Science Questions

My husband is colour blind and we recently had a baby girl. I know that colour blindness is an X-linked recessive trait and that one X-chromosome is randomly switched off in females because females have two X-chromosomes and only need one. Does this mean that our daughter, as a carrier, could be colour blind in one eye or in some of her cone cells but not all of them? Jessica in South Carolina

The answer is that your daughter is mosaic, as all women are. I don't mean that all women are little bits of ceramic pottery, but because we have two X-chromosomes as women, one of them gets switched off in every cell. In healthy people, this is a completely random process. Colour blindness is caused when there is a faulty gene on one of the X-chromosomes, and if you're a man then you don't have a counter balance to that (ie: another 'non-faulty' X-chromosome.) If you're a woman and one of your X-chromosomes is dodgy, you may have it switched on in some cells and off in others. The fact it's a mixture means that the healthy cells are going to compensate for that, so you're not going to be colour blind in just one eye. Some of the cells will have the colour blindness problem but there will be enough healthy ones to compensate.

February 2007