X Inactivation Hypothesis
- proposed by Mary Lyons
- British geneticist
Barr Bodies


When one of the X
chromosomes
inactivates, it
condenses into a black
spot outside of the
nucleus
This inactivation
occurs in a random
pattern
How many Barr Bodies?

XY – no barr bodies

XX – 1 barr body

XXY – 1 barr body

XXX – 2 barr bodies
Practical Example

Calico Cats


They carry 2 different
alleles for coat color
Random X inactivation
during embryonic
development causes the
patchwork colored coat
in some female cats