Looking at a very strange cell from a person who has 3X chromosomes, how many Barr bodies would you expect?

Answer 1

Two. Barr bodies are inactivated X chromosomes and typically we only keep one active X chromosome per cell, so with three X chromosomes, two will be inactivated. However, which two are picked seems to random from cell to cell

Answer 2

The previous responder is correct in saying there would be 2 Barr bodies, but I will elaborate on his description of X-inactivation (Barr body) formation as random.

As germ cells form gametes, the Barr body in oocytes is reactivated, and thus the gametes form either a male zygote with one X chromosome inherited from his mother, or a female zygote with one X inherited from her mother, and one from her father.

In a male zygote, there is only one X chromosome obviously, and inactivation will not occur, thus Barr bodies will not be formed. When a female zygote inherits her X chromosomes, they are both inherited in the activate form, and during development, one of these X chromosomes is tagged with a blocking sequences randomly, so it will not get methylated (methylated = shut off). So let's say that in about half of an early embryo's somatic cells, the maternal X is inactivated, and in the other half the paternal X is inactivated.

Remember that in every cell division from this point, THE SAME X CHROMOSOME WILL BE IMPRINTED (another word for inactivated). I can use the popular example of the Calico Cat where an active paternal X chromsome in females leads to an black coat, and an active maternal X chromsome in females leads to an orange coat. Well, if this imprinting process were 100% random through cell divisions, you would just see a black/orange mixed coat (like a gray) on the cat. But we see big distinct sections of black and big sections of orange, meaning that all the cells in a particular section of coat came from the same cell lineage, and thus have inactivated the same copy of the X chromosome (all paternal X chromosomes in orange spots, and all maternal X chromosomes in black spots)!

Another situation in which this inactivation will not appear as random is a female with Duchenne Muscular Dystrophy. This is called skewed inactivation, where all of the cells will appear to have either the maternal X active, or all the cells will have the paternal X active. This is because those cells with the OTHER X copy active could not survive due to some mutation (Duchenne Muscular Dystrophy for example), and thus only those without the mutation will survive.