Nondisjunction
I. Nondisjunction
A.  When chromosomes
do not separate
normally during
meiosis
1.  Can happen at any
point in meiosis (I or
II)
2.  Results in gametes
with missing or extra
chromosomes
II. 
Two examples:
A.  A nondisjunction of the 21st
chromosome
Nondisjunction
+1
Normal
-1
Trisomy 21 Karyotype
B. A nondisjunction of the X
chromosomes of a female
Nondisjunction
+1
Normal
-1
Triple X Syndrome Karyotype
Turner Syndrome Karyotype
III. Nondisjunction Vocabulary
A.  Monosomy
One chromosome of a pair instead of
the usual two.
B.  Trisomy
Three chromosomes of a pair instead of
the usual two.
IV. Examples in Humans
A.  Down Syndrome
Trisomy 21
B.  Trisomy 16
common cause of miscarriage
C.  Klinefelter Syndrome
XXY in males
D.  Triple X Syndrome
XXX in females
E.  Turner Syndrome
The only viable monosomy in humans – females with
only one X chromosome