GRE Subject Test: Biology : Understanding Knockouts

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Knockouts

Which of the following is not a methodology to obtain either transient or stable knockout/ knockdown of a gene?

Possible Answers:

Morpholino

GAL4-UAS system

Homologous recombination

Crispr-Cas9

shRNA interference

Correct answer:

GAL4-UAS system

Explanation:

Homologous recombination and Crispr-Cas9 create stable gene knockouts whereas shRNA and morpholino interference transiently knockdown a gene of interest. The GAL4-UAS system acutally is used to overexpress a gene of interest. By using a cell type specifc promoter, researchers are able to overexpress a specific protein in a desired cell type in a whole organism. 

Example Question #2 : Understanding Knockouts

Genetically altering an animal to reduce the expression of a gene of interest can be a labor intensive process that does not necessarily produce complete loss of a gene. A genetic null is an animal in which the gene has been completely (or nearly completely) excised and as such, no protein is produced. A genetic hypomorph is an animal in which only part of a gene has been deleted and as such, a lower amount of protein or a dysfunctional protein is produced, but it is still there. Hypomorphs can be close to null, perhaps only retaining 5-10% of normal function, or they can be close to wild-type, retaining 80-90% of gene function and having mild mutant phenotypes.

You are studying Gene H, a gene that regulates head size, and the more Gene H is expressed, the larger the organism's head is. You have one animal that is null for Gene H, and one that is a hypomorph for Gene H. You compare the head sizes of these animals. Which of the following results is least likely to be true from your experiment? You can assume that a wild-type animal will have the largest head size of the three. 

Possible Answers:

Both the null and hypomorph have smaller heads than the wild-type, and the null is smaller than the hypomorph, but only by about 5%. 

Both the null and hypomorph have smaller heads than the wild-type, but the null is the smallest by a very large factor, nearly 90% smaller than the hypomorph. 

The null and the hypomorph cause smaller head size early in development, but the hypomorph head growth accelerates later in development. 

The null has a larger head than the hypomorph by about 10%.

The null and the hypomorph have roughly the same head size, because the hypomorph is dysfunctional enough to disturb head size development. 

Correct answer:

The null has a larger head than the hypomorph by about 10%.

Explanation:

The only result that would be very unlikely is the one in which the null has a larger head size than the hypomorph. All of the other cases could be true since the nature of a hypomorph is often hard to ascertain, but given that more protein = bigger head, a true genetic null would have the least amount of protein (i.e. no protein) and represents the absolute smallest a head can get. 

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