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AP Biology
Mendelian Math
Solve these genetics problems. Be sure to complete the Punnett square to show how you
derived your solution and include genotypic as well as phenotypic outcomes whenever possible.
Assume that all traits are on separate chromosomes. SHOW ALL CALCULATIONS!!!
PRACTICING THE BASICS (Monohybrid Crosses):
1. In humans the allele for albinism is recessive to the allele for normal skin pigmentation. If
two heterozygotes have children, what are the probabilities for genotype and phenotype
for the possible offspring?
Genotypes:
Pheno:
If the child is normal, what is the probability that it is
a carrier (heterozygous) for the albino allele? (Be CAREFUL!)
2. In purple people eaters, one-horn is dominant and no horns is recessive. Show the cross of
a purple people eater that is heterozygous for horns with a purple people eater that does not
have horns. Give the probability of the genotypes & phenotypes of the all possible offspring.
3. In humans, the brown-eye (B) allele is dominant to the blue-eye allele (b). If two
heterozygotes mate, what will be the genotype and phenotype probabilities of all offspring?
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Dihybrid Crosses and BEYOND!
4.
In pea plants, the green color allele (G) is dominant over yellow color allele (g) for seed color
and tall (T) is the dominant allele in plant height. Parents heterozygous for both traits are
cross-pollinated. Determine the frequency for the genotypes and phenotypes of the offspring.
Parent #1 & #2 Genotypes_________________
Possible Gametes (remember that each gamete MUST have one allele for EACH trait)
________
________
________
_________
Phenotypic Probablilites:
Tall plant, green seeds:
Tall plant, yellow seeds:
Short plant, green seeds:
Short plant, yellow seeds:
Genotypes:
5. Now let’s try a mathematical shortcut for solving that same dihybrid cross. Because of
Mendel’s Law of Independent Assortment, you can work with the color gene and the height
gene separately… so set up separate monohybrid crosses for each trait from above:
Now use the laws of probability to calculate your frequencies of each trait alone and combined:
Recall that when adding probabilities, you must MULTIPLY them! Whenever possible use /4’s
P=probability
color
Green seeds, Tall plant
Yellow seeds, Tall plant
/4
height
/4
=P
color
/16 Green seeds, Short plant
Yellow seeds, Short plant
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height
=P
ALL following problems must be done mathematically and you must show your work!
Now use the same idea to figure out the probability for getting specific genotypes from the same cross.
Given the parents’ genotype for height is Tt x Tt, what is the probability of getting offspring with TT?
First, figure out what the probability of getting a “T” from the first parent, then of getting a “T” from the
second parent and then combine the probabilities. You may use the Punnett squares on the previous page to confirm your answer
Given the parents’ genotype for color is Gg x Gg, what is the probability of getting offspring with Gg?
First, figure out what the probability of getting a “G” from the one parent, then of getting a “g” from the
second parent and then combine the probabilities. Note that it is possible to get the G or g from either
parent, so you will have to figure the probability of getting G from the first parent and g from the second, AND
the probability of getting g from the first parent and G from the second, then combine those two probabilities
by adding the two probabilities together. You SHOULD use the Punnett squares on the previous page to confirm your answer
Combine the two probabilities above to determine the probability of getting an offspring with TTGg
EXTENSION PROBLEMS (you must show ALL MATH work or you will NOT receive credit!):
6. a. Now let’s try to use the math you have put together to solve some more complex problems, namely
three factor crosses. Keep in mind the traits above, but include the fact that purple flowers are dominant
to white flowers. If we cross TtGgPp with ttGgPP, what is the probability that we will have offspring with
the genotype TtGGPP?
b. What is the probability that the above cross will yield offspring with the genotype TtGgpp? Explain your
answer!
c. What is the probability that the same cross will yield offspring that are Tall, Green-seeded, and Purpleflowered?
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7. Do you REALLY UNDERSTAND? (if not, see me for help!)
Given the following P generation cross: AaBBCcDdee x AAbbccddEE
What is the probability of having offspring with the genotype AABbCcDdEe?
Modified to the point of being barely recognizable from Lisa Miller http://www.saddlespace.org/millerlisa/apbiology/cms_page/view/1461255
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