Down syndrome, the most common birth defect affecting children in the US, is caused by chromosomal abnormalities and is most often associated with babies born to mothers age 35 and older. A team of research scientists has recently discovered a new clue in the mystery of why chromosomal birth defects, like those that cause Down syndrome, are more likely to occur in older mothers. Their findings point to a process called meiosis.

Chromosomes and Cell Division

Almost every cell in the human body contains 46 chromosomes aligned in 23 pairs; each pair consists of 23 chromosomes (genes) from the father and 23 from the mother. When cells divide and reproduce as a child grows or when cells are replaced to maintain health, all 46 chromosomes are passed from one cell to the next in a process called mitosis.

Germ cells (egg and sperm) are different, however; each germ cell contains only 23 chromosomes that are not paired. The 23 chromosomes in the egg contain genetic material from the woman’s mother and father; the 23 chromosomes in the sperm contain genetic material from the father’s parents.


Germ cells multiply in a process called meiosis. The genetic material in the egg is shuffled (recombined) in a unique way each time a new egg matures; recombination also happens in sperm cells. When the egg is fertilized, the chromosomes in the egg and sperm unite to form a unique set of 23 pairs of chromosomes. Recombination ensures no two humans are ever exactly the same genetically.

Recombination Events

Researchers from the Albert Einstein College of Medicine of Yeshiva University in New York City analyzed the recombination events occurring in the genetic material of 4,200 families that included at least two children (a total of 645,000 recombination events). They found that recombination events increase in number and occur more closely to each other in older women.

This increase in recombination events represents a weakening of the regulatory process, according to the research team led by Dr. Adam Auton, an assistant professor of genetics and epidemiology and population health. The weakened regulatory system is prone to mistakes and errors that can result in chromosomal birth defects. No weakening of the recombination process was noted in the sperm cells of older men.

The study was funded by grants from the US National Institutes of Health and the genetic testing company, 23andMe. All genetic material analyzed in the study came with informed consent from the 23andMe customers who supplied the samples.

At this time, there is no way to apply the study’s findings to a clinical situation but Auton said, “Our study adds to the understanding of the basic biology of meiosis and how recombination shapes the evolution of the human species.” It is expected that the findings of the study may help increase the understanding of how birth defects such as Down syndrome occur.

Down Syndrome and Mother’s Age

According to the US Centers for Disease Control and Prevention (CDC), the percentage of mothers having babies with Down syndrome (the risk) is higher at age 35 and older. The greatest number of babies born with the defect, however, are born to women younger than 35. The CDC attributes this finding to the fact that more babies in general are born to women 35 and younger.


  1. Auton, Adam, et al. "Escape from crossover interference increases with maternal age." Scientific American / Nature Communications. Nature Publishing Group / Macmillan Publishers Limited, 19 Feb. 2015. Web. 24 Feb. 2015.
  2. "Sex and Genetic Shuffling: The Details." Evolution 101. University of California, Berkeley, n.d. Dolan DNA Learning Center. Web. 24 Feb. 2015.
  3. "Facts about Down Syndrome." CDC / Centers for Disease Control and Prevention. US Department of Health and Human Services, Oct. 2014. Web. 24 Feb. 2015.