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.

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 XbY male fruit fly.
Second lane is mRNA from an XBXb female fruit fly.
Third lane is mRNA from an XBXb female fruit fly.
B. First lane is mRNA from an XBY male mouse.
Second lane is mRNA from an XBXb female mouse.*
Third lane is mRNA from an XBXB female mouse.*
C. First lane is mRNA from an XB0 male C. elegans.
Second lane is mRNA from an XBXb hermaphrodite C. elegans.
Third lane is mRNA from an XBXB 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.

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Northem blot 2 3 800 600