Download Notes 21

4/18/02
Reminder: Research seminars
Today, Multimedia Room 1, 12:30, Reconstruction of gene regulatory
networks, Math/Biology joint seminar, Biever guest lecture.
Tuesday, April 23, MO 157, 12:30, hepatic stem cells
Third exam, one week - April 25
Final remarks on allotetraploids
Origins of Aneuploidy
• Normally arises by nondisjunction at anaphase I or II of meiosis or
sometimes anaphase of mitosis (Fig. 18-16 animated on CD)
• If just one chromosome is
involved, the outcome is a
gamete (if meiosis) with n + 1
and a gamete with n - 1
chromosomes.
• Monosomic - (2n -1) results
if the n - 1 gamete fuses with
a normal gamete
• Trisomic - (2n + 1) results if
the n + 1 gamete fuses with a
normal gamete
• Double trisomic - (2n +1 +1)
results if two nondisjuctions
occur simultaneously
involving nonhomologous
chromosomes or two n + 1
gametes fuse.
• Tetrasomic - (2n + 2) results
if two nondisjunctions of the same chromosome occur in both
parents and the two n + 1 gametes fuse.
• Monosomics in humans - Turner's syndrome (2n -1 = 45, XO).
Occurs in 1/5000 female births (Fig. 18-17)
All other monosomics die in utero.
• Trisomics in humans
• Klinefelter syndrome (2n +1 = 47,
XXY)
• Down syndrome (2n +1 = 47, +21)
• Edwards syndrome (2n +1 = 47,
+18)
• Patau syndrome (2n +1 = 47, +13)
• XXX condition
• XYY condition
Klinefelter syndrome occurs in 1/1000
male births (Fig. 18-20)
Down syndrome occurs in
15/1000 births. (Fig. 18-21)
Mean life expectancy is 17
Edwards syndrome occurs in
1/8000 births with a life
expectancy of a few weeks.
Patau syndrome occurs in
1/20,000 births and has a mean
life expectancy of 130 days.
• XXX condition - occurs in
1/1200 female births are
often normal sometimes with
underdeveloped sex
characteristics
• XYY condition - occurs in
1/500 male births and usually
normal (sometimes
undetected)
• Why sex chromosome
trisomics so prevalent?
Dosage compensation. Sry
gene on Y chromosome.
• Why 13, 18 and 21?
• Significant deficit of genes
on chromosomes 4, 13, 18,
21, and X (1996)