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Nondisjunction during meiosis primarily leads to the failure of homologous chromosomes to separate properly during the first meiotic division or sister chromatids during the second division. This error can result in gametes (sperm or egg cells) that contain an abnormal number of chromosomes—either an extra chromosome (trisomy) or a missing chromosome (monosomy)—when fertilization occurs.

This is significant because the resulting zygote can have genetic imbalances, which frequently lead to developmental disorders or conditions such as Down syndrome (trisomy 21) or Turner syndrome (monosomy X). The process of homologous chromosomes separating correctly is crucial during meiosis to ensure that each gamete receives an equal and appropriate complement of chromosomes.

The other options do not accurately describe the consequence of nondisjunction. Chromosomal segments being deleted pertains to different types of chromosomal abnormalities, chromosomes being duplicated refers to processes like DNA replication or mitotic errors, and increased sets of chromosomes would relate to polyploidy rather than the specific failure to separate chromosomes during meiosis.