Chapter 11.3 – Nondisjunction I.Nondisjunction – the failure of homologous chromosomes to separate properly during meiosis A.Normal Meiosis – one chromosome of each homologous pair moves toward opposite sides of the cell B.Nondisjunction Meiosis – both chromosomes in 1 or more homologous pair move to the same side of the cell

1.Forms of Nondisjunction – 2 different types of gametes (egg or sperm) a.+1 chromosome – human gamete with 24 chromosomes instead of the normal 23 b.-1 chromosome – human gamete with 22 chromosomes instead of the normal 23

II.Karyotype – an arrangement of homologous chromosomes  normal karyotype shows 23 pairs of each chromosomes #1-23 (46 total) A.Procedure: 1.Amniocentesis – a large needle is inserted into mother’s placenta to collect amniotic fluid, which contains baby’s cells

2.Baby’s cells are searched for cells in metaphase 3.Metaphase cell is photographed & enlarged on film

4.Chromosomes are arranged in homologous pairs, according to size & structure B.Autosomes – chromosomes 1-22 C.Sex Chromosomes – 23 rd pair  Female = XX, Male = XY

III.Disorders Seen on Karyotypes DisorderConditionTotal #Effects Down Syndrome3 of #2147Mental disability, heart defects, respiratory problems, early death (40s) Edwards’ Syndrome 3 of #1847Severe mental disabilty, deformed skull, pelvis, feet, die in early infancy Patau’s Syndrome3 of #1347Severe mental disabilty, deformed face, die in early infancy Turner’s Syndrome1 X in females 45sterile, webbed neck, short, slight mental disability Klinefelter’s Syndrome Extra X in males 47Normal appearance, tall, sterile XYY SyndromeExtra Y in males 47Normal appearance, low mental ability XXX SyndromeExtra X in females 47Normal appearance, only some are sterile

Down syndrome Turner’s syndrome Patau’s syndrome Edwards’ syndrome Klinefelter’s syndrome