Here's the classic, if overly simplistic, example: Children inherit sets of chromosomes from each of their parents, with each chromosome containing the genes for various traits. A blue-eyed child has to inherit the blue-eyed gene from both the mother and the father. Otherwise, the dominant brown-eyed gene trumps the recessive blue-eyed gene.
In reality, eye color is determined by more than one gene. But the same principle applies to genetic defects such as muscular dystrophy: Even if you inherit the mutated gene for muscular dystrophy from one parent, the normal gene from the other parent can compensate and keep you from getting the disease.
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The downside for genetic engineers is that the mechanism makes it harder to introduce desired mutations. Mutagenic chain reaction, or MCR, makes the job easier. The researchers behind the Science study tweaked the CRISPR genome-editing procedure in fruit flies to make a mutation that's generated on one copy of a chromosome spread automatically to the other copy. Thus, both copies of the gene carry the mutation.
"MCR is remarkably active in all cells of the body, with one result being that such mutations are transmitted to offspring via the germline with 95 percent efficiency," study lead author Valentino Gantz, a graduate student at the University of California at San Diego, said in a news release.