You have been hired to create a cat that will not cause allergic reactions in cat-lovers

Question

Your coworkers have cloned the gene encoding a protein found in cat saliva, expressed the protein in bacteria, and shown that it causes violent allergic reactions in people. But you soon realize that even if you succeed in making a knockout cat lacking this gene, anyone who buys one will easily be able to make more hypoallergenic cats just by breeding them. Which of the following will ensure that people will always have to buy their hypoallergenic cats from you?
(a) Inject the modified embryonic stem (ES) cells into embryos that have a genetic defect to prevent the mature adult from reproducing.
(b) Implant the injected embryos into a female cat that is sterile as a result of a genetic defect.
(c) Sell only the offspring from the first litter of the female cat implanted with the injected embryos.
(d) Surgically remove the sexual organs of all the knockouts before you sell them.

Answer 1

(d) Neutering all the knockout animals that you sell is the only option of the four listed that will prevent happy pet owners from becoming happy pet breeders. The situation described in choice (a) will not allow you to make any knockout cats because the first litter (which will at best have a few mosaics in which one copy of the gene has been knocked out in the germ cells) will be sterile and you will not be able to mate them. The genotype of the female cat in which you implant the embryos has no effect on the genotype of the embryos, which is why choice (b) is incorrect. Choice (c) is incorrect because the first litter will yield mosaic cats that still have one copy of the allergen-producing gene in their cells and are therefore not hypoallergenic.