Part A: monohybrid cross
Kernels of 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.
1. Select 10 rows on your cob, and count the total number of purple (or brownish) and yellow kernels.
2. 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.)
3. Which is the dominant trait, purple or yellow?
4. 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.)
Yellow phenotype = genotype
Purple phenotype = genotype
5. 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?
6. Use one or more Punnett squares to confirm your conclusion in #5.
7. With these parents, what would be the expected ratios of each phenotype?
8. Do your results in #2 agree with the expected ratios? Why might they be different?
Part B: dihybrid cross
9. Count the numbers of each kind of kernel in ten rows of the cob labeled "B". There are four phenotypes:
Purple smooth
Purple wrinkled
Yellow smooth
Yellow wrinkled
10. 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.
11. Which two traits are dominant? Which two traits are recessive?
12. Using the appropriate upper and lower case letters, give the possible genotypes for each phenotype.
Purple smooth
Purple wrinkled
Yellow smooth
Yellow wrinkled
13. What must the genotypes of the parent plants have been?
14. With these parents, what would be the expected ratios of each phenotype?
15. Do your results in #9 to #10 agree with the expected ratios? Why might they be different?
Part C: human traits
Other factors 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):
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Genotype
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Phenotype
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BB
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Brown eyes
|
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Bb
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Brown eyes
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bb
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Blue eyes
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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.)