2. SEX CHROMOSOMES
A sex chromosome also referred as allosomes,
heterochromosome.
In organisms that have separate sex a specific
pair of chromosomes are present which
determines the sex of an organisms.
The other chromosomes are referred as
autosomes or somatic chromosomes.
3. Discovery of Sex Chromosomes
Sex chromosomes in the form of X- was first seen
in fire flies by Heinking in 1891. the chromosome
was without any homologous in male .
Mcclung in 1902 discovered that the single X-
chromosome determines the sex of squash bug.
Mc clung in 1902 observed 24 chromosomes in
female grasshopper & 23chromosomes in male
grasshopper.
Miss Stevens in 1902 discovered Y- chromosome.
Edmund Beecher Willson also contributed towards the
discovery of sex chromosomes.
4. Polymorphism in sex chromosomes
Homogametic and Heterogametic
The organism having homomorphic sex chromosome
i.e. XX , ZZ produces only one kind of gamates all
with X or Z and these organisms are said to
HOMOGAMETIC.
The organism having heteromorhic sex chromosomes
i.e. XY or ZO produces two types of gametes half
with X and half with Y .
The sex chromosomes have two region : a.
Homologous
b. Non
homologous or Differential
The differential region carry completely sex linked
gene and doesn’t undergoes
crossing over but the homologous region carry same
genes and goes under crossing over..
The genes present in the differential region of the Y-
chromosome are called as Holoandric genes
(Holo means whole & Andros means male) .
5. Non allosomic sex determination : in this type of sex determination of an
individual factors other than the allosomes are taken into consideration.
Hormonal
Haplo Diploidy mechanism
Environmental
Parasitic mode of life
There are different methods operate in the determination of sex of an
individual.
Allosomic sex determination: in this type of sex determination method
sex chromosomes are taken into consideration for the determination of
sex of an individual.
eg. XX-XY, ZZ-ZW, ZZ-ZO
6. Allosomic determination of sex
Heterogametic Male: Males produces two
types of gametes .
XX-XY , XX-XO
XX-XY Type : found in mammals, including human beings,
fruitflies.
Parents : Father Mother
Genotype: 44A+XY 44A+XX
Gamates 22A+X +22A+Y 22A+X+22A+X
7. XX-XO type : this type is usually seen in some
insects.
Parents: Male Female
Genotype: AA+XO AA+XX
Gametes: 22A+X+22A+O 22A+X+22A+X
8. ZZ-ZW type: found in certain
insects,
and vertebrates like fishes, reptiles,
birds and also in some plants.
Parents: Father Mother
Genotype: AA+ZZ AA+ZW
Gamates : A+Z,A+Z A+Z,A+W
Hetrogametic Female: Usually seen in
birds and in reptiles. In thus case
females are heteromorphic and males
are homomorphic.
ZZ+ZO , ZZ+ZW
9. ZZ-ZO type: usually seen in moths and butterflies.
Parents : Male Female
Genotype: AA+ZZ AA+ZO
Gamates : A+Z , A+Z A+Z , A+O
12. Hormonal control of sex determination
Formation of free martin: in cattles, when twin calves of
different sex occurs the female is usually sterile known as
freemartin. This usually occurs due to the hormones of other
calf which is of different sex.
13. Haplodiploidy mechanism of sex determination
•Hymenopterans like honey bee, wasps, sawflies,
&ants shows a unique phenomena in which
unfertilized eggs develops into male.
•The female is diploid(2n) and the male is
haploid(n).
Eggs are formed by meiosis and sperm by
mitosis.
Fertilization restores the diploid number of
chromosomes in zygote.
In honey bee quality of food determine the fate
of diploid individual ; if the larvae feed on royal
jelly it will became the fertile queen and if on the
nectar it will became sterile female or worker.
Development of a haploid egg into a haploid
individual (male) without fertilization is known
as ARRHENOTOKUS PARTHENOGENESIS or
PARTHENOGENESIS or ARRHENOTOKY
(ARHEN means MALE & TOKUS
means BIRTH)
14. DIFFERENT FACTORS INVOLVED IN SEX
DETERMINATION
SIZE OF ANIMAL: The marine annelid Ophryotrocha
differentiate into a sperm producing male as a young animal and
then changes into an egg laying female when it gets older.
SIZE OF EGG : Marine archannelida Dinophilus.
Small egg – Male
Large egg - Female
AGE OF AN ORGANISM: seen in a mollusc belonging to
genus Crepidula.
young organism is functional male while it becomes
female when it matures.
RATE OF METABOLISM: A.F.Shull & D.D.Whitney explain
that by increasing the metabolic rate in rotifers the occurrence
of male increases than female.
15. Temperature determining sex of an organism
•Temperature plays an imp role in sex determination.
• For turtles eggs incubate at temperature
less than 28 degree Celsius – males
above 31 degree Celsius – females
• for Snapping turtles, Macroclemys
• females develop if the temp is below 22 degree or
above 28 degree Celsius.
• males will develop if the temp lies between 22—
28 degree Celsius.
• in American alligators a temp below 28 degree
celsius produces males while above 33 degree
celsius produce female and temp between 28—33
degree produces male and female in equal proportion.
•For lizard the organism in cool nest are male
irrespective of their genetic composition.
•European pond turtle, Emys obicularis
Temp below 25 degree develops into males
temp above 30 degree develops into females
16. Parasitic sex determination
An invasive barnacle (Loxothylacus panopaei) that hijacks mud’s crab
reproductive system and impregnates it with parasitic larvae even if
the crab is male.
17. Sex determination in C. elegans
• Two sexual phenotypes are seen in this
worm- the male having only one functional
testis and hermaphrodite bearing two gonads
producing both egg and sperm.
•The signal that determines maleness is
provided by the expression of gene present
on X- chromosome & autosomes.
•Hermaphrodite have two X chromosomes
while males have only one X-chromosome.
•Ratio of one results in the organism being
hermaphrodite while that of 0.5 result in
male worm.
18. Sex determination in humans
•Maleness is due the dominant effect
of Y chromosome.
•The effect of Y chromosome is
manifested Early in the development
when it directs the primordial gonads
to develop into testis.
•Testis-Determining factor(TDF)
product of gene called
SRY(Sex determining region Y ).
• A syndrome called Testicular
feminization results from a mutation
in an X-linked gene , Tfm which
encodes the testosterone receptor.
