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.

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:

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.

parasites can detrimentally affect their host, but also must not allow their host to die before they can complete their life cycle. some parasites have complex life cycles with many different hosts, and have had to find ways of persisting in many different environments during the change from one host to another. ectoparasites live outside the body of the host, and can more easily  disperse, but are  more exposed to predators, parasites, parasitoids. endoparasites live inside the body of the host, and are therefore protected from the outside environment. but they can be  attacked by the host's immune system. they have therefore developed  mechanisms for dispersal such as  complex life cycles.

what are the options