Final answer:
Individuals with Down Syndrome, caused by an extra chromosome 21, are more numerous than individuals with an extra chromosome 3 or chromosome 16 because the extra chromosome 21 allows them to survive to adulthood. Trisomy 21 disrupts the balance of gene dosage and can cause severe developmental problems.
Step-by-step explanation:
Individuals with an extra chromosome 21, also known as Down syndrome, are more numerous than individuals with an extra chromosome 3 or chromosome 16 because the extra chromosome 21 allows them to survive to adulthood, while most embryos with extra chromosomes do not survive. Chromosomal abnormalities, such as monosomy (loss of one chromosome) or trisomy (gain of an extra chromosome), disrupt the balance of gene dosage and can cause severe developmental problems. While Down syndrome, caused by trisomy 21, still presents challenges and impairments, individuals with this condition have a higher chance of survival compared to other trisomies.