Final answer:
Down syndrome is best described as a condition with an extra chromosome 21, known as trisomy 21, leading to mental and physical impairments. Klinefelter syndrome, caused by an extra X chromosome in males, and Turner syndrome, with only one X chromosome in females, also involve atypical chromosome numbers.
Step-by-step explanation:
The best description for a human born with an extra chromosome is Down syndrome, which is caused by having three copies of chromosome 21 (trisomy 21). Down syndrome gives rise to mental disabilities, physical deformities, and is also known as Trisomy 21. With 47 chromosomes in total, individuals with Down syndrome exhibit features such as short stature, stunted digits, a distinct facial appearance, and developmental delays which can include both growth and cognition challenges. This condition is more likely to occur in children born to older mothers, due to the increased likelihood of chromosomal nondisjunction during meiosis as maternal age rises.
Another condition caused by an extra chromosome is Klinefelter syndrome, where a male is born with an additional X chromosome, resulting in an XXY genotype. These individuals can have underdeveloped sex organs, elongated limbs, and may face learning difficulties.
However, Turner syndrome is different, as it involves having only one X chromosome (monosomy) rather than an extra one. Turner syndrome affects females, who may have underdeveloped sexual organs and can be sterile.