Oculocutaneous albinism (OCA) is a type of albinism. There are many different forms of OCA. OCA1A is one form of OCA, caused by a recessive mutation in the autosomal gene, TYR, coding for the enzyme tyrosinase. This enzyme is involved in the biosynthetic pathway that results in the production of melanin, the pigment responsible for the colour of hair, skin and eyes. A person with OCA1A has white hair, very pale skin and pink eye colour.

1. (a)Draw a genetic diagram to show the probability of a child having OCA1A, if both parents are carriers. Use the symbols A and a for the alleles. parental genotypes gametes offspring genotypes offspring phenotypes probability[4]
2. (b)(i)Fig. 4.1 shows the biosynthetic pathway involving tyrosinase. [Figure 4.1] There are a number of different mutations of the TYR gene that can result in an absence of melanin and cause OCA1A. These include: a missense mutation, caused by a base substitution, is most common a nonsense mutation, caused by a base substitution, is less common an insertion mutation, which is extremely rare. A missense mutation results in a complete polypeptide chain that does not fold properly to form the functioning enzyme. A nonsense mutation results in a shortened polypeptide. Explain why a missense mutation results in a different product from a nonsense mutation.[2]
3. (b)(ii)Explain how an insertion mutation in TYR can lead to a lack of melanin in a person with OCA1A.[4]
4. (c)Worldwide, 1 in 17000 people are born with OCA. This compares with 1 in 165 people among the Guna people of Panama. The Guna people of Panama have a small population and mostly live on many small islands off the coast of Panama. Suggest reasons why the Guna population of Panama has a relatively high number of cases of OCA.[2]