Test identifies `safe' human eggs

Genes are located in chromosomes, and the cells of any speciescontain a characteristic number of chromosomes - 46 for humans. Eggsand sperm have only half that number so that one 23-chromosome eggplus one 23-chromosome sperm yields one 46-chromosome baby withtraits from each parent.

The polar body test is only useful for families with a historyof inherited ailments, and it depends on the fact that abnormal genesare recessive. That means an individual must have two bad genes tohave the disease. Thus, a disease-free woman from such a family canbe, at worst, a carrier with only one bad gene.

When eggs are formed, only half of the 46 chromosomes are used,and the eggs jettison the unused 23 wrapped in the polar body.

Therefore, if the bad gene is in the polar body, the egg isuntainted. And the reverse is true: If the good gene is in the polarbody, the bad one is presumed to be in the egg, which is discarded.If only healthy eggs are fertilized and returned to the mother, theresulting child would be safe from the disease.

Should such an egg receive a disease gene from the father'ssperm, the baby would be a carrier but would not have the disease.