Down syndrome and other disorders where there are extra or missing copies of chromosomes are the consequence of: crossing over. nondisjunction. fertilization. mitosis. sex determination.

The correct answer is **nondisjunction**. Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division (meiosis or mitosis). This can result in gametes with an abnormal number of chromosomes, leading to conditions such as Down syndrome, which is caused by an extra copy of chromosome 21 (trisomy 21). Here's a brief explanation of the other options: - **Crossing over**: This is a process that occurs during meiosis where homologous chromosomes exchange genetic material, but it does not lead to an abnormal number of chromosomes. - **Fertilization**: This is the process of combining sperm and egg, which does not directly cause chromosomal abnormalities. - **Mitosis**: This is the process of cell division that results in two identical daughter cells, and while errors can occur, it is primarily nondisjunction during meiosis that leads to disorders like Down syndrome. - **Sex determination**: This refers to the biological mechanisms that determine the sex of an organism, which is not directly related to the presence of extra or missing chromosomes. Thus, the primary cause of disorders like Down syndrome is nondisjunction.