The balancer chromosome CyO carries a dominant mutation, Cy, which causes curly
ID: 179313 • Letter: T
Question
The balancer chromosome CyO carries a dominant mutation, Cy, which causes curly wings. CyO is balancer for the second chromosome. The balancer chromosome TM3 carries a dominant mutation, Sb, stubble, which causes the bristles to be short. TM3 is a balancer for the third chromosome. You obtain two Drosophila stocks.
You cross the first stock, with genotype, CyO / lethal-2 so --> (CyO / lethal-2 ; + / + )to a stock in your laboratory described as CyO / Df58B, which carries a deletion of the 58B region.
4A) [5 points] Suppose that lethal-2 maps within the 58B region. In that case, what fraction of the progeny from this cross will have curly wings? (your explanation would benefit from a Punnett square. It should be specific and succinct).
4B) [5 points] Now suppose that lethal-2 does not map within the 58B region (it maps to the 54C region). In that case, what fraction of the progeny from this cross will have curly wings? Explain (your explanation would benefit from a Punnett square. It should be specific and succinct).
please solve and explain. Thank You!
(edit: this is all the information I am given to solve the problem. As for the 58B and 54C regions, I'm pretty sure they are just random places on the chromosome where a deletion does and doesn't exist, respectively.)
Explanation / Answer
4A.
The Cyo is a dominant mutation.
Cross is CyO / lethal-2 crossed to CyO / Df58B
Results will be CyO / lethal-2; ++ ,CyO / Df58B (parental types); and CyO / Df58B, CyO / lethal-2 (recombinants)
Gametes of CyO / lethal-2; ++ will be Cyo/+, +/lethal-2, ++, and Cyo/lethal-2.
Gametes of CyO / Df58B will be same.
The results should be 1:1:1:1 of all four genotypes. Lethal-2 is a dosage compensation gene. Cyo is the balancer. It will balance the dose of lethal-2. So, CyO +/Cyo Df58B will be non- curly. + lethal-2/ CyO Df58B will be curly i.e. 25% of total. Cyo lethal-2/CyO Df58B will also be curly, that is also 25% of total.
So, 50% of all will be curly.
B.
When lethal-2 does not map within 58B region, then dosage will not be compensated. In such a case,
+ lethal-2/CyO Df58B will be non-curly.
Similarly Cyo lethal-2/CyO Df58B will also be non-curly.
So, there will be no curly phenotypes despite present of dominant Cyo balancer.
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