At nearly a quarter of the way through perhaps it’s useful to take stock. We still don’t have flying cars or a cure for cancer, but what about designer babies? Can we choose some traits that might give our offspring advantages later in life, or at least protect them from disease? The answer to this question today remains an unsatisfying “kind-of”.
The fact is that with rare exception humans have been practicing selective breeding by mate selection for millennia. It’s a low-tech version of embryo trait selection - trait selection 1.0 - if you see the traits you like in a member of the opposite sex, mate with them and see what you get! Until the early 2000’s this rolling of the genetic dice was the only game in town.
Preimplantation Genetic Testing (PGT) is used to detect aneuploidies (too many or too few chromosomes) as well as single-gene mutations and chromosomal rearrangements. The former is strongly correlated with maternal age while the latter two are heritable from parent to offspring. PGT requires In-vitro fertilization (IVF) and is used in up to 40% of IVF cycles in the United States. PGT is a reductive embryo selection tool - trait selection 2.0 - allowing patients and their physicians to select against embryos that carry genetic anomalies by transferring only low-risk embryos.
Of course, chromosome numeration also reveals the gender of each embryo tested which allows for gender-based selection, often referred to as “family balancing”. A study published in 2017 found that 83.5% of surveyed IVF clinics offer gender selection for family balancing to otherwise fertile couples that can conceive naturally.
Identifying the genes responsible for so-called polygenic or multi-gene traits such as height, intelligence, and cancer risk, has proven difficult with current methods and would require a large number of embryos in order to have embryos identified as low-risk available to transfer. Ovulation induction in a typical IVF cycle yields 5-15 oocytes with approximately half of those completing normal fertilization and blastocyst formation and therefore being available for PGT, far less than would be needed for complex trait selection as described above.
Genetic engineering by genome editing involves techniques that offer tremendous potential for improving lives and curing disease. However, their application to human reproduction requires humility and caution in the face of yet-unknown consequences of germline (egg and sperm cell) manipulations. Trait selection 3.0 will rely upon these technologies in the setting of IVF treatment and will represent an “event horizon” whereby we progress from passive deselection to active addition of genetic elements into our collective genetic legacy.