This presentation intends to explore the sex-linked characters along with some fatal diseases of human beings, their cause, consequences and other issues.

1. SEX LINKAGE : UNIQUE
HERITAGE

2. PRESENTED BY
Dr. N. Sannigrahi, Associate Professor
Department of Botany
Nistarini College, Purulia (W.B) India

3. The study of genetics is not only fascinating but also it explores
a new world of human health and hygienic attributes and these
may become a great concern to all of us. The fragility of society
is not only the measurement of the economic affairs bit also it
deserve the understanding of the health index of the society. We
very often suffer form some issues and it apparently do not
convey any cause and consequences. But if the scientific
investigations are done, reasons appeared and proper safeguards
can ensure the degree of the damage and sufferings in the
forthcoming generations. Hemophilia, Red-green color
blindness. Anhydrotic ectoderma, Night blindness, Myopia,
Juvenile glaucoma, Inhthyosis hystrix gravis, Xeroderrma
pigmentosa, Retinitis pigmentosa, baldness and its pattern,
muscular dystrophy etc are some of the concern of great medical
importance and its understanding is solicited with the
enlightenment of sex linked inheritance.

5. INTRODUCTION
Have you heard about bleeder’s disease which is biologically
known as hemophilia? The unfortunate phenomenon where the
coagulation of the blood does not take place due to some reason
of the absence of the blood coagulating agent. Do you aware
about the fate of some royal families across the globe like in
Russia, England, Sweden and elsewhere? The answer lies in the
understanding of the unfortunate incidents of some hidden
genetically traits. Boldness is very common but it becomes a
little bit of inferiority beauty complex mostly in cases of the
male counterpart. Syndrome like Down’s , Klinefelter’s are very
common. The beauty lies in the biology of the sex determination
and the balance of the sex ration is being maintained by the
equal distribution of sex chromosome during the fertilization
process. More number of incidents can increase your appetite to
have the pleasure of the magic of the reality of sciences. The
understanding lies in the exposure of the beauty of sex
determination.

6. SEX LINKAGE
Almost in all the cases. The F1 & F2 generations from
reciprocal crosses yield identical results and it does not matter if
the female or male parent had the recessive character. Further,
both males and females in the progeny show identical ratios. But
this general rule becomes exception in case of the interesting
genetically phenomenon commonly known as sex linkage. This
kind of genetic feature is produced if the characters undergone
consideration is associated with the sex of the individual. Sex
linkage is the consequence of a gene being located in the X or
sex chromosome. The human sex determination is mainly of two
types-male as heterogametic having XY and the homogametic
female having XX chromosomes. The Y chromosome in male is
almost bears seldom genes for the inheritance of any characters.
So, the credit or discredit for the undesired genetically trait goes
in favor of the X chromosomes.

7. Sex linked genes may be grouped into three types-
lethal, Semi lethal and non-lethal.
The sex linked genes cause the death of the individual from zygote
to sexually mature adult stage is termed as lethal.
When the sex linked gene can cause death of some individuals but
the individual could survive and get maturity and reproduce are
termed as semi-lethal.
Non-lethal does not harm the individual in course of the life cycle.
As far as the nature of the distribution of the genes on sex
chromosome, sex linked genes could be classified into three types-
X linked-Genes localized on the homologous sections of X
chromosome and that have no alleles on the Y chromosome,
Y-linked- These genes are localized on the sections of Y
chromosome and are commonly called holandric genes.
X-Y linked-Sex linked inheritance performed by those genes which
are localized in homologous sections of X & Y chromosomes.

8. CHARACTERISTICS OF SEX-LINKED INHERITANCE
Sex linked traits are characterized generally by the following
features-
The frequency of individuals showing a recessive sex linked traits
is markedly higher in the heterogametic sex as in male drosophila
and humans along with female birds than that of the homogametic
sex like female Drosophila & human and male birds.
Ordinarily, genes governing sex-linked traits an man and
Drosophila are not transmitted from male parents directly to their
male progeny. White eye gene (w) is not transmitted from male
Drosophila to its male progeny.
A male Drosophila or man transmits its sex linked genes to all
daughters and the daughters transmit these genes to their male
children . Thus, a sex-linked gene passes from male to female then
back to male ; such kind of inheritance pattern is called criss-cross
inheritance pattern in genetics.

