1. Down Syndrome
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This set of slides contains information on:
Clinical features of Down Syndrome
Karyotypes of Trisomy 21
Meiotic Non disjunction
FISH images

2. Fig. 2.2 ©Scion Publishing Ltd
A child with Down syndrome
Fig. 2.2 ©Scion Publishing Ltd

3. 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

4. Three different patterns of chromosomes can cause Down syndrome
95% people have three separate copies of chromosome 21 - trisomy 21
4% have the extra copy of chromosome 21 because of a Robertsonian translocation
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

5. Trisomies appear to be associated with an increase in maternal age
eggs held at crossing-over stage in meiosis from approx 6 months gestation
so “wear and tear” with increasing maternal age in machinery for cell division thought to be a major component (plus other factors)
The trisomy 21 type of Down syndrome is the result of an error in meiosis, and has a recurrence risk of about 1 in 100.

6. three separate copies of chromosome 21
Trisomy 21: 47,XX,+21
three separate copies of chromosome 21

7. Fig. 2.11 ©Scion Publishing Ltd
Karyotype showing trisomy 21 (47,XX,+21)
Fig ©Scion Publishing Ltd

8. Fig. 4.15 ©Scion Publishing Ltd
Interphase FISH test for trisomy 21
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.
Fig ©Scion Publishing Ltd

9. 47,XX,+21

11. Non-disjunction in meiosis I resulting in trisomy 21 Down syndrome
Animation from Tokyo Medical University Genetics Study Group Hironao NUMABE, M.D

12. Meiotic Non-disjunction
(Trisomy 21: 75% meiosis 1)
Trisomy
Monosomy (lethal)

13. Incidence of trisomy 21 at the time of chorionic villus sampling (10-11 weeks), amniocentesis (16 weeks) and term.
The incidence of trisomy 21 increases with increasing maternal age.

14. Trisomy 21 amniocyte