Dr. David C. Page, a Howard Hughes Medical Institute researcher at the Whitehead Institute, said that about 10 percent of all men who can produce little or no sperm suffer from a genetic defect in a part of the Y chromosome.
When these men father children through in vitro fertilization, or IVF, techniques, said Page, their sons tend to inherit the genetic defect. Daughters are not affected since females receive from their fathers an X chromosome instead of the Y chromosomes.
This means, said Page, that although IVF may help otherwise infertile men produce sons, it will not ensure that there eventually will be grandchildren.
Infertility affects 10 percent to 15 percent of all American couples. Men are responsible for about half of infertility problems, with an inadequate number of sperm being the most common problem. Some men with low sperm counts have a genetic flaw in a part of the Y chromosome known as the AZFc.
A normal sperm count is 50 million to 200 million per cubic centimeter of ejaculate. Men with the AZFc genetic mutation may have sperm counts of zero to only 5 million.
Page said the AZFc part of the Y chromosome plays a key role in the production of sperm. For reasons not understood, he said, some men develop a mutation of the AZFc. Since the condition causes infertility, the mutation is not thought to be inherited.
Before IVF, such men were unlikely to father children. However, an IVF technique called intracytoplasmic sperm injection, or ICSI, enables test tube conception with a single sperm.
The ICSI technique involves injecting a single sperm into an egg in a laboratory dish. When an embryo develops, it is transferred into the fallopian tube of the mother and can result in a normal pregnancy.
Daughters born from this technique are normal, but Page said that a genetic study of four sons born to men with flawed AZFc genes were found to share the flaw. The study is published this week in the journal Human Reproduction.
"This study is the first to demonstrate that ICSI does result in some men passing the genetic flaw responsible for their infertility on to their sons," said Page.
The discovery may lead prospective parents to ask that their doctor determine the gender of an embryo, said Page. Only female embryos would be implanted into the mother. This could ensure that the baby would be a fertile daughter instead of an infertile son.
Page said it is possible that the AZFc flaw may cause the sperm count to drop only with age, and that men with the genetic flaw may have normal sperm counts during puberty and early adulthood.
If that is the case, he said, sperm could be collected in the male's youth and then frozen until he is ready to start a family.