9. Sex linked genes are not present in Y-chromosome.
Consequently, the heterogametic sex as in human male and
Drosophila along with female birds as stated earlier is
hemizygous for such genes. It has only one allele of sex-linked
genes and this always become functional as in male either
dominant or recessive irrespective its nature of .
It has been observed that a region of the Y chromosome is
homologous to a region in the X chromosome. For example, the
long arm of the human Y chromosome is homologous to the short
arm of the X chromosome. Similarly one third of the Drosophila
X chromosome is homologous to a region of the Y chromosome.
Genes located in such regions of X chromosome do not show sex
linked inheritance.
Thus, the above characteristics features are considered for the
understanding and analysis of the inheritance of sex linked
characters of the higher organisms.

10. SEX LINKAGE IN Drosophila
Drosophila sp in the animal world possess a significant result as
far as sex linked inheritance pattern is concerned. Hare are some
examples to explore the beauty of biology.
Inheritance of X-linked gene for eye color in Drosophila sp.
In Drosophila, the gene for eye color is X –linked and recessive
to another X-linked dominant gene for red color. It was
discovered by Morgan ( 1910) and explored by the criss-cross
inheritance pattern.
w= white eye color (recessive)
W= Red color (dominant)
a.Red eyed female X white eyed male
From the cross, the 1st generations flies are red eyed in both
sexes. When these bred together, white reappears in quarter of the
F2 offspring indicating that red and white eye colors due to an
allelic pair of genes which red acts as the dominant. However, of
the F2 offspring all the females are red, whereas half the males

11. are red and half are white. The females are two kinds genotypic
ally. Half of them give nothing but the red offspring, half must carry
the recessive white, for in their offspring and half the males are
white eyed. So, in Drosophila, sex linked traits such as eye color
follow a criss cross inheritance. The male (grand-father) transmits
his sex linked traits to his grand sons through his daughters , never
or through his sons.
b. Red eyed male X white eyed female: When a red eyed male is
bred to a white eyed female, quite different result is obtained.
Among the F1 offspring, all the females are red-eyed and all the
males are white eyed. When these red bred together, their F2
offspring consist of red eyed and white eyed individuals in equal,
numbers in both the sexes.
Thus, from the above observation, it has become crystal clear that
the females play a very crucial role as far as the inheritance of the
characters which are sex linked in general and X linked in
particular.

13. X-Y linked inheritance:
The sex-linked genes contained in the X-chromosome of
drosophila have no alleles in the Y-chromosome. This is the
reason why a male that carries a single dose of a recessive sex-
linked gene shows its effects in the phenotype. The Y-
chromosome is therefore genetically inert or empty. One gene
bobbed , however , has alleles both in the X-chromosome and in
the Y-chromosome. The recessive mutant allele is bobbed , when
present in both X-chromosomes of a female., causes bristles on
the body of the fly to the shorter and slender than the normal. A
male carrying bobbed in the X-chromosome but normal allele in
the gene in the Y-has normal bristles. If such a male is crossed
with bobbed females, all the sons have normal bristles and all the
daughters are bobbed. The normal bristles are transmitted from
father to sons.

14. PRIMARY NON-DISJUNCTION OF X CHROMOSOME
Non-disjunction is a genetically irregularities when the
homologous chromosomes fain to pair during the meiotic cell
division. This kind of exceptional things happened in X-
chromosome in Drosophila that was by Bridges (1916). Normally,
the white –eyed Drosophila females crossed with red-eyed males
produce red eyed daughters and white eyed sons in the F1
generation. However, there are some exceptions in F1 offspring has
the unexpected eye color , red in males and white in females. This
appearance contradicts the Morgan’s assumption. Bridges solved
this problem as follows.
The white eyed exceptional females must carry two X
chromosomes , since their sex is female and they must have
inherited both their X chromosome from their mothers , since the
color of their eyes is white. Similarly, the red eyed exceptional
males must have one X chromosome which must have come from
their father since their eyes are red.

