Meiosis occurs repeatedly,
as testes or ovaries produce gametes. Almost always, the meiotic spindle
distributes chromosomes to daughter cells without an error. But occasionally
there is an accident, called a nondisjunction, in which the members of a
chromosome pair fail to separate. Figure here, A and B illustrate two ways that
nondisjunction can occur. For simplicity, we use a hypothetical organism whose
diploid chromosome number is 4. In both figures, the cell at the top is diploid
(2n), with two pairs of homologous chromosomes undergoing meiosis I.
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| Non-Disjunction |
Sometimes, as in Figure, a
pair of homologous chromosomes does not separate during meiosis I. In this
case, even though the rest of meiosis occurs normally, all the resulting
gametes end up with abnormal numbers of chromosome. Two of the gametes have
three chromosomes, two of which are the same; the other two gametes have only
one chromosome each. In Figure B, meiosis I is normal, but a pair of sister
chromatids fail to move apart in one of the pair of the cells during meiosis
IT. In this case, two gametes have the normal complement of two chromosomes
each, but the other two gametes have the abnormal complement.
Figure shows what happens
when an abnormal gamete produced by nondisjunction unites with a normal gamete
in fertilization. Here, an egg cell with two copies of one of its chromosomes
(a total of n + 1 chromosomes) is fertilized by a normal sperm cell (n). The
resulting zygote has an extra chromosome (a total of 2n + 1 chromosomes).
Mitosis will then transmit the abnormality to all embryonic cells. If this were
a real organism and if survived, it would have an abnormal karyotype and
probably a syndrome of disorders caused by the abnormal number of genes.
Nondisjunction can lead to
an abnormal chromosome number in either sex of any sexually reproducing, diploid
organism, including humans. Three chromosome disorders of interest are Down
syndrome, Klinefelter Syndrome and Turner syndrome.
