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 1-22C. Sex Chromosomes – 23rd pair

Female = XX, Male = XY

III. Disorders Seen on KaryotypesDisorder Condition Total # Effects

Down Syndrome 3 of #21 47 Mental disability, heart defects, respiratory

problems, early death (40s)

Edwards’ Syndrome

3 of #18 47 Severe mental disabilty, deformed skull, pelvis, feet,

die in early infancy

Patau’s Syndrome

3 of #13 47 Severe mental disabilty, deformed face, die in early

infancy

Turner’s Syndrome

1 X in females

45 sterile, webbed neck, short, slight mental disability

Klinefelter’s Syndrome

Extra X in males

47 Normal appearance, tall, sterile

XYY Syndrome Extra Y in males

47 Normal appearance, low mental ability

XXX Syndrome Extra X in females

47 Normal appearance, only some are sterile

Down syndrome

Turner’s syndrome

Patau’s syndrome

Edwards’ syndrome

Klinefelter’s syndrome