2. Warfarin is a substance that stops blood coagulation and is used to prevent strokes and heart attacks. Given in sufficient amounts warfarin will cause animals to bleed to death. For this reason is has been used as a poison to eliminate rats and mice. Warfarin works by binding to and interfering with the activity of an enzyme that produces some of the clotting factors that are essential for blood clotting.

3. In 1959, rats resistant to warfarin were found in an area in central Wales. It was found that this resistance was determined by a single gene, with two co-dominant alleles. Wr and Ws. The three following genotypes have the following characteristics:

4. Rats with this genotype were susceptible to warfarin they only require small amounts of vitamin K in their diet. Rats with this genotype are resistant to warfarin. They require more vitamin K in their diet than rats with genotype WsWs. Rats with this genotype are also resistant to warfarin but as they need EXTREMELY large amounts of vitamin K in their diets they cannot survive under natural conditions. EEEEEKK!!

5. Receptor binding Their genotype results in the rats having slight differences in the enzymes they possess and some have a lower affinity for warfarin. So some individuals will continue to produce clotting factors and will therefore be resistant to warfarin.

6. Stabilising Selection The selection is stabilising. -If warfarin is present then resistant heterozygous rats are likely to survive and reproduce, WsWs are killed due to warfarin and WrWr cannot survive in the wild as they require high vitamin K levels. -When warfarin is no longer used WsWs genotypes are more successful as there is no threat from warfarin and high vitamin K levels are not needed as are for WsWr and WrWr.