Down syndrome, the most common genetic condition in the united states, is also called trisomy 21. what does this mean? a. a person with down syndrome has two copies of chromosome 21 but they're fused together. during early embryo development, these chromosomes didn't segregate properly. b. a person with down syndrome has only one copy of chromosome 21 instead of two copies. either the sperm cell or the egg cell was lacking this chromosome. c. a person with down syndrome has three copies of chromosome 21 instead of two copies. an extra chromosome 21 was present in either the sperm or the egg cell. d. a person with down syndrome has three copies of chromosome 21 instead of two copies. two sperm cells simultaneously fertilized the egg cell.

The correct answer option is C. A person with Down syndrome has three copies of chromosome 21 instead of two copies. An extra chromosome 21 was present in either the sperm or the egg cell.

Explanation:

Down Syndrome is the most genetic condition in the United States caused by the error in the cell division called 'disjunction'.

A normal person has two copies of chromosome 21 while a person with down syndrome has three copies of it in either the sperm or the egg which fails to separate.

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