Chromosomes and Karyotypes
• Chromosomes are comprised of a single,
uninterrupted DNA molecule and proteins
(histones)
• A karyotype is a
visual display of the
chromosomes of an
individual.
Karyotype
A picture of a complete diploid set of
chromosomes arranged in pairs of
decreasing size
Amniocentesis
16-22 Weeks
Chronic Villus Sampling (CVS)
10-12 Weeks
How Do We Look at the
Chromosomes?
 Cells are treated with Colchicine - WHY?
 Prevents Spindle Fibers form forming
 Cells will be stuck in Metaphase
 Chromosomes will be condensed and VISIBLE
 Cells are placed in pure water (hypotonic) – WHY?
Color or Black and White?
Black & White – Most Common
Each chromosome has a characteristic banding pattern
that helps to identify them; both chromosomes in a pair
will have the same banding pattern.
Let’s Make It Pretty - Way to Go
Photoshop
Karyotypes are arranged with the short arm of the chromosome on
top, and the long arm on the bottom, called p and q, respectively.
• Sex Chromosomes• determine an individual’s sex
»X and Y chromosome
• Autosomes-the other 44
chromosomes
Normal Female Karyotype
Normal Male Karyotype
Let’s Make It Pretty - Way to Go
Photoshop
So, whose Karyotype is this?
What is wrong with this picture?
Nondisjunction
• Occurs when homologous
chromosomes or sister
chromatids fail to
separate during meiosis
• Results in gametes with
an abnormal number of
chromosomes (missing
or extra)
Meiosis I
Meiosis II
Results in
Trisomy
Results in
Monosomy
Nondisjunction
Does this remind you of something?
Hint: What happened when we played
– Mutated Tumor suppressor gene?
Nondisjunction Consequences
• Nondisjunction produces abnormal
gametes (sperm and egg)
• Fertilization wit an abnormal gamete can
result in Monosomy or Trisomy
• Monosomy – missing 1 chromosome
• Trisomy – having 1 extra
• Most monosomies and trisomies do not
survive to birth
Down Syndrome
Trisomy 21
•
•
•
•
1 in 700
babies (6000
babies/year)
400,000
Americans
Rate
increases
with
maternal age
5% of cases
traced to
father
Turner Syndrome (XO)
• 1 in 2500 females
• Monosomy of X
chromosome
• 10% of all
miscarriages
• Highly variable
Turner Syndrome (X0)
Kleinfelter’s Syndrome (XXY)
• 1 in 700-1000 males
• Many times they may
not know