OCR A-Level Biology: Genetics, Cellular Control and Inheritance

The bacterium Escherichia coli uses the lac operon to control whether the enzymes for lactose metabolism are made.

Describe and explain how the lac operon controls the expression of the structural genes when lactose is absent and when lactose is present. In your answer you should refer to the regulatory gene, the operator, the repressor protein and the inducer, and you should make clear why this arrangement is an economical way for the cell to behave.

(6 marks)

A plant breeder crossed two pea plants that were both heterozygous for two unlinked genes: seed shape (round R dominant to wrinkled r) and seed colour (yellow Y dominant to green y). If the genes assort independently, the offspring are expected to show a 9 : 3 : 3 : 1 phenotypic ratio.

The breeder grew 320 offspring and recorded the four phenotypes.

Phenotype	Observed number (O)
Round yellow	172
Round green	64
Wrinkled yellow	70
Wrinkled green	14

The breeder used a chi-squared (χ²) goodness-of-fit test to decide whether the results fit a 9 : 3 : 3 : 1 ratio:

$\chi^2 = \sum \frac{(O - E)^2}{E}$χ2=∑E(O−E)2​

The critical value of χ² at p = 0.05 for 3 degrees of freedom is 7.82.

(a) State the null hypothesis, calculate the expected numbers, and calculate the value of χ². Show your working. (4 marks)

(b) Using your value of χ², state and explain the conclusion the breeder should reach. (2 marks)

In humans the ABO blood group is determined by a single gene with three alleles: Iᴬ, Iᴮ and Iᴼ. The alleles Iᴬ and Iᴮ are codominant with each other, and both are dominant to Iᴼ.

A man with blood group AB and a woman with blood group O have several children together.

(a) State the genotypes of the man and the woman, and complete a genetic cross to predict the blood groups of their possible children and the ratio in which they are expected. (4 marks)

(b) The couple are surprised that none of their children has blood group AB or blood group O. Explain why this outcome is expected. (1 mark)

In one woodland population of a land snail, shell colour is controlled by a single gene. The brown allele (B) is dominant to the yellow allele (b), so yellow snails are homozygous recessive.

A student sampled 400 snails from this population and found that 36 of them were yellow.

Assume the population is in Hardy-Weinberg equilibrium, where:

$p + q = 1 \qquad \text{and} \qquad p^2 + 2pq + q^2 = 1$p+q=1andp2+2pq+q2=1

(a) Calculate the frequency of the yellow allele (q) and the frequency of the brown allele (p) in this population. Show your working. (2 marks)

(b) Calculate the predicted number of snails in this sample of 400 that are heterozygous (Bb). Show your working. (3 marks)

Petal colour in a species of garden flower is controlled by two genes that act in the same pigment pathway.

• Gene P controls the first step. Plants that are homozygous recessive (pp) cannot make the colourless precursor, so they have white petals whatever alleles they carry at the second gene.
• In pigmented plants (those carrying at least one P allele), gene C controls the final step: C_ gives purple petals and cc gives red petals.

A breeder crossed two plants that were both heterozygous at both genes (PpCc × PpCc).

Predict the ratio of petal colours expected in the offspring, and explain why this differs from the standard 9 : 3 : 3 : 1 dihybrid ratio. (4 marks)

Red-green colour blindness is caused by a recessive allele carried on the X chromosome. It is much more common in males than in females.

Explain why this X-linked recessive condition affects far more males than females. (3 marks)