The result of normal meiosis , consisting
of two cell divisions (not shown here), is
a normal distribution and a haploid chromosome
complement. With abnormal distribution
(nondisjunction either in meiosis I or inmeiosis
II), one gamete is formed with an additional
chromosome, whereas the other is missing a
chromosome. After fertilization, the respective
zygote contains either three copies of one chromosome
(trisomy) or only a single chromosome
of a pair (monosomy). Abnormal chromosome
distribution can occur during oogenesis (maternal
nondisjunction) or during spermatogenesis
(paternal nondisjunction).
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