Medical Diagnosis


Diagnostic screening allows medical practitioners to identify diseases caused by defective chromosomes, genes or proteins

This screening can occur pre-natally (before birth), pre-symptomatically (before symptoms develop) or be used to confirm a suspected diagnosis


  • A karyotype is a visual profile of all the chromosomes in a cell
  • The chromosomes are arranged into homologous pairs and displayed according to their structural characteristics
  • Pre-natal karyotyping is often used to test for chromosomal abnormalities (e.g. aneuploidies resulting from non-disjunction)

Human Male Karyotype

Karyotyping involves:

  • Harvesting cells (usually from foetus or white blood cells of adults)
  • Chemically inducing cell division, then halting it during mitosis when chromosomes are condensed and thus visible
    • The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids
  • Staining and photographing chromosomes, before arranging them according to structure

Collecting Cells for Pre-Natal Testing

Gel Electrophoresis

Gel electrophoresis is a technique which is used to separate fragments of DNA according to size

  • Samples of fragmented DNA are placed in the wells of an agarose gel
  • The gel is placed in a buffering solution and an electrical current is passed across the gel
  • DNA, being negatively charged (due to phosphate), moves to the positive terminus (anode)
  • Smaller fragments are less impeded by the gel matrix and move faster through the gel
  • The fragments are thus separated according to size 
  • Different alleles will be different sizes and thus be used to identify genotypes and carriers
  • Disorders like Huntington's disease, which is late onset and caused by a trinucleotide CAG repeat, can be resolved by gel electrophoresis before symptoms appear

DNA Microarrays

  • Genetic disorders can be screened using a DNA microarray - a collection DNA spots / probes (representing genes) embedded on a solid matrix
  • A single microarray can contain 10,000 or more probes and can hold representative fragments from the entire genome
  • To determine which genes are being expressed by cells (including those responsible for disease), the following steps are conducted:
    • mRNA from the cell is converted into copy DNA fragments (cDNA) by the enzyme reverse transcriptase
    • The cDNA fragments are attached to a fluorescent dye and added to the microarray 
    • cDNA fragments will bind to their specific probe (via complementary base pairing) and remain fixed when unattached fragments are washed away 
    • The wells that still fluoresce after washing contain fragments that represent mRNA being expressed within the cell
    • Comparing the genes being expressed in a normal individual versus a sick individual can provide clues as to the cause of disease

Overview of DNA Microarrays