Trait selection for 'designer babies' remains limited

Ever since in vitro fertilisation became a reality and scientists began pre-screening embryos for genetic disorders, the fantasy of creating ‘designer babies’ has taken hold of the public’s imagination. But just how far has reproductive science developed when it comes to selecting for traits such as height and IQ?

Currently, the most practical approach to genetic ‘enhancement’ of embryos is preimplantation genetic screening of IVF embryos. Selecting embryos for eye colour or sex is relatively easy for a scientist to perform, as it involves only one or very few genes, as is screening for genetic diseases caused by a single mutation. But according to a recent study, led by Dr Shai Carmi at The Hebrew University of Jerusalem and published in the journal Cell, the ability to select for traits that are brought about by multiple genes is more complicated than people probably realise.

Dr Carmi’s team looked at the feasibility of selecting embryos based on each of two traits caused by multiple genes — IQ and height — running virtual experiments based on real-life genomic data to answer the question: what would happen if we took 10 embryos from one pair of parents, rated each embryo for height or IQ, and implanted embryos with the highest score? They ran computer simulations using gene sequences from real people to create profiles of hypothetical embryos that would result from pairings of those people. They then predicted the adult height or IQ for each of the embryos based on the gene variants present in their genomes.

What they found was that the expected advantages for ‘top-scoring embryos’ were relatively small. For height, the gain was 3 cm above the average embryo in the batch; for IQ, the gain was three points. With five embryos to choose from, the gain was 2.5 cm and 2.5 IQ points. When the team widened the scope to see what would happen if they could choose from a batch of 50 embryos (a near-impossible biological feat for most couples) the highest gain was 4.5 cm for height and 4.5 IQ points.

To corroborate their findings, the researchers also used real-world data to demonstrate that trait predictions based on currently known gene variants are not guaranteed. They looked at the genetics of 28 families with 10 or more adult-age children. Based on the genomic make-up of each child, they selected those with the top score for height. However, in 75% of the families, the child that the scientists had ‘selected’ was not the tallest sibling, even though their genomic data had predicted that they would be.

For those who argue that even nominal height and IQ improvements may warrant embryo selection, Dr Carmi cautions that not only are their desired outcomes not guaranteed, there are also pitfalls involved in such a procedure at this time. The nature of gene variants is that one may select for one outcome but increase the risk for another, less desirable outcome. For example, the group of genes that is linked to a high IQ is also somewhat linked to anorexia. Additionally, attempts to select for several traits at once — for instance, an embryo that is tall, smart and thin — would make embryo selection far more complicated; an embryo that ranked highest for IQ may rank lowest for the desired BMI, for example.

“Our current knowledge of the genetic make-up of certain traits may not be enough to generate a substantial increase in the desired traits in an embryo selection scenario,” Dr Carmi said. “The crucial roles of nurture and unknown genetic factors are also at play.”

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