Question 1

Why can't the order of three genes on a chromosome be determined using only two-gene crosses?

Accepted Answer

Because distances between two genes can be measured but not their orientation on the chromosome.

Question 2

In a three-point testcross, which offspring classes are expected to be the least frequent?

Accepted Answer

Double recombinant classes

Question 3

Given the following offspring numbers from a three-point testcross: 420 (P Z D), 415 (p z d), 37 (P z D), 38 (p Z d), 1 (P z d), 4 (p Z D), 41 (P Z d), 44 (p z D), which genotypes represent the double recombinants?

Accepted Answer

P z d and p Z D

Question 4

What is the correct gene order for the genes P, Z, and D based on the double recombinant genotypes P z d and p Z D?

Accepted Answer

Z D P

Question 5

Why is the tester's gamete ignored when analyzing a three-point testcross?

Accepted Answer

The tester provides the same alleles in every offspring.

Question 6

If the recombination frequency between genes A and B is 10.1%, what is the map distance between these genes?

Accepted Answer

$$10.1\ \mathrm{cM}$$

Question 7

In a three-point testcross, why must double recombination events be included when calculating map distances between two genes?

Accepted Answer

Because double crossovers also result in recombination between the two genes of interest.

Question 8

Given the following offspring numbers from a three-point testcross: 420 (P Z D), 415 (p z d), 37 (P z D), 38 (p Z d), 1 (P z d), 4 (p Z D), 41 (P Z d), 44 (p z D), calculate the recombination frequency between P and D. Use the formula $$	ext{RF} = \frac{	ext{Number of recombinants}}{	ext{Total offspring}} 	imes 100.$$

Accepted Answer

$$8.5\ \mathrm{cM}$$

Question 9

Which of the following statements best describes the relationship between map distance and recombination frequency?

Accepted Answer

1% recombination equals 1 map unit (1 cM).

Question 10

A trihybrid testcross is performed with a triple heterozygote (A B C / a b c) and a tester (a b c / a b c). Which of the following offspring genotypes would be considered a single recombinant between genes A and B?

Accepted Answer

A b C / a b c

Question 11

In a three-point testcross, what is the first step in determining the order of the three genes?

Accepted Answer

Identify the two offspring classes that are non-recombinant (parental).

Question 12

If the gene order is A–C–B and the distances are 36 cM between A and C, and 7 cM between C and B, what is the total map distance between A and B?

Accepted Answer

$$43\ \mathrm{cM}$$

Question 13

Which of the following is a key feature of double recombinant genotypes in a three-point testcross?

Accepted Answer

The allele pair for the middle gene will appear 'flipped' compared to the parental genotypes.

Question 14

A three-point testcross yields the following offspring numbers: 351 (C A B), 354 (c a b), 3 (C a B), 2 (c A b). Which gene is in the middle?

Accepted Answer

A

Question 15

Why is it important to include both single and double recombinants when calculating the distance between two genes in a three-point testcross?

Accepted Answer

Because both types of recombinants represent crossover events between the two genes of interest.

Question 16

If a three-point testcross produces 1000 offspring, and 90 of them are recombinant between genes Z and D, what is the map distance between Z and D?

Accepted Answer

$$9\ \mathrm{cM}$$

Question 17

Which of the following best describes the purpose of a three-point testcross?

Accepted Answer

To determine if three genes are linked, their order, and the distances between them.

Trihybrid Mapping and Three-Point Testcrosses in Chromosome Mapping

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