 Occurs when chromosomes fail to separate during meiosis  EITHER  Homologues chromosomes during anaphase I  OR  Sister chromatids during anaphase

 Normal gametes n  Some gametes will have 2 copies of a chromosome  n + 1  Some gametes will not have any copies of that chromosome  n-1

TRISOMIC CELLMONOSOMIC CELL  Has one extra chromosome  2n + 1  Has one missing chromosome  2n - 1

 Normal Human fertilized egg (zygote)  46 chromosomes (2n)

Normal human sperm cell 23 chromosomes (n) Trisomic human fertilized egg (zygote) 47 chromosomes (2n + 1)

Abnormal human sperm cell 22 chromosomes Monosomic human fertilized egg (zygote) 45 chromosomes (2n – 1)

 Nondisjunction occurs frequently  If fertilized:  Results are usually devastating to zygote  Miscarriage occurs very early in the pregnancy  If zygote survives:  Results in syndrome  Caused by the abnormal dose of chromosomes

 Picture of organized chromosomes  Homologous chromosomes are paired  Shown :  From longest (#1) to shortest (#22)  Sex chromosomes at end

 Down’s Syndrome  Patau Syndrome (Trisomy 13)  Edward’s Syndrome (Trisomy 18)  Klinefelter Syndrome  Jacob’s Syndrome (XYY)  Trisomy X  Turner Syndrome (Monosomy X)

 Extra Chromosome 21

 Characteristic facial features  Round, full face  Large forehead  Slanted eyes  Short height  Enlarged tongue  Heart defects  Shorter life span  Wide range of mental handicap

 AKA Trisomy 13  Extra chromosome 13

 Serious eye, brain and circulatory defects  Cleft palate  Rarely live more than a few months

 AKA Trisomy 18  Extra chromosome 18

 Affects almost every organ  Mental handicap  Many physical deformities  Do not live more than a few months

 Sex chromosomes : XXY  Male (XY) with and extra X chromosome

 Male sex organs  Usually small testis  Sterile  Can not have children  Enlarged breasts  High pitched voice  May have mental handicap

 Sex chromosomes: XYY  Male (XY) with an extra Y chromosome

 Male sex organs  Taller than average  Below normal intelligence  Once thought to be associated with criminal and aggressive behaviour Proven untrue

 Sex chromosomes: XXX  Female (XX) with an extra X chromsome

 Female sex organs  May be taller than average  May have delayed speech or learning disabilities  Reproductively healthy  Fertile  Female usually unaware of condition

 AKA Monosomy X  Sex chromosomes: X  Missing 2 nd sex chromosome

 Female sex organs, but underdeveloped  Do not mature sexually  Sterile  Shorter than average  Thick neck  Normal intelligence  98 of fetuses die before birth