A human parent’s biggest fear is having a child with a genetic disorder (though the most commonly expressed fear on Facebook is of Donald Trump winning a second term!).
Technology is bringing us pretty close to eliminating this fear and giving us a bewildering array of options.
She Has Her Mother’s Laugh: The Powers, Perversions, and Potential of Heredity by Carl Zimmer reminds the reader of the diversity in sperm and egg cells due to meiosis:
In men, meiosis takes place within a labyrinth of tubes coiled within the testicles. The tube walls are lined with sperm precursor cells, each carrying two copies of each chromosome—one from the man’s mother, the other from his father. When these cells divide, they copy all their DNA, so that now they have four copies of each chromosome. Rather than drawing apart from each other, however, the chromosomes stay together. A maternal and paternal copy of each chromosome line up alongside each other. Proteins descend on them and slice the chromosomes, making cuts at precisely the same spots. As the cell repairs these self-inflicted wounds, a remarkable exchange can take place. A piece of DNA from one chromosome may get moved to the same position in the other, its own place taken by its counterpart. This molecular surgery cannot be rushed. All told, a cell may need three weeks to finish meiosis. Once it’s done, its chromosomes pull away from each other. The cell then divides twice, to make four new sperm cells. Each of the four cells inherits a single copy of all twenty-three chromosomes. But each sperm cell contains a different assembly of DNA. One source of this difference comes from how the pairs of chromosomes get separated. A sperm might contain the version of chromosome 1 that a man inherited from his father, chromosome 2 from his mother, and so on. Another sperm might have a different combination. At the same time, some chromosomes in a sperm are hybrids. Thanks to meiosis, a sperm cell’s copy of chromosome 1 might be a combination of DNA from both his mother and father.
A particular child of two parents, therefore, is just one choice from a near-infinite array of genetic possibilities assembled from the four grandparents. That’s what comes out when a baby is conceived naturally. What if parents were given the opportunity to choose from hundreds of possible outcomes?
In 2012, the Japanese biologist Katsuhiko Hayashi managed to coax induced pluripotent stem cells to develop into the progenitors of eggs. If he implanted them in the ovaries of female mice, they could finish maturing. Over the next few years, Hayashi perfected the procedure, transforming mouse skin cells into eggs entirely in a dish. When he fertilized the eggs, some of them developed into healthy mouse pups. Other researchers have figured out how to make sperm from skin cells taken from adult mice.
Nevertheless, the success that Yamanaka and other researchers have had with animals is grounds for optimism—or worry, depending on what you think about how we might make use of this technology. It’s entirely possible that, before long, scientists will learn how to swab the inside of people’s cheeks and transform their cells into sperm or eggs, ready for in vitro fertilization. If scientists can perfect this process—called in vitro gametogenesis—it will probably be snapped up by fertility doctors. Harvesting mature eggs from women remains a difficult, painful undertaking. It would be far easier for women to reprogram one of their skin cells into an egg. It would also mean that both women and men who can’t make any sex cells at all wouldn’t need a donor to have a child.
Today, parents who use in vitro fertilization can choose from about half a dozen embryos. In vitro gametogenesis might offer them a hundred or more. Shuffling combinations of genes together so many times could produce a much bigger range of possibilities.
But the implications of in vitro gametogenesis go far beyond these familiar scenarios—to ones that Hermann Muller never would have thought of. Induced pluripotent stem cells have depths of possibilities that scientists have just started to investigate. Men, for instance, might be able to produce eggs. A homosexual couple might someday be able to combine gametes, producing children who inherited DNA from both of them. One man might produce both eggs and sperm, combining them to produce a family—not a family of clones, but one in which each child draws a different combination of alleles. It would give the term single-parent family a whole new meaning.
Here’s a yet more science fiction-y possibility… The highest fertility among Americans is in the lowest income mothers, i.e., those who are on welfare. The government will be paying for 100 percent of the costs of any children produced by these mothers: housing, health care, food, education, etc. Once grown up, these children are likely to be low earners and therefore on welfare themselves (see The Son Also Rises). What if the government begins to run out of borrowing capacity and decides that it needs to fund future taxpayers, not future welfare recipients? The tendency to work and pay taxes is as heritable as anything else. So the government offers financial inducements to mothers who agree to abort children conceived with low-income men and instead incubate embryos provided by the government. Said embryos to be carefully screened such that the moms are almost guaranteed to have a physically and mentally healthy child and the government is almost guaranteed to get an adult that enjoys working and paying taxes.
Readers: What do you think? In 2040 or 2050 will there be anyone willing to roll the genetic dice by having sex and seeing what kind of baby comes out? Full post, including comments