Sex Linked Inheritance in Man

1. SEX LINKED
INHERITANCE

2. INTRODUCTION
Reproduction is the production
of alike offsprings by parents. Sexual
reproduction include fusion of
haploid male and female gametes to
form zygote. The chromosomes in the
gametes transfer parental characters
into offspring, this is called as
inheritance.

3. SEXES IN HUMAN
 MALE – Heterogametic and have testes
and produce sperms.
 FEMALE – Homogametic and have
ovary and produce ovum.
 TRANSGENDER – Generally sterile and
may have testes / ovary as primary sex
organs.
As Transgenders are sterile, only
male and female are considered
for inheritance

4. CHROMOSOMES IN HUMAN
 AUTOSOMES – These are 22
homologous pairs of chromosomes
numbered from 1 to 22 and 24 to 45. They
bear genes for somatic or body
characters.
 ALLOSOMES / SEX CHROMOSOMES
– These are one pair of chromosomes
numbered 23 and 46. They bear genes for
sex determination and also some somatic
genes. They may be of two types :- X and
Y. Y chromosome is J-shaped.

5. ALLELES OF A GENE
 Each gene is located at a fixed position on
a chromosome, called LOCUS / LOCI.
 Each gene has two alleles, either both
dominant or both recessive (cis
arrangement) and one dominant and
other recessive (trans arrangement).
 The two alleles are located at same locus,
one on each of homologous pair of
chromosomes.
 As Y chromosome is curved, so X and Y
have some different types of genes.

6. ALLELES OF A GENE
 Out of two alleles
 Dominant allele :- This is the allele which
is expressed in both homozygotic and
heterozygotic conditions.
 Recessive allele :- This is the allele, which
is expressed only in heterozygous
condition.
 Homozygous Condition :- If both the
alleles are dominant or both recessive.
 Heterozygous Condition :- If one of the
alleles is dominant or and other is
recessive.

7. INHERITANCE BY ALLOSOMES
 Generally, X and Y chromosomes
contains genes for sex determination,
but also contain some somatic / body
genes.
 Definition :- The inheritance of somatic
genes located on sex chromosomes by
them is called as sex linked inheritance.
 Inheritance :- It is the transfer of
parental characters or genes by
chromosomes into offspring(s).

8. TYPES OF INHERITANCE BY ALLOSOMES
X – linked / Sex linked / Recessive
inheritance :- Genes on X chromosomes
are called as X – linked / Sex linked /
Recessive genes and their inheritance is
called so. e.g. sickle cell anaemia,
haemophilia, red – green colour
blindness etc.
Y – linked / Holandric inheritance :-
Genes on Y chromosome are called as Y
– linked / Holandric genes and their

9. TYPES OF INHERITANCE BY ALLOSOMES
inheritance is called as Y – linked /
Holandric inheritance. e.g. patterned
baldness, hypertrychosis.
Incomplete sex linked inheritance :-
Genes on both X and Y chromosomes
are called as Incomplete sex linked
genes and their inheritance is called as
Incomplete sex linked inheritance.

10. TYPES AND DOMINANT OF ALLOSOMES
Normal X chromosome :- Contain
normal allele of gene under study.
Affected X chromosome :- Contain
affected or mutant allele of gene under
study.
Normal Y chromosome :- Not contain
any allele of gene under study.
ORDER OF DOMINANCE ( Y > X > Xa)
Y is dominant to Normal and Affected X
Normal X is dominant to Affected X

11. CHARACTERS OF SEX LINKED INHERITANCE
This is inheritance of somatic genes by X
chromosomes.
As it is recessive to Y, it is also called as
recessive inheritance. But in case of males
it is expressed as Y lacks the
complementary genes for the said
inheritance.
Normal X is dominant to affected X.
X chromosome is inherited from father to
daughter to grandson, so it is called cris-
cross inheritance.

12. CRIS CROSS INHERITANCE
FATHER
DAUGHTER
GRAND SON
MOTHER
SON
GRAND
DAUGHTER

13. CHARACTERS OF SEX LINKED INHERITANCE
If female contain two affected X
chromosomes, effect is expressed and she
is called as a victim.
If female contains one normal X and one
affected X, effect is not expressed but
hidden or she is normal in phenotype but
has a tendency to inherit the same to her
son so she is called as a carrier.
Male containing affected X, express the
effect as it has only one X and Y lacks
body genes.

