Dear Science,

How close are we to being able to tell fetuses what kind of adults they should become? Like, genetically? I'm asking because I want to have a baby that looks exactly like Gabriel García Márquez, but the problem is that I am white and look nothing like him.

Eugenicist for Gabriel

Quite close, actually.

First, let's talk about what "like, genetically" actually means. Genes are the dishes the body must make. The actual recipes for each dish are called alleles; humans get exactly two alleles for almost every gene, one from each parent. Genetics is all about following alleles around.

Let's go for the Gattaca strategy of creating a bunch of embryos in a lab, screening them genetically to find the "best," and implanting only those embryos. Enter the in vitro fertilization clinic! A largely unregulated industry! Eggs meet sperm in the lab and the fertilized eggs start to grow, first into a solid ball of eight or so cells, and finally into a blastocyst of about 150. Pluck a cell from these early embryos—the eventual fetus won't miss the single cell—freeze the embryo for later, and "grow up" that one cell a little bit. Collect the DNA.

Next, figure out all the alleles in the embryo's cells—what recipes the embryo has for each dish. Reading all the embryo's DNA, all three billion base pairs, is the brute-force way to identify all the alleles present. Until quite recently, this cost about a dollar per base pair. Thanks to a few technical breakthroughs, the cost is rapidly dropping to about $1,000 per genome. Yay! Another trick is to use something called a single nucleotide polymorphism microarray, screening for tiny differences in the DNA sequence between different alleles—also around $1,000 per embryo.

So, we can create embryos, get DNA from them safely, and even figure out what alleles the embryo has. Here comes the rub: For most genes, we've only now begun to associate a given allele with a given trait. Puppies are helping, thanks to the huge variety in dog breeds. For example, a Chihuahua male paired with a Great Dane female? Puppies! (The reverse, with a Chihuahua female, would be more like Alien than Gattaca.) By comparing the alleles in a Chihuahua to a Great Dane, we can get a pretty good sense of what alleles it takes to make a big or small dog. By looking at the alleles in the second-generation children of such a pairing, we can really nail down the key alleles. Thanks to evolution, knowing what alleles mean in dogs pretty much tells us what they mean in humans. The alleles behind a handful of serious illnesses are already worked out. With time, your wish probably will come true.

Discriminatingly Yours,


Send your science questions to