With the help of new genetic markers, Johns Hopkins Hospital researchers have found evidence that the chromosome largely responsible for Down's syndrome comes from the mother's egg in 95 percent of children with the disorder.
Down's syndrome, which is the most common genetic cause of mental retardation, results from an extra, or third, 21st chromosome contributed at conception. The syndrome occurs in one in 700 births in the United States. Studies have shown that older mothers are at greater risk.
Dr. Stylianos Antonarakis, a researcher at Hopkins Center for Medical Genetics, says that DNA polymorphisms, the new markers, have shown that the origin of the Down's syndrome chromosome is maternal more often than was previously thought.
Until now, subjective studies suggested that about 80 percent of the third 21st chromosome came from the mother's egg and 20 percent came from the father's sperm cells.
"There are no ambiguities with DNA polymorphisms, which allow us to look more closely at the DNA," Antonarakis said in a report that was to be presented today at the American Society for Human Genetics meeting in Cincinnati.
"The polymorphism appears as a band on X-ray film. Either it's there in the mother or it's not," Antonarakis said.
Antonarakis said that in the past, geneticists looked at pictures of the family's chromosomes and decided whether the child's extra chromosome looked more like his mother's or the father's.
Normally, every person has two 21st chromosomes, but the Down's syndrome child has three. Antonarakis' research shows the mother gives two and the father gives one in 95 percent of the cases. In 5 percent of the cases, the father gives two and the mother gives one. Geneticists have been studying the origin of the extra 21st chromosome for the last 20 years.
"This research will help scientists understand what happens to a mother's eggs when she has a child with Down syndrome," Antonarakis said in an interview earlier this week. "It will give us a better understanding of the mechanism of the disorder -- why a female gives two chromosomes. And, perhaps it will help us with prevention.
"But, please stress that prevention is in the distant future. We're not anywhere near that."
Antonarakis believes the communication system between cells in the mother's two 21st chromosomes goes awry and that is what produces a child with Down's syndrome. His is one of many theories that have been proposed, he said.
Antonarakis said the research, which took three years to complete, looked at the DNA of 193 children with Down's syndrome and determined the origin of the extra 21st chromosome was maternal in 184 of the children and paternal in nine of the children.
The scientists further studied the DNA of 34 families, a subset of the 193, using both the old and new methods. The subjective chromosome analysis showed the origin was maternal in 26 children (84 percent) and paternal in five children (16 percent). The DNA analysis of chromosomes disclosed the origin was maternal in 29 (94 percent) of the children and paternal in two (six percent) of the children.