Question 1

In a dihybrid testcross (AaBb x aabb), the following progeny numbers are observed: 54 AaBb, 56 aabb, 42 Aabb, 48 aaBb. Which of the following conclusions is most supported by these results?

Accepted Answer

The A and B loci are unlinked and assort independently.

Question 2

If two genes are located 16 map units apart on the same chromosome, what proportion of the gametes produced by a heterozygote will be recombinant for these two genes?

Accepted Answer

$$0.16 \div 2 = 0.08$$ for each recombinant type, so $$0.16$$ total recombinant gametes

Question 3

Which of the following statements about genetic map units (m.u.) is correct?

Accepted Answer

One map unit corresponds to a 1% recombination frequency.

Question 4

A testcross between a plant heterozygous for two linked genes (in coupling configuration) and a homozygous recessive plant produces 80% non-recombinant and 20% recombinant progeny. What is the recombination frequency between the two genes?

Accepted Answer

$$0.20$$

Question 5

In a three-point testcross, the two most frequent progeny classes represent:

Accepted Answer

Non-recombinant (parental) progeny

Question 6

Why does the observed recombination frequency between two genes underestimate the actual physical distance when the genes are far apart?

Accepted Answer

Because double crossovers restore the parental combination, making some recombinants undetectable.

Question 7

A double crossover between two genes on the same chromosome can result in:

Accepted Answer

No detectable recombination if the crossovers involve the same chromatids

Question 8

In a testcross AaBb x aabb, the following progeny are observed: 10 AaBb, 40 Aabb, 40 aaBb, 10 aabb. What is the configuration of the alleles in the AaBb parent?

Accepted Answer

Repulsion (trans)

Question 9

Which of the following is a key difference between independent assortment and complete linkage?

Accepted Answer

Independent assortment produces equal numbers of recombinant and non-recombinant gametes, while complete linkage produces only non-recombinant gametes.

Question 10

A geneticist maps three genes (A, B, and C) and finds the following distances: A–B = 5 m.u., B–C = 10 m.u., A–C = 15 m.u. What is the most likely order of these genes on the chromosome?

Accepted Answer

A–B–C

Question 11

If a crossover occurs between two loci during meiosis, what is the expected frequency of recombinant gametes produced from that crossover event?

Accepted Answer

$$0.5$$

Question 12

In a three-point testcross, which progeny class is expected to be the least frequent?

Accepted Answer

Double crossover progeny

Question 13

Which of the following best explains why the maximum observable recombination frequency between two genes is 50%?

Accepted Answer

Because at 50% recombination, genes assort as if they are on different chromosomes.

Question 14

A plant heterozygous for two linked genes (TtDd) in coupling configuration is testcrossed with a double recessive (ttdd). If the recombination frequency is 16%, what proportion of the progeny will be warty, shiny (T d)?

Accepted Answer

$$0.08$$

Question 15

Which of the following is true regarding the effect of a double crossover between two distant genes?

Accepted Answer

It can restore the original parental combination, making some recombinants undetectable.

Question 16

In a three-point testcross, how can the order of the three genes be determined?

Accepted Answer

By comparing the double crossover progeny to the parental types.

Question 17

Which of the following statements about coupling and repulsion configurations is correct?

Accepted Answer

In coupling, both wild-type alleles are on the same chromosome; in repulsion, each chromosome has one wild-type and one mutant allele.

Question 18

A two-point testcross between genes a and b yields a recombination frequency of 50%. What does this indicate about the relationship between these genes?

Accepted Answer

They are on different chromosomes or far apart on the same chromosome.

Question 19

Which of the following best describes the relationship between physical distance and genetic map distance for genes that are far apart on the same chromosome?

Accepted Answer

Genetic map distance is an underestimate of the actual physical distance.

Linkage, Recombination, and Genetic Mapping: Study Guide

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