This document summarizes different mechanisms of sex determination in animals. It discusses chromosomal sex determination mechanisms including XX-XO type (found in grasshoppers and aphids), XX-XY type (found in humans), ZZ-ZW type (found in birds), and haplodiploid type. It also covers environmental sex determination influenced by factors like temperature (in crocodiles and turtles) and location (in Bonellia viridis). Sex determination in Drosophila depends on the ratio of X chromosomes to autosomes. The document concludes that the key to maleness and femaleness ultimately lies in the control of gene expression.

1. DIFFERENT MECHANISMS OF SEX
DETERMINATION IN ANIMALS
Name- Sonali Priya Singh
Register Number-220705180038
Subject- Genetics and Epigenetics
Branch- M.Sc. 1st year Zoology
School of Applied Sciences
CUTM,BBSR Guided By- Dr. Gagan Kumar Panigrahi

2. CONTENTS
• Introduction
• XX-XO type sex determination
• XX-XY type sex determination
• ZZ-ZW type sex determination
• Haplodiploid sex determination
• Sex determination in Drosophila
• Environmental sex determination
• Conclusion
• References

3. INTODUCTION
• Sex determination is the biological mechanism by which sex (female
or male) of an individual is established.
• The term sex refers to the sexual phenotype of a person. Most
organism have two sexual phenotypes namely male or female.
• Sex determination mechanisms is of two types
i. Chromosomal sex determination
ii. Environmental sex determination
• Chromosomal sex determination include XX-XO , XX-XY, ZZ-ZW
and Haplo-Diploid type of sex determination

4. XX-XO TYPE SEX DETERMINATION
• In some insect orders, the
female has two similar sex
chromosomes (XX) thus it
is homogametic while male
has only one (XO)
• The letter O signifies the
absence of a sex
chromosome.
• This type of determination
occurs in Grasshoppers,
Aphids etc.
Fig1- XX-XO type sex determination in Grasshopper
Fig2- Grasshopper, A-female B-male
Source- Idrissa et.al,(2008)

5. XX-XY TYPE SEX DETERMINATION
• In many species, the cells
of males and females have
same number of
chromosomes, but females
have two X chromosomes
and male have a single X
chromosome and a smaller
sex chromosome called Y
chromosome.
• Thus the sex of an
individual in this case
depends upon the male
gamete.
• Examples include humans.
Fig3- XX-XY type sex determination in humans
Source-Hake,L and O’Conner,C.(2008)

6. ZZ-ZW TYPE SEX DETERMINATION
• In this case, the female is
heterogametic and the male is
homogametic. Thus female
has a Z chromosome and a W
chromosome while the male
has two Z chromosomes.
• It is reverse of XX-XY sex
determination, here sex of
individual depends on female
gamete.
• It mainly occurs in Birds.
Fig4- ZZ-ZW type sex determination
Source- Hake,L and O’Conner,C.(2008)

7. HAPLODIPLOID SEX DETERMINATION
• In this case, all the
fertilized eggs develops
in to female organisms
while the unfertilized
eggs develops in to male.
• Hence, the males are
haploid and the females
are diploid.
• The unfertilized eggs
develops in to male by a
process called
parthenogenesis.
Fig5- Haplodiploid sex determination
Source- Hake,L and O’Conner,C.(2008)

8. SEX DETERMINATION IN DROSOPHILA
• Sex in Drosophila
melanogaster is
determined by genic
balance mechanism.
• It depends upon X : A
ratio that is number of X
chromosomes divided by
number of haploid sets of
autosomes.
• X : A ratio
Female= 1.0
Male=0.5
Intersex= in between
0.5 to 1.0 Fig6- X:A sex determination
Source- Hake,L and O’Conner,C.(2008)

9. ENVIRONMENTAL SEX DETERMINATION
• In a number of organisms, sex is determined fully or in part by
environmental factors.
• In crocodiles and turtles, sex is influenced by temperature at which the
eggs are incubated. At higher temperature of incubation eggs develops
in to female while at lower temperature eggs develops in to male.
• In Bonellia viridis, sex is influenced by location of landing of the
larvae. If the larvae land on the bare ocean floor then they turn in to
female on the other hand if they land on the body of female they
develops as male.

10. Fig7- Central bearded dragon
Source- Howard (2019)
Fig8- Crepidula fornicata (common slipper
limpet)
Source- Hake,L and O’Conner,C.(2008)

11. CONCLUSION
In all sex determining systems, the sex is actually determined by
individual genes. However, if we look deeply, the key to maleness and
femaleness does not even lies in the genes , but in the control of their
expression.

12. REFERENCES
• Idrissa, M., Lecoq, M. and Kooyman, C.(2008) Ecology and
Management of the Senegalese Grasshopper, Oedaleus senegalensis,
Annales-Societe Entomologique de France 44(3):271-288
• Hake, L. and O’Conner, C.(2008) Genetic Mechanisms of Sex
Determination, Nature Education1(1):25
• Howard, J. (2019)Ecology of the Central Bearded Dragon(Pogona
Vitticeps), Australian Veterinary Association Unusal and Exotic Pets
Conference2019. At: Melbourne, Australia.

13. ACKNOWLEDGEMENT
I do hereby acknowledge and sincerely thanks Dr. Gagan Kumar
Panigrahi for the help extended by them in the form of literature for the
preparation of this presentation “ fertilisation in fishes”. I also sincerely
express my profound gratitude to my parents and friends for their
continuous support and encouragement.

14. THANK
YOU