Once the technique of in vitro fertilization transformed the act of conception into a rather simple laboratory procedure, it became feasible to subject early-stage human embryos to DNA sequence analysis just like any other biological sample. Since each parent passes down only 50 percent of his or her DNA to offspring, the particular constellation of chromosomes and genes that a child inherits is essentially random. But the ability to generate multiple embryos in the laboratory using multiple eggs and sperm changed all that. Instead of implanting random embryos into the mother, IVF doctors
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