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.
KaryotypeA picture of a complete diploid set of chromosomes arranged in pairs of decreasing size
16-22 Weeks
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 inTrisomy
Results inMonosomy
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 SyndromeTrisomy 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