A true-breeding black mouse is crossed with an albino mouse, producing F1 offspr
ID: 312294 • Letter: A
Question
A true-breeding black mouse is crossed with an albino mouse, producing F1 offspring that are all agouti (grizzled fur). The F1s are undercrossed, producing F2s with the following phenotypes: 48 agouti- 28 albinos, and 18 black. Actual research has revealed that fur color in this population of mice is controlled by two genes. What is the observed phenotypic ratio for the F2 generation? The researchers believe that dominant epistasis is occurring in this cross. Use the chi test to test their hypothesis. Arc they correct? Do you think another type of epistasis is occurring? Test your hypothesis with another chi square. Are you correct?Explanation / Answer
1. Observed phenotypic ratio is Agouti : albino : black :: 48 : 28: 18
OR Agouti : albino : black :: 24 : 14: 9
2. Expected frequency of dominant epistasis is 12 : 3 : 1
Observed frequency is 48:28 :18
Divide observed frequency with 5.875 ( 94/16)
Array: Obs Expected
Agouti 8.170213 12
Albino 4.65957 3
Black 3.06383 1
Total 16 16
Chi Table
Agouti (8.170213-12)2/12
Albino (4.65957-3)2 /3
Black (3.06383-1)2/1
Chi Table
Agouti 1.222273
Albino 1.039535
Black 4.259393
6.521201
The p value for this chi square at df 2.0 is much smaller than 0.05
Thus, these frequencies does not match.
Conclusion: This is not a case of dominant epistasis
3. Expected frequency of recessive epistasis is 9:4:3
Observed frequency is 48:28 :18
Divide observed frequency with 5.875 ( 94/16)
Array: Obs Expected
Agouti 8.170213 9
Albino 4.65957 4
Black 3.06383 3
Total 16 16
Chi Table
Agouti (8.170213-9)2/9
Albino (4.65957-4)2 /4
Black (3.06383-3)2/3
Chi Table
Agouti 0.076505
Albino 0.146673
Black 0.001358
0.224536
The p value for this chi square at df 2.0 is nearly 90%
Thus, observed and expected frequencies match.
Conclusion: This is a case of recessive epistasis
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