2. What is sex determination?
• The method by which distinction between male and female is
established in an organism under genetic control.
• The sex chromosomes are responsible for determination of
separate sexes.
• It is a biological system that determines the development of
sexual characteristics in an organisms.
Traditionally, the symbol ♂ designates male
and the symbol ♀ designates female.
3. The XX-XY type of chromosomal
sex determinations
• Since the female are XX , as a result of meiosis during the
formation of gametes the egg cell contain only one type of
egg.
• The males consist of XY, and produce two type of sperm. Out
of which half of the sperm consist of X chromosome and the
other half consist of Y chromosome.
• During fertilization, if the sperm containing X chromosomes
fuses with the egg, the zygote XX develops into female.
• While the sperm containing Y chromosomes fuses with an egg
to produce a male with XY chromosomes.
4.
5. • Homogametic: The female is called the homogametic sex
because only one type of gamete(X-bearing) is produced.
• Heterogametic: The male is called as heterogametic sex
because two different gametes(X and Y – bearing) are
produced.
6. • Sex ratio: The ratio of sex at fertilization is expected to be 1:1
because males produce equal number of X bearing and Y
bearing sperm.
7. Sex determination in Various
Organisms
Genotype of female Genotype of male
1. Insects XX XO
2. Drosophila XX XY
3.Birds,fish and
butterflies
WZ WW
4. Humans and other
mammals
XX XY
8. Mechanism of Sex
Determination:
• 1. The X-O system of sex determination operates in insects.
The male have only one X chromosome designated as XO
while the female have two X chromosomes(XX).
• 2. Drosophila melanogaster have four pairs of
chromosomes. The female is XX and the male is XY. The Y is
characteristically J shape.
9. • For both insect and Drosophilsa the ratio of the X
chromosome to the set of autosomes plays a key role in sex
determination.
• 3. The Z-W system is a form of sex determination in birds and
butterflies and some fishes. The genotype of the female is
referred as WZ and WW for male. In this system the male is
homogametic and female is heterogametic.
• 4. The XX/XY system is the most common sex determination
system found in humans and mammals. The presence of Sry
gene on the Y chromosome causes maleness.
10. • Some organisms depends on the environmental factors such
as temperature. Example: American alligator
11. C0NCLUSION:
• After the gametes fuse with each other during fertilization,
what factors determine whether the resulting zygote
developed into male or female? Researchers have studied the
process of sex determination in a wide range of species and
discovered several mechanism of sex determination. They
conclude that the difference in chromosome constitution is a
mechanism for determining sex at the time of fertilization.
12. Reference
• 1.Genetics : Analysis of Genes and Genomes by Daniel Hartl
and Elizabeth Joness
• 2. A Textbook of Biology by Sarita Aggarwal
• 3. Chromosomal Sex Determination in Mammals -NCBI
