04 In a study of tooth decay, researcher checked the dental records of 100 pairs of identical twins. They recorded the number of fillings for each twin and found the following data:

Use your knowledge of genotype and phenotype to explain the data in Table 1. [4 marks]

Suggested Answer: Genotype is a person’s actual genetic makeup, and identical twins would have the same genotype. Therefore, based on a person’s genotype we would expect all the identical twins to demonstrate tooth decay in the same way.

Phenotype is the physical expression of a person’s genotype, and therefore identical twins can display different phenotypes if they experience different environments. 52 of the twin pairs have different phenotypes because they have different fillings and therefore this is likely to be the result of the environment because one twin could have eaten more sugary foods than the other twin.

Exam Hint: While students generally understand the concepts of genotype and phenotype, applying these concepts to this extract was tricky. Students need to understand that identical twins have the same genotype; however, they can display different phenotypes (expression) as a result of the environment.