Interpreting and evaluating experimental results

Two genes in a flowering plant

Unit 5
9 points
About 25 minutes

You can use a calculator on this question, just like on exam day.

A long question built around a real-world experiment and its data in a table or graph. You describe the biology behind it, pick out the experiment's variables and controls, analyze the data or do a calculation, and make and justify a prediction. On the exam: Question 1 of 6 in Section II (90 minutes, 50% of the exam score); about 25 minutes suggested.

The question and its sources

In a species of flowering plant, the allele for purple stems (P) is dominant to the allele for green stems (p), and the allele for smooth leaves (S) is dominant to the allele for hairy leaves (s). Both genes are on autosomes.

Crosses

Researchers crossed a true-breeding plant with purple stems and smooth leaves (PPSS) with a true-breeding plant with green stems and hairy leaves (ppss). All F₁ offspring had purple stems and smooth leaves.

The researchers then crossed F₁ plants with plants that had green stems and hairy leaves (ppss) and recorded the phenotypes of 400 offspring (Table 1).

Source: Hypothetical study

Table 1. Offspring of the F₁ × ppss cross

Phenotype	Number of offspring
Purple stem, smooth leaves	168
Green stem, hairy leaves	160
Purple stem, hairy leaves	38
Green stem, smooth leaves	34
Total	400

Source: Hypothetical data

Suggested time: 25 minutes

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Part (a)

1 point

Describe the event during meiosis I that results in independent assortment of genes located on different chromosomes.

Your answer to part (a)

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Part (b)

3 points

(i) State the null hypothesis for the F₁ × ppss cross, including the expected phenotypic ratio. (ii) Justify the researchers' choice to cross the F₁ plants with ppss plants rather than with each other. (iii) Identify the two parental (nonrecombinant) phenotypes among the offspring.

Your answer to part (b)

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Part (c)

3 points

(i) Calculate the chi-square (χ²) value for the data in Table 1. (ii) Using the critical value at p = 0.05, determine whether the null hypothesis can be rejected. Support your answer by identifying the degrees of freedom and the critical value. (iii) Calculate the recombination frequency between the two genes, as a percent.

Your answer to part (c)

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Part (d)

2 points

Researchers find a third gene on the same chromosome that is located farther from the stem-color gene than the leaf gene is. (i) Predict how the percent of recombinant offspring from a testcross involving the stem-color gene and this third gene would compare with the percent you calculated in part (c)(iii). (ii) Justify your prediction.

Your answer to part (d)

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