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Clinical Knowledge


Basic genetic terminology

  • Gene: basic unit of genetic information.
      • It is made up of a DNA.
      • DNA helps makes an RNA product that is either directly functional or makes a protein.
  • Chromosomes – the protein strands around which DNA (and hence genes) is anchored.
      • Each human cell contains 23 pairs of homologous (identical) chromosomes. 
      • So, human somatic cells contain 46 chromosomes, i.e., 46 DNA molecules, of various lengths, which form 23 pairs of homologous chromosomes. 
      • 1 of these 23 pairs is ONE pair of sex chromosome – which consists of either 2 X chromosomes (female genotype) or one X and one Y chromosome (male genotype)  
      • A chromosome pair contains one chromosome inherited from each parent.
  • Germ cells (sperm cells/ova) only contain 23 SINGLE chromosomes (no chromosome pairs!)
      • In other words, they are a haploid chromosome set with only one copy of each chromosome.   (NB every other cell in the body has a diploid set of chromosomes)
      • When a couple have a baby, one part of the pair of each of the 23 chromosomes comes from each parent. 
      • This enables diversity as population generations progress.
  • Expressivity is the extent of expression of a given genotype at the phenotypic level.  In other words, to what extent does the abnormal gene express and manifests itself as a disease?  Patients with Marfan syndrome present with highly variable manifestations of the disease.  Some have mild Marfan’s (arachnodactyly) to a whole variety of presentations including the severe life-threatening (aortic aneurysm) form. 

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