Dihybrid cross

For each of the given crosses
1. Mate parent flies to F2 generation. Insert computer generated results.
2. Hypothesise the mode of inheritance of each mutation (autosomal or
sex- linked, dominant, recessive or lethal)
3. Explain your reasoning for your proposed mode of inheritance.
4. Model the pattern of inheritance from parents to F2.
5. Predict the phenotypic ratio in the F2 (or F1 in the case of lethal genes)
6. Do a chi-square test of your hypothesis on the F2 data (computer
generated)
7. Accept or reject your hypothesis.
The crosses are:

white eyes female x vestigial wing male
lobe eyes female x crossveinless wing male

Sample. Dihybrid cross
1. apterous wings female x brown eyes male
Parents F1 cross
Female, AP X Male, BW Female, + X Male, +
F1 Progeny F2 Progeny
Phenotype Number Ratio Phenotype Number Ratio
Female, + 5087 1.006 + 5569 8.953
Total 10143 AP 1868 3.003
BW 1880 3.023
AP,BW 622 1
Total 9939

2. Hypothesis apterous wings is autosomal recessive
brown eyes is autosomal recessive
3. Reasoning all F1 are wild ? both apterous wings and brown eyes are recessive
ratios are the same for both males and females ? both are autosomal
4. Model Parents apterous wings female x brown eyes male
ap ap ++ x + + bw bw
Gametes ap + x + bw
F1 +ap + bw
F1 cross + ap + bw x + ap + bw
Gametes ++, + bw, ap+, ap bw x ++, + bw, ap+, ap bw
Punnet square
++ + bw ap+ ap bw
++

+ bw

ap+

ap bw ++ ++ ++ +bw +ap ++ +ap +bw
++ +bw ++ bw bw +ap +bw +ap bw bw
+ ap ++ + ap +bw ap ap ++
ap ap +bw
+ ap +bw + ap bw bw apap +bw apap bw bw

5. Predict
F2 phenotype ratio should be 9 wild ? 3 brown eyes ? 3 apterous wings ? 3 brown eyes and apterous wings.
6. Chi-square test (computer generated)
Phenotype Observed numbers Expected ratio
+ 5569 9
BW 1868 3
AP 1880 3
BW AP 622 1
Chi-square = 0.24 df = 3

7. ?2 is less than 7.81 and F2 ratio is as predicted by model ? hypothesis is accepted.
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