Corn Genetics and Chi Square Analysis Name ______Partner ______

Background: Each kernel on a cob of corn is a separate offspring (zygote) resulting from a cross between a male (sperm-pollen) and female (egg) parent. The colored layer of the seed that is visible is the endosperm. If the endosperm is colored (P – purple colored) it is visible, if it is not colored it appears cream (p – creamy yellow). Kernels that are smooth outside contain starch (S – smooth) and those that are wrinkled outside contain sugar (s - sweet).

The breeder started by establishing pure breeding parents that were homozygous dominant (PPSS) and homozygous recessive (ppss). The P1 generation crossed the two pure breeding strains (PPSS x ppss). The resulting F1 generation then was crossed to produce the corncob you have in front of you.

Dihybrid Cross:

F1 genotypes: PpSs x PpSs F1 phenotype: ______

  1. What are all the possible gametes for PpSs? ______
  2. Complete the Punnett square for this cross.
  1. What are the phenotype ratios and percentages for the following:
  1. purple and smooth ______c. yellow and smooth ______
  2. purple and wrinkled ______d. yellow and wrinkled ______
  1. Now mark the ends of 5 rows of kernels with the overhead marker. DO NOT remove the plastic!!

Count the number of kernels of each phenotype in these 5 rows and record in the table below.

Phenotype / # kernels counted / %: (# counted/total)x100
purple/smooth
purple/wrinkled
yellow/smooth
yellow/wrinkled
TOTAL
  1. Compare your corncob percentages (#4) with the predicted percentages (#3). Did you obtain a

9:3:3:1 ratio? Use your data to support your answer.

  1. To determine whether the differences between your observed and predicted kernel counts are due to chance alone OR that they are truly different, you will need to conduct a chi square test.

Phenotype / Expected # (e) / Observed # / Deviation (d) / d2 / d2/e
Purple/smooth / Total x 9/16 =
Purple/wrinkled / Total x 3/16 =
Yellow/smooth / Total x 3/16 =
Yellow/wrinkled / Total x 1/16 =
Chi square value = sum of all d2/e values 
  1. Now determine whether your observed data is a good fit with the predicted ratios. First calculate the degrees of freedom (df). # phenotypes: ______df = # phenotypes – 1 ______. Find the number in the Chi square table below in the correct df row that is closest to your calculated chi square value…..circle it!

  1. Explain what it means for your observed ratios to have a “good fit” or a “poor fit”. Explain how you determined whether your calculated chi square value supports or does not support the hypothesis that the parental generation was PpSs.