# GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Gene Mapping

## Example Questions

### Example Question #1 : Help With Gene Mapping

If the recombination frequency between two genes is __________ then the genes are most likely __________

high . . . far apart

zero . . . far apart

low . . . far apart

high . . . far apart

Explanation:

Recombination frequencies are used to map genes on chromosomes by determining their relative distances from other genes. If a distance is large, there is a higher chance that a recombination event can occur between these two genes. Linkage occurs when genes are so close to one another that the genes always segregate together (recombination never occurs between them). The only answer that makes sense is that high recombination frequencies lead to the conclusion that two genes are far apart.

A low recombination frequency would indicate that the gene loci are close together, and a recombination frequency of zero would indicate linked genes.

### Example Question #2 : Help With Gene Mapping

A scientist performs a series of experiments to determine the recombination frequencies between the following genes. He acquires the following data:

W-X: 3%

X-Y: 2%

Y-Z: 13%

Z-W: 8%

Which of the following choices places the genes in the correct order relative to one another?

Z, W, X, Y

W, Y, Z, X

W, X, Y, Z

X, W, Z, Y

Z, W, X, Y

Explanation:

The larger the recombination frequency, the larger the distance between two genes. By looking at the data, we know that genes W and X are close to one another. Also, genes X and Y are close to one another. Gene Z, however, seems to be far away from both W and Y (but closer to W). We can represent these distances relatively in a picture:

W - - - X (3)

X - - Y (2)

Y - - - - - - - - - - - - - Z (13)

Z - - - - - - - - W (8)

The most likely explanation is that W, X, and Y are close to one another and Z is located slightly farther away on whichever side W is closest. A spatial map would look something like this:

Z - - - - - - - - - - W - - - X - - Y

### Example Question #3 : Help With Gene Mapping

If two genes are found to have a recombination frequency of 25%, what does this mean about the location of the two genes?

The genes are located on the same chromosome and neither extremely close, nor extremely far apart

The genes are located on the same chromosome and are on opposite ends of the chromosome

The genes are on different chromosomes

The genes are located on the same chromosome and neither extremely close, nor extremely far apart

Explanation:

If the genes were linked, there would be an incredibly small recombination frequency. If the genes were on opposite ends of the chromosome or on separate chromosomes, the recombination frequency would approach the maximum of 50%.

Because the recombination frequency is relatively intermediate, we can conclude that the distance between the genes does not fall at either extreme. The genes are neither very close, nor very far apart.