When describing down syndrome, it's important to note that a. it can be classified as a single-gene disorder b. it is caused when red blood cells cannot properly form hemoglobin c. it causes infected males to have low testosterone levels d. it is usually caused by three copies of chromosome 21

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D.  It is usually caused by three copies of chromosome 21.

Explanation:

Down syndrome is a genetic disorder. It is caused by the trisomy of chromosome number 21 that means the presence of a third copy of chromosome number 21 is responsible for this disorder. This is caused by the abnormal cell division during gamete formation that may be sperm or egg cells in parents.

This disorder is associated with the intellectual disability, delay in growth, and development of characteristic facial features.

a progressive heart disease that affects pumping action of the heart muscles.

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explanation: