Question

Intersex Drosophila has
(a) Two sets of autosomes and two X-chromosomes
(b) Three sets of autosomes and two X-chromosomes
(c) One set of autosomes and two X-chromosomes
(d) Three sets of autosomes and two Y-chromosomes

Answer 1

The theory of sex determination tells you that the number of X-chromosomes to the total autosomal sets present, i.e., (X/A ) must fall in the range of 0.5 and 1 to be able to mark the presence of intersex phenotype.

Complete Step by Step Answer:
We first investigate the options that are given. The first option tells us that there is a presence of two sets of autosomes and two of X-chromosomes and following the theory of sex determination we find out that the ratio, ${ X }/{ A }={ 2 }/{ 2 }=1$, and hence the resulting sex would be a female and not an intersex because for intersex that ratio must be in the range of 0.5 to 1. Then let us go to the second option. In option two we see that there are three sets of autosomes and two X-chromosomes which give us the ratio, ${ X }/{ A }={ 2 }/{ 3 }=0.67$.
Now, 0.67 lies in the range of 0.5 to 1, hence this marks the presence of Intersex phenotype. Hence, intersex drosophila has Three sets of autosomes and two X-chromosomes. Yet we can find out the ratio of the rest of the options too. The third option fetches us the result, ${ X }/{ A }={ 2 }/{ 1 }=2$, which will result in a superfemale and as Y-chromosome plays no part in the sex determination of drosophila, therefore, the fourth option is also incorrect.

Additional Information:
- Drosophila is known to have four pairs, viz., three autosomes and an X or Y- pair. The fourth out of the three autosomes present in drosophila is too small and is insignificant, other than one of the important traits of the eyeless gene.
- Y-chromosome does not play any part in the determination of sex in drosophila.
- In drosophila, the Y-chromosome does not determine the presence of male characteristics rather it is used for gene encoding which is necessary for the production of sperm.
- In Drosophila, there is a presence of gynandromorphs, a trait in which the body of the drosophila partly develops male and partly develops female characteristics.
- The presence of major genes like sex-lethal, dead-pan, and sisterless generally decides upon the matter of sex determination of drosophila.
So, the correct answer is, “Three sets of autosomes and two X-chromosomes”.

Note:
- In the case of sex determination of drosophila you must always go for the ${ X }/{ A }$ ratio because the question is based on phenotype or sex determination generally the ${ X }/{ A }$ gives out the answer easily.
- Also, you must keep in mind the ratio which determines the type of phenotype, for example, in this question for the intersex phenotype of drosophila the ration lies in the range 0.5 to 1.