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:

Homologous recombination

Crispr-Cas9

shRNA interference

GAL4-UAS system

Morpholino

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 : Genetic Manipulation

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, but the null is the smallest by a very large factor, nearly 90% smaller than the hypomorph. 

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%. 

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

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%.

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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