Genetics: Analysis and Principles 5th Edition by Robert Brooker

Exercise 18

Chapter 20 describes a blotting method known as Northern blotting that is used to determine the amount of mRNA produced by a particular gene. In this method, the amount of a specific mRNA produced by cells is detected as a band on a gel. If one type of cell produces twice as much of a particular mRNA as another cell, the band will appear twice as dark. Also, sometimes mutations affect the length of mRNA that is transcribed from a gene. For example, a small deletion within a gene may shorten an mRNA. Northern blotting also can discern the sizes of mRNAs.
a.
Lane 1 is a Northern blot of mRNA from cell type A that is 800 nucleotides long.Lane 2 is a Northern blot of the same mRNA from cell type B. (Cell type B produces twice as much of this RNA as cell type A.)
Lane 3 shows a heterozygote in which one of the two genes has a deletion, which shortens the mRNA by 200 nucleotides.Here is the question. Suppose an X-linked gene exists as two alleles: B and b. Allele B encodes an mRNA that is 750 nucleotides long, and allele b encodes a shorter mRNA that is 675 nucleotides long. Draw the expected results of a Northern blot using mRNA isolated from the same type of somatic cells taken from the following individuals:
a. First lane is mRNA from an X b Y male fruit fly. Second lane is mRNA from an X b X b female fruit fly. Third lane is mRNA from an X B X b female fruit fly
b. First lane is mRNA from an X B Y male mouse. Second lane is mRNA from an X B X b female mouse. *
Third lane is mRNA from an X B X B female mouse. *
C. First lane is mRNA from an X B 0 male C. elegans. Second lane is mRNA from an X B X b hermaphrodite C. elegans. Third lane is mRNA from an X B X B hermaphrodite C. elegans.
* The sample is taken from an adult female mouse. It is not a clone of cells. It is a tissue sample, like the one described in the experiment of Figure 5.6.FIGURE 5.6 Evidence that adult female mammals contain one X chromosome that has been permanently inactivated.
Starting material: Several small skin samples taken from a woman who was heterozygous for the fast and slow alleles of G-6-PD.