Chromosomal theory best explains human sex determination. Females are XX and produce X-bearing eggs, making them homogametic. Males are XY and produce both X- and Y-bearing sperm, making them heterogametic. The presence of a Y chromosome results in male development due to Y-linked genes, while its absence results in female development. Other factors like genetic balance and hormones can influence sex development but not determination.

1. SEX
DETERMINATIION
IN
HUMAN

2. INTRODUCTION
Human beings are most
evolved showing well marked
sexual dimorphism. Puberty is the
maturity age marked by separate
male or female secondary sex
characters due to secretion of
respective sex hormones by
gonads.

3. SEXES IN HUMAN
MALE – Heterogametic and have
testes and produce sperms.
FEMALE – Homogametic and
have ovary and produce ovum.
TRANSGENDER – Generally
sterile and may have testes / ovary
as primary sex organs.

4. CHROMOSOMES IN HUMAN
 AUTOSOMES – These are 22
homologous pairs of chromosomes
numbered from 1 to 22 and 24 to 45. They
bear genes for somatic or body
characters.
 ALLOSOMES / SEX CHROMOSOMES
– These are one pair of chromosomes
numbered 23 and 46. They bear genes for
sex determination and also some somatic
genes. They may be of two types :- X and
Y. Y chromosome is J-shaped.

5. ALLELES OF A GENE
 Each gene is located at a fixed position on
a chromosome, called LOCUS / LOCI.
 Each gene has two alleles, either both
dominant or both recessive (cis
arrangement) and one dominant and
other recessive (trans arrangement).
 The two alleles are located at same locus,
one on each of homologous pair of
chromosomes.
 As Y chromosome is curved, it has some
maleness and other genes not found on X.

6. ALLELES OF A GENE
 Out of two alleles
 Dominant allele :- This is the allele which
is expressed in both homozygotic and
heterozygotic conditions.
 Recessive allele :- This is the allele, which
is expressed only in heterozygous
condition.
 Homozygous Condition :- If both the
alleles are dominant or both recessive.
 Heterozygous Condition :- If one of the
alleles is dominant or and other is
recessive.

7. LAW OF SEGREGATION
 As per law of segregation or law of
independent assortment, alleles of a gene
segregate or separate during gamete
formation and reunite in the zygote.
 In human, each gamete normally gets one
set (22) of autosomes and one sex
chromosome.
 Homogametic – if both the gametes get
same type of chromosomes.
 Heterogametic – if both the gametes get
different types of chromosomes.

8. THEORIES OF SEX
DETERMINATIION IN HUMAN
 Chromosomal Theory – Stevens – 1905
 Genic balance Theory – Bridges – 1921
 Hormonal Theory – Crew
 Environmental Theory – Blatzer – 1935

9. CHROMOSOMALTHEORY
OF SEX DETERMINATION
Its details was given by STEVENS (1905),
supported by WILSON, BRIDGE,
GOLDSCHMIDT & WHITINGS.
As per this theory :-
Y chromosome carries genes for
maleness and X chromosome for
femaleness.
Males contain 22 pairs of autosomes
and one X and one Y chromosome.

10. Females contain 22 pairs of autosomes
and two X chromosomes.
So it is also called as XX – XY type of
sex determination in human.
Homogametic Female :- As female
contains 22 pairs of autosomes and one
pair of X chromosomes, its gametes
contain 22 autosomes and one X
chromosome or both female gametes
have same type of chromosomes and
called isogametes and females are called
as homogametic.

11. Heterogametic Male :- As male contains
22 pairs of autosomes, one X and one Y
chromosome, its gametes contain 22
autosomes and any one of X or Y
chromosome or both male gametes have
different types of chromosomes and
called anisogametes and males are
called as heterogametic.
Y chromosome is dominant to X.
In heterozygotic condition, Y is
expresssed.
In XY condition, Y is expressed.

12. In XX condition, X is expressed and
other X remain as barr body till gamete
formation.
As Y chromosome contains genes for
maleness, presence of at least one Y
chromosome make the zygote into a
male individual and absence of Y makes
female individual.
Female produce two gametes, each with
one X chromosome.
Male produce two gametes, one with
one X and other one Y chromosome.

13. If X containing ovum of female unite
with X containing sperm of male, zygote
become a female or daughter.
If X containing ovum of female unite
with Y containing sperm of male, zygote
become a male or son.
The cross may be described as below :-

14. GENERATIONS CROSSMALE
FEMALE
PARENTS
GAMETES
F 1 GENERATION
XX XX
XY XY
X
Y
1 – FEMALE
2 – FEMALE
3 – MALE
4 - MALE
X
X

15. GENERATIONS CROSSMALE
FEMALE
PARENTS
GAMETES
F 1
X XY

16. GENIC BALANCE THEORY
It was proposed by CELVIN BRIDGES
(1921).
It says, X chromosome carries genes for
femaleness and Autosomes for maleness
while Y lacks body genes and does not take
part in sex determination.
Sex of and individual is determined by ratio
of number of X chromosomes to number of
sets of Autosomes.
Generally we have 2 sets of Autosomes. Male
has one and female two X chromosomes.

17. S. N. Ratio Sex
1 0.5 Male
2 0.5 to 1.0 Intersex
3 0.33 Super male
4 1 Female
5 1.5 Super female

18. HORMONAL THEORY
In many instances hormones modifies sex
instead of determining, as follows :-
Observations of CREW in chicks :- A hen
laid fertile eggs, accidentally lost its ovary.
Production of ovarian hormones stopped
egg laying. The rudimentary testis
(remaining in all female birds), got chance
to develop, produced male hormones and
hen developed male characters as comb
and male plumage and became a cock.
Such a cock produced sperm and became

19. HORMONAL THEORY
a father of two chicks.
Freemartin :- In cattle when twin calves of
opposite sex are in womb, male produce
its hormone earlier in development, they
pass into under developed female through
circulation and causes its partial sex
reversal. This female is called as
freemartin.

20. ENVIRONMENTAL THEORY
BLATZER (1935) observed in Bonellia :-
Female is several inches long and male is
very small living in reproductive tract of
female.
The newly hatched young worm, when
reared in isolation, became a female but
when in water containing mature females,
some attach to proboscis of female and
became males.

21. CONCLUSION
From above descriptions, we conclude
that, the sex chromosomes determine the sex
of human beings but autosomes and
hormones modify their sex.