How are chromosome inherited?
A child receives one of each pair of chromosomes from the mother in the egg and one of each pair from the father in the sperm. Normally, the egg and the sperm each have 23 chromosomes so that the first cell of the child has a total of 46 chromosomes – half from the mother and half from the father. Sometimes during the formation of the egg or sperm an error can occur, resulting in an egg or sperm with an abnormal number of chromosomes. This type of error is called ‘chromosome non-disjunction’. The result is aneuploidy in the embryo. Having an extra chromosome is called ‘trisomy’; a missing chromosome is called ‘monosomy’. The vast majority of embryos with aneuploidy do not implant in the uterus or are lost in early miscarriage often before a woman even realizes she’s been pregnant. In fact, over half of all early miscarriages are due to aneuploidy. In some cases a baby can be born with an abnormal number of chromosomes, a situation usually associated with mental retardation and birth de
