Chi-squared testAQA A-Level Biology: Mind map

Exam questions on Chi-squared test

1. A student crosses two heterozygous pea plants and expects a 3 : 1 ratio of round to wrinkled seeds in the offspring. She counts 400 seeds: 295 are round and 105 are wrinkled. She uses a chi-squared (χ²) test to compare her observed numbers with the numbers expected.
   Calculate χ² for her results. Use the formula χ² = Σ (O − E)² / E and give your answer to two decimal places.2 marks
2. A researcher crosses two fruit flies that are heterozygous for body colour and wing length. He expects the offspring to show a 9 : 3 : 3 : 1 phenotypic ratio. Among 160 offspring he counts 79 grey normal-winged, 40 grey vestigial-winged, 29 ebony normal-winged and 12 ebony vestigial-winged flies. He calculates χ² = 5.11. The critical value of χ² at p = 0.05 with 3 degrees of freedom is 7.82.
   State and explain the conclusion the researcher should draw from his result.2 marks
3. A student test crosses a fly heterozygous for two genes with a fly that is homozygous recessive for both genes. If the genes assort independently, a 1 : 1 : 1 : 1 ratio of four phenotypes is expected. Among 160 offspring there are 62 of the first phenotype, 58 of the second, 22 of the third and 18 of the fourth. The critical value of χ² at p = 0.05 with 3 degrees of freedom is 7.82.
   State a null hypothesis for this investigation and explain how the number of degrees of freedom is found.3 marks