This theory was proposed by a Drosophilist Calvin Blackman Bridges in 1921. This theory fully explains the sex determination in Drosophila. This theory shows that the Maleness - Autosomes Femaleness -X chromosomes.

1. Submitted by
SHIVASHINI R
ID NO.: 2017021065
Course Teacher
Dr. M. Kanimoli Mathivathana
Assistant Professor
Department of Plant Breeding & Genetics
COLLEGE OF AGRICULTURAL TECHNOLOGY
(Affiliated to Tamil Nadu Agricultural University,Coimbatore-3)
(Accredited by Indian Council of Agricultural Research, New Delhi)
Kullapuram (Po), Via Vaigai Dam, Theni - 625 562.

2. This theory was proposed by a
Drosophilist Calvin Blackman Bridges in
1921.
This theory fully explains
the sex determination in
Drosophila.
This theory shows that the
Maleness Autosomes
Femaleness X chromosomes.

3. Sex is determined by the composition of
chromosomes in the zygote.
If the sex is determined by the SEX
chromosome, Chromosomal
determination of sex.
If the sex is by the AUTOSOMES,
Genetic determination of sex
Genic Balance theory – Balance
between them.

4. They have rapid reproductive rate.
i.e., many offspring at a time.
Contains only 4 pairs of Homologous
chromosomes.
Easy & inexpensive to maintain.
They are tiny.
Easily examine the
changes (Phenotypically
or Genotypically)

5. Fruit flies are
Diploid organisms
Homologous pairs
of chromosomes
For a simple phenotypic
trait, there’ll be 2 copies
of gene
i.e., one on
each
chromosome

6. In 1916, Bridges discovered XXY females & XO
males in Drosophila while studying the
inheritance of vermilion eye gene located in the X
chromosome.
He showed that
XX Femaleness
XY Maleness
Y chromosome didn’t play a role in sex
determination.
BUT Y chromosome MALE FERTILITY
Without Y, males are Sterile.

7. It states that sex of an individual
is determined by a ratio called
sex index.
Sex Index = No. of X chromosomes = X
Autosomal sets A
Autosomal sets diploid or triploid or
tetraploid.

8. Sex of that individual
Greater than 1 Super females/Meta females
Equal to 1 Normal Females
Between 1 & 0.5 Intersex
Equal to 0.5 Normal males
Less than 0.5 Super males/Meta males

9. Sxl gene - Sex lethal gene
Tra gene 1 & 2 -Transformer genes
Dsx gene - Double sex gene
Ix gene - Intersex gene
Fru gene - Fruitless gene

11. Sex lethal gene: Transformer gene 1&2:
Master regulatory gene
controlling sex
differentiation.
X- linked gene.
Present in both male &
female.
Causes Lethality when
Loss of function Female
Gain of function Male
Sxl must be active in
FEMALE.
Tra 1& 2 are
Feminizing genes.
Tra 1 is present both in
male & female.
Tra 1 is active in
FEMALES.
But Tra 2 is absent in
Males.

12. Double sex gene:
Dsx gene present in both
male & female.
Dsx gene suppresses the
opposite sex trait in an
individual.
Dsx(F) gene present in
female, suppresses the male
character.
Dsx(M) gene present in
male, suppresses the female
character.

13. Intersex gene: Fruitless gene:
Ix is the terminally
positioned gene.
Ix gene functions only
with the Dsx(F) gene
to implement Female
sex differentiation.
Fru gene is required for
courtship in males.
Fru gene is functions only
with Dsx(M) gene.

16. These are the organisms that contain both female &
male parts in a single individual.
Gynandromorphs derived from the Greek words.
Gyne = Woman
Andro = Man
Morph = Form
These gynanders
are sterile.
Have less life period than normal ones.

18. Lobster Bilateral Gynandromorph Butterfly Bilateral Gynandromorph

19. Sex piebalds in Lobster

20. Gynanders in Drosophila are the
mosaics (an organism composed of cells
of 2 genetically different types) for X
chromosome.
Part with Female phenotype XX
Part with Male phenotype XO
Gynandromorphs arise from the XX
zygotes.

21. Mitotic chromosomal loss (X) Female losing one x chromosome during first mitotic
division after fertilization

22. Bilateral gynandromorph in
drosophila

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25. Thank u so much!