The sexually reproducing organisms are classified into two types such as monoecious (hermaphrodite) and dioecious. In monoecious organisms, both male and female gametes (sex cells) are produced by a single individual. The organisms in which both male and female gametes are produced by different individuals are called dioecious. Living organisms, with a very few exceptions, are differentiated into male and female individuals. The sexes of the individuals are genetically determined. The biological system that determines the development of sexual characteristics in an organism is called sex determination. There are two different systems of sex determination- Chromosomal sex determination and Non-genetic sex determination.

1. Sem. V, Paper X
Genetics
Dr. Seema A. Gaikwad
Dept. of Botany
Vidnyan Mahavidyalaya, Sangola

2. Unit 3
Sex-determination and Sex-linked Inheritance
Sex determination is the natural event by which an
individual of a dioecious species becomes male or
female. The mechanism by which sex is established is
termed sex determination. We define the sex of an
individual organism in reference to its phenotype.
Sometimes an individual organism has chromosomes
or genes that are normally associated with one sex but
a morphology corresponding to the opposite sex.

3. Unit 3- Mechanism of Sex-determination
Sexual Reproduction is the formation of offspring that are
genetically distinct from their parents; most often, two
parents contribute to their offspring and the genes are
assorted into new combinations through meiosis.
Among most eukaryotes, sexual reproduction consists of
two processes that lead to an alternation of haploid and
diploid cells; meiosis produces haploid gametes(spores in
plants), and fertilization produces diploid zygotes.

4. Unit 3
Mechanism of Sex-determination
The term Sex refers to sexual phenotype. Most
organisms have only two sexual phenotypes:
male and female. The fundamental difference
between males and females is gamete size:
males produce small gametes; female produces
relatively large gametes.

5. Unit 3
Mechanism of Sex-determination
Sex determination mechanism include-
A. Chromosomal determination mechanism
B. Genetically controlled sex determination
C. Hormonally controlled sex determination
D. Environmentally controlled sex determination

6. Unit 3 - Sex-determination and Sex-linked Inheritance
• Chromosomal Sex determination: Sex determination is associated
with sex chromosomes that are different between male and female
individuals. Many species have chromosomal sex- determining
systems. As Stevens and Wilson found for insects, sex in many
organisms is determined by a pair of chromosomes, the sex
chromosomes, which differ between males and female.
Chromosomal determination is found in,
• XX/XO mechanism
• XX/XY mechanism
• ZW/ZZ mechanism

7. Unit 3
Mechanism of Sex-determination
• Genetic sex determination: Sex is determined at
fertilization by the combination of genes that the zygote
receives.
• Hormonal sex determination: It refers to the extent of the
body coordination system and its effects.
• Environmental sex determination: In some species, sex is
determined after fertilization by environmental factors like
temperature, population size, or sex of others.

8. Autosomes and sex chromosomes
• During the cell division, chromatin in the nucleus shrinks to
a thread like structures named chromosomes. Two major
types of chromosomes can be found in eukaryotic cells.
They are autosomes and sex chromosomes. Humans have
22 homologous pairs of autosomes and one pair of sex
chromosomes. The main difference between autosomes
and sex chromosomes is that autosomes are involved in
determining the somatic characters of an individual and
sex chromosomes are involved in determining the sex
and the sex-related hormonal traits.

9. Autosomes and sex chromosomes
What are Autosomes?: Non-sex chromosomes which determine the trait of
an organism is identified as autosomes. They are also known as somatic
chromosomes since they determine the somatic characters of an individual.
A genome mainly consists of autosomes. For example, human body
contains 46 chromosomes within its genome and 44 chromosomes of them
are autosomes. Autosomes exist as homologous pairs and 22 autosome pairs
can be identified in the human genome.
Both autosomal chromosomes contain the same genes, which are arranged
in the same order. But an autosomal chromosome pair differs from other
autosomal chromosome pairs within the same genome. These pairs are
labeled from 1 to 22, according to the base pair sizes contained in each
chromosome.

10. Autosomes and sex chromosomes
What are Sex Chromosomes?
Sex chromosomes are referred to as allosomes. They determine the sex
of an individual. The sex determination also happens in most animals
and many plants. Humans have only 2 sex chromosomes in their genome
which are labeled as X chromosome and Y chromosome. A female
individual is determined by XX and a male individual is determined by
XY. A female contains the same two copies of sex determining genes
arranged in the same order in both X chromosomes (homomorphic).
Therefore the sex chromosomes in a female are homologous to each
other. In male’s, the two sex chromosomes contain different genes
(heteromorphic).

11. Autosomes and sex chromosomes
Sr.
No.
Autosomes Sex Chromosomes
1 Most of the chromosomes
within a genome are autosomes
A few of the chromosomes within a
genome are sex chromosomes
2 The 22 pairs of autosomes are
homologous in humans
Female sex chromosomes (XX) are
homologous (homomorphic) while male
sex chromosomes (XY) are non-
homologous (heteromorphic)
3 Since autosomes are
homomorphic, the position of
the centromere is identical
Since the male sex chromosomes are
heteromorphic, the position of the
centromere is not identical. The position
of the centromere in female sex
chromosomes is identical
4 Autosomes contain the number
of genes varying from 200 to
2000. Chromosome 1 which is
the largest, carries about 2800
genes in humans
X chromosome contains more than 300
genes while Y chromosome contains
only a few genes since it is small in size
5 Autosomal disorders show
Mendelian inheritance
Sex-linked disorders show Non-
Mendelian inheritance
Difference Between Autosomes and Sex Chromosomes:

12. Autosomes and sex chromosomes

13. Sex chromosomes in Drosophila
The fruit fly. Drosophila melanogaster has eight
chromosomes. These chromosomes are present in the form of
four homologous pairs. T.H. Morgan in 1911 found difference
in the chromosomes of male and female Drosophila. The
chromosomes of the three homologous pairs were similar in
both of the sexes. But the fourth was heteromorphic pair and
it had different structures. The female has two similar rod
shaped X-chromosomes in the fourth pair. But male has one
rod shaped X-chromosome but the other a morphologically
different. J-shaped Y chromosome in the pair.

