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?