Sex is a cumulative characters differentiating individuals as two partners for the perpetuation of life and has genetic basis in all the animals

1. K.K.Gupta
University Department of Zoology
VBU, Hazaribag

2. What is Sex ?
Sex refers to differentiation
of individuals as male or
female or other sex variants
Like intersex,
gynandromorph , super male,
meta female etc.
Female Male
Large size Small size
Pointed
abdomen(boat
shaped)
Blunt abdominal
tip
No sex comb on
prothracic leg
Sex comb om
prothoracic leg
Abdominal tip
not black
Abdominal tip
black

5. Basis of Sex determination
• Initiated just after the 2hrs of fertilization
• Classical way was
• Chromosomal Basis and X:A ratio pointed by Bridge
• Phenotypic differentiation of either sex is not merely chromosomal rather
it is the coordinated action of many genes located on both X
chromosomes and autosomes in a very regulative manner during the
development
Basis of Sex
Determination
Chromosomal X:A Ratio Genetics

6. XX as Female and XY as male
primary sex-determining signal, which depends on the ratio of X chromosomes relative to
autosomes (X:A ratio).

7. Genetic Basis
X linked
• Sxl- Main gene
• Sis a
• Sis b
• Runt
• Dpn
Autosomal linked
• Da- Deadpan
• Emc- Extramacrochaetae
• These proteins block the
binding or activity of the
numerator proteins
• Tra-3L
Bind to PE of Sxl gene to
promote transcription
after 2hrs of fertilization
It appears, then, that the X:A ratio is measured by competition between X-
encoded activators and autosomally encoded repressors of the promoter of
the Sxl gene.

8. X:A i.e Numerator : Denominator
Numerator –sis a, sis b runt dpn
Denominator –da emc

9. SXL differentially expressed as per X:A ratio
Alternate splicing
there are two targets for the female-specific Sxl protein. One of these targets is the pre-
mRNA of Sxl itself. The second is the pre-mRNA of the next gene on the
pathway, transformer. And regulate somatic sex determination

10. TRA – Transformer gene , Autosomal on 3L
2nd exon containing nonsense codon is retained in male while it is removed in
female.
For this, the female-specific protein from the Sxl gene activates a female-
specific 3´ splice site in the transformer pre-mRNA, causing it to be processed in
a way that splices out the second exon.
The female-specific tra product also acts in concert with the product of
the transformer-2 (tra2) gene to help generate the female phenotype.
Gene No. of initial Exon No. of final exon in m RNA
Male female
Sxl 8 4 with UGA codon in 3rd Exon 7 no 3rd exon
Tra 4 4 with UGSA in 2nd Exon 3 no 2nd exon
Dsx 6 3 exon , no 4,5 &6 exon 4 no 5 & exon

11. Alternate splicing and
sex differentiation

12. Doublesex: The switch gene of sex
determination
• The doublesex (dsx) gene is active in both males and females
• Its primary transcript is processed in a sex-specific manner bt alternate splicing .
• This alternative RNA processing appears to be the result of the action of
the transformer gene products on the dsx gene.
• If the Tra2 and female-specific Tra proteins are both present, the dsx transcript is
processed in a female-specific manner.
• And produces a female-specific protein that activates female-specific genes (such
as those of the yolk proteins) and inhibits male development.
• If functional Tra is not produced, a male-specific transcript of dsx is made. This
transcript encodes an active protein that inhibits female traits and promotes male
traits.
• Male and female DSX isoforms both encode zinc finger proteins that are identical
at the amino terminus where the DNA binding domain is found but differ at their
carboxy termini
• DSX is responsible for almost all somatic sex determination throughout
the Drosophila body including functions in the CNS

13. Summary

14. Conclusion
• Thus , the result of the sex determination depends
upon the type of mRNA leading to final expression of
male and female specific Dsx.
• If the X:A ratio is 1, then Sxl makes a female-specific
splicing factor that causes the tra gene transcript to be
spliced in a female-specific manner. This female-
specific protein interacts with the Tra2 splicing factor to
cause the doublesex pre-mRNA to be spliced in a
female-specific manner. If the doublesex transcript is
not acted on in this way, it will be processed in a
“default” manner to make the male-specific message.