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Supporting Genetics Education for Healthwww.geneticseducation.nhs.uk
Down syndrome
• 1 in 700 live births• >60% spontaneously aborted• 20% stillborn
• Facial appearance permits diagnosis• Marked muscle hypotonia as baby• Single palmar crease may be present• Learning difficulty (IQ usually <50)• Congenital heart malformations (40%)• Many other associated features
Supporting Genetics Education for Healthwww.geneticseducation.nhs.uk
Three different patterns of chromosomes can cause Down syndrome
• 95% people have three separate copies of chromosome 21 - trisomy 21
Non-disjunction
Non-disjunction
• 1% have mosaicism with normal and trisomy 21 cell lines (and usually have much milder features because of the presence of the normal cells); - occurs postzygotically
• 4% have the extra copy of chromosome 21 because of a Robertsonian translocation
The chromosome 21 probe is labelled with a red fluorochrome and a control probe (for chromosome 18) is labelled in green. The two green dots show that the hybridization has worked for this cell, and the three red dots show that there are three copies of chromosome 21. The clinical report is based on examining a large number of cells. For prenatal diagnosis a mix of differently coloured probes from chromosomes 13, 18, 21, X and Y is often used.