Download Monohybrid Crosses: Corn Lab

Monohybrid Crosses: Corn Lab
A cross between individuals that involves one trait is called a monohybrid cross. First, we will
use Punnett square diagrams to predict the results of various monohybrid crosses. Afterward, we
will look at ears of corn that have purple and yellow kernels. In hybrid corns, the colour purple is
dominant (P) over the colour yellow (p). We will be making observations and assumptions for
both the genotype and the phenotype.
PART A: Theoretical Test Crosses:
1. Fill in the Punnett square to show the result of a cross between a homozygous dominant
parent and a homozygous recessive parent.
2. Fill in the Punnett square to show the result of a cross between a homozygous dominant
parent and a heterozygous parent.
3. Fill in the Punnett square to show the result of a cross between a heterozygous parent and
another heterozygous parent.
4. Fill in the Punnett square to show the result of a cross between a homozygous recessive
parent and a heterozygous parent.
PART B: Actual Test Crosses
1. Obtain an ear of corn that is the result of heterozygous x heterozygous.
2. Count and record the number of purple and yellow kernels below.
3. Also record the total number of kernels.
TOTAL purple: __________ TOTAL yellow: __________ TOTAL all kernels: __________
4. Find the percent of purple and yellow kernels. To find the percent of purple, divide the total
purple by the total for all kernels, then multiply by 100.
Percent purple: __________%
Percent yellow: __________%
5. Compare your results with the theoretical answers you obtained for the heterozygous x
heterozygous cross:
6. Results for genetic crosses are often recorded as ratios. Calculate the ratio of purple to
yellow. To do this, use their totals. The smaller total, you can call “1”, and write it in the
appropriate space. Then divide the larger one by the smaller one, and round to the nearest
whole number. Record this number in the appropriate space.
Ratio of phenotypes for this cross.
PURPLE to YELLOW
_______ : _______
PART C: Actual Test Crosses (Only do this if teacher says so)
7. Now repeat steps 5 through 7 for an ear of corn that is the result of a test cross between a
homozygous recessive parent and a heterozygous parent.
TOTAL purple: __________ TOTAL yellow: __________ TOTAL all kernels: __________
Percent purple: __________%
Percent yellow: __________%
8. Compare your results with the theoretical answers you obtained for the heterozygous x
heterozygous cross:
9. Ratio of phenotypes for this cross.
PURPLE to YELLOW
_______ : _______
Conclusion – In the space below, write an overall conclusion with respect to the lab. In your
answer, also discuss possible factors that may influence the results (in some cases, your actual
crosses will differ from the theoretical crosses).
Assignment: Genetic Problems
Answer neatly on a separate piece of paper. Show all of your work (Punnett squares +
calculations) using the format we have used in class.
1. In sheep, the gene for white wool is dominant over the gene for black wool.
a. When a homozygous white ram is mated with a black ewe, what are the expected
genotype and phenotype ratios in the offspring?
b. What will be the expected phenotype and genotype ratios if an offspring of this
mating is bred to a black sheep?
2. A tall pea plant is crossed with a dwarf pea plant. All the offspring are tall. What can be
concluded for this? Show how you came to this conclusion.
3. Wild red foxes occasionally have a silver fox in their litters. When two of these silver foxes
are mated, they produce only silver-coloured offspring. Assume that colour is controlled by
one pair of allele (eg. RR, Rr, or rr). Determine:
a. 3 crosses in which no silver-coloured foxes would appear in the offspring.
b. 2 crosses in which silver-coloured offspring could appear.
4. In the four o’clock flower, red is homozygous dominant and white is homozygous recessive.
Offspring of a cross between purebred red and purebred white four o’clock flowers yields all
pink flowers, which is heterozygous. What will be the result of the following crosses:
a. pink x pink
b. white x pink
c. pink x red
5. In tomatoes, red fruit (R) is dominant over yellow fruit (r). A plant that is homozygous for
red fruit is crossed with a plant that has yellow fruit.
a. What would be the genotypes and phenotypes (and their ratios) of the P1 and F1
generations?
b. If two of the F1 generation from the above cross were mated, what would be the
genotypes and phenotypes (and their ratios) of the F2 generation?