Numerical Chromosomal Aberrations:
Aneuoploidy
A chromosome profile with fewer or greater than the normal diploid number where the total is not a multiple of 23.
n If an extra chromosome is present (presence of three copies of the chromosome) : trisomy for that particular chromosome e.g. trisomy 21, trisomy 18, trisomy 13.
Trisomies are the most common numerical chromosomal anomalies found in humans.
Most autosomal aneuploidies are fatal in utero.
Three are survivable to term: Trisomy 13, 18 and 21.
Children with trisomy 21 (Down’s syndrome) are believed to have the most favorable outcome, as chromosome 21 has fewer genes than the other autosomes (it is short and has a low gene density)
Mechanism: Non disjunction (Failure of the two chromosomes of one pair to disjoin).
n If a chromosome is missing: monosomy for that chromosome e.g. Turnersyndrome (monosomy X chromosome).
Mechanism:
ü Non-dysjunction.
ü Anaphase lag; failure of the chromosome to move quickly to be incorporated in the daughter cell, so it will be lost.