variation

• during crossing over, chromatids of homologous pairs may cross, break and exchange segments
• crossing over occurs during prophase 1
• the chiasma is where two chromatids cross 
• crossing over result in new combination of alleles along chromosome (recombination), so chromosomes passed on are not identical to parents 
• non-disjunction occurs in anaphase 1 or 2

  • First meiotic division : one or more chromosome pair fails to separate 
  • Second mitotic division : one or more chromatids fails to separate
• non-disjunction results in one daughter cell receiving an extra chromosome and other lacking one, therefore gametes will have either 24 or 22 chromosomes 
• a zygote formed from a gamete with non-disjunction will have either 47 or 45 chromosomes, this can cause severe and distinctive birth defect and miscarriages often occur early in pregnancy
• Trisomy is a condition in which an individual inherits an extra copy of a chromosome
• Trisomy 21 or Down syndrome :

  • Relative frequently in children of older mothers 
  • Characteristic facial appearance
  • Variable degrees of intellectual disabilities and physical abnormalities
• Trisomy 13 or Patau syndrome :

  • Intellectual disability
  • Small head
  • Extra fingers or toes
  • Cleft palate and/or cleft lip
  • Malformations of ear and eyes
• Trisomy 18 or Edwards syndrome :

  • Intellectual disability
  • Defects in eyes, ears, hands and head
• Partial trisomy is when part of an extra chromosome is attached to one of the other chromosomes
• Monosomy is a condition in which an individual is missing a chromosome. it usually results in severe malformations and miscarriages
• Partial monosomy is when part of a chromosome is missing
• random or independent assortment - when pairs of chromosomes separate, they do so randomly. the way one pair of chromosomes separates is unaffected by the way other pairs separate (chromosomes move apart independently)