sex Determination in animals

2. SEX DETERMINATION IN ANIMALS
Submitted By:
• Damor Kalpeshkumar M.
• Reg.No:- 04-AGRMA-01983-2019
• Dept. Genetics and Plant Breeding
• C. P. College Of Agriculture, SDAU,
S.K. Nagar, Dantiwada.
Submitted To:
• Dr. N. V. Soni
• Assistant Professor,
• Dept. Genetics and Plant Breeding
• C. P. College Of Agriculture,SDAU,
S.K. Nagar, Dantiwada.

3. INTRODUCTION
 The word SEX is derived from the Latin word sexus meaning separation.
 Sex is the morphological ,physiological & behavioral difference observed among egg
producing organism and sperm producing organism.
Y
X

4. WHAT IS SEX DETERMINATION?
• Sex determination system is a biological
system that determines the development of
sexual characteristics in an organism.
• Most organisms that create offspring using
sexual reproduction have two sexes.
• There are hermaphrodites in place of one or
both sexes.
• There are also some species that have only
one sex due to parthenogenesis (female
reproducing without fertilization.)

5. HISTORY
• 1891 – H.Henking
• 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”.
• First experimental evidence that led to
discovery of sex chromosomes.

6. • 1902 – C. E. McClung
• He 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.
• 1905 – E. B. Wilson & N. Stevens
• Noted that females of Proteron have 7 pairs of
chromosome & male have 6 & an unpaired
chromosome. They called it X – chromosome.
• 1908 – Discovered Y chromosome and so named
by Stevens.

7. PRIMARYAND SECONDARY SEX CHARACTERS
In most animals, male and female individuals differ for many characteristic features, which
may be grouped into two categories:
1. Primary sex characters
2. Secondary sex characters
1.Primary sex characters
• Primary sex characters are related to the gonads or the gamete producing organs of male
and female individuals.
• The development of these characters depends on genes present in the zygote.

8. 2. Secondary Sex Characters
These include all those characters that show consistent differences between male and
female individuals of a species, these include genital ducts, genetalia, and many other body
characteristics, e.g., facial hairs, mammary gland development, pitch of voice, etc. in
humans.
The development of secondary sex character is included by hormones produced by the
somatic elements of gonads.

9. MECHANISM OF SEX DETERMINATION
• The mechanism of sex determination may be classified into the following three main
groups.
1) Environmental sex determination
2) Chromosomal sex determination
3) Genic mechanisms of sex determination

10. 1. ENVIRONMENTAL SEX DETERMINATION
 Sex determination in some organisms such as sea worm (Bonellia and Dinophilus) and
horse tail plant is governed by environmental conditions and also includes some
hormonal effects.
 In Bonellia, the larvae which remain free in the sea water and settle on the sea bottom
are differentiated into females while those larvae which settle on the proboscis of female
develop into tiny males.
Undifferentiated Larvae Free living larvae Develop into female
Larvae attach to proboscis of female Develop into male

11. In some animals,e.g. Turtles, Alligators,
Crocodiles, some Lizards, etc., the temperature
at which eggs are included has a decisive effect
on the sex of animals that hatch from them.
These effects are of the following three types.
1. Only Females at High (30-35°C) and only
Males at Low (23-28 °C) Temperature.
Incubation of eggs at intermediate
temperatures (29-30°C) produces both males
and females. This situation is founding most
species of turtles.

12. 1. Only Wales a High (>30 °C) and Only
Females at Low (<30 °C) Temperatures.
Intermediate temperature (30-33°C) produce
both males and females. This phenomenon is
prevalent in many species of crocodiles and
alligators, and in some species of lizards.
2. Only Females at High (30-35°C) and Low
(25°C) Temperature. At intermediate
temperatures, both males and females are
produced, but at some intermediate
temperatures, only males are produced. This
pattern is known in Australian crocodile,
snapping turtle and some other turtle species.

13. CHROMOSOMAL SEX DETERMINATION
• Sex expression is governed by chromosomes and genes. In unisexual animals,
chromosomes are of two types, viz., autosomes and allosomes.
• Autosomes - Chromosomes which do not differ in morphology and number in male and
female.
• Allosomes or sex chromosomes - Chromosomes which differ in morphology and
number in male and female and contain genes that determine sex.
• Unisexual diploid individuals have two sex chromosomes and rest are autosomes.

14. • The different mechanisms of chromosomal sex determination may be grouped into
five classes:
1. XX Female, XO Male
2. XO Female, XX Male
3. XX Female, XY Male
4. XX Male, XY Female
5. Diploid (2n) female and haploid (n) male.

15. 1. XX FEMALE, XO MALE
• XX-XO is a type of heterogametic male sex determination.
• In this type of sex determination females will posses two X- chromosomes [XX]
there by are known as Homogametic Females.
• Whereas males posses only one X-chromosome and are Heterogametic Males.
• Heterogametic males contain XO chromosomes in which O or zero represents the
absences of another X-chromosomes.
• O will not determine sex of organisms.

16. MaleFemale
Female Male Female Male
XX
XX XX
X
XX X
X
X
X
Parents
Gametes
Possible
fertilizations
Offspring
Sex:
• In some insect orders, the female has two
similar sex chromosomes (XX) while the
male only has one (XO).
• In the sperm produced by males, there is a
50% chance that it will have a sex
chromosome and create a female offspring
when it fertilizes an egg.
• XO sex determination occurs in:
• Grasshoppers
• Aphids
• Honey bees
• Hemiptera (bugs)

17. 2. XO FEMALE, XX MALE
• This system of sex determination is known in a few insects species. E.g., Fumea.
• In such spices, females have only one X chromosome; as a result, they produce two type of
eggs, half having an X chromosome and the other half having none.
• The males on the other hand , have two X chromosome; all the sperms, therefor, have one X
chromosome.
• In fumea, females are the heterogametic and males are the homogametic sex.
• The union of a sperm with an X chromosome containing egg produces an XX zygote such
zygotes develop into males. But fertilization of an egg devoid of an X chromosome with a
sperm gives rise to an XO zygote; such zygote develop into females.

