Trends and technologies in sex selection.

"MicroSort technology tags sperm bearing X chromosomes (those which determine females) and sperm bearing Y chromosomes (those which determine males) with a fluorescent dye so that they can be segregated into different batches. The dye harmlessly attaches to the DNA molecules that make up genes. Female-determining X chromosomes are much bigger than male-determining Y chromosomes, which means that human sperm carrying X chromosomes have 2.8 percent more DNA than do sperm with Y chromosomes."

(via)