Lecture 21: 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. Briefly summarize the conventions that are used for naming genetic loci in Drosophila and explain how they differ from the conventions used to describe genetic loci in peas.
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.
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 answerthis part. Please indicate what you have assdumed.
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?
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