This document provides an overview of sex determination systems in animals. It discusses the main types of sex determination including environmental/non-genetic (influenced by temperature or location), chromosomal (XX-XO, XX-XY, etc.), and genic systems. For chromosomal sex determination, it describes the different mechanisms like XX-XO system found in grasshoppers and XY system common in humans and mice. It also discusses rare systems like haplodiploidy in bees and wasps.

1. DARSHANA A S
2018-11-039
SEX DETERMINATION IN
ANIMALS

2. INTRODUCTION
• Sex determination system is a biological system
that determines the development of sexual
characteristics in an organism.
• Most organisms that create offspring using
sexual reproduction have two sexes.
• There are hermaphrodites in place of one or
both sexes.
• There are also some species that have only one
sex due to parthenogenesis (female
reproducing without fertilization.)

3. HISTORY
• 1891 – H.Henking
While studying spermatogenesis
of wasp noted a particular
nuclear structure. Half of the
sperm receives this structure &
half didnot. He didnot speculate
on the significance of this body
but called it “X-body”. First
experimental evidence that led
to discovery of sex
chromosomes.

4. • 1902 – C. E. McClung
Made extensive observations
of spermatogenesis in
Grasshopper & suggested that
“X-body was involved in
determination of sex”. He
reported that somatic cells of
female grasshopper contains
24 chromosomes & male have
23 chromosomes.

5. • 1905 – E. B. Wilson & N. Stevens
Noted that females of Protenor
have 7 pairs of chromosome & male
have 6 & an unpaired chromosome.
They called it X – chromosome.
• 1908 – Discovered Y chromosome
and so named by Stevens.

6. SEX DETERMINATION
• Establishment of male and female individuals or
male and female organs of an individual is called
sex determination.
• Mechanism of sex determination-3 types
Environmental
Chromosomal
Genic

7. • Zygotes do not differ genetically.
• Sex differentiation determined by environmental
factors.
• 2 types
Temperature dependent
Location dependent
1. ENVIRONMENTAL / NON-
GENETIC SEX DETERMINATION

8. 1. Temperature-dependent sex
determination
In some animals (turtles, alligators, crocodiles,
some lizards etc.) the temperature at which eggs
are incubated has a decisive effect on sex of
animals that hatch from them. These effects are
of 3 types:
• Only females at high (30-35°C) and only males
at low (23-28°C). Incubation of eggs at
intermediate temperatures(28-30°C) produce
both males and females.
Eg: most species of turtles

9. • Only males at high(>33°C) and only females at
low(<30°C) temperatures. Intermediate
temperatures(30-33°C) produces both males and
females
Eg: many species of crocodiles and alligators and
in some species of lizards
• Only females at high(30-35°C) and low(<25°C)
temperatures. At intermediate temperatures,
both males and females are produced, but at
some intermediate temperatures, only males are
produced.
Eg: Australian crocodile, snapping turtles

10. 2. Location-dependent sex
determination
Eg: Bonellia viridis – marine
(green spoon worm)
• ♂ and ♀ have same chromosomes.
• If a young worm is raised from an
isolated larva it becomes a female.
• If larva comes in contact with female
then larva turns into male and
eventually migrates to female
reproductive tract.
• Possibly a hormone is secreted from
the proboscis of female to initiate
male sexual differentiation.

11. 2. CHROMOSOMAL SEX
DETERMINATION
• Male and female individuals differ from each
other in respect to either the number or
morphology of the homologues of one
chromosome pair, referred to as sex
chromosome or allosome – X & Y
• X chromosome found in both males(only one)
and females(has two X chr.)
• Y chromosome occurs only in one of two sexes
of a species.
• Chromosomes whose number and morphology
do not differ between males and females of a
species are called autosomes.

12. Mechanisms of chromosomal sex determination:
1. XX-XO system [XX female, XO male
XO female, XX male]
2. XX-XY system [XX female, XY male
XY female, XX male]
3. ZZ-ZW system
4. Haplodiploidy system [Diploid (2n) female,
Haploid (n) male]

13. XX FEMALE, XO MALE
• Females possess two X- chromosomes (XX) -
homogametic females.
• Males possess only one X-chromosome -
heterogametic males.
• O or zero in XO chromosome →absence of
another X- chromosome.
• Found in grasshoppers, Protenor and many other
insects, esp. those belonging to Orthoptera.

