Sex determination is controlled by sex chromosomes. In humans and many other species, females have two X chromosomes (XX) while males have one X and one Y chromosome (XY). The presence of a Y chromosome determines maleness, while its absence results in femaleness. There are two main systems - heterogametic males which include humans and heterogametic females found in some insects and fish. The ratio between X chromosomes and autosomes also influences sex determination in some species through a genic balance mechanism.

1. SEX
DETERMINATION

2. SEX DETERMINATION
■ A type of chromosome in the genome that is involved
in determination of sex as well as development of
sexual characteristic of an organisms.
■ Sex chromosome is either a pair of chromosomes that
determine whether an individual is male or female.
■ Humans have 23 pairs of chromosomes among which
22 pairs are autosomes and one pair of allosomes in
which individuals having XX chromosomes are females
and individuals with XY chromosomes are male.

3. MECHANISMOF SEX DETERMINATION
■ The fruit fly Drosophila melanogaster and humans
have been important in the development of genetic
concepts.
■ Many higher individuals exhibit one of sex
phenotypes- females or males, where female produce
female gametes and male produces male gametes.
■ Species with separation of the sexes in different
individuals are called dioecious.

4. ■ Species in which both male and female are produced by
each individuals are called monoecious organisms.
■ There are two systems of sex chromosomal
determination:-
1. Heterogametic males.
2. Heterogametic females.

5. HETEROGAMETIC MALES
In this type the female sex has two X chromosomes,
while the male sex has only one X chromosome.
They are of two types:
1. XX-XY type:
In man, other mammals and certain insects including
Drosophila, the female has two X chromosomes and being
homogametic, produce one kind of eggs, each with one X
chromosome. Since the males are heterogametic they
produce two types of sperms, half with X chromosome and
half with Y chromosome.

6. Sex is determined by the type of sperm fertilizes the egg.
If an egg is fertilized by an X-type sperm, the resulting
individual is a female and if it is Y-type sperm, resulting
individual is male.

7. 2. XX-XO TYPE
In certain plants and in insects –bugs and grasshoppers
the female having X chromosomes (XX) and are thus
homogametic, while the male having only one X
chromosome (XO).
The male lacking one X chromosome produces 2 types of
sperms half with X chromosome and half with out X
chromosomes.

9. HETEROGAMETIC FEMALES:
In this type, the male sex possesses two chromosomes
and is homogametic and produce single gametes where as the
females are heterogametic and produce a single gamete X or
one X chromosome and one Y chromosome.
There are two types:
1. ZO-ZZ system:
This type is found in certain moths and butterflies. In this
case , the female possesses single z chromosome and produce

10. two kinds of eggs half with a Z chromosomes and half
without any Z chromosome. The male possesses two Z
chromosomes and produce single type of sperms. The sex
of the offspring depends on the kind of egg.

11. 2. ZW-ZZ SYSTEM:
This system of sex determination is found in certain
insects and vertebrates such as fishes, reptiles and birds.
Here the females sex as one Z chromosome and one W
chromosome and it produces two types of ova, 50% with
Z chromosome and 50% W chromosome.
The male sex has two homogametic Z
chromosomes producing single type of sperms. The sex
of the offspring depend on the kind of egg.

13. GENIC BALANCE MECHANISM
Extensive experimentation of different scientists
Wilson, Bridges, Goldschmidt have worked on the fact
that most organisms generally have inherent potentialities
for both sexes, intermediate between male and female
sexes.
SEX DETERMINATION IN Drosophila:
■ In Drosophila, the presence of Y chromosome has been
found essential for the fertility of male sex.

14. ■ If the sex determining genes were so distributed that
the net effect results in the autosomes determining
maleness and the X chromosomes femaleness.
■ The sex of an individual depends upon the ratio of X
chromosome to autosome.
■ Let A represent haploid set of autosome. If each
haploid set of autosomes carries factors with male
determining value equal to one. Then each X
chromosome carries factors with a female determining
value of one and half.

15. ■ For example in a normal male (AAXY)the male and female
determinants are in the ratio 2:11
2 and therefore the genic
balance is in the favor of maleness.
■ This conditions is also seen in chickens, birds and also in
the flowering plants.
■ It has been supported following two findings:
1. POLYPLOID FLIES:
o Bridges crossed experimentally produce triploid female
drosophila (3A:3X) to diploid males (2A:XY).
o From this cross he obtained normal diploid males, triploid
females, intersexes, super males and super females. This
indicates autosomes also carry genes for sex determination.

17. 2. GYNANDROMORPHS:
o Concepts of sex determination has developed for
drosophila based on occurrence of gynandromorphs
which are individuals in which part of the body
express male character and other part female character.
o It is a kind of mosaic or organisms made up of tissues
of male and female genotype.
o This may result loss of X chromosome forever in the
daughter nucleus during cell development.

19. SEX DETERMINATION IN MAN:
■ In man like Drosophila XX-XY type sex determining
mechanism occurs.
■ Here the Y chromosome is a determiner of fertility
and sex of male individual came from certain
abnormal condition which contain aneuploidic sex
chromosomal abnormalities.
■ Turner’s syndromes (XO):Sterile females having
abnormalities such as short stature, shield chest,
webbed neck, short fourth, metacarpal, color
blindness etc.

20. ■ Klinefelter’s syndromes (XXY): A person with extra
one X and Y chromosome display true hermaphroditism
having both ovarian and testicular tissues and variable
degrees of intersexual development of the genitalia.

21. SEX DETERMINATION IN PLANTS:
■ Most flowering plants are monoecius and do not have
sex chromosomes.
■ Dioecious plants are found to be under the genetic
control of a single gene.
Sex determination in Asparagus:
It is a dioecious plant, in which females bear
rudimentary anthers and male bears rudimentary pistils.
When the seeds of this plants were raised the male
and female plants were found to be present in 3:1ratio.

22. Rare male plant
Pp
self (Pp x Pp)
PP Pp pp
(25%) (50%) (25%)
Male (75%) Female (25%)

23. DOSAGE COMPENSATION OF GENES:
It was coined by Muller and this was originally
discovered in drosophila.
X Chromosome Inactivation In Mammals:
• It has been demonstrated that homogametic XX female
individuals, one X chromosomes gets characteristically
condense and inactivated such chromatin material is
called facultative heterochromatin.
• The phenomenon of inactivation of X chromosome
was confirm by the observation of Barr body.

24. • Barr body was first discovered in the nerve cells of
female cat which was absent in male. It is also called
as sex chromatin.
• The human females are called sex chromatin positive
and males are called as sex chromatin negative.

25. HYPERACTIVITY OF X CHROMOSOME IN MALE
Drosophila:
• The phenomenon of dosage compensation in drosophila is due
to hyperactivity of one X chromosome in male Drosophila.
• In mosaic individuals with XX and XO cells, it could be
shown that X chromosome in XO cells was always
hyperactive.
• Due to this fact mutant and Y type flies showed same intensity
of eye color in male and female flies.
• Several enzymes including six phosphogluconate
dehydrogenase, fumarase, tryptophan pyrrolase were also
found to be similar.