Role of chromosomes

• Chromosomes

  Full set of 23 pairs in human body cells
• Sex cells (gametes)

  Contain half the full set of chromosomes (23 single chromosomes)
• Fertilisation

  1. Nucleus of male sperm joins with nucleus of female egg

  2. Cells of baby have complete set of chromosome pairs (one from each parent)
• Karyotype

  A set of chromosomes separated from its cell and spread out on a microscope slide
• Karyotypes show X-shaped chromosomes, which is how they look part way through replicating
• Sex chromosomes

  X and Y chromosomes that determine gender
• Sex chromosomes are usually labelled as pair 23
• Females are 'XX' and males are 'XY'
• Genetic mutation

  An error in an individual's genetic material
• Chromosome mutations

  • Person has 45 or 47 chromosomes instead of 46
  • Individual has three chromosomes instead of a pair (trisomy)
• Trisomy

  An individual has three chromosomes instead of a pair
• Genetic disorders caused by chromosome mutations

  • Down's syndrome (trisomy 21)
• Gene mutations

  • Change in the sequence of bases in the DNA strand
  • Affects proteins made from the gene
• Genetic disorders caused by gene mutations

  • Cystic fibrosis
  • Sickle cell anaemia
• Mutated genes can be inherited and may result in a condition, although sometimes we can have the faulty gene without even realising it
• Polydactyly is a condition that results in extra fingers or toes, caused by a gene mutation