Question

Exercise 2: Heredity and DNA replication

1. In Mendel’s experiments with pea plants, name and describe the experiment which demonstrated that emasculation was not the reason why the wrinkled seed shape disappeared in the F₁ generation. (4 marks)

2. In Mendel’s experiments with pea plants, test crosses used a plant which was homozygous recessive to determine the genotype of the F2 plant. Why did the test crosses not use plants which was homozygous for the dominant allele? (2 marks)

3. Some people with Marfan syndrome have mild symptoms and others have life threatening symptoms. Is this an example of variable penetrance or variable expressivity? (1 mark)

4. Meselson and Stahl’s experiments examined DNA after 0, 20 and 40 minutes. Complete the table below to show the percentage of the DNA density after 60 minutes, assuming a) conservative replication and b) semi-conservative replication. (2 marks)

	Conservative replication		Semi-conservative replication
Time (mins)	40	60	40	60
14N / 14N (light)	75%		50%
14N / 15N (intermediate)			50%
15N / 15N (heavy)	25%

5. What temperatures (^oC) are typically used for each step of PCR? (3 marks)

Denaturation

Anneal primers

DNA synthesis

6. Jo is completing their undergraduate research project and has run out of Taq polymerase. They remember that, in humans, DNA is synthesised by DNA polymerase III and so adds some of that to the reaction. When the PCR products are analysed, the results show that the amplification failed. Why did it fail? (2 marks)