Question

What is sex determination? How can we determine the sex in Drosophila?

Answer 1

Sex determination in Drosophila:

• Sex is determined in Drosophila by the ratio of X chromosomes to sets of autosomes (sets of autosomes simply refers to the ploidy of the fly).
• When the ratio is 1.0 or greater flies are female. When it is 0.5 or less flies are male Intermediate values give rise to intersex flies.
• Some typical examples are given in the below table. Extreme ratios such as 0.33 and 1.5 give rise to flies that are called meta males or meta females.
• Although clearly of their respective sex these flies are poorly developed and have a shortened life-span.
• The fact that sex determination is a result of a balance of X chromosomes and autosomes suggests that genes that cause female development are clustered on the X chromosome and genes for maleness on the autosomes.
• One important point to note concerns the Y chromosome.
• The data above indicate that it has no role in sex determination in Drosophila.
• This is correct, but although flies that lack a Y chromosome may be male, they are infertile because a gene on the Y chromosome is essential for the development of functional sperm.
• A similar genetic balance mechanism regulates sex determination in other species such as the nematode Caenorhabditis elegans (round worm).
• However this is slightly more complex as male and hermaphrodite individuals exist in this species.
• One significant feature of sex determination in Drosophila is the presence of abnormal flies known as gynandromorphs.
• These are the result of non disjunction in the somatic cells of the flies.
• If this results in a change in the number of X chromosomes in a cell the X: autosome ratio will be changed and may affect the sex of the cell.
• This can occur because in flies sex is determined autonomously in every cell.
• As the cell continues to divide its descendants will form a patch of cells (clone) which, depending on their position in the organism, may differentiate to form structures of the opposite sex to that of the rest of the fly.
• In the most extreme case, loss of an X chromosome in the first division after fertilization can result in a fly which develops bilaterally into two halves, one male and the other female.
• This type of event is determined hormonally. is not found in mammals where the production of secondary sexual characters is determined hormonally.

 

Number of X chromosomes(X)	Number of sets of autosomes(A)	X:A ratio	Sex
3	2	1.5	Female
3	3	1.0	Female
2	2	1.0	Female
2	3	0.67	Intersex
1	2	0.5	Male
1	3	0.33	Male

Sex Determination:

The sex-determination system is a biological system which determines the development of sexual characteristics in an organism.