Time to be "Corny" - A Genetics Exercise  

    Maize can either self-pollinate or cross-pollinate.  Because the male and female flowers are separate, maize is an easy plant for breeders to study.  Many mutants of maize are known.  We will study two mutations that are visible in the grains.

    One mutant has purple pigmented grains rather than the wild-type yellow coloration.  The other mutant, has wrinkled kernels instead of the wild-type smooth kernels.  The purple pigmentation is caused by production of a pigment in the aleurone layer of the grain.  Wrinkled kernels arise when the endosperm contains more sugar than starch.   To complete this exercise you will need to print, in color, the figure of maize ears (CLICK HERE).

I. MONOHYBRID CROSS:    Wild-type plants have yellow-pigmented grains; mutant individuals have purple grains.   Consider a cross between a true-breeding yellow parent and a true-breeding purple parent.   See Figure 1 or the ear of corn at your desk labeled P.

      Q1. Since both parents were true-breeding, they must be _____________  (homozygous or heterozygous).

      Q2. If the yellow parent self-pollinated, what would you expect one of its ears to look like?

At your desk is an ear with F1 offspring (or see Fig 2) from a cross between a true-breeding yellow and true-breeding purple plants.  Record your observations:

             Purple grains _________   Yellow grains ____________

             % Purple  _____________   % Yellow ______________

      Q3.  Were the results of this cross as you expected?  Explain.

      Q4.  Which allele is dominant? ______________ (purple or yellow)

     Q5.  What is the evidence to support your conclusion?

      Q6.  The F1 offspring are: ______________ (homozygous/heterozygous)

     Q7.  Using the letter P, write the genotype of the purple parent ______

     Q8.  Write the genotype of the yellow parent _________

     Q9.  Write the genotype(s) of the F1 offspring __________

    Q10.  Is the recessive phenotype visible in the F1? _______

Assume that one of the F1 grains is planted, grown to maturity and allowed to self-pollinate.  The resulting ears will bear the F2 offspring.

     Q11.  Write the genotypes for this cross:  _____  x   _____

     Q12.  What phenotype(s) do you expect to appear in the F2 generation  ______________________

     Q13.  What percentage of the F2 do you expect to be purple? _______

     Q14.  What percentage of the F2 do you expect to be yellow? _______

     Q15.  What ratio of purple : yellow do you expect? ___ : ___

Examine an F2 ear (or see Fig 3).   Count the number of yellow and purple grains on the F2 ear and record your data below.  (If you have a ear, use tape to mark rows.  Do not unwrap the ear and handle it with care.)

      purple grains  ______    yellow grains   ______     total grains (individual) ______

      Q16.  What percent of the F2 were purple?  _______

     Q17.  What percent of the F2 were yellow?  _______

     Q18.  What is the ratio of purple : yellow? ___ : ___

     Q19.  Do your data (Q18) support your predictions (Q15)?  (Hint:  You will need to perform a chi square analysis)

                    x² =  _______                       p  =  _______

     Q20.  The genotype of the yellow grains in the F2 generation is_______

     Q21.  The two possible genotypes of the purple grains are: ________  and  _________

To distinguish between the two possible genotypes in the purple-grained F2's, a geneticist would conduct a test cross.  In a test cross, a homozygous recessive individual is crossed with an individual showing the dominant phenotype.  Thus, in our case, a yellow plant (homozygous recessive) will be crossed with a purple one.

      Q22. If the purple plant is homozygous dominant, what percent of the offspring from the test cross do you expect to be purple? ________

      Q23. If the purple plant is heterozygous, what percent of the offspring do you expect to be yellow? ______   purple? _______  and thus, the ratio of purple : yellow offspring will be  ___ : ___.

You have been given an ear of corn (or see Fig 4) that has resulted from a test cross between a yellow and a purple parent. 

      Q24.  The suspected genotypes of the parent for this cross are:  _______  x  _______

Count the number of yellow and purple grains on the test cross ear.  Record your data below:

          Purple grains ________    Yellow grains __________  Total grains __________

      Q25. What percent of the grains were purple? _____

     Q26. What percent of the grains were yellow? _____

     Q27. The ratio of purple : yellow is   ___ : ___.    

      Q28.  Do your data (Q27) support your predictions (Q23)?  (Hint:  You will need to perform a chi square analysis)

                    x² =  _______                        p  =  _______

II. DIHYBRID CROSS:  Now, we will consider a cross involving two traits, grain color (purple or yellow) and seed shape (wrinkled or smooth).  Grains that contain high concentrations of starch are smooth and those that contain mostly sugar will be wrinkled.  Assume that a cross is made between one two true-breeding plants, one that has purple smooth grains and the other with yellow wrinkled grains.  All the F1's are purple smooth. 

     Q29.  Which is allele is dominant? _____________  recessive? ___________

     Q30.  The genotype of the purple smooth parent is _____________.  (Use the letter S to refer to the seed shape trait.)

     Q31.  The genotype of the yellow wrinkled parent is ___________.

     Q32.  The genotype of the F1's is ____________________.

     Q33.  The F1 can produce gametes with four possible genotypes. These are:

               __________   _________   _________   _________

Assume that a seed from the F1 is planted, grown to maturity, allowed to self pollinate and the resultant F2 ears collected.  One of them is at your desk or see Fig. 5.

      Q34.  Write the genotype for this cross  ___________  x  ___________

     Q35.  List the four possible phenotypes that could result in the F2:  

     Q36.  What percent of the F2 do you expect for each phenotype?  And, what ratio of phenotypes do you expect?  Complete the table above.

Now, count the F2 grains being careful to count them all and to not damage the ear.  Record your data in the table above. 

     Q37.   Do your data support your predictions (Q35-36)?  Support your answer with a chi square analysis.

                        x²=  ______________                         p  =  ______________

      Q38.  Are the genes for kernel color and kernel texture linked?  Explain. 

Last updated: July 14, 2009     � Copyright by SG Saupe