# GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Genomics

## Example Questions

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### Example Question #1 : Genomics

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 #21 : Gene Regulation And Genomics

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?

W, Y, Z, X

W, X, Y, Z

X, W, Z, Y

Z, W, X, 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 #1 : Genomics

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.

### Example Question #1 : Help With Linkage

What does it mean to say that two genes are linked?

The genes overlap one another

The genes almost always segregate together during meiosis

The genes code for the same mRNA

The genes share a promoter

The genes almost always segregate together during meiosis

Explanation:

Genetic linkages are determined by frequencies of recombination. These are measures of how often chromosomal crossovers will take place between two genes. The closer the loci of the two genes are on a chromosome, the less likely a crossover event will separate the two genes. If the recombination frequencies are sufficiently low, the genes are considered to be linked.

Genetic linkage has nothing to do with genes coding for the same mRNA, sharing a promoter, or overlapping one another. Linked genes still code for distinctly separate traits/proteins and have different loci (don't overlap).

### Example Question #2 : Genomics

Which of the following is true of gene families?

I. They likely arose from gene duplications

II. They are only seen in eukaryotic genomes

III. Members of the same family have identical DNA and amino acid sequences

I only

I and II

II and III

I, II, and III

I only

Explanation:

Gene families consist of several copies of genes that encode very similar proteins. These likely arose due to gene duplications, and were altered by mutation over time to generate separate similar proteins. It is not a requirement that members of gene families have identical DNA or amino acid sequences. Both prokaryotes and eukaryotes have gene families.

A common example is the homeobox, or Hox, gene family, which codes for several proteins that are essential for developmental timing and orientation.

### Example Question #61 : Molecular Biology And Genetics

Which of the following is not true about transposable elements?

Transposable elements often move around the genome

Transposable elements are primarily considered non-coding DNA

Transposable elements are only found in eukaryotes

Transposable elements can cause disease

Transposable elements are only found in eukaryotes

Explanation:

Transposable elements are portions of the DNA that are free to move around the genome and are generally considered non-coding DNA. This can be potentially dangerous, however. Transposable elements can insert themselves in the coding regions of genes, thus making them non-functional. This can lead to disease. Both eukaryotic and prokaryotic genomes contain transposable elements.

### Example Question #62 : Molecular Biology And Genetics

Transposable elements, or transposons, are separated into two classes. Which of these categories of life have class I transposons in their genomes?

I. Bacteria

II. Yeast

III. Eukaryotes

I, II, and III

None of these

II and III

III only

I and II

II and III

Explanation:

Class I transposable elements are RNA-mediated elements of a single evolutionary origin, and are found in yeast, which only have class I elements, and in eukaryotes, which have both class I and class II elements. Bacteria only have class II elements, and hence are not included in the correct answer to this question.

### Example Question #62 : Molecular Biology And Genetics

What differentiates a LTR retrotransposon and a retrovirus?

Retroviruses are the only ones present in eukaryotes

None of these are correct

LTR retrotransposons cannot move between organisms

Retroviruses do not insert DNA into their host

Retroviruses encode an envelope protein

Retroviruses encode an envelope protein

Explanation:

The only difference between most LTR retrotransposons and retroviruses are that retroviruses can encode an envelope protein. Phylogenetic analyses have shown that retrotransposons and retroviruses are extremely closely related, and may be direct ancestors of one another.

### Example Question #63 : Molecular Biology And Genetics

The hybrid dysgenesis phenomenon was observed in Drosophila flies. It was determined that this was caused by a transposon no longer under control in wild type - lab strain crosses. What are transposons commonly controlled by in their hosts?

Other types of transposons

The immune system

RNAi and piRNAs

None of these

A transposase inhibitor

RNAi and piRNAs

Explanation:

Movement of transposons is very commonly controlled by RNA interference. The RNAi system cuts up problematic RNAs, and uses these small pieces to target transposons for destruction.

### Example Question #1 : Genomics

How do transposons rapidly propogate through and between species?

None of these are correct

Transposons cannot move between species

Horizontal transfer

Bacterial infections

Vertical transfer