Drosophila has 8 chromosomes: three pairs of autosomes and 1 pair of sex chromosomes. It inherited 1 haploid set of autosomes and 1 sex chromosomes from each parent. The Y chromosome does not determine maleness instead, each fly’s sex is determined by a balance between genes on autosomes and genes on X-chromosome. This type of sex determination is called genic balance system. The X-chromosome contains genes with female producing effects, whereas the autosomes contain a gene with male-producing effects. Thus, a fly’s sex is determined by X: A ratio, = the no. Of X-chromosome/ the no. Of the haploid set of autosomal chromosome.
X:A = 1 (female) ; X:A= 0.5 (male) ; X:A = <0.5. (male phenotype but sterile, metamales); X:A = 1.0 between 0.5 (intersex fly, both features); X:A = >1.0 female phenotype (metafemale)
Normal females contain (XX,AA) (1.0)
Normal males (XY,AA) (0.5)
(XXY,AA) (fully fertile female)
(XO,AA) (sterile male)
Thus, Y chromosome does not determine sex.
|
XX
|
AA
|
1
|
Female
|
|
XXY
|
AA
|
1
|
Female
|
|
XY
|
AA
|
0.5
|
Male
|
|
XO
|
AA
|
0.5
|
Male
|
|
XXX
|
AA
|
1.5
|
Metafemale
|
|
XXXY
|
AA
|
1.5
|
Metafemale
|
|
XX
|
AAA
|
0.67
|
Intersex
|
|
XO
|
AAA
|
0.33
|
Metamale
|
|
XXXX
|
AAA
|
1.3
|
Metafemale
|
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