Question

In Drosophila melanogaster, cherub wings $\left(ch\right($, black body $\left(b\right)$ and cinnabar eyes $\left(cn\right)$ are recessive to their corresponding alleles (represented as $ch+,b+$ and $cn+$, respectively) and are all located on chromosomes $2$. Homozygous wild type flies were mated with cherub, black and cinnabar flies and the resulting $F1$ females were test crossed with cherub, black and cinnabar males. The following progeny were produced from the test cross:

$chb+cn$	$110$
$ch+b+cn+$	$780$
$ch+bcn$	$70$
$ch+b+cn$	$6$
$chbcn$	$769$
$chb+cn+$	$60$
$ch+bcn+$	$111$
$chbcn+$	$9$
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

The correct option is A The locus that determines cherub wings

The parental combinations are in highest frequency and the double cross over offsprings are of lowest frequency. The genotype of the double cross-over offsprings is synonymous to the order of the gene arrangement on the chromosome. So, from the table we can see that ch+b+cn (6) and ch b cn+ (9) indicates that cn is in the middle of the triad and gets interchanged or recombined during random and independent gamete formation.

So, the correct option is 'The locus that determines cinnabar eyes'