The virus's harmful genes are stripped out and a therapeutic gene inserted in their place.

In gene therapy, the virus's harmful genes are excised and a corrective human gene is inserted in their place.

One danger is that inserted genes might activate other harmful genes, like cancer-causing oncogenes.

The harmful genes need an environment conducive to carrying out their dirty work.

It also advised breeders to avoid inbreeding to reduce the chance of doubling up on the harmful gene.

Genetic load is the probability that an average individual will die or fail to reproduce because of its harmful genes.

Any harmful hidden genes that might once have been able to destroy so incestuous a colony have long since been weeded out.

Old families tended to inbreed to the point where harmful genes became predominant, and great Houses were the worst offenders.

Using new 'viral engineering' techniques, researchers would use the protein 'coat'of the Aids virus but remove harmful genes wrapped inside it.

They occur because of the presence of harmful genes, which may be impossible to detect, but are again rarely inherited.