As our understanding of genetics grows, so does the potential for people to use it to have babies with particular characteristics. In theory, DNA analysis could help prospective parents choose which IVF embryos to implant on the basis of characteristics like intelligence. But now a study estimates that such practices are likely to have only a small effect.
In recent years, it has become possible to sequence the entire genome of embryos. This means it is feasible to look at thousands of genetic variants that each have a tiny effect on traits such as height. Geneticists can add all of these together, producing a combined “polygenic score” for that characteristic. This raises the prospect of being able to calculate polygenic scores for each embryo produced by IVF, and letting parents choose which ones they want to use.
But would selecting embryos in this way really make much of a difference? To find out, Shai Carmi at the Hebrew University of Jerusalem and his colleagues fed data from detailed, large-scale genetic studies into a computer model to estimate the maximum potential effect of selecting IVF embryos on the basis of polygenic scores for IQ or height.
They found that the approach could only increase height by 3 centimetres at most, and IQ by an average of only 3 points. “The gain is not guaranteed,” says Carmi. “This is the average over all families.”
“It’s not nearly as predictive as we think it might be,” says Frances Flinter at Guy’s and St Thomas’ NHS Foundation Trust in the UK. “It’s ascribing undue and inappropriate importance to the genetics, and I say that as a geneticist.”
Non-genetic factors such as the environment, diet and education can have a much greater effect than genetic variants, she says.
Carmi’s team says there are many limitations that mean even the small potential gains identified might be unachievable in practice. For instance, the study assumes that prospective parents have 10 embryos to choose among, but in reality many people end up with far fewer.
What’s more, the polygenic scores weren’t highly predictive. The team’s model took in detailed genetic data from around 1000 individuals, but the tallest of these people didn’t have the highest polygenic scores for height. Some of those with the highest scores were shorter than the average.
In 20 years or so, when we know much more about genetic variants, it might be possible to achieve bigger gains, particularly for IQ, says Carmi.
Flinter, however, is sceptical. “Even if we know far more, I think the contribution of these variants will still be fairly small,” she says.
In some countries, including the UK, embryo screening can be used only to prevent children inheriting serious diseases. However, certain other countries lack specific regulations or guidelines.
Journal reference: Cell, DOI: https://doi.org/10.1016/j.cell.2019.10.033
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