Question

In Drosophila melanogaster, cherub wings (ch). black body (b) and cinnabar eyes (cn) are recessive to their corresponding alleles (represented as ch+, b+, and cn+, respectively) and are all located on chromosome 2.Homozygous wild type flies were mated with cherub, black, and cinnabar flies and the resulting F1 females were tests crossed with cherub, black and cinnabar males. The following progeny were produced from the testcross:
ch b + cn 110
ch + b + cn + 780
ch + b cn 70
ch + b + cn 6
ch b cn 769
ch b + cn + 60
ch + b cn + 111
ch b cn + 9
Total 1915
Of these three genes, which one is in the middle?
(a) The locus that determines cherub wings.
(b) The locus that determines cinnabar eyes.
(c) The locus that determines black body.
(d) Cannot be determined from the given data.

Answer 1

Hint: On a chromosome, a number of genes are located in a specific or fixed position, and in the above question the required genes that are located in the middle position are related or responsible for the development of drosophila’s locomotory organ.

Complete answer:
In Drosophila melanogaster, cherub wings (ch). black body (b) and cinnabar eyes (cn) are recessive to their corresponding alleles (represented as ch+, b+, and cn+, respectively) and are all located on chromosome 2.
Homozygous wild-type flies were mated with cherub, black, and cinnabar flies and the resulting F1 females were tested crossed with cherub, black and cinnabar males.
In the progeny produced from the testcross, the genes in the middle out of these three genes is the locus that determines cherub wings.
In the above-mentioned test cross, the frequency of parental combination is on the higher side and that of the double crossover offspring is on the lower side.
Here the order of the arrangement of the gene on the chromosome is synonymous with the genotype of the double crossover offspring. In the table provided with the question, it is clearly shown that ch+b+cn (6) and ch b cn+ (9) indicates that cn is in the middle of the triad.
During random and independent gamete formation this combination gets interchanged or recombined.

So, the correct answer is, ’The locus that determines cherub wings’.

Note:
-Drosophila melanogaster is also known as the common fruit fly. It is the most commonly used model organism for biomedical science that is why it is also called “Cinderella of Genetics”.
-Drosophila is the most used insect for genetic experiments because of its short life cycles, easy and inexpensive method of culturing, large progeny, availability of a number of mutant stocks, and genomes of few species.
-Drosophila melanogaster found worldwide on every continent and most islands. It is mainly associated with a wide array of decaying vegetables and other plant matter.