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  • the human genome consists of ~3 billion (3×10^9) base pairs of DNA organized as 23 chromosomes of which:
    • 3% of which encodes genes:
      • “coding genes” can produce mRNA which is used to produce protein
      • “noncoding” genes may encode snRNA or miRNA.
  • mitochondria have a distinct genome consisting of 16,589 base pairs which encodes 37 genes, including 13 proteins
    • mitochondria are transmitted only in the egg, and thus all mitochondrial DNA is maternally inherited
  • genes are localised on chromosomes according to a convention Zp/qAB.C where z is the chromosome number (or X or Y), p/q specifies which arm (p is the short arm ie. p for “petite”, while q is the long arm), and A,B,C are integers.
  • most familial traits and diseases are polygenic and are further influenced by environmental factors.

genetic variation

  • there are 3 main types:
    • DNA sequence variation
    • structural cytogenetic variation
    • epigenetic variation

DNA sequence variation

  • single nucleotide polymorphisms (SNPs)
    • eg. nonsense, missense, splice site, and silent mutations as well as regulatory polymorphisms
  • indels
    • insertions and deletions
  • triplet-repeat expansion

structural cytogenetic variation

  • affect large numbers of bases, usually more than 1000, or even whole chromosomes
  • eg. copy number variations and chromosome translocations and inversions

epigenetic variation

  • modifications of DNA or chromatin that do not alter the DNA base sequence
  • eg. methylation, acetylation
  • these can be programmed prior to fertilization, in utero, or acquired during lifetime in response to environmental changes

modes of inheritance

Mendelian inheritance

  • single gene disorders
  • autosomal dominant
  • autosomal recessive
  • X-linked dominant
  • X-linked recessive
  • Y-linked inheritance

phenotypic variations in Mendelian inheritance

  • penetrance
    • the proportion of persons carrying the gene who demonstrate the phenotype or disease
    • may be age-related or gender-related
    • if a disease is only expressed in males, then presence of male-to-male transmission excludes X-linked recessive inheritance
  • mosaicism
    • the heterogenous expression of a disease at the cellular or tissue level, resulting from cell-specific differences in the expression of a mutation or the presence of a chromosome aberration
  • anticipation
    • repeated sequences may increase with each generation and result in increased disease severity
  • imprinting
    • different phenotypic expression depending upon whether transmitted from mother or father
genetics.txt · Last modified: 2019/07/12 15:14 by wh