BIOQ 1099 Lab
exercise: genetics of corn kernels
A: monohybrid cross
corn on the same cob are all “siblings”, offspring of the same two
parents. The phenotypes of the parents are known: both parent plants had only dark-coloured
kernels (usually called “purple”). Their
offspring have a mix of both purple and yellow kernels. These corn kernels can therefore be used to
determine the genotypes of the offspring,
their parent plants, and how their genes are carried from one generation to the
next. Gregor Mendel used this kind of
genetic exercise to formulate his three laws of genetics.
- Select 10 rows on your cob, and count
the total number of purple (or brownish) and yellow kernels.
- Express the numbers from #1 as a
ratio. Then divide both numbers by
the smaller number to find a reduced ratio. (For example, if your ratio was 53 to 8,
divide both numbers by 8 to get 6.625 to 1.)
- Which is the dominant trait, purple or yellow?
- List the possible genotypes for each
phenotype (purple or yellow), using upper case letters for dominant and
lower case for recessive. Remember
that genotypes consist of pairs
of the same letters. (Example: if yellow is dominant, then use “Y” for
yellow alleles and “y” for purple alleles.)
phenotype = genotype
phenotype = genotype
- How could two parents with purple
kernels have offspring with a mix of purple and yellow kernels? What must the genotypes of the parent
plants have been?
- Use one or more Punnett squares
to confirm your conclusion in #5.
- With these parents, what would be
the expected ratios of each phenotype?
- Do your results in #2 agree with
the expected ratios? Why might they
Part B: dihybrid cross
- Count the numbers of each kind of
kernel in ten rows of the cob labeled “B”.
There are four phenotypes:
- Express your results as a four-part ratio. (For example: 98 to 32 to 29 to 16) Then divide all four numbers by the
smallest of the numbers to get a reduced ratio.
- Which two traits are
dominant? Which two traits are
- Using the appropriate upper and lower case letters, give the
possible genotypes for each phenotype.
- What must the genotypes of the parent plants have been?
- With these parents, what would be the expected ratios of
- Do your results in #9 to #10 agree with the expected ratios? Why might they be different?
Part C: human traits
being equal, blue eyes occur in humans when an individual carries two recessive
genes for eye colour, and brown eyes occur when an individual carries one or
two dominant genes for eye colour. This
means that there are three possible genotypes, but only two possible phenotypes,
as follows (“B” represents a brown-eyed allele, and “b” represents a blue-eyed allele):
16. What possible gametes could be produced
by a blue-eyed individual? (Remember
that gametes are haploid.)
17. What possible gametes could be produced
by a brown-eyed individual?
18. Draw one or more Punnett squares to show a
cross between a brown-eyed mother and a blue-eyed father.
19. What is the probability that a child of the
parents in #18 will be brown-eyed?
Blue-eyed? (If there is more than
one possible situation in #18, treat each one separately.)
20. Draw one or more Punnett squares to show a
cross between two brown-eyed individuals.
21. What is the probability that a child of the
parents in #20 will be brown-eyed?
Blue-eyed? (If there is more than one possible situation in #20, treat
each one separately.)