Final answer:
An XY individual with a third chromosome 21 has both Klinefelter syndrome and Down syndrome. This combination of an extra sex chromosome and an extra chromosome 21 leads to a mix of features from both conditions.
Step-by-step explanation:
A person who is XY and has a third chromosome 21 is considered to have Klinefelter syndrome and Down syndrome. Klinefelter syndrome typically occurs in males who have two X chromosomes and one Y chromosome (XXY), resulting in features like small testes and reduced body hair.
Down syndrome is characterized by the presence of an extra chromosome 21, which is known as trisomy 21. Therefore, an individual with both conditions would have an extra chromosome 21 in addition to the XY sex chromosomes, leading to a combination of symptoms and physical characteristics from both syndromes.