aneuploidy
The occurrence of one or more extra or missing chromosomes leading to an unbalanced chromosome complement, or any chromosome number that is not an exact multiple of the haploid number (which is 23).
(Types)
• nullisomy (2N-2): A pair of homologous chromosomes is missing. 2 chromosomes are missing, 44 chromosomes in total.
• monosomy (2N-1): Missing a copy of a chromosome. 1 chromosome is missing, 45 chromosomes in total.
• trisomy (2N+1): An extra copy of a chromosome. 47 chromosomes in total.
• tetrasomy (2N+2): Extra 2 chromosomes of the same type. 48 chromosomes in total. E.g. 4 sex chromosomes.
• pentasomy(2N+3): Extra 3 chromosomes. 49 chromosomes in total. E.g. 5 sex chromosomes.
Trisomy can arise as a result of non-disjunction, when homologous chromosomes fail to separate at meiosis resulting in a germ cell containing 24 chromosomes rather than 23.
For tetrasomy and pentasomy, the extra sex chromosomes do not accentuate sexual characteristics.
(Examples of monosomy)
• Turner syndrome (45,X, 45,X0, monosomy X): One X chromosome only and the other is missing.
• Cri-du-Chat syndrome: The end of the short p arm of chromosome 5 is missing.
• 1p36 deletion syndrome: The end of the short p arm of chromosome 1 is missing.
(Examples of trisomy)
• trisomy 13 (Patau syndrome): An extra copy of chromosome 13.
• trisomy 18 (Edwards syndrome): An extra copy of chromosome 18.
• trisomy 21 (Down syndrome): An extra copy of chromosome 21.
• Klinefelter syndrome (47,XXY): An extra copy of the X chromosome.
(Examples of tetrasomy)
• 48,XXXX syndrome: Female. A genetic disorder of a person with 4 copies of the X chromosome instead of 2.
• 48,XXXY syndrome: Male. Extra 2 copies of the X chromosome.
• 48,XXYY syndrome: Male. An extra copy of the X chromosome and an extra copy of the Y chromosome.
(Examples of pentasomy)
• 49,XXXXX syndrome: Female. A genetic disorder of a person with 5 copies of the X chromosome instead of 2.
• 49,XXXXY syndrome: Male. Extra 3 copies of the X chromosome.
• 49,XXXYY syndrome: Male. Extra 2 copies of the X chromosome and an extra copy of the Y chromosome.