1. Types of genetic tests
1. Cytogenetic
2. DNA
3. Metabolic

2. Karyotype
Picture of the chromosomes in a cell used to check for abnormalities
Prenatal diagnosis: trisomy 21 (Down’s syndrome)

3. Postnatal diagnosis:

4. Postnatal diagnosis: detecting cancer

5. Preparing a karyotype Harvest cells (from where?)
Postnatal diagnostic karyotype
Prenatal diagnostic karyotype

6. Preparing a karyotype Harvest cells Postnatal diagnostic karyotype
tumor biopsy
skin cells from mouth (ie for non-cancer related diagnoses)
Prenatal diagnostic karyotype
chorionic villi sampling (CVS)
amniocentesis

7. Who is offered amniocentesis or CVS?
Maternal age (women 35 or older)
Risk of Down’s syndrome:
mother in 20s 1/ % OK
mother at / % OK
mother at / % OK

8. Down’s syndrome: how does it happen?
Chromosomal non-disjunction during meiosis of eggs and sperm.
1. Chromosomes
replicate
2. Homologous chromosomes separate
3. Chromatids from each chromosome separate

9. Who is offered amniocentesis or CVS?
Maternal age (women 35 or older)
Risk of Down’s syndrome:
mother in 20s 1/ % OK
mother at / % OK
mother at /100 99% OK
A previous child or pregnancy with a birth defect
Screening test with a positive result
Other family history

10. Prenatal diagnosis: amniocentesis
Sampling cells from amniotic fluid
Usually done ~ 15–18 weeks

11. Prenatal diagnosis: chorionic villi sampling (CVS)
Sampling cells from placenta
Usually done 10–12 weeks

12. Preparing a karyotype Harvest cells Culture cells 1–2 days
Arrest cells in metaphase with colchicine
metaphase

13. Mitosis metaphase chromosomes condense DNA replication
nuclear envelope breaks down
metaphase
chromosomes aligned on spindle fibres

14. Preparing a karyotype Harvest cells Culture cells 1–2 days
Arrest cells in metaphase with colchicine
‘Spread’ cells on slide and stain
Count chromosomes in 20 representative cells
Capture image of five ‘best’ cells and construct karyotypes for each
metaphase

15. FISH analysis of chromosomes: Fluorescent In Situ Hybridization
Metaphase spread chromosomes stained with DAPI, a fluorescing stain that specifically binds double-stranded DNA

16. FISH Expose DAPI-stained metaphase chromosomes to fluorescent probes
red = control probe for centromere of the X chromosome and another probe for end of chromosome X
green = probe for the end of chromosome 4

17. DiGeorge syndrome/CATCH22
Microdeletion on chromosome 22
Birth defect that affects the immune system
Absence or underdevelopment of the thymus and parathyroid glands
Facial features include low-set ears, wide-set eyes, small jaw and bowing up of upper lip

18. FISH tests: DiGeorge syndrome
Expose DAPI-stained chromosomes to mixture of fluorescent probes
green = control probe for chromosome 22
red = probe for DiGeorge region on long arm of chromosome 22

19. FISH tests: Painting chromosomes
Expose chromosomes to fluorescent probes that highlight entire chromosomes.

20. FISH tests: Painting chromosomes
Expose chromosomes to fluorescent probes that highlight chromosomes 13, 18, 21, X and Y.
nuclei from the same foetus
green = chromosome 13
red = chromosome 21
aqua = chromosome 18
green = X chromosome
red = Y chromosome

21. Trait A physical characteristic that is determined
by genes, eg eye colour.

22. Human traits Thumb shape hh Hh or HH Earlobe attachment AA or Aa aa
‘Hitchhiker’s thumb’
Earlobe attachment
AA or Aa aa
unattached attached

23. Human traits Genotype vs phenotype Thumb shape hh Hh or HH
‘Hitchhiker’s thumb’
Genotype vs phenotype
Genotype = specific allelic make-up of an individual, eg HH, Hh or hh
Phenotype = an observable physical or measurable biochemical
characteristic, eg thumb shape or lack of a particular enzyme

24. Punnett squares Remember these?? Used to determine the probability
of an offspring having a particular
genotype hh
Hh or HH
H H
‘Hitchhiker’s thumb’ H h
H allele = dominant
h allele = recessive

25. Punnett squares Now try it backwards hh Hh or HH Hh Hh
‘Hitchhiker’s thumb’ Hh Hh
H allele = dominant
h allele = recessive hh hh

26. Recombination: Shuffling the deck
DNA crossovers in chromosome pairs that result in children receiving a different combination of genes than either parent