15. W=white eye (recessive), W= red eye (dominant)
Bridges supposed that the two x chromosomes present in a female
may occasionally fail to disjoin at the meiotic division as they
would normally do. This primary non-disjunction leads to
production of eggs with two X chromosomes and eggs with no X
chromosomes.
Such exceptional gametes (eggs) of white females would be
fertilized by the normal spermatozoa of red eyed males. This
fertilization would possible following 6 combinations –
i. White eyed females ( XX)
ii. Red eyed males (XY),
iii. With three X chromosomes red eyed super females, usually
dies
iv. With two X chromosomes and a Y chromosomes, white female,
v. With only one X-chromosome red female
vi. With only one Y-chromosome with no X chromosomes , not
viable , dies

16. SECONDARY NON-DISJUNCTION OF X CHROMOSOME
The exceptional males without Y-chromosomes are sterile but
they are quite normal male in their appearance and behavior. The
white eyed XXY females are normal and fertile .Bridges crossed
them to normal red-eyed males and observed their offspring
which is called secondary non-disjunction. In their progeny, 96%
of their daughters have red eyes and 4% have white eyes. Among
the sons, 96% are white eyed and 4% are red.
During meiosis in XXY females, the two X chromosomes disjoin
in about 92% of the oocytes but pass together either in the polar
body or in the egg nucleus in about 8% of the oocytes giving four
kind of eggs.
i. With a single X chromosome,
ii. With an X & Y chromosome,
iii. With two X chromosome,
iv. With a Y chromosome.

17. During the fertilization by normal X or Y containing spermatozoa,
8 kinds of zygotes are formed but the zygotes will not occur in
equal frequencies but offer following kind of outcomes as stated
below:
i. All the white eyed females and some of the red eyed females
must carry not only two X chromosomes but also a Y
chromosomes,
ii. The red eyed males in contrast to those arising from primary
non-disjunction most have a Y chromosome and as par natural
become fertile,
iii. Some of the white eyed males must have one X and two Y
chromosomes.
iv. This observation also hold good the chromosome theory of
inheritance.
In addition n to these, attached X chromosome in drosophila
along with the position effect variegation (PEV) in Droso[phila
also reflects interesting genetic outcomes.

18. SEX LINKED INHERITANCE IN MAN
Man is the measure of all things” irrespective of its nature and as
far as the biological research is concerned, the human genetically
behavior is always a pleasure to the biologist. In man, more than 50
sex-linked genes have been reported . The most important and
common X-linked genes are-
i. Red-green color blindness,
ii. Hemophilia,
iii. Anhydrotic ectoderma (non-functional sweat glands)
iv. Night blindness
v. Myopia (short sightedness)
vi. Juvenile glaucoma ( hardness of eyeball)
vii. White forked lock etc.
viii.These are associated with X-linked recessive genes and are
most common. Certain X-linked diseases such as defective
tooth enamel having wearing down of teeth early is caused by
dominant genes.

19. Case I: COLOR BLINDNESS
Do you have the problem to distinguish between red and green
color ? Do you suffer from the stigma of color blindness? If so,
your color sensitive retinal cells are being recessive one and
cause this kind of serious health hazard. The X-linked recessive
gene is able to express itself in males due to absence its any
allelic gene in the Y chromosome. The pattern of the inheritance
can be observed in the following two types marriage:
i. Marriage between color blind man with normal visional
women
ii. Marriage between normal visioned man and color blind
female. But the second one chance seldom has occurrence
due to social structure.
The first consequence can be studied by the following
genetically observation.
C= colorblind gene (recessive)
+= Normal vision (dominant)

