MIDTERMBIOs325Answers - 1 GENETICS BIOs325 Summer 2008...

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1 GENETICS – BIOs325 Summer 2008 Prof: Janice Fischer MIDTERM EXAM July 29, 2008 Name ANSWERS Helpful Advice: • Read my questions carefully and answer the ones that you can answer. • Answering a different question that you invent and you like better than mine will get you nowhere. • Throwing “key words” around will get you nowhere. Words need to be in sentences that make sense. • PLEASE TEAR OFF LAST PAGE AND USE IT FOR REFERENCE • There are 20 questions, each of which is worth 5 points. • There is a bonus question at the back in case you have extra time.
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2 1. In the dihybrid cross shown below, genes A and B are closely linked with 1 map unit between them. Allele A is completely dominant to a, allele B is completely dominant to b, and the A and B genes affect different traits. P AABB x aabb F1 AaBb (self) F2 A. What are the gamete types produced by the F1 and at what frequency are they produced? There are four gamete types: AB, ab, aB, Ab The parental, AB and ab, are 99% of the total, and the recombinants, aB and Ab, are 1% of the total. As the parentals and recombinants are each present at equal frequencies, the frequencies are: AB (49.5%), ab (49.5%), Ab (0.5%), aB (0.5%). B. What is the phenotypic ratio observed in the F2? It’s not anything like 9:3:3:1, because that is based on the four gamete types being present at equal frequencies, and here the recombinants are much less frequent than the parentals. The easiest way to figure out the answer is to draw a Punnett square, with the frequencies of each gamete indicated:
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3 2. A pure-breeding plant with striped leaves is crossed with a pure-breeding plant with dotted leaves, and the F1 are all striped and dotted. The F1 are selfed, and in all cases the phenotypic ratio observed in the F2 is: 1 striped : 2 striped and dotted : 1 dotted. A. Provide a model for the mode of inheritance of striping and dotting. Invent symbols for genes and alleles, and diagram all three generations of the cross. Be sure to indicate (with words as well as symbols) the dominance relationships between the various alleles. Striping and dotting appear to be controlled by alternate codominant alleles of a single gene. Let’s call that gene A. A1 = stripes A2 = dots P A1 A1 X A2 A2 F1 A1 A2 (self) F2 1 A1 A1 : 2 A1 A2 : 1 A2 A2 B. Answer A again assuming that the F2 showed the following phenotypic ratio: 9 dotted and striped : 3 dotted : 3 striped : 1 no dots or stripes. Striping and dotting appear to be controlled by two different
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MIDTERMBIOs325Answers - 1 GENETICS BIOs325 Summer 2008...

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