Question

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 only one copy of chromosome 21 instead of two copies. Either the sperm cell or the egg cell was lacking this chromosome.
B. 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.
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.

Answer 1

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.

Step-by-step explanation:

Trisomy refers to presence of three copies of particular chromosome in genome of organisms. Since Down syndrome is caused by trisomy 21, the affected person has three copies of chromosome 21, instead of two. Here, either male or female gamete could have two copies of chromosome 21. Fusion of gamete with extra copy of chromosome 21 with a normal gamete led to development of zygote with trisomy 21.