Final answer:
Tetrasomy is the form of aneuploidy where four copies of a chromosome are present, often caused by nondisjunction during meiosis resulting in gene dosage imbalance.
Step-by-step explanation:
The form of aneuploidy in which four members of the same homologous pair are present is known as tetrasomy. In this condition, instead of the normal two copies of a chromosome, there are four copies, which can lead to a variety of developmental and functional difficulties due to an imbalance in gene dosage. Aneuploidy can occur as a result of nondisjunction during meiosis, where chromosomes fail to separate properly, leading to gametes with an abnormal number of chromosomes.