Question

When disrupting a mouse gene by knockout, why is it desirable to breed mice until offspring homozygous \((-/-)\) for the knockout target gene are obtained?

Short answer

Answer: Obtaining homozygous \((-/-)\) offspring is crucial for complete gene inactivation, observing the effects of the missing gene on specific phenotypes, reducing experiment variability, and allowing accurate control group comparison with wild-type \((+/+)\) counterparts. This helps researchers to clearly study the functions and effects of the knockout target gene with minimal interference and variability.

Step-by-step solution

Understanding gene knockout

Gene knockout is a technique used in research to study the functions of a specific gene, by deleting or inactivating the gene of interest in a living organism. For mice, this is often done through breeding, where two mice heterozygous \((+/-)\) for the knockout target gene are crossed to produce offspring. One goal of this breeding is to obtain homozygous \((−/−)\) offspring, which have both copies of the knockout target gene disrupted.

Explaining the importance of obtaining homozygous offspring

Obtaining homozygous \((−/−)\) offspring is crucial for several reasons: 1. Complete gene inactivation: Homozygous \((−/−)\) mice have both copies of the target gene inactivated or deleted, ensuring that the gene is completely non-functional. This allows researchers to clearly pinpoint the effects and functions of the knockout gene, without any interference from an active copy of the gene. 2. Phenotype observation: If the knockout target gene affects a specific phenotype in the mouse, homozygous \((−/−)\) offspring will allow researchers to observe the complete effect of the missing gene on that phenotype. 3. Reducing variability: By obtaining homozygous \((−/−)\) offspring, researchers can minimize the variability in their experiment results, as all of the mice being studied have the same genetic background and gene inactivation. 4. Control group comparison: Homozygous \((−/−)\) knockout mice can be used in conjunction with their wild-type \((+/+)\) counterparts as valid control groups. This allows researchers to make accurate comparisons and draw conclusions about the effects of the knockout gene.

Breeding strategy to obtain homozygous knockout mice

To obtain homozygous \((−/−)\) offspring for the knockout target gene, researchers will typically cross two heterozygous \((+/-)\) mice. These heterozygous mice have one active copy and one disrupted copy of the target gene. According to Mendelian inheritance, the offspring will have the following distribution: - 25% homozygous wild-type \((+/+)\) - 50% heterozygous \((+/-)\) - 25% homozygous knockout \((−/−)\) Continued breeding of heterozygous mice will increase the chances of generating homozygous knockout \((−/−)\) offspring, allowing researchers to study the full effects of the disrupted gene.