Some children are born with the wrong number of chromosomes, either too many or too few. The most common case of this is Down syndrome, shown here, where individuals have an extra copy of chromosome 21. Cases such as this are the result of nondisjunction, where chromosomes fail to separate in meiosis. This can happen if homologues fail to separate during meiosis I. The result will be that some gametes will be missing a chromosome and others will have an extra one. This same result can occur if chromatids fail to separate during meiosis II. In most cases, where an embryo has the wrong number of chromosomes, the consequences are so severe that proper development is disrupted and a miscarriage results.