Mendel’s laws are not applicable on those genes which
are exclusively located either on X or Y gene chromosome. It has been observed
that the genes only in X chromosomes are represented twice in females and once
in male (because male have only one X-chromosomes). The sex linked inheritance
can be classified into three types in XY type organisms.
1.
X-linked:
here the linked genes are exclusively located on the non-homologous sections of
X-chromosome, and do not have any corresponding allele on the Y-chromosome.
These are called commonly as X-linked genes.
2.
Y-linked:
the Y-Linked genes are localized on the non-homologous sections of
Y-chromosome, and have no corresponding allele on the X-chromosome. The
Y-linked genes are commonly known as Holandric genes.
3.
XY-linked:
the XY linked genes are located on the homologous sections of X and Y
chromosomes.
In human beings more than 150 confirmed or highly
probable X-linked traits are known; most of these are recessives. Certain well
known example of X-linked recessive genes in humares are those for red-green
colour blindness and haemophilia.
Inheritance of X-linked genes:
1.
these genes
whether dominant or recessive show there effects in male phenotypes.
2.
It shows
criss-cross inheritance pattern (X-linked recessive genes are transmitted from
P1 male to F1 female and from F1 female to F2 male)
3.
Usually
none of the offspring of the affected male will be affected, but all of his
daughters will carry the gene in masked heterozygous conditions.
Example: colour blindness, haemophilia.
Inheritance of Y-linked genes: the genes linked to Y-chromosomes are also called
holandric genes. They are always transmitted from father to son.
Ex: the gene responsible for icthyosis hystrix gravis hypertrichosis (excessive development of pinnae hair) are transmitted from father to son only, because only the male posses the Y-Chromosomes.
INHERITENCE OF X-Y-LINKED GENES: the genes which occur in homologous sections of X and
Y chromosomes have inheritance like an autosomal genes. .
