Final answer:
A genetic disorder can occur due to nondisjunction during meiosis, leading to gametes with abnormal chromosome numbers, which results in physical and mental abnormalities or lethal genetic imbalances in offspring.
Step-by-step explanation:
A genetic disorder can occur when both members of the gene pair—one from each parent—are abnormal. This may happen as a result of nondisjunction, a type of chromosomal error during cell division. During meiosis, nondisjunction can happen when homologous chromosomes fail to separate properly. This leads to the formation of gametes with an abnormal number of chromosomes, a condition known as aneuploidy. When these gametes are involved in fertilization, it can lead to offspring with physical and mental abnormalities, various syndromes, or other genetic imbalances that are often lethal to a diploid organism.