20.2 Monogenic inheritance

Question 1

Describe the steps needed to gain full marks on a genetic cross diagram question. (6 marks)

Answer 1

State phenotypes of parents
State genotypes of parents
State gametes (draw in circles) of parents
Complete Punnett square
State proportion of each
State phenotypes of offspring

Question 2

What is codominance? (2 marks)

Answer 2

When two different alleles occur for a gene- both are equally dominant, so both alleles are expressed in the phenotype.

Question 3

Give an example of codominance. (1 mark)

Answer 3

Flowers:
Red and white allele –> pink flower

Question 4

What does it mean when genes are ‘sex linked’? (1 mark)

Answer 4

The genes are carried on the sex chromosomes.

Question 5

Explain why the significance of the difference in size between X and Y chromosomes. (2 marks)

Answer 5

The Y chromosome is much smaller than the X chromosome, so cannot carry genetic information.

Question 6

Explain why males are more prone to certain diseases. (2 marks)

Answer 6

Males only have one copy of a X chromosome, so only have on allele on this (Y cannot carry an allele), so any disease caused by a recessive allele on the X chromosome occurs more frequently in males (they cannot be carriers, just positive or negative).

Question 7

Give an example of a sex-linked genetic disorder. (1 mark)

Answer 7

Haemophilia

Question 8

What is haemophilia? (2 marks)

Answer 8

Do not have protein blood-clotting factor, leads to blood clotting very slowly. Injury can result in prolonged bleeding, which can lead to death.

Question 9

Why can females be carriers of haemophilia, but males cannot? (2 marks)

Answer 9

If the X chromosome in a male is carrying the recessive allele (affected), then they do not have another X chromosome that can carry the dominant (healthy) allele.
Whereas, females have two X chromosomes, so can be a carrier, they do not have haemophilia, but they do carry the gene, so could pass it on to their children.