Answer the following question given this paragraph: 

Outside of chromosome 21 and the sex chromosomes, most embryos with extra chromosomes do not usually survive. Because chromosomes carry many, many genes, a disruption of a chromosome can cause severe problems with the development of a fetus. Individuals with one (or more) fewer chromosome usually don't survive either. Can you explain why?.


Q: Embryos usually don't survive if they have extra or too few of these genetic units?


A:
chromosomes