Editor—With respect to the news item by Hargreaves,1 much of the concern about the possibility of “designer babies” is misplaced. The barriers to successful selection of “superior” babies are likely to remain insurmountable, outside the context of selection for babies unaffected by genetic disease (which is generally not controversial). 
Traits such as intelligence and aspects of personality are genetically complex. Any one gene is unlikely to be shown to contribute more than a small amount to the variation in these characteristics in the population. To select at pre-implantation diagnosis for high intelligence (for example) with any realistic chance of success, several, perhaps many, genes would probably need to be tested. The problem with this is that each additional gene tested reduces the number of embryos available for implantation.
Suppose that there are three genes of interest, each of which has a favourable allele that confers an advantage in a heterozygous state and that one of the parents carries the advantageous allele for each of the three genes. The chance of an embryo carrying all three favoured alleles is one in eight. Each additional gene in this model halves the number of suitable embryos. But in practice, the availability of embryos for testing at pre-implantation diagnosis is usually quite limited—almost always fewer than 20, and often only a handful. Some embryos will not be suitable for implantation. Also, even if problems such as allele dropout can be eliminated, a selection process that reduces the availability of embryos eightfold (or 16-fold for four alleles, 32-fold for five, and so on) will render the chances of a successful pregnancy very small.
This is a fairly conservative model; if the beneficial allele is recessive and both parents are heterozygous, testing for just one gene will reduce the number of available embryos fourfold.
Designer babies are thus an interesting topic for theoretical debate, but not a practical proposition and not likely to become one.
References
- 1.Hargreaves S. Safeguards needed now to prevent unethical genetic selection in future. BMJ. 2002;325:733. doi: 10.1136/bmj.325.7367.733. . (5 October.) [DOI] [PMC free article] [PubMed] [Google Scholar]