BIO 325 - Fall 2009 - TTh 1230 to 200 - Exam 2 - Key-1

BIO 325 - Fall 2009 - TTh 1230 to 200 - Exam 2 - Key-1 -...

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GENETICS (BIO 325) Fall 2009 T, Th 12:30 – 2:00 PM section Key - Exam 2 __________________________________________________________________________________ 1. Genotypes Phenotypic ratio 9 A/- ; B/- 9 3 A/- ; b/b 7 3 a/a ; B/- 1 a/a ; b/b 2. (a) 70% penetrance corresponds to the percentage of individuals that show the phenotype associated with the allele. In other words, in a population of 100 individuals that have the allele associated with the phenotype, only 70 individuals show the phenotype. Polydactyly is an example of a trait in humans that shows incomplete penetrance. (b) Variable expressivity can be observed in traits that are completely penetrant or incompletely penetrant. Variable expressivity represents the level of expression of the phenotype. 3. Chargaff’s rules state that the amount of purines is equal to the amount of pyrimidines, and specifically the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C) in DNA. Watson and Crick used these rules for base pairing of A with T, and G with C in their model of the double-stranded structure of DNA 4. Semi-conservative 5. (a) False (b) False (c) True (d) False (e) True 6. (a) Without the β clamp, the DNA polymerase will frequently fall off the DNA template. In principle, it can bind and continue, but the continual falling off and rebinding will be
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time-consuming and will greatly slow down DNA replication. (b) In the absence of primase, RNA primers cannot begin on either the leading or the lagging strand. DNA replication therefore cannot begin. 7. (a) It would a very long time (more than 8 days) to replicate the chromosome. (b) It would take double the time than what is required in part (a). 8. DNA polymerases that replicate chromosomes utilize single-stranded DNA as the template, whereas telomerases utilize RNA as the template.
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