A variety of pure-breeding snapdragons produces a tall plant with green seeds (WWSS). A second variety of pure-breeding snapdragons produce a short plant with white seeds (wwss). The two varieties are crossed, and the resulting seeds are collected. All of the seeds are green and when planted, they produce all tall plants (WwSs). These tall F1 plants are allowed to self-fertilize. The results for seeds and plant height in the F2 generations are as follows. What is the ?2 value, and the degrees of freedom for this experiment?
F2 Plant phenotype |
Observed # |
Green seed, tall |
600 |
White seed, tall |
175 |
Green seed, short |
165 |
White seed, short |
60 |
Total |
1000 |
?2 = 6.1, df = 4 |
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?2 = 6.1, df = 1 |
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?2 = 0.4, df = 3 |
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?2 = 0.4, df = 4 |
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?2 = 6.1, df = 3 |
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?2 = 0.4, df = 1 |
Answer: ?2 = 6.1, df = 3
Explanation:
During F2 generation of Dihybrid cross, progeny are produced in the following manner.
Both dominant phenotypes = 9/16 –green seed, tall
First dominant phenotype and second is recessive phenotype = 3/16—green seed, short
First recessive phenotype and second is dominant phenotype = 3/16---white seed, tall
Both recessive phenotypes = 1/16---white seed, short
Phenotype | Observed(O) | Expected (E) | O-E | (O-E)2 | (O-E)2/E |
green seed, tall | 600 | 562.50 | 37.50 | 1406.25 | 2.50 |
green seed, short | 165 | 187.50 | -22.50 | 506.25 | 2.70 |
white seed, tall | 175 | 187.50 | -12.50 | 156.25 | 0.83 |
white seed, short | 60 | 62.50 | -2.50 | 6.25 | 0.10 |
Total | 1000 | 1000.00 | 6.13 |
X2 value = 6.1
Degrees of freedom = number of phenotype class -1 = 4-1 = 3
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