What is the source of the extra chromosome 21 in an individual with Down syndrome?

d, because Down syndrome occurs when an egg or sperm with an extra number 21 chromosome fuses with a normal gamete.

The best answer is (E).  For reasons not clearly understood, the age of the mother seems to increase chances of nondisjunction of chromosome #21.

B) Nondisjunction or translocation in either parent

While the mother is most likely, especially in nondisjunction, a paternal source can occur.

The cause of Down syndrome is one of three types of abnormal cell division involving chromosome 21. All three abnormalities result in extra genetic material from chromosome 21, which is responsible for the characteristic features and developmental problems of Down syndrome. The three genetic variations that can cause Down syndrome include:

so B) Nondisjunction or translocation in either parent
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?Trisomy 21. More than 90 percent of cases of Down syndrome are caused by trisomy 21. A child with trisomy 21 has three copies of chromosome 21 ? instead of the usual two copies ? in all of his or her cells. This form of Down syndrome is caused by abnormal cell division during the development of the sperm cell or the egg cell.
?Mosaic Down syndrome. In this rare form of Down syndrome, children have some cells with an extra copy of chromosome 21. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.
?Translocation Down syndrome. Down syndrome can also occur when part of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception. Children with translocation Down syndrome have the usual two copies of chromosome 21, but they also have additional material from chromosome 21 stuck to the translocated chromosome. This form of Down syndrome is uncommon.
There are no known behavioral or environmental factors that cause Down syndrome.


Most cases of Down syndrome aren't inherited. They're caused by a mistake in cell division during the development of the egg, sperm or embryo.

Translocation Down syndrome is the only form of the disorder that can be passed from parent to child. However, only about 4 percent of children with Down syndrome have translocation. And only about half of these cases are inherited from one of the parents.

In these translocation cases, the mother or father is a balanced carrier of the translocation, which means he or she has some rearranged genetic material, but no extra genetic material. A balanced carrier has no signs or symptoms of Down syndrome, but he or she can pass the translocation on to children.

The chance of passing on the translocation depends on the sex of the parent who carries the rearranged chromosome 21:

?If the father is the carrier, the risk is about 3 percent.
?If the mother is the carrier, the risk is about 12 percent.