Year 10 Science · Unit 1 · Lesson 4

Genes, Alleles and Punnett Squares

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Learning Goals

Complete Punnett squares for monohybrid crosses and state genotype/phenotype ratios
Calculate probabilities of offspring genotypes from a carrier cross
Apply inheritance patterns to cystic fibrosis as a real-world example

Warm Up

Punnett square practice

Complete each Punnett square and fill in the phenotype ratio. Use T = tall pea stem (dominant) and t = short pea stem (recessive); C = normal allele, c = cystic fibrosis allele.

Cross 1: TT × tt (tall × short)
	t	t
T
T
Phenotype ratio:

Cross 2: Tt × Tt (two heterozygous tall plants)
	T	t
T
t
Phenotype ratio:

Cross 3: Cc × cc (carrier × affected, CF)
	C	c
c
c
Phenotype ratio:

Cross 4: Cc × Cc (two CF carriers)
	C	c
C
c
Phenotype ratio:

Your Turn

Real-world context

Cystic fibrosis (CF) is the most common life-threatening genetic disorder in Australia, affecting approximately 1 in 2,500 newborns. About 1 in 25 Australians carries a single recessive CF allele (c) without showing symptoms. Two carrier parents (Cc × Cc) have a chance of having an affected child (cc) with each pregnancy.

(a) Using the Punnett square from Cross 4 above, state the probability (as a fraction and a percentage) that a child of two CF carriers will: (i) have CF; (ii) be a carrier; (iii) be unaffected and not a carrier.

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(b) A CF-affected child is born to parents who both appear healthy. Explain why neither parent showed symptoms of CF, even though they each passed on a CF allele. Use the terms dominant, recessive, genotype and phenotype in your answer.

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Show What You Know

1. Huntington's disease is caused by a dominant allele (H). A person with genotype Hh has the disease. Construct a Punnett square for Hh × hh and state the probability that a child will inherit Huntington's disease.

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2. Blood group AB results from codominance, neither the A nor the B allele is dominant or recessive. A person with blood group AB has one A allele and one B allele. Explain how codominance is different from dominant–recessive inheritance, using blood groups as your example.

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Wrap Up

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