18. MaleFemale
Male Female
XO
XX XO
X
XX
XXParents
Gametes
Possible
fertilizations
Offspring
Sex:
FUMEA

19. 3. XX FEMALE, XY MALE
• In the XY type, sex determination is based on the
presence or absence of the Y chromosome;
without it, an individual will develop into a
female.
• XY sex determination occurs in:
• Mammals (including humans)
• Fruit fly Drosophila
• Some dioecious (separate male and female)
plants such as kiwifruit.
• Females are homogametic with two similar sex
chromosomes(XX). The male has two unlike
chromosomes (XY) and is heterogametic.
• Primary sex characteristics are initiated by genes
on the X. ‘Maleness’ is determined by the Y.
MaleFemale
Parents
Gametes
Possible
fertilizations
Offspring
Sex
:
Female Male Female Male
XX
XX XXXY XY
X
XY
X YX X

20.  Fertilization of egg by a sperm having X chromosome -XX zygote female.
 Union of egg with sperm having Y chromosome –XY zygote -males.

21. • Found in birds, reptiles, some insects, e.g.,
silk worm, etc.
• Females have XY chromosome constitution
- heterogametic sex -half the eggs have X,
rest have Y chromosome.
• Males have two X chromosomes (XX)
homogametic sex -all sperms have one X
chromosome.
• Fertilization of X containing egg with sperm
–XX zygote-male.
• XY zygote -when Y containing egg is
fertilized by a sperm-female.
MaleFemale
Male Female
XY
XX XY
X
XX Y
XX
Parents
Gametes
Possible
fertilizations
Offspring
Sex
4. XY FEMALE, XX MALE

22. 5. DIPLOID FEMALE, HAPLOID MALE
• Found in Hymenoptera(honeybees, ants, termites, etc.)
• An individual’s sex determined by number of sets of chromosomes.
• First demonstrated by Dzierzon in 1850.
• Somatic chromosome no. of females is diploid, males is only haploid.
• When an egg is fertilized by a sperm, the developed organism will be diploid
(2n)- female.
• If egg is not fertilized, organism is haploid(n) - male.

24. Monogenic Sex Determination
 Expression of sex is influenced by a single gene.
 In Drosophila, a transformer gene (tra) which is present on autosomes plays an
important role in sex expression.
 Transformer gene is recessive and hence does not have any effect in
heterozygous condition (Tra/tra) on either sex.
 In homozygous condition (tra/tra), this gene transforms the normal diploid
females into sterile males.

25.  A similar SRY gene is found in human. The gene is probably present on
autosome and transforms the normal male (XY) into female.
 Such males have feminine (female) characters. They have internal
degenerated testes and are sterile. This condition is known as testicular
feminization.
 Similar effects of single recessive gene on expression of sex have been found
in some other animals such as goats, pigs, dogs, etc.

26. Genic Balance System
 This system was developed by Bridges (1922) in Drosoplaila.
 According to this theory “the sex of an individual is determined by a balance between
the genes for maleness and those for femaleness present in the individual,”
 Genic balance theory states that sex determining genes are present on both X
chromosome as well as autosomes.
 The male sex determining genes are present on autosomes and female sex determining
genes on X chromosome.
 The sex expression is determined by the balance of genes on autosomes and X
chromosome.
 In other words, the expression of sex depends on the ratio of X chromosomes to that of
autosomes. This ratio is represented as X/A ratio.

27.  The genic balance is governed by X/A index.
 Individuals with index of 1 develop into female and those with sex index of 0.5 into
male.
 If the sex index is between 1 and 0.5, the resulting individual will be neither a female
nor a male, but have an intermediate sex expression and is called inter sex.
 The sex index of 1.5, which is higher than the sex index of normal female gives rise
to super female.
 A sex index of 0.33, which is lower than the sex index of normal male gives rise to
super male.

28. Sex expression in Drosophila in relation to X/A ratio Sex
Index
Ploidy
Level
Number Of X
chromosomes
(=X)
Number of
Autosomal sets
(=A)
Sex Index
(X/A)
Sex expression
2n 3 2 3/2 = 1.5 Super Female
3n 4 3 4/3 = 1.33 Super Female
4n 4 4 4/4 = 1.0 Female
3n 3 3 3/3 = 1.0 Female
2n 2 2 2/2 = 1.0 Female
4n 3 4 3/4 = 0.75 Intersex
3n 2 3 2/3 = 0.67 Intersex
2n 1 2 1/2 = 0.5 Male
4n 2 4 2/4 = 0.5 Male
3n 1 3 1/3 = 0.33 Super Male

29.  Sex Index = Number of X chromosomes = X/A
Number of Autosomal sets
 Individuals with the sex index of 1.0 are normal females (XX: Diploid
female, XXX: Triploid fly, XXXX: Tetraploid fly)
 Flies having the sex index of 0.5 are normal males (XO diploid flies,
XXOO: Tetraploid fly)

30. EVOLUTION OF X AND Y CHROMOSOME

31. HUMAN Y CHROMOSOME

32. REFERENCES
• Genetics : B. D. Singh- 3rd Edition (Kalyani Publication) page no: (246-
256)