The wolf, Canis lupus, lives in North America. Wolves may have a grey or a black coat colour. The colour of an individual wolf depends on the DNA it inherits at the CPD103 gene locus. • Wolves inherit two copies of CPD103, one from each parent. • Wolves that inherit one copy of the black form of the CPD103 gene have a black coat. (a) State the term used to describe: • an organism that has two copies of each gene • a form of a gene • a form of a gene that gives a phenotypic effect in a heterozygote. In addition to producing black coat colour, the protein coded for by the CPD103 gene also defends against infectious lung disease. Canine distemper virus (CDV) causes serious lung disease in wolves. Wolves that have been previously infected by CDV have antibodies against CDV (anti-CDV antibodies) in their blood. (b) CDV can be passed from domestic dogs to wolves. • Domestic dogs are more numerous in the southern part of the area occupied by wolves. • Domestic dogs are less numerous in the northern part of the area occupied by wolves. • The relative frequency of black wolves compared to grey wolves increases from the north to the south of the area they occupy. Explain how natural selection causes this trend in the distribution of black wolves. (c) Several different populations of wolves were compared. [Figure 4.1] shows the relationship between the percentage of wolves in a population that have anti-CDV antibodies in their blood and the percentage of wolves in that population that are black. The line of best fit was calculated after comparing the different populations of wolves. With reference to Fig. 4.1, state the relationship between the percentage of wolves with anti-CDV antibodies and wolf coat colour, and suggest reasons for this relationship.