Alterations of chromosome number or structure cause some genetic disorders * Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders * Plants tolerate such genetic changes better than animals do Abnormal Chromosome Number * In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis * As a result, one gamete receives 2 of the same type of chromosome, and another gamete receives no copy * Aneuploidy results from the fertilization of gametes in which nondisjunction occurred * Offspring with this condition have an abnormal number of a particular chromosome * A monosomic zygote has only 1 copy of a particular chromosome