Lecture 40

Prof Duncan Shaw

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 configuration– Includes translocations, deletions, inversions

• Only a few are compatible with life

AneuploidiesSyndrome Abnormality % paternal % maternal

Patau’s Trisomy 13 15 85

Edward’s Trisomy 18 10 90

Down’s Trisomy 21 5 95

Turner’s 45,X 80 20

Klinefelter’s 47,XXY 45 55

XYY male 47,XYY 100 0

The cause of aneuploidy

• Non-disjunction - failure of pairs of chromosomes to separate during meiosis I or II

• Probably usually due to ageing of oocyte, hence maternal age effect

• Environmental factors may also be important, e.g. radiation

Structural abnormalities• Translocation - transfer of part of one chromosome

to another– Reciprocal translocation

– Robertsonian translocation

• Deletions - loss of material from within a chromosome (usually lethal)

• Insertions - extra material added within a chromosome

• Inversion - section within a chromosome is turned around