Download Independent Assortment Lab

Biology
Mr. Galego
Independent Assortment Lab
The color and texture of corn kernels (corn embryos) are inherited characteristics that are easily observed.
Purple color and smooth texture are dominant traits, whereas yellow color and wrinkled texture are recessive
traits. You will be observing these traits in order to test the hypothesis that the corn parents were heterozygous
for both traits and that the two traits assort independently during meiosis.
1. Predict the expected ratios of the possible phenotypes, under the assumption that the parents were
heterozygous. Justify your prediction with a dihybrid cross.
2. Mark your starting row, and then carefully count the number of purple smooth, purple wrinkled, yellow
smooth, and yellow wrinkled kernels.
3. Record the numbers of each phenotype (in the first ten rows) in the Observed Count column of the data
table below.
Kernel Phenotype
Observed Count (for ten rows)
Expected Count (for ten rows)
purple/smooth
purple/wrinkled
yellow/smooth
yellow/wrinkled
Total
4. Calculate the expected ratio of each phenotype to the total. Record the numbers in the Expected Count
column of the data table below.
5. Did you obtain the ratio that you predicted?
6. To determine if the deviations from your observed data are due to chance alone or if the data is
significantly different, you need to use a chi square test. A chi square test is a statistical measure to
determine how meaningful a difference is from an expected value. Calculate your chi-square value by
filling out the table below.
Trait
Observed
Expected
(observed-expected)2/expected
purple/smooth
purple/wrinkled
yellow/smooth
yellow/wrinkled
Sum of last column =
chi square value
7. To determine whether your data fits, you will compare it to chi square values on the chart. Your degrees
of freedom (df) is the number of possible phenotypes minus 1. In this case, 4 - 1 = 3. Using the chart
below, find the number in the "df=3" row that is closest to your chi square value and circle it. A "good fit"
indicates that any deviation in the observed ratio from the predicted ratio is due to chance. A "poor fit"
indicates that the deviation is significant and not due to chance.
Analysis:
1. Does your chi square analysis of corn data support the hypothesis that the parental generation was
heterozygous for both traits and that the color and texture genes assort independently?
2. What can we infer about the location of the color and texture genes on chromosomes?
3. If the color and texture genes were close together on the same chromosome (meaning that crossing over is
unlikely to separate them), how would your results differ?
4. If the color and texture genes were far apart on the same chromosome (meaning that crossing over is likely
to separate them – occasionally) how would your results differ?