1. Cytogenetic

2. DNA

3. Metabolic

Types of genetic testsTypes of genetic tests

Prenatal diagnosis: trisomy 21 (Down’s syndrome)

Karyotype

•Picture of the chromosomes in a cell used to check for abnormalities

Postnatal diagnosis:

Postnatal diagnosis: detecting cancer

Preparing a karyotype

• Harvest cells (from where?)

Postnatal diagnostic karyotype

Prenatal diagnostic karyotype

Preparing a karyotype

• Harvest cells

Postnatal diagnostic karyotype tumor biopsy skin cells from mouth (ie for non-cancer related diagnoses)

Prenatal diagnostic karyotypechorionic villi sampling (CVS)amniocentesis

Who is offered amniocentesis or CVS?

• Maternal age (women 35 or older)Risk of Down’s syndrome: mother in 20s 1/1250 99.92% OKmother at 35 1/400 99.75% OKmother at 40 1/100 99% OK

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

Who is offered amniocentesis or CVS?

• Maternal age (women 35 or older)Risk of Down’s syndrome: mother in 20s 1/1250 99.92% OKmother at 35 1/400 99.75% OKmother at 40 1/100 99% OK

• A previous child or pregnancy with a birth defect• Screening test with a positive result• Other family history

Prenatal diagnosis: amniocentesis

• Sampling cells from amniotic fluid• Usually done ~ 15–18 weeks

Prenatal diagnosis: chorionic villi sampling (CVS)

• Sampling cells from placenta• Usually done 10–12 weeks

Preparing a karyotype

• Harvest cells• Culture cells 1–2 days• Arrest cells in metaphase with colchicine

metaphase

metaphase

Mitosis

DNA replication

chromosomes condense

nuclear envelope breaks down

chromosomes aligned on spindle fibres

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

Metaphase spread chromosomes stained with DAPI, a fluorescing stain that specifically binds double-stranded DNA

FISH analysis of chromosomes: Fluorescent In Situ Hybridization

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

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

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

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

FISH tests: Painting chromosomes

green = chromosome 13red = chromosome 21

aqua = chromosome 18green = X chromosomered = Y chromosome

nuclei from the same foetus

Expose chromosomes to fluorescent probes that highlight chromosomes 13, 18, 21, X and Y.

TraitA physical characteristic that is determined by genes, eg eye colour.

Thumb shape

Earlobe attachment

Human traits

hh Hh or HH‘Hitchhiker’s thumb’

AA or Aa aaunattached attached

Thumb shape

Human traits

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

Punnett squaresRemember these??Used to determine the probability of an offspring having a particular genotype

H allele = dominanth allele = recessive

H H

H

h

hh Hh or HH‘Hitchhiker’s thumb’

Punnett squaresNow try it backwards

H allele = dominanth allele = recessive

hh Hh or HH‘Hitchhiker’s thumb’

hh

Hh Hh

hh

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