The article on human in vitro eugenics by Sparrow is provocative and pertinent.1 Nonetheless, practical limitations to the technique of creating human gametes from stem cells have not been considered. Those limitations are relevant as they lead to ethical complications of higher magnitude than those presented in the paper.

One practical limitation to the technique is that, no matter how a pluripotent cell is created, it is still a diploid cell. In order to make gametes out of such cells, they must be induced to undergo meiosis, which will turn them into haploid cells. Only haploid gametes could fuse to generate a true zygote. Mitosis is distinct from meiosis; in the former, segregation of DNA is equal throughout all cell generations, but this does not apply to the latter. In meiosis, heterozygous genes segregate differently to form different gametes. Moreover, numerous other processes become activated during meiosis in order to provide the individual with the largest possible array of genetically diverse gametes. Processes such as homologous recombination and crossing-over are quite frequent during meiosis, causing small pieces of DNA to be exchanged among chromosomes.2 Mammalian spermatogenesis, for instance, is divided into three phases: first, primitive diploid germ cells undergo mitotic divisions to increase their numbers; next, they undergo meiosis to produce haploid spermatids; and, finally, they differentiate into true gametes …