Down Syndrome is a neurological disorder that produces mild cognitive impairment

Question

Down Syndrome is a neurological disorder that produces mild cognitive impairment along with distinctive facial features that allows one to immediately discern the phenotype simply by looking at the face of an individual with this disorder. Children who are genetically unrelated but who have Down Syndrome tend to look very similar. The genetic defect is not inherited by a child from its parents; however, it affects the children of older mothers [40 years +] more commonly than offspring born to younger women.

Is Down Syndrome a form of Mendelian inheritance? Why or why not?

Is there anything in the above description that suggests that Down Syndrome may be due to an inherited genetic defect?

Explanation / Answer

Down Syndrome is not a form of Mendelian inheritance because it is caused due to chromosomal aberration but not due to genes or alleles.

What Causes Down Syndrome?

A fertilized egg normally has 23 pairs of chromosomes. However, most of the people who have Down Syndrome have an extra (or third) copy of chromosome 21.

Having this extra copy of chromosome 21 changes the body’s and brain’s normal development.

In most cases Down Syndrome is caused by a random error in cell division that happens during formation of the mother’s egg or father’s sperm. Because of this error, when fertilization does take place, the embryo has an additional third chromosome 21 or “trisomy 21.”

It is not believed that Down Syndrome is a result of the parents’ behavior or environmental reasons.

Down Syndrome Risk Factors:-

Researchers have found that it is more likely for an older mother to have reproductive cells with an extra copy of chromosome 21.

So an older mother is more likely to have a baby with Down Syndrome than a younger mother.

Most of the babies who have Down Syndrome (about 75%) are born to mothers who are 35 or younger. This is because older mothers tend to have fewer babies. (Only about nine percent of total births occur in mothers over age 35 – but about 25% of babies with Down Syndrome are born to women in this age group.)

**The likelihood that a woman under age 30 who becomes pregnant will have a baby with Down syndrome is less than one in 1,000, but the chance of having a baby with Down Syndrome increases to 1 in 400 for women who become pregnant at age 35. The likelihood of Down Syndrome continues to increase as a woman ages, so that by age 42, the chance is one in 60 that a pregnant woman will have a baby with Down syndrome, and by age 49, the chance is one in 12.

**Because the chances of having a baby with Down Syndrome increase with the age of the mother, many health care providers recommend that women over age 35 have pre-birth testing for the condition.

**Testing the baby before it is born to see if he or she is likely to have Down Syndrome allows parents and families to prepare for the baby’s special needs.

**Parents who have already have a baby with Down Syndrome or who have abnormalities in their own chromosome 21 are also at higher risk for having a baby with Down Syndrome.

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