Final answer:
The loss of a single chromosome in an organism, known as monosomy, usually results in a lethal condition in humans due to the loss of essential genes. It is often the result of nondisjunction during meiosis. The given answer to the student's multiple-choice question is monosomy.
Step-by-step explanation:
The loss of a single chromosome in an otherwise diploid organism is known as monosomy. This condition arises when aneuploid gametes are produced due to a failure in chromosome segregation during meiosis, an error known as nondisjunction. Monosomy is a type of genetic imbalance that is usually lethal in humans because individuals lack essential genes required for normal development.
The most familiar type of aneuploidy that results in a viable individual is the gain of an extra chromosome, specifically chromosome 21, leading to trisomy 21 or Down syndrome. This condition is correlated with maternal age, with older women having a higher chance of having a child with Down syndrome. The extra genetic material alters the balance of gene products, causing various developmental and physiological challenges.