15 points total possible

Background information

Exactly how different are men and women? Each has 46 chromosomes in every one of their cells.  Each cell has two of each of 22 alleles and one pair of sex chromosomes. Both males and females have at least one X chromosome on which such important genes as glucose - 6 - phosphate dehydrogenase resides. The other sex chromosome is either another full X chromosome containing another full complement of genes or a much smaller Y chromosome with essentially no genes which are allelic to the X chromosome. Much of the Y consists of redundant multi-copy non-expressing genetic material with a few sexual characteristic genes thrown in for good measure.

If all the genes on the X chromosome were concerned with sex this arrangement might be
acceptable. However, it is apparent that most X chromosome genes have nothing whatsoever to do with sexuality but instead are functional and common in both sexes. Do females then have twice as much of enzymes such as glucose - 6 - dehydrogenase? The answer is no because one of the X chromosomes in females in inactivated a mechanism known as “dosage compensation”.

In 1949 Murray L. Barr and E.G. Bertram described a small darkly staining body that was present in the somatic cells of female cats. Further studies found this dark staining body to be found in the somatic (body) cells of both human and feline females. It was subsequently named the “Barr Body” and could be observed most easily in cells scraped from the lining of the mouth (buccal cells). It was found to be an X chromosome which was inactivated and not actively transcribing.  It was observed in all cells except the sex cells. (Presumably it is necessary for two active X chromosomes for proper oocyte development.)

In 1962, Mary Lyon proposed an explanation for this phenomenon. According to her hypothesis, in the process of early development (about the 32 cell stage) an X chromosome becomes randomly inactivated and stays inactivated for the rest of the cell’s life. In addition, any daughter cells derived from that original cell also maintain the X as inactivated. This X chromosome is called “heterochromatin” as opposed to the term “euchromatin” which refers to the chromosome on which actively transcribing genes are located. As to which of the two X alleles is inactivated it appears that it is a completely random process which happens by chance alone. In some cells the inactive X may be paternal (from the father) in origin and in some cells it may be maternal (from the mother) in origin.

Since each female is made of a mixture of cells with different active X chromosomes then every female is in reality a mosaic for all the genes which are actively being expressed on the X chromosomes. Different alleles will in all probability reside on different X chromosomes.
Evidence that the condensed, presumably inactive, X was truly inactive for gene expression was first found in mice heterozygous for the dominant and recessive coat color alleles, wild type (brown) and albino (white). On cytogenetic analysis it was discovered that, although the gene for mouse color is located on an autosome other than the X chromosome a mutation had occurred. A portion of the chromosome with the mouse color gene on it had broken and reattached (translocated) to one of the X chromosomes.

Therefore when the X chromosome with the mouse color gene was inactivated, the autosomal
gene was inactivated as well. Since the inactivated gene was the dominant gene in those cells it was the recessive trait which began to be expressed. In cells where the normal X was inactivated the dominant gene was able to express.  Since inactivation did not occur until about the 32 cell stage the mouse was mosaic for color producing a coat some of which was albino and some of which was the wild type brown color.  This phenomenon produced a strain referred to as the “Tortoise” color.  In this activity you will explore the probability of any particular cell and the progeny of that cell carrying the active dominant gene.

Materials Needed:
1 coin for flipping
brown and yellow (substitutes for white) markers or colors

Directions and Questions:
You are a scientist working in a laboratory that has just received a rare mouse embryo known as the “tortoise” variety. You know the karyotype of this embryo has the gene for wild type brown color translocated to one of the X chromosomes. You do not know the sex of the embryo.

1. What color will the resulting mouse be if the embryo is Male?
2. How do you know this?

This particular mouse type experiences random X inactivation at the 16 cell stage. At this stage of embryo division either the normal X will inactivate and become unable to express its genes or the abnormal X will inactivate.

3. If the abnormal X inactivates what color fur will the clone of cells from this original cell be?
4. If the normal X inactivates what color fur will the clone of cells from this original cell be?

The mice on the following page are divided into 16 sections. These represent the 16 cells of the trophoblast that were present when inactivation occurred. (We are assuming the inactivation occurred at 16 cells for the simplicity of the activity - it probably occurs a little later.) Since inactivation of any specific X chromosome is a random process we will simulate this with the flip of a coin. Determine the gene which is being expressed in each of the 16 original cells which will be come clones of cells within the mouse and give it a mottled appearance by flipping the coin. If heads then the normal X is inactivated. If tails then the abnormal X is inactivated.

Lets Look at These Funky Mice
(2 points)
The mice drawn in below are divided into 16 parts. Color these mice in accordance with the coin
flipping which you have just completed to visualize the mottled color pattern of the mice.

Questions and Further Thinking:
5. What is the probability of having an all white (albino) mouse that carries the abnormal X
chromosome? (Please show your calculations)

6. What is the probability of an all brown mouse which carries the abnormal X chromosome?
(Please show you calculations)

7. When this mouse (Tortoise color) is bred to a normal mouse which is heterozygous for the
dominant wild type allele what is the ratio of possible offspring? (Hint: Do a Punnet Square).

8. If this translocated gene was instead of color, a gene critical for survival (the cells would die if it was not there), what percent of a female’s cells would have the X with this gene inactivated and why?