produced at the same frequency

Question 9

In Drosophila melanogaster, Genes A and B are separated by 1/1000th the length of a chromosome. Which cross do you expect to detect in recombinant progeny in the next generation?

1) AB/ab (female) x ab/bb (male)
2) AB/ab (male) x ab/ab (female)
3) AB/AB (female) x ab/ab (male)

Answer 9

Only females undergo recombination
Answer: AB/ab (female) x ab/bb (male)
Because females cannot be homozygous for both because of the recombination

Question 10

How do you calculate the recombination frequency (r)?

Answer 10

r = (number of recombinants) / (total number of progeny)

Question 11

What is recombination frequency a function of?

Answer 11

The physical distance between two genes

- the greater the distance, the more recombination observed

Question 12

What is 10.7% recombination in centimorgans?

Answer 12

10.7 centimorgans

Question 13

If a cross produces 30% recombinant offspring, each parental class type (P1 and P2) must represent approximately _____% of the progeny.

Answer 13

Total parental percentage = 70%

70/2 for each parent = 35%

Question 14

Why is gene mapping used?

Answer 14

To determine the order and distance between genes on a chromosome.

Question 15

True breeding parents (P= Ab/Ab and aB/aB) are crossed. A test cross of the resultant F1 produces the progeny ratios: 11% AB, 13% ab, 38% Ab, and 38% aB. How far apart are the A and B linked genes?

Answer 15

Add the AB and ab

11% + 13% = 24%

Question 16

Single stranded crossover event

Answer 16

Four dsDNA molecules (chromatids) in all combinations, half the gametes are recombinants

Question 17

Double Crossover between two genes

Answer 17

Where there are two crossover events, occurs much less frequently

Question 18

When is a chromosome considered recombinant?

Answer 18

If the alleles don’t look like the parents.

Example: Parents have AB and ab, if the gametes look like aB or Ab, then those are recombinant

Question 19

How do you tell which phenotypes are the parental, single recombination, or double recombination based on the number observed for a 3 point cross?

Answer 19

Parental - largest number
Single - intermediate number
Double - smallest number

Question 20

How can you tell which gene is in the middle using the number observed in a 3 point cross?

Answer 20

Look at the double crossover events.
See which gene differs from the parentals
Example: Parents are vga and VGA, double cross over shows vGa and VgA
Therefore, G is the gene in the middle