Lecture 22: Dihybrid crosses, independent assortment
1. A true-breeding strain of peas exhibiting unlinked dominant traits A and B is crossed with a true-breeding strain exhibiting both recessive forms.
a. What will be the phenotype of the F1 progeny?
b. What will be the genotype of the F1 progeny?
c. What genotypes will be present and in what ratios in the F2 generation?
d. What phenotypes will be present and in what ratios in the F2 generation?
e. What phenotypic ratio will be obtained in a test cross of the F1 generation? Include in your answer a description of each of the phenotypes you expect.
f. What fraction of the peas from the F2 generation that exhibit both dominant traits will be found to be true-breeding?
g. What fraction of all of the peas in the F2 generation will be found to be true-breeding? (Think about this one carefully before you answer!)
h. What fraction of the peas from the F2 generation are heterozygous for both traits?
2. Parents that are heterozygous for four unlinked genes (AaBbCcDd) are crossed. Assume that you are working with a species that produces large numbers of progeny. Use the forked line method (probability is not introduced until the next lecture) to arrive at the answers to the following questions. (Hint: to keep things from getting messy, draw only those parts of the diagram that you actually need).
a. What fraction of the progeny will be homozygous for all four dominant alleles?3. Use a diagram to demonstrate the origin of the 9:3:3:1 phenotypic ratio that is characteristic of the F2 generation from a dihybrid cross with full dominance. (Note that there are two distinctly different approaches you can take).
b. What fraction of the progeny will be homozygous for all four recessive alleles
c. What fraction of the progeny will be heterozygous at locus A.
d. What fraction of the progeny will be recessive for the genetic trait controlled at locus B?
e. What fraction of the progeny will express at least one dominant allele of one of the four genes? (Hint: this is best done by subtracting).
f. What fraction of the progeny will express the dominant phenotypes for genes A and D?
g. What fraction of the progeny will express all four dominant phenotypes.
4. What is the difference between the principle of equal segregation and the principle of independent assortment?
5. How does the nomenclature used to describe dominant and recessive genetic relationships differ between Drosophila and peas?
6. What is the most common reason for an allele to exhibit recessive behavior?
7. What is the biochemical basis for the difference between the round and wrinkled alleles that Mendel studied?
8. Explain how the 9:3:3:1 phenotypic ratio of a typical dihybrid cross is related to the expected 3:1 phenotypic ratio for each of the genetic loci that are involved.
9. If two pea plants that were both heterozygous for all seven of the traits that Mendel studied were crossed, what fraction of the progeny would be recessive for all seven traits? Assume fully independent assortment of all seven trailts.
10. In question 9, what fraction of the progeny would be tall plants with constricted yellow pods containing wrinkled peas? (Hint: Table 12.1 summarizes the dominance relationships for all seven of the traits Mendel studied).
11. Punnett squares become impractical when more than 3 loci are involved.
a. How many different types of gametes would you expect the heterozygous parental plants in question 9 to produce?12. What changes in biology between 1866 and 1900 made Mendel's work more popular when it was rediscovered than when it was first published?
b. How many squares would be contained within a Punnett square used to analyze that mating?
c. How many different phenotypes of offspring would you expect?
d. How many different genotypes of offspring would you expect?
e. What fraction of the progeny would you expect to be heterozygous at all seven loci?
13. What genetic phenomenon does each of the following ratios bring to mind? Note that in some cases there may be more than one correct answer.
a. 1:114. Draw a human pedigree using correct symbols that demonstrates the occurrence of a homozygous recessive disease resulting from the marriage of first cousins. Trace the recessive gene back through the pedigree to its origin from a single individual. (First cousins are defined as the children of siblings married to unrelated people).
b. 2:1
c. 1:2:1
d. 3:1
e. 9:3:3:1
f. 1:1:1:1
g. 27:9:9:9:3:3:3:1
15. In problem 14, what is the probability that a healthy sibling of the diseased individual is not a carrier of the recessive gene?
16. Identify the pedigree symbol for each of the following:
a. male
b. female
c. an afflicted individual
d. a consanguineous marriage.
e. twins
17. Draw a human pedigree using correct symbols that demonstrates the inheritance of a dominant genetic trait through at least three generations.
18. Draw a human pedigree using correct symbols that demonstrates independent assortment over three generations of two unlinked dominant traits, both of which were introduced into the pedigree from the same individual. (You will have to invent a notation to show which of the two traits each individual displays).
19. Draw a pedigree demonstrating the use of brother-sister matings to obtain a true-breeding strain of laboratory mice carrying a recessive allele, starting with a colony of wild-type mice and a single individual known to carry the recessive allele.
Go to Review Questions for the Following Lecture
Return to Index of Review Questions
Go to Review Questions for the Previous Lecture
Return to Lecture 22