14. Sex chromosomes in Drosophila
The genome of D. melanogaster contains four pairs of chromosomes – an
X/Y pair, and three autosomes labeled 2, 3, and 4. The fourth chromosome
is relatively very small and therefore often ignored, aside from its
important eyeless gene.

15. Sex chromosomes in man
Human body cells have 46 chromosomes: 22 homologous pairs of autosomes
plus 2 sex chromosomes. In females, there is a pair of identical sex
chromosomes called the X chromosomes. In males, there is a non-identical
pair, consisting of one X and one Y. The Y chromosome is considerably
shorter than the X. At meiosis in females, the two X chromosomes pair and
segregate like autosomes so that each egg receives one X chromosome. Hence
the female is said to be the homogametic sex. At meiosis in males, the X and
the Y pair over a short region, which ensures that the X and Y separate so that
half the sperm cells receive X and the other half receive Y. Therefore the male
is called the heterogametic sex.

16. Sex chromosomes in man
The human sex determination mechanism to a larger extent
resembles XX - XY type of Drosophila. However, the Y chromosome
contains male determining genes and it is the determiner of fertility and
sex of male individual. Thus in human beings, the presence of Y
chromosome determines maleness and its absence determines
femaleness.

17. Balance concept of sex determination in Drosophila- Bridge’s Experiment

18. Balance concept of sex determination in Drosophila- Bridge’s Experiment

19. Sex linked inheritance in man
• Sex linked inheritance is the inheritance of the genes present on the sex
chromosomes like X or Y or both XY, due to which some traits are transmitted to
the next generation. This transmitted trait may be expressed phenotypically in one or
both the sexes and sometimes in some generations only. During the inheritance of
genes sex chromosomes carry not only the gene related to sex determination of the
organism but also the genes related to some other characteristics. When any
mutation occurs in this gene it leads to genetic disorders and are transmitted. Based
on the linking and transmission of the genes sex-linked inheritance is classified into
three types as
• I) X-Linked inheritance
• II)Y-Linked inheritance
• III)XY-Linked inheritance

20. X- linked inheritance
• This is the inheritance where the genes are carried from one
generation to the next generation through X chromosome. In humans
approximately fifty X – linked disorders have been identified.

21. Haemophilia

22. Haemophilia
• Haemophilia is an X – linked recessive disorder. It is of two types
Haemophilia-A and Haemophilia -B. they are caused by lethal
genes
• 1)Haemophilia – A is also known as royal disease as it was first
identified in royal family of Queen Victoria and some other
families. This is due to lack of blood protein called clotting factor
VIII (AHG- anti haemophilic globulin).
• 2)Haemophilia - B is also known as bleeder’s disease because the
person suffering with this disease has an inability of their blood to
clot normally. This is due to the lack of blood protein called
clotting factor IX (Christmas factor).

23. Haemophilia
• In the inheritance pattern if the female is possessing one recessive X
chromosome for haemophilia (XH) she will not be affected by the
disease but she will be a carrier to the disease. She will be affected
only when she possess both X recessive chromosomes (XHXH). in
case of males if he possess one X recessive chromosome for
haemophilia (XH) he will be affected by the disease. He passes this
character to his grandson through his daughter
• If the mother is normal and the father is haemophilic then all the
daughter will be carriers and all the sons will be normal

24. Colour blindness
• Colour blindness is another common X-linked recessive disease in which the people
have a defect in the cones of retina. Lack of colourable pigment in retinal cones is
known as Deuteranopia or deutan colour blindness. Lack of erythrolable pigment for
differentiating the red and green is known as Protanopia or protan colour blindness.
Colour blindness is detected by Ishihara’s test introduced by Dr. Shinobu Ishihara
The Ishihara Colour Test

25. Colour blindness
Red-Green Colour Blindness
Red-green colour blindness is a genetic disorder whereby an individual
fail to discriminate between red and green hues
 This condition is caused by a mutation to the red or green retinal
photoreceptors, which are located on the X chromosome
 Red-green colour blindness can be diagnosed using the Ishihara
colour test

26. Y- linked inheritance
• Genes that are present on the non-homologous region of the Y
chromosome and are inherited from father to the grandson through
son is known as Y-linked inheritance.

27. Holandric genes
This pattern of inheritance is also known as Holandric Inheritance and
the genes responsible for this inheritance are called Holandric genes. In
humans holandric inheritance is signified by disease called
Hypertrichosis or Ichthyosis hystrix gravis hypertrycosis. The disease is
identified by the presence of excess hair on the body completely or
restricted to a particular region like external pinna.

28. Holandric genes