Genetics of Sex Determination.

1. SEX DETERMINATION AND
SEX
CHROMOSOMES
SUKANTA SARKAR
M.Sc. BOTANY 3rd SEM.
GURU GHASIDAS VISHWAVIDYALAYA
BILASPUR C.G. 495009 INDIA

2. SEX DETERMINATION MECHANISMS
Genetically controlled Sex Determination
Mechanism
Metabolically controlled Sex Determination Mechanism
Hormonally controlled Sex Determination Mechanism
Environmentally controlled Sex Determination
Mechanism

3. GENETICALLY CONTROLLED SEX DETERMINATION
MECHANISMS
1.Sex chromosome mechanism or
heterogamesis
2.Genic balance mechanism
3.Male haploidy or haplodiploidy mechanism
4.Single gene control of Sex

4. DISCOVERY OF SEX CHROMOSOME
H.Henking (1891) While studying spermatogenesis of Wasp noted a
particular nuclear structure. Half of the sperm receives this structure &
half did not. He did not speculate on the significance of this body but
called it “X-body”.
C.E. McClung(1902) 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.
E.B. Wilson(1905) Noted that females of Protenor have 7 pairs of
chromosome & male have 6 & an unpaired chromosome. He called it X-
chromosome.

5. STRUCTURE OF X-CHROMOSOME
•The X & Y chromosome exhibit structural differences.
Cytological study show that –
1. X-chromosome of most organism are straight rod like &
comparatively longer than Y-chromosome.
2. X have large euchromatin and small amount of
heterochromatin. Euchromatin have large amount of DNA
& hence much genetic information. Y- chromosome
contain small euchromatin & large heterochromatin hence
it has little genetic information, sometimes referred as
genetically inert or inactive.

7. SEX
DETERMINATION
IN HUMANS
In humans sex is
determined by a
dominant effect of
the SRY gene on
the Y chromosome;
the

8. TYPES OF SEX CHROMOSOMAL
MECHANISM OF SEX DETERMINATION
Heterogametic
Males
• XX-XO Type
• XX-XY Type
Heterogametic
Females
• ZO-ZZ System
• ZW-ZZ
System

9. XX-XO SYSTEM
Plants – Vallisneria & insects of order Hemiptera & Orthptera
female have 2 X-Chromosomes(XX) and male have only one X-
Chromosomes(XO).
Male produce two types of sperms half with X & half without X.
The presence of one unpaired X-Chromosomes determine the
masculine sex.

10. XX-XY TYPE
Found in mammals including humans, insects like
Drosophila & in certain angiospermic plants –
Melandrium, Humulus, Coccinia Indica etc. .
Female have 2 homomorphic X-Chromosomes in
their body & produce same kind of egg.
Male possess one X & one Y chromosome (XY). So
two kinds of sperms are produced.
The sex of the embryo depends upon the kind of
sperm.

13. ZO-ZZ SYSTEM
Found in certain moth & butterflies.
Female possess single Z-Chromosomes in its body
hence called ZO (Heterogametic). Produce 2 kinds of
egg.
Male possess two Z-Chromosomes ( referred as ZZ) &
produce 1 type of sperm.
The sex of offspring depends upon the kind of egg.

14. ZW-ZZ SYSTEM
Occur in certain insects & vertebrates like –
amphibians,reptiles, birds & plants –Fragaris
elatior.
Female have one Z and one W chromosome. So it
produce two types of egg.
Male have two homomorphic Z-Chromosomes.
The sex of the offsprings depends upon the kind
of egg.
Z bearing egg produce male & W bearing egg
produce female.

15. GENIC BALANCE IN DROSOPHILA
When geneticist Calvin Bridges, working with Drosophila, crossed a
triploid (3n) female with a normal male, he observed many combinations
of autosomes and sex chromosomes in the offspring.
From his results,Bridges suggested in 1921 that sex in Drosophila is
determined by the balance between (ratio of ) autosomal alleles that
favor maleness and alleles on the X chromosomes That favor
femaleness. He calculated a ratio of X chromosomes to autosomal sets
to see if this ratio would predict the sex of a fly. An autosomal set (A) in
Drosophila consists of one chromosome from each autosomal pair, or
three chromosomes. (An autosomal set in human beings consists of
twenty-two chromosomes.)

17. MALE HAPLOIDY OR HAPLODIPLOIDY
MECHANISM
Particularly common in insects as bees, ants, saw flies & wasps.
In these insects fertilized egg develop into diploid female &
unfertilized ones into haploid male.
Meiosis is normal in females but crossing over and reductional
division fail to occur during spermatogenesis in male due to
haploidy.
Example – see next slide

18. EXAMPLE OF MALE HAPLOIDY OR HAPLODIPLOIDY
MECHANISM
Honey bee Queen (2n=32) can produce two types of egg.
Fertilized egg (Diploid zygote) having 32 chromosome
Unfertilized egg (haploid zygote) having 16 chromosome develops into a
male.
 The diploid zygote can differentiate into either workers (sterile) & queens
(fertile) depending on the diet they consumed during development.

19. SINGLE GENE CONTROL OF SEX
Single genes are found to be responsible for
the determination or expression of sex in
Chlamydomonas, Neurospora, Yeast,
Asparagus, maize, Drosophila.

20. MONOGENIC SEX
DETERMINATION IN MAIZE
#MAIZE is a monoecious plant with male inflorescence
(tassel) and female inflorescence (silk) located on the
same plant.
#A gene called tassel seeds (ts) convert the tassel into
seed bearing inflorescence.
#Another gene silkless(sk) is responsible for absence
of silk .
#A plant with genotype sk/sk will be effectively a male
plant & plant with genotype ts/ts will be effectively a
female plant. So the individual single geners sk/ts can
impose bisexuality in maize.

21. MONOGENIC SEX DETERMINATION IN
DROSOPHILA
In Drosophila a transformer gene (tra) has been recoginsed.
Which when present in homozygous condition (tra/tra) transforms
a female fly into a sterile male. But does not act upon normal male
individual.
Therefore XX female with tra/tra genotype will be sterile male but
XX male with tra/tra genotype will still a normal male.

22. REFERENCES
1. Snustad & Simmons Principles of GENETICS 6th
Edition John Wiley & Sons, Inc 2012
2. Tamarin R.H. Principles of Genetics 7th Edition The
McGraw−Hill Companies, 2001
3. Clugg & Cummings Concepts of Genetics Benzamin
Cummings 2008