If a gene segregates randomly with the dominant mutations located on chromosome 2 (Cy/Pm) but not with those on chromosome 3 (D/Sb), it is located on:
Chromosome 1 (the X) |
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Chromosome 2 |
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Chromosome 3 |
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Chromosome 4 |
1.Chromosome 1 (the X)
Reason:Balanced marker stock
It is relatively easy to determine the chromosome on which a mutation is located using the balanced marker stock Curly/Plum (2); Dichaete/Stubble (3). This stock is perpetually heterozygous for the four dominant mutant genes Cy, Pm, D and Sb which are lethal when homozygous. Cy (Curly wing) and Pm (Plum eye colour) are located on chromosome 2, while D (Dichaete wing - affects wing shape) and Sb (Stubble bristles) are third chromosome mutants.
No crossing over will occur between Cy and Pm, nor between D and Sb because these genes are associated with chromosome inversions. Cy/Pm D/Sb is a balanced lethal stock; the flies breed true when crossed among themselves. All flies of this strain are therefore heterozygous at these loci.
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