Chances of a Defective Child

Until recently, the parents of one defective child had only two courses to follow if they wanted another baby. One was to trust to luck, worry throughout the pregnancy, and blame themselves if a defective child was born. The other was to have no child. Now, by using charts of probabilities worked out by Dr. J. A. Fraser Roberts of London's famed Guy's Hospital, a geneticist can give parents an accurate appraisal of what their chances are of producing a second defective.

Though genetics is an arcanely intricate science, the risks of unfavorable inheritance fall sharply and neatly into two groups. Dr. Fraser Roberts labels them, simply, the bad and the good. In the bad-risk group, the chance that a couple's next child will be born defective is at least one in ten. In the good-risk group, it is no more than one in 25. For remote mathematical reasons, there are few in-between cases.

One of the most common inherited diseases in Britain, as in the U.S., is cystic fibrosis, which occurs once in about 2,500 births.* The pattern of inheritance is Mendelian recessive--the gene carrying the defect is weak and is overpowered by a corresponding normal gene, so that a child with one normal parent does not develop the disease. But if both parents carry the abnormal gene, and it is proved by the birth of one child with cystic fibrosis, there is a one-in-four risk that any subsequent child will also be afflicted. Other diseases in the bad-risk group: some forms of mental retardation, deafness, muscular dystrophy and hemophilia-like bleeding disorders. The counselor's advice to parents with one child suffering from these has to be: if you want more children, adopt them.

Rather Cheerful. Other abnormal conditions seen at birth are associated with defective genes, but the pattern is more complicated than Mendelian recessive. They produce hydrocephalus (water on the brain), spina bifida (failure of the spinal column to close), harelip and clubfoot. When a couple has had one child with one of these defects, the chance that a later child will have it is in the good-risk range, or about one in 25. "You may think that is rather serious," said Dr. Fraser Roberts, "but we think it is really something rather cheerful. You have to remember there is a one-in-33 chance that any pregnancy will end with some severe malformation or developmental error."

Most of the couples counseled at Guy's Hospital have heeded the geneticists' advice. Only a few in the poor-risk group have had more children. For them, the gloomy predictions were borne out, with severe defects in two cases out of eleven. But of 81 children born to couples in the good-risk group, only one inherited a major defect. That is a better showing than even "normal" couples can expect.

* Red-green color blindness, transmitted by the x (female) chromosome, is far more common, affecting 8% of men and slightly more than .5% of women, but Dr. Fraser Roberts considers this "a harmless physiological variation."

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