Science: Sex by Sedimentation

Once scientists learned the technique of artificial insemination in both animals and humans, they seemed within possible reach of an exciting goal: selection of the sex of an infant before conception. But how to separate sperm that produce female offspring from those that produce males? Various methods, such as the use of electric fields or care ful temperature control, produced only minimal results. Then, in India, Zoologist Bhairab Chandra Bhattacharya noticed that the upper portion of a sperm sample tended to breed more bulls; the lower portion gave more cows. Apparently this was because the sperm that produce female offspring are heavier than those that produce males and sank to the bottom of the solution.

To improve the process, Bhattacharya moved to the Max Planck Institute for Animal Breeding at Hagen, Germany, where he went to work under the direction of Zoologist Gham Gottschewski. Using rabbits, which are not only cheaper than cattle but much quicker to breed, he inseminated thousands of does with sperm that had been allowed to settle under varying conditions. His early results were not promising, but after three years of experimentation he hit on a winning combination. He mixed rabbit sperm with egg yolk and glycol, and stored the solution for twelve hours in a refrigerator at slightly above the freezing point to keep the sperm from swimming and allow them to separate by sedimentation. Then he used the sample to inseminate 176 rabbits. Those that got the upper portion produced 77.4% male offspring, while those impregnated with the lower portion produced 72% females. The middle portion gave a near-normal result: 55% males and 45% females. A control group of 182 rabbits inseminated with unsedimented sperm produced the standard fifty-fifty percentage of males and females.

No one except a rabbit cares much whether rabbits are male or female, but with dairy cattle, bred for their milk-producing ability, sex is a vital difference. Bhattacharya has now moved to a big cattle-breeding establishment in Schleswig-Holstein to find out whether his simple system can reduce the number of low-value male calves born to German dairy cows. Theoretically it should work the same for all mammals, including humans.