What condition does non-disjunction lead to in chromosome separation?

Non-disjunction refers to the failure of chromosomes or sister chromatids to separate properly during cell division, specifically during meiosis or mitosis. When non-disjunction occurs, it can result in one of the following scenarios: sister chromatids fail to separate during anaphase of mitosis, or homologous chromosomes do not separate during anaphase I of meiosis. This failure can lead to gametes that contain an abnormal number of chromosomes.

When the resulting gametes fuse during fertilization, they can lead to zygotes with an abnormal chromosome number, a condition which is often associated with various genetic disorders. For example, Down syndrome is caused by the presence of an extra copy of chromosome 21, a result of non-disjunction.

Other options provided do not accurately reflect the consequences of non-disjunction. Non-disjunction does not result in all chromosomes separating correctly, nor does it merely cause a reduction in chromosome number or duplication of homologs without separation. The key aspect of non-disjunction is the improper separation of either sister chromatids or homologous chromosomes, directly leading to an imbalance in chromosome distribution.