2 Chromosome abnormalities Numerical abnormalities - gain or loss of one or more whole chromosome(s)Includes aneuploidy (trisomy, monosomy)Structural abnormalities - chromosome breakage and rejoining in a different configurationIncludes translocations, deletions, inversionsOnly a few are compatible with life
4 The cause of aneuploidy Non-disjunction - failure of pairs of chromosomes to separate during meiosis I or IIProbably usually due to ageing of oocyte, hence maternal age effectEnvironmental factors may also be important, e.g. radiation
7 Structural abnormalities Translocation - transfer of part of one chromosome to anotherReciprocal translocationRobertsonian translocationDeletions - loss of material from within a chromosome (usually lethal)Insertions - extra material added within a chromosomeInversion - section within a chromosome is turned around
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