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 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.

Answer 1

Trisomy=3 chromosomes; 1 more than the required pair (2)

21=the type of chromosome

So, here we can eliminate A and B.

Now we have C and D.

I know that when sperm cell(s) simultaneously fertilize [an] egg, a class of identical twins will form. Therefore, we can eliminate D, leaving us with merely C. Henceforth, C is your final answer.

Answer 2

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.

Step-by-step 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.