Mutations [1] occur usually during meiosis [2] when chromosomes fail to
separate (non-disjunction) [3] either in part or in whole.
Polyploidy, a condition in which cells contain 3 or more sets of
chromosomes, can occur naturally in animals such as flatworms and
earthworms. It is common in many plants including those cultivated such as
seedless oranges and strains of wheat. It is an abnormality in humans and
embryos usually abort.
Aneuploidy results from the non disjunction or non-separation of
homologous [4] chromosomes during the reduction phase of meiosis. Failure
of chromosomes to separate means that some gametes will have more or less
than the normal somatic chromosome number (2n). The most common form of
aneuploidy is of chromosome number 21. As eggs form in the mother's ovaries
(seldom in sperm formation) the homologous number 21 chromosomes fail to
separate. The resulting condition of trisomy (x3) of chromosome 21 in the
child is Down syndrome.
Aneuploidy of sex chromosomes [5] in human sperm or eggs can result in a
variety of disorders such as Kleinfelter's syndrome _(XXY, XXXY_ or
_XXXXY),_ Turner's syndrome_ (XO)_ and Jacob's syndrome _(XYY,_ or_ XYYY)_
Click here to view a video showing Dr. Malcolm Simons discussing human
disease and the role of the immunogeneticist.
