see also:

• sex determination
• DNA and genes
• genomics

• 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.

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

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

• 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 the exposome

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