How crossovers map genes

Science · Genetics & Evolution · ages 18-19

Name ______________________ Date ____________

Genes close together on one chromosome tend to be inherited as a unit.
Circle one: True False
A test cross gives 200 offspring and 20 are recombinant. What is the recombination frequency?
- 5 percent
- 20 percent
- 10 percent
What does one map unit mean?
- One percent recombination
- One crossover per cell
- One gene per chromosome
Pairwise distances are A to B 8, B to C 5, and A to C 13. Which gene sits in the middle?
- Gene A
- Gene C
- Gene B
A cross gives 800 offspring and 96 are recombinant. Type the recombination frequency as a percent.
Answer: ______________
The outer distance of three genes comes out shorter than the sum of the two short ones. Why?
- The middle gene moved away
- Some offspring were sterile
- Double crossovers pass unseen
A recombination frequency of 49 percent proves two genes are far apart on the same chromosome.
Circle one: True False
Two genes show 48 percent recombinants. What should you conclude?
- They sit 48 map units apart
- They are effectively unlinked
- The cross failed

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · Linkage keeps neighbours together unless a crossover splits them.
10 percent · Divide 20 by 200 to get 0.10, which is 10 percent.
One percent recombination · One map unit equals 1 percent recombination, also called a centimorgan.
Gene B · B appears in both short distances, so it sits between A and C.
12 · 96 divided by 800 is 0.12, which is 12 percent.
Double crossovers pass unseen · Double crossovers restore the parental mix, so those events are missed.
False · It only shows they behave as unlinked; they may sit on different chromosomes.
They are effectively unlinked · Near 50 percent means linkage information has run out.