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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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