The question
Name: ______________________ Date: ____________ Two dihybrid crosses tested with chi-square (9 points)
Data table
F2 plants from two dihybrid crosses in a fast-growing mustard
Researchers studied three genes in a fast-growing mustard plant. Allele A gives a purple stem and is dominant to a (green stem). Allele Y gives dark green leaves and is dominant to y (yellow-green leaves). Allele H gives hairy stems and is dominant to h (hairless). Seedlings were grown under the same bright lights and scored at 7 days.
Experiment 1: true-breeding purple, dark green plants (AAYY) were crossed with true-breeding green-stemmed, yellow-green plants (aayy). The F1 plants (AaYy) were crossed with each other to give an F2.
Experiment 2: true-breeding purple, hairy plants (AAHH) were crossed with true-breeding green-stemmed, hairless plants (aahh), and the F1 plants (AaHh) were crossed with each other to give an F2.
| Phenotype class | Experiment 1 (stem color, leaf color) | Experiment 2 (stem color, hairs) |
|---|---|---|
| Both dominant traits | 412 purple, dark green | 498 purple, hairy |
| Purple stem, recessive second trait | 129 purple, yellow-green | 42 purple, hairless |
| Green stem, dominant second trait | 145 green stem, dark green | 46 green stem, hairy |
| Both recessive traits | 34 green stem, yellow-green | 134 green stem, hairless |
| Total | 720 | 720 |
(a) (i) Describe the ratio of F2 phenotypes expected in experiment 1 if genes A and Y assort independently.
(ii) Explain how events in meiosis in the F1 plants would produce that ratio. [2 points](b) (i) State the null hypothesis for a chi-square test of the experiment 1 data.
(ii) Calculate the chi-square value for experiment 1. Show your work.
(iii) The critical value for 3 degrees of freedom at p = 0.05 is 7.82. Determine whether the null hypothesis should be rejected, and state what this means for genes A and Y. [3 points](c) The chi-square value for experiment 2, against a 9:3:3:1 ratio, is about 320.
(i) Identify the two phenotype classes that are larger than a 9:3:3:1 ratio predicts, and explain what this indicates about the locations of genes A and H.
(ii) Explain why the purple hairless and green-stemmed hairy plants appear at all. [2 points](d) (i) Genes A and H are about 14 map units apart. An F1 plant from experiment 2 (AaHh) is crossed with a green-stemmed, hairless plant (aahh). Predict the percentage of offspring in each of the four phenotype classes.
(ii) Stem color depends on a purple pigment that plants make more of in bright light. Justify growing every plant under the same bright light before stem color is scored. [2 points]