14. • If X carrying ovum fertilized
by X carrying sperm -zygote
develop into a female.
• If sperm containing no
chromosome unites with
ovum -zygote formed is XO-
develop into male.
• Maternal gametes always
contain an X chromosome -
sex of offspring depends on
whether a sex chromosome
is present in male gamete.

15. XO FEMALE, XX MALE
• Known in a few insect species, e.g., Fumea
• Females are heterogametic (producing two kinds
of eggs, half with a X chromosome and half
without any X chromosome) and males are
homogametic (producing single type of sperms,
each carrying a single X chromosome).
• Union of a sperm with X chromosome containing
egg -XX zygote -males.
• Fertilization of an egg devoid of X chromosome
with a sperm -XO zygote -females.

16. XX FEMALE, XY MALE
• Most common among animals
• Found in humans, mice,
Diptera(Drosophila, house fly,
etc.), some fishes, some
amphibia, etc.
• Females are homogametic (XX)
produce one kind of eggs, each
with one X chromosome.
• Males are heterogametic (XY)
produce two kinds of sperms :
half with X chromosome and half
with Y chromosome.

17. • Fertilization of egg by a
sperm having X
chromosome -XX zygote-
female.
• Union of egg with sperm
having Y chromosome -XY
zygote -males.

18. XY FEMALE, XX MALE
• Found in birds, reptiles, some
insects, e.g., silk worm, etc.
• Females have XY chromosome
constitution - heterogametic sex -
half the eggs have X, rest have Y
chromosome.
• Males have two X chromosomes
(XX) homogametic sex -all sperms
have one X chromosome.

19. • Fertilization of X containing egg with sperm -XX
zygote-male.
• XY zygote -when Y containing egg is fertilized by
a sperm-female.

20. ZW FEMALE, ZZ MALE
• Occur in certain insects & vertebrates like
amphibians, reptiles, birds & plants
• Female have one Z and one W chromosome-
produce two types eggs.
• Male - two homomorphic Z chromosomes.

21. • Sex of offsprings depends upon the
kind of egg
• Z bearing egg produce male & W
bearing egg produce female.

22. • Found in Hymenoptera(honeybees, ants, termites,
etc)
• An individual’s sex determined by number of sets
of chromosomes.
• First demonstrated by Dzierzon in 1850
• Somatic chromosome no. of females is diploid,
males is only haploid
• When an egg is fertilized by a sperm, the
developed organism will be diploid (2n)- female
• If egg is not fertilized, organism is haploid(n) -
male.
DIPLOID FEMALE, HAPLOID MALE

23. • During spermatogenesis, males
produce haploid sperms
• Normal meiosis during oogenesis
produces all haploid eggs
• Fertilization of eggs produces
diploid zygotes - develop into
diploid larvae - give rise to
workers which are sterile
females
• Diploid larvae fed on royal jelly
develop into fertile females
called queen
• Unfertilized eggs develop
parthenogenically - produce
haploid larvae - fully fertile
haploid males called drones

24. • Some specific genes located in X and Y
chromosomes are involved in sex determination.
• Explained by Genic balance theory of sex
determination in Drosophila proposed by Bridges
in 1921.
• Theory states that sex of an individual is
determined by a balance between the genes for
maleness and those for femaleness present in
the individual
• In Drosophila, genes for maleness are present in
autosomes and those of femaleness are located
in X chromosome
3. GENIC SEX DETERMINATION

25. • ie, sex of an individual is determined by the ratio
of no. of its X chromosome and that of its
autosomal sets, termed as sex index
Sex index = Number of X chromosomes(=X) = X/A
Number of autosomal sets (=A)
• Sex index = 1 normal females
= 0.5 normal males
= 1.0 – 0.5 intersex
= > 1.0 super females/meta females
= < 0.5 super males

26. GYNANDROMORPHS
• Individuals showing male characteristics
in a part of their body and remaining
parts show female phenotype
• In Drosophila gynandromorphs are
always mosaics for X chromosome;
parts with male phenotype are
always XO and female phenotype
are XX
• They arise from XX zygotes.