14. EXAMPLES OF SEX LINKED INHERITANCE
Red – green colour blindness :- The victim
of this disease is unable to distinguish
between red and green colours at low
resolution.
Haemophilia / Bleeder’s Disease :- In the
victim of this disease, automatic blood
clotting is impossible due to absence of
Antihaemophilic Factor A, necessary for
blood clotting. So any injury, cut or
menstruation cause severe bleeding and
may lead to death.

15. EXAMPLES OF SEX LINKED INHERITANCE
The genes of above two diseases are
located on X chromosome.
Victim :- a diseased individual.
Normal :- a normal individual.
Carrier :- as these are recessive
inheritance, in case of female having one
normal and one affected X chromosomes,
normal X is expressed. So she is normal in
phenotype but probable to inherit the
affected X to offsprings resulting in victim
sons and victim or carrier daughters.

16. SIGNS USED
PARTICULAR
COLOUR
BLINDNESS
HAEMOPHILIA
NORMAL – Y Y Y
NORMAL - X X X
AFFECTED – X XC Xh
NORMAL MALE X Y X Y
VICTIM MALE XC Y Xh Y
NORMAL FEMALE X X X X
VICTIM FEMALE XC XC Xh Xh
CARRIER FEMALE X XC X Xh

17. CASE STUDIES – COLOUR BLINDNESS
S. N. FATHER MOTHER
1 NORMAL X Y NORMAL X X
2 NORMAL X Y VICTIM XC XC
3 NORMAL X Y CARRIER X XC
4 VICTIM XC Y NORMAL X X
5 VICTIM XC Y VICTIM XC XC
6 VICTIM XC Y CARRIER X XC

18. RESULT OF F1– COLOUR BLINDNESS
S. N. FATHER MOTHER F1 PHENOTYPE
1 NORMAL NORMAL ALL NORMAL
2 NORMAL VICTIM S – VICTIM, D - CARRIER
3 NORMAL CARRIER
S – NORMAL / VICTIM
D – NORMAL / CARRIER
4 VICTIM NORMAL S – NORMAL, D – CARRIER
5 VICTIM VICTIM ALL VICTIM
6 VICTIM CARRIER
S – NORMAL
D – CARRIER / VICTIM
** S = SON, D = DAUGHTER

19. CASE STUDIES – HAEMOPHILIA
S. N. FATHER MOTHER
1 NORMAL X Y NORMAL X X
2 NORMAL X Y CARRIER X Xh
3 VICTIM Xh Y NORMAL X X
4 VICTIM Xh Y CARRIER X Xh
IN HAEMOPHILIA :- VICTIM MOTHER CAN NOT BE
TAKEN AS SHE WILL DIE AT HER FIRST
MENSTRUATION DUE TO SEVERE BLEEDING.

20. RESULT OF F1– HAEMOPHILIA
S. N. FATHER MOTHER F1 PHENOTYPE
1 NORMAL NORMAL ALL NORMAL
2 NORMAL CARRIER
S – NORMAL / VICTIM
D – NORMAL / CARRIER
3 VICTIM NORMAL S – NORMAL, D – CARRIER
4 VICTIM CARRIER
S – NORMAL
D – CARRIER / VICTIM
** S = SON, D = DAUGHTER
BOTH FATHER & MOTHER NORMAL CASES SHOULD
BE AVIDED TO DESCRIBE DISEASE INHERITANCE.

21. VICTIM FATHER CARRIER MOTHER
PARENTS
GAMETES
F 1 GENERATION
1 X XC
2 XC XC
3 X Y 4 XC Y
XC
Y
1 – CARRIER DAUGHTER
2 – VICTIM DAUGHTER
3 – NORMAL SON
4 – VICTIM SON
X
XC

22. CONCLUSION
In above crosses I have described that, the
probable phenotype and genotype of offspring (son /
daughter) may be told. Here percentage should not
be used as we are telling the type of offspring going to
be born during the study.
In above case study with victim father and
carrier mother, one daughter was victim and one was
carrier and one son was normal and other victim. It
does not mean that 50% of daughter will be carrier
and 50% will be victim or 50% of sons normal and
50% son victim rather it means the offspring going to
be born may have any one of the possible genotype
and phenotype depending on its sex.