Lecture 09: Continuous Variation, Heretability,
Penetrance, Expressivity, Imprinting
(Old Lecture 14)
1. Describe the difference between discontinuous and continuous variation.
2. What characteristics of wheat make it particularly useful in studies on quantitative genetics?
3. Distinguish between additive and non-additive alleles and
explain how their interaction determines phenotype.
4. Explain how five different shades of color can be achieved with just two alleles each at two loci.
5. What characteristics must be possessed by a set of genetic loci in order for the principles of quantitative genetics to be applied to them?
6. How would you determine the theoretical number of different phenotypes and their phenotypic ratios for n different additive genetic loci?
7. What factors usually limit the number of different phenotypes that can be distinguished in polygenic inheritance?
8. What is meant by the concept of heritability when it is applied to a phenotypic trait that is influenced by polygenic inheritance?
9. What prevents application of the concepts of quantitative genetics to the study of traits that are controlled by dominant and recessive alleles?
10. Distinguish between maternal imprinting and paternal imprinting in a manner that makes it clear that you understand what each is and how they differ.
11. What is the result of replacing the male pronucleus in a newly fertilized mouse egg with a second female pronucleus. What explanation is offered for the phenomenon that is observed? What name is applied to a zygote that has been manipulated in this manner?
12. Same as question 3 except that the female pronucleus is replaced with a second male pronucleus.
13. How has the IGF2 gene been used to study genetic imprinting? Why is this gene particularly valuable for such studies?
14. What phenotype would you expect from each of the following
genotypes.
IGF2- = loss of function mutation of the IGF2 gene;
IGF2+ = wild type allele of IGF2 gene.
a. Maternal IGF2-, paternal IGF2+
b. Paternal IGF2-, maternal IGF2+
c. Paternal IGF2-, maternal IGF2-
d. Paternal IGF2+, maternal IGF2+
e. Disomic for mouse chromosome 11 maternally inherited.
f. Disomic for mouse chromosome 11 paternally inherited.
15. Paternal inheritance of autosomal dominant Huntington disease tends to result in earlier onset than maternal inheritance. Is it valid to refer to this phenomenon as "imprinting"? Justify your answer.
16. What happens to maternal imprinting during spermatogenesis? How can this be demonstrated?
17. What happens to maternal imprinting during oogenesis? How can this be demonstrated.
18. Distinguish between penetrance and expressivity in a manner that makes it clear you know what each is and how they differ.
19. Describe the phenomenon of genetic suppression. Why do you usually need two genes (or at least two mutations) to observe suppression?
20. What is meant by the term "position effect" and how is it manifested?
21. Why are temperature-sensitive mutations considered to be such a valuable genetic tool?
22. Cite a well known example of a temperature sensitive mutation that is not lethal at the elevated temperature.
23. What is an auxotrophic mutation?
24. Why is it not appropriate to refer to differences in severity of triplet repeat diseases inherited paternally vs. maternally as imprinting? What is the actual mechanism that is involved?
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