Chapter 5, Problem 52P

Two important methods for understanding the genetic basis for development are mitotic crossing-over and the use of the gene from jellyfish called GFP (for green fluorescent protein) that makes these animals glow in the dark. By recombinant DNA techniques described later in the book, you can insert the jellyfish GFP gene anywhere into the genome of organisms like Drosophila or mice. Cells expressing this GFP gene will glow green in the microscope, while those without the GFP gene will not glow green.

Mice homozygous for the recessive mutation small cells (smc) die as early embryos because their cells divide prematurely before they reach normal size.

You want to design a mouse carrying one copy of the GFP gene and heterozygous for smc in which you could generate clones in adult mice by mitotic recombination. In this designer mouse, every cell in every clone that is not green would be homozygous for the smc mutation. The figure below shows a field of epithelial cells in the mouse you design. You will see some cells that are normal size and other cells that are small. You will also see cells of three different colors: blank, weakly glowing cells (light green), and brightly glowing cells (dark green). Most of the cells in the epithelium of this mouse are of normal size and weakly glowing. The epithelium also contains three clones of cells (1, 2, and 3) that have unusual appearances due to the occurrence of mitotic recombination.

a.	Show the chromosomes and centromeres, the alleles smc+ and smc, and GFP+ (GFP gene present) and GFP- (GFP gene absent) in your designer mouse. (As a reminder, this mouse will carry one copy of the GFP gene and will be heterozygous for smc. Every cell in every clone generated by mitotic recombination that is not green should be homozygous for the smc mutation.)
b.	Why do you need to use mitotic recombination to study the function of smc+ in adult mice?
c.	Why do you see cells of three different colors?
d.	Why are clones 1 and 2 next to each other?
e.	On your map in part (a), place an arrow to show the position of a mitotic recombination event that could give rise to clones 1 and 2..
f.	Why do more cells exist in clone 1 than in clone 2?
g.	On your map in part (a), place an arrow to show the position of a mitotic crossover that could give rise to clone 3.