Year 10 Science · Unit 1 · Lesson 4

Genes, Alleles and Punnett Squares

Challenge Worksheet

Name

Date

Class

Learning Goals

Predict how CF inheritance patterns would change if the allele were dominant
Explain the concept of carriers and why they would not exist for a dominant condition
Use a Punnett square to illustrate a changed inheritance scenario

Go Deeper

What if…?

Scenario

Cystic fibrosis (CF) is currently caused by a recessive allele (c). About 1 in 25 Australians carries the allele without having the disease. Imagine that the CF allele suddenly became dominant instead of recessive, we'll call this new dominant allele C^CF. Anyone inheriting even one copy of C^CF would now develop CF.

(a) Construct a Punnett square showing a cross between a person with one copy of the dominant CF allele (C^CFc) and an unaffected person (cc). State the genotype and phenotype ratios. Compare this to the recessive cross Cc × Cc from your previous worksheet.

Challenge 3 marks

(b) If CF were dominant, could carriers still exist (people who carry the allele but show no symptoms)? Explain your reasoning using your knowledge of how dominant alleles are expressed.

Challenge 2 marks

(c) How would making CF dominant change its frequency in the population over many generations? Consider what natural selection does to dominant versus recessive harmful alleles.

Challenge 3 marks

Show What You Know

1. Colour blindness is a sex-linked recessive trait carried on the X chromosome (X^b). A carrier mother (X^BX^b) and an unaffected father (X^BY) have children. Construct a Punnett square and calculate the probability that: (a) a son is colour blind; (b) a daughter is a carrier. Explain why colour blindness is more common in males than females.

Challenge 5 marks

2. Huntington's disease is caused by a dominant allele and does not appear until middle age (40–50 years). Explain why natural selection has not eliminated this dominant allele from the population, even though it is harmful. What does this suggest about the limits of natural selection?

Challenge 3 marks

Wrap Up

In one sentence, what was the main idea of this lesson?