The jaguar, Panthera onca, is a large cat that lives mainly in South America. The majority of jaguars have light brown fur with black spots, as shown in [Figure 1.1]. Some jaguars have completely black fur, as shown in [Figure 1.2]. (a) The pigments involved in fur colour are produced as a result of biochemical pathways that take place in cells called melanocytes. These pathways are similar to those that occur in human melanocytes. The melanocortin 1 receptor (MC1R) is located on the cell surface membrane of melanocytes and is coded for by the MC1R gene. [Figure 1.3] outlines the processes that occur in jaguar melanocytes.

1. (a)(i)Name the substance represented by X.[1]
2. (a)(ii)When MC1R is activated a second messenger is produced in the cell. Give an example of a second messenger.[1]
3. (a)(iii)Substance X is also produced in humans, but a mutation of the TYR gene can result in substance X not being produced. Describe the phenotype of a person with this mutation.[2]
4. (b)The MC1R gene has two alleles and is located on an autosome. • When two jaguars with light brown fur mate all the offspring have light brown fur. • When two jaguars with black fur mate either all the offspring will have black fur or some offspring will have black fur and some will have light brown fur. Using symbols, construct a genetic diagram to show how two jaguars with black fur can produce some offspring with black fur and some offspring with light brown fur.[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.