Nuriddinov
Bio2296: Jaloliddin Genetics Lab
TA: Taufiqul Huque
2/17/2012
Dominant and recessive epistasis in corn ears CG-35 and CG-65. Linkage of A & Sh genes
on the corn ear CG-70.
Objective:
The experiment is being done in order to see the actual proof of theoretical epistatic ratios
of F2 offspring. Also, the null hypothesis about whether or not genes involved in epistatic
interaction, are linked, is clarified. The second part of the experiment is about linkage of two
genes: A and Sh; and the distance between these two genes is determined.
Methods:
The experimental data is determined by counting each corn kernel on corn ear. Since,
each kernel is one unique individual the amount of each kind of kernel on the corn ear is
recorded. The ratio of the corn kernel types shows the type of epistasis. It is normal that ratio is
not exactly same as written in the book. More important thing is the Chi square value, since it
clarifies if the phenotypic values are close to real or not.
Count the corn kernels on the corn ear CG-35 and CG-65. This can be done by 2 people
or 2 pairs at the same time. It will be easier if you count the number of kernels on the each row
of corn ear and add the similar groups up at the end. There are 3 kinds of kernels both for
dominant and recessive epistasis. For dominant epistasis there are purple, yellow and white corn
kernels, while for recessive epistasis purple, yellow and red. Make a mark to the row with which
you started so you do not confuse. While counting kernels first count the more abundant group of
corn kernel types and then less abundant group is counted. Tabulating recording is good practice,
because it will help when you are calculating chi square value.
For the Linkage of A & Sh genes on the corn ear CG-70 part the number of corn
kernels is counted again. This time there are 4 different kinds of kernels on the corn ear CG-70.
The A gene determines if the color of aleurone is red or yellow and the Sh gene determines if
endosperm is shrunken or smooth. Red color dominates to the yellow and smooth consistency
dominates to the shrunken one. The linkage between these two genes determines what kind of
corn kernels we get. There are red smooth, yellow shrunken, red shrunken and yellow smooth
corn kernels are observed. Last two types are recombinant types so their number is not going to
be that great. Again while counting corn kernels same methods are used as first part of this lab
experiment. Data for the while class is recorded in order to get bigger number counts which will
increase accuracy when we calculate the distance between the genes given above.
Results:
Corn ear CG-35: Y & R genes
Kernal Color
Purple
Yellow
White
Total
Observed
472
124
14
610
Expected
458
114
38
610
Difference
14
10
24
70
Difference2 / Expected
0.42
0.88
15.16
2 = 16.46
Overall apparent ratio of phenotypes:
472 purple: 124 yellow: 14 white 12:3:0.3
2 (Chi square) analysis:
The initial null hypothesis that states the resulting ratio is close to real is rejected, because
2=16.46 with df=2. If we check the Critical Chi-square values table in the Genetics; from genes
to genomes book we will see that null hypothesis we made is less than 0.1% close to reality, so
the hypothesis can be rejected. According to the theory, the 12:3:1 ratio should have been
observed, if it was dominant epistasis, but our 12:3:0.3 ratio barely corresponds to that.
Corn ear CG-65: Pr & R genes
Kernal Color
Purple
Red
Yellow
Total
Observed
350
86
145
581
Expected
327
109
145
581
Difference
23
33
0
46
Difference2 Expected
1.62
4.85
0
2= / 6.47
Overall apparent ratio of phenotypes:
350 purple: 86 red: 145 yellow 9:2.3:4
2 (Chi square) analysis:
According to the same table used for previous part, the 2=6.47 and df=2 values show that null
hypothesis can be rejected. Our 9:2.3:4 ratio is close to the actual value but 2.3 makes it far from
the true value with 1 to 5% of closeness to the real value.
Linkage of A & Sh genes in corn ear CG-70.
Kernal Phenotype/
Red
Yellow
Red
Yellow
Name
Smooth
Shrunken
Shrunke
Smooth
397
135
181
216
253
390
219
263
2054
n
2
0
2
0
0
4
2
7
17
3
2
0
1
0
4
2
6
18
My Data
Total
412
159
217
242
210
382
380
229
2231
Total number of test cross offspring = 4320
Corn kernels with parental phenotype: red smooth and yellow shrunken
The recombinant kernels and their amount: red shrunken, yellow smooth; 35 recombinant
individuals
The crossover of parental chromosomes that gave rise to recombinant type corn kernels:
The map distance between A& Sh genes=
Duscussion:
The experiment was done in order to see if the theoretical hypothesis about epistasis and
gene linkage is really true in practice. The two epistasis experiments did not give satisfactory
results which can be seen by looking at the chi-square values of both experiments. As given in
the textbook chi-square value shows us the closeness of experimental value to the reality. It can
be compared as experimental error in chemistry and physics labs.
If we look at first experiment: the corn ear CG-35, we can see from the results that our
experimental ratio of F2 phenotypes is somewhat close to the theoretical 12:3:1 ratio. With the
2=16.46 and df=2 the results are less than 0.1% is true. That is why we can reject the null
hypothesis which said Y & R genes are epistatic to each other. However, considering the fact that
there were practical errors, such as missing some corn kernels on the corn ear null hypothesis is
under different condition. Missing kernels might significantly change the results. Since equal
amount from each group is not missing but unknown amount is missing we cannot determine
which phenotypic group is short of kernels.
The second experiment which was about the epistatic interaction in corn ear CG-65 also
did not give satisfactory results. With the 2=6.47 and df=2 the results deviate from expected
value. According to the Chi-Square table our result: 9:2.3:4 ratio is 1-5% close to expected value.
It is not 9:3:4 ratio but very close. Again this also can be result of missing corn kernels on the
corn ear.
The last part was the determination of linkage between genes A & Sh on the corn ear CG70. We had satisfactory results since we had big pool of results. Total 4320 corn kernels were
observed and classified to phenotypic classes which we had 4 of them. The more abundant,
parent-like phenotypic groups were red smooth and yellow shrunken individuals while
recombinant individuals such as red shrunken and yellow smooth were few. This is in accordance
with what we expected. As theory says in linkage of genes parent-like phenotypic groups are way
more abundant than recombinant types. When we calculated the map distance between A and Sh
genes it was 0.8 map units. 0.8 map units is sufficient enough distance crossover to occur.
Reference:
Hartwell, L. H. et al. 2011. Genetics: from genes to genomes, 4th ed. McGraw-Hill.