This document provides an overview of sex determination in humans and other organisms using the XX-XY system. It discusses that: 1) Sex determination involves the factors that determine whether an individual develops as male or female, while sex differentiation is the actual developmental processes leading to sex organs and secondary sex characteristics. 2) In humans and other mammals, females have two X chromosomes (XX) and are called the homogametic sex, while males have one X and one Y chromosome (XY) and are called the heterogametic sex. 3) Fertilization determines sex, with an X-bearing sperm resulting in an XX female zygote and a Y-bearing sperm resulting in an XY

1. GENETICS ASSIGNMENT
SEX DETERMINATION: XX-XY
Submitted to: Dr. Shefali Raizada
By: Akshatha.S.Ravi
2015RSS85005

2. INTRODUCTION
 Sex determination in most plants and animals is concerned with the study of factors which
are responsible for making an individual male, female and hermaphrodite.
 In the past, mechanisms of sex determination were explained purely on the basis of sex
chromosome, the constitution of which generally differed in male and female.
 In recent years, however a distinction has been made,
a) firstly between sex determination and sex differentiation
b) secondly between the roles played by the chromosome constitution and specific genes
(located on both sex chromosomes and autosomes)in achieving sexual dimorphism.
 SEX DETERMINATION is a mere signal initiating male and female development patterns.
 SEX DIFFERENTIATION involves the actual pathway of events leading to the development of
not only male and female organs but also the secondary sex characters.

3. XX-XYsex determination
 The XX-XY sex determination system was not discovered
until 1905 . Edmund Beecher Wilson and Nettie Stevens
are credited with the discovery.
 XX-XY type of sex determination occurs in insects and mammals including humans.
 Sex gets determined at the time of fertilisation and depends on the type of sperm
that fuses with ovum.
 Females have 2 of the same kind of sex chromosome
(XX) and are called the homogametic sex. Males have two
distinct sex chromosomes (XY) and are called
the heterogametic sex.
 A man's semen contain approximately 50%
of female carrying X sperm cells which is called gynosperms
and 50% male carrying Y sperm cells called androsperms.

4.  If the X carrying ovum is fertilised by a x carrying sperm, the resulting zygote will
have a sex chromosome composition of XX. Such a zygote develops into a female.
 If the X carrying ovum is fertilised by a Y carrying sperm, the resulting zygote will
have a sex chromosome composition of XY. Such a zygote develops into a male.

5.  Although the role of the Y chromosome in mammalian sex determination has been
known since the twentieth century, it was not until 1959 that scientists were able to
identify the region of the Y chromosome that controlled sex determination.
 Subsequent experiments narrowed down this region and was found that the sex
determining region of Y or SRY was the master regulator of sex determination.
 The presence of just this region from the Y chromosome is thus sufficient to cause
male development.

6.  Drosophila melanogaster sex determination differs from mammalian sex
determination in several ways. Sex in Drosophila is determined at the stage of
fertilisation.
 Primarily sex is determined by the ratio of number of X chromosomes to the
number of set of autosome [X:A ratio].

7. THANK YOU