The genes short legs (sl), plexus (px) and brown eyes (bn) are linked on the sec

Question

The genes short legs (sl), plexus (px) and brown eyes (bn) are linked on the second chromosome of Drosophila. In a cross of bn sl px / + + + females with bn sl px / bn sl px males, the following progeny were counted.

The genotypes of the nonrecombinant parental type are

A. bn sl px / bn sl px and bn + + / bn sl px

B. bn sl px / bn sl px and + + px / bn sl px

C. + + + / bn sl px and bn sl px / bn sl px

D. bn + px / bn sl px and + sl + / bn sl px

E. None of the above

The genotypes of the double crossover are A. bn sl px / bn sl px and bn + + / bn sl px

B. bn sl px / bn sl px and + + px / bn sl px

C. + + + / bn sl px and bn sl px / bn sl px

D. bn + px / bn sl px and + sl + / bn sl px

E. None of the above

The gene order is

A. bn – sl – px

B. sl – bn – px

C. px – sl – bn

D. Either answer A or answer C. The data do not distinguish between the two possibilities.

E. None of the above is correct

Explanation / Answer

Answer:

1). C). + + + / bn sl px and bn sl px / bn sl px

The nonrecombinant parental type are always more in number than the any recombinants.

2). D). bn + px / bn sl px and + sl + / bn sl px

Doble recombinant types are always small in number than any other genotype in three point test cross.

3). D. Either answer A or answer C. The data do not distinguish between the two possibilities.

The gene for short legs (sl) is middle between plexus (px) and brown eyes (bn) genes.

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