Cytogenetics

Study of Chromosomes

Chromosomes• 23 pairs• Numbered

and arranged by size and position of centromere

Chromosomes• Can be displayed on a

karyotype• This shows extra,

missing, or fragmented chromosomes

Chromosomes

• Coiled DNA, with some RNA and histone proteins

• DNA strand coils around histones, which look like beads

• Histones can alter the activity of the DNA

• (turn genes on or off)

Obtaining Cells

• Obtain cells by 4 methods:

• Amniocentesis• Chorionic villus

sampling• Fetal Cell sorting• Maternal blood

screening

Chromosomes

• Staining techniques point out bands

• FISH allows more bands to be seen

• Can fish for a particular piece

Chromosomes

• Symbols show aberrations

• 46, XY normal male• 46,XX normal female• 45,XO Turner

syndrome• 47, XX 21 Trisomy 21

Wrong Number

• Polyploidy: an entire set is affected; one or more extra sets

• Could be triploid• Aneuploidy: one extra

or missing chromosome

• As in Down Syndrome (Trisomy 21)

• Caused by nondisjunction or translocation

• In translocation, chromosomes break and swap ends

Triploidy

Changes in chromosome structure

• Major categories include duplication, inversion, translocation, deletion

Duplication

• Certain gene sequences are repeated.

• May be just once, may be thousands of times

• The more copies, the more severe the disorder

May get worse with each generation

Inversion

• No genes are lost, but order is changed

• A piece becomes oriented in the reverse direction

• Detaches, flips around, and reattaches.

Translocation• Can have no affect

if all parts are there

• Causes problems when a sperm or egg gets the normal chromosome and the extra attached to another

Deletion• Loses a segment of the chromosome

• Most serious; often lethal

• Missing 3 is less serious than missing one. Why?

• Could cause a frameshift

A point mutation

• Only involves a single base change• Not a total chromosome problem• Can still cause a problem. Why?