Lecture 20: Monohybrid Crosses, Genetic Segregation, Dominance
1. Model systems are widely used in the study of genetics.
a. What properties are desirable in a genetic model system and why?2. Mendel spent his first two years verifying that his variants were true-breeding. Why was this important?
b. What model system did Mendel use for his studies?
c. What desirable properties did Mendel's model system have?
d. By modern standards, what weaknesses did Mendel's model system have?
e. Some of Mendel's markers yielded more rapid results than others. Explain the difference.
3. What experiment disproved the then-popular blending theory of inheritance?
4. Which of Mendel's experimental results provided evidence for the particulate nature of inheritance. Describe the evidence.
5. Distinguish between genotypic ratio and phenotypic ratio. Include in your answer values for both ratios for the F2 generation of a monohybrid cross in which there is full dominance.
6. What result would you expect for a test cross of each of the following:
a. The F1 hybrid formed by crossing a true-breeding round pea with a true breeding wrinkled.7. Define each of the following terms and explain its significance in the study of genetics.
b. A true-breeding tall pea plant.
c. A true-breeding short pea plant.
d. The F2 progeny of a monohybrid cross in which there is full dominance when those that exhibit the dominant phenotype are tested one at a time.
e. The F2 progeny of a monohybrid cross in which there is full dominance when those that exhibit the recessive phenotype are tested one at a time.
a. Allele8. What is a Punnett square and what is it used for. Include in your answer an example of a Punnett square for an F2 generation in which there is full dominance.
b. Genetic locus
c. Self-fertilization
d. Equal Segregation
f. F2
g. Test cross
h. Back cross
i. Phenotype
j. Genotype
k. Heterozygous.
9. Explain how Mendel used predictions and experimental testing of predictions to test his postulates about the nature of inheritance in monohybrid crosses.
10. What convention is used to distinguish between dominant and
recessive alleles in the naming of genes in peas? What major area
of ambiguity remains in the naming of genes in peas?
11. Rather diverse systems of notation are used to depict alleles in various species.
a. Briefly summarize the conventions that Mendel used to depict dominant and recessive alleles.
b. Round is dominant over wrinkled in peas. What are the symbols used to depict round, wrinkled, and heterozygous in cases where the locus is represented with the letter "r" for round.
c. Same as b except the locus is represented with the letter "w" for wrinkled.
d. What is the rationale for using the recessive phenotype as the basis for naming a genetic locus?
e. What is the rationale for using the dominant phenotype as the basis for naming a genetic locus?
12. From a molecular perspective, what is the most likely reason for an allele to be recessive?
13. When during the course of meiosis does the equal segregation of heterozygous alleles into individual gametes occur?
14. The answer to this question requires you to project what has been presented in this lecture to a different situation. A diploid organism requires two copies of essential allele E to support a a fully normal phenotype. At least one functional copy is required for viability and fertility. Do not make any assumptions about dominance based on the gene designation. I have deliberately used capital letters for both the wild type and the mutant alleles.
a. Would you expect a frameshift mutation E* that caused total loss of function to behave as a recessive mutation or a dominant mutation? Explain the reasoning behind your answer.
b. Two heterozygous individuals E/E* are mated. What phenotypic ratio would you expect among their progeny? Explain the reasoning behind your answer.
c. A heterozygous individual E/E* is mated with a wild-type individual E/E. What phenotypic ratio whould you expect among the progeny?
d. How would your answer to part b differ if survival and fertility did not require a functional copy of allele E, but other properties stayed the same? You willl need to make an additional assumption to answer this part. Please indicate what you have assumed.
e. Would your answers to any of the parts of this question have been different if the loss of function were caused by a missense mutation?
15. One of the important features of the scientific method is to make predictions based on a model and then perform experiments designed to test those predictions. Identify three successful predictions that Mendel made based on his model for inheritance and briefly summarize the evidence that verified each of the three predictions.
16. Modern biochemical tests have shown that heterozygotes often have reduced amounts of the molecular species responsible for the dominant phenotype relative to individuals with a homozygous dominant genotype. Does this invalidate the concept of dominance? Explain the reasoning behind your answer.
17. Explain why a test cross is such a valuable genetic tool.
18. Explain why a cross with a homozygous dominant parent is not as useful as a test cross.
19. Explain how Mendel was able to achieve controlled matings among the various strains of peas that he studied.
20. From a molecular point of view, identify as many possible mechanisms as you can that might result in a mutation that would behave in a dominant fashion relative to wild type in F1 and F2 crosses of true breeding parents. This calls for speculation. You have not yet been given an answer.
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 20