21. When a color blind man marries with a normal visioned woman,
then they produce normal visioned male and female individuals in
F1. The marriage between a F1 normal visioned woman and a
normal visioned male, will produce in F2, two normal visioned
females and a normal visioned male and one color blind male. This
kind of inheritance pattern is called criss-cross inheritance pattern.
But when color blind woman marries a normal man, they produce
normal visioned female individuals in F1. The marriage between
F1 normal visioned woman and colorblind male because male
receives an X-linked recessive gene for color blindness from color
blind mother . The daughter receives an X –linked dominant gene
for normal vision from father and one X-linked recessive gene for
color blindness from the mother. If the daughter again married to a
color blind male then they produce a colorblind homozygous
daughter, a normal visioned daughter , a normal homozygous son
and a homozygous color blind son.

23. Another important disease is hemophilia and it has been most
common in royal families of Russia and England. Queen Victoria
and her further generations became the victims of the same and if it
did not happen both in Russia & England, power politics across the
globe might be different .
INHERITANCE OF Y-LINKED GENES IN MAN
Genes in the non-homologous region of Y chromosome pass directly
from male to male. In man, Y linked on holandric genes such as
ichthyosis hystrix gravis , hypertrichosis (excessive development of
hair on pinna of ear) and they are directly transmitted from the father
to the sons almost in all the cases.
SEX LINKED LETHALS IN MAN
Certain sex-linked lethal genes are lethal and may cause death from
egg up to sexually mature adult stage. The sex linked lethal of man
are genes for hemophilia, pseudo hypertrophic muscular dystrophy
etc and these are major concern of the inheritable genetic disorders
of human.

24. INHERITANCE OF X-Y LINKED GENES IN MAN
The genes which occur in the homologous section of X and Y
chromosomes have inheritance like that of autosomal genes. The X-
Y linked genes are partially or incompletely sex linked, because
sometimes, crossing over may occur in the homologous section of
X and Y chromosomes. In human, several diseases are X-Y linked.
Certain X-Y linked genes of man are total color blindness,
Xeroderma pigmentosum, Epiper-molysis bullosa, Retinitis
pigmentosa, spastic paraplegia etc.
In addition to those aforesaid ideas, some autosomes expression are
also influenced by the sex chromosomes called sex influenced gene,
however, the pattern of expression differs based on sex. The pattern
of baldness, forelock, absence of upper lateral incisor teeth, a type
of the enlargement of terminal joint finger, harelip, fissure in upper
lip, cleft palate are some of the instances deserve mentioning.

25. CONCLUSION
After having the exposure of the reality of the biological so called
paradox, it has become quiet crystal clear about the occurrence of
some genetic defects and this can be forecasted through the proper
analysis of the pedigree of the incumbents. In addition to these,
the telomere regions called pseudoautosomal regions ( PAR)-
PAR I & PAR II are also important and the genes in X
chromosomes are also important as because the inheritance
pattern of the genes of these regions resembles the autosomal
genes . This is called partial sex linkage and the regions involved
is called pseudoautosomal regions. In partial sex linkage, the gene
located in X chromosome shows autosomal inheritance pattern.
Thus, it has been quite clear that the foundation of the Bridges
experiment open a new world of biomedical research and the sex
linked inheritance pattern undoubtedly a new world that needs
huge research for further exploration. You , the young and
talented students may take your pleasure of research to contribute
your inquisitiveness for the best of human welfare.

27. References:
1. Google for images,
2. Different WebPages for content,
3. Principles of Genetics- Basu & Hossain,
4. A textbook of Botany (Vol III) Ghosh, Bhattacharya, Hait
5. Fundamentals of Genetics- B.D. Singh,
6.A Textbook of genetics- Ajoy Paul
DISCLAIMER:
This presentation has been made to enrich open source of
information without any financial interest. The presenter
acknowledges Google for images and other open sources of
knowledge to develop this PPT.