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