Sex
Linked Inheritance
The inheritance of a
trait that is determined by a gene located on one of the sex chromosomes is
called sex linked inheritance. Genes present on the differential region of X or
Y chromosomes are called sex linked genes. The genes present in the
differential region of “X” chromosome are called X linked genes. The X–linked
genes have no corresponding alleles in the Y chromosome. The genes present in
the differential region of Y chromosome are called Y- linked or holandric
genes. The Y linked genes have no corresponding allele in X
chromosome. The Y linked genes inherit along with Y chromosome and they
phenotypically express only in the male sex. Sex linked inherited traits are
more common in males than females because, males are hemizygous and therefore
express the trait when they inherit one mutant allele.
The X – linked and Y –
linked genes in the differential region (non–homologus region) do not undergo
pairing or crossing over during meiosis. The inheritance of X or Y linked genes
is called sex-linked inheritance.
Red-green colour
blindness or daltonism, haemophilia and Duchenne’s muscular dystrophy are
examples of X-linked gene inheritance in humans.
Haemophilia is commonly
known as bleeder’s disease, which is more common in men than women. This
hereditary disease was first reported by John Cotto in 1803. Haemophilia is
caused by a recessive X-linked gene. A person with a recessive gene for
haemophilia lacks a normal clotting substance (thromboplastin) in blood, hence
minor injuries cause continuous bleeding, leading to death. The females are
carriers of the disease and would transmit the disease to 50% of their sons
even if the male parent is normal. Haemophilia follows the characteristic criss
- cross pattern of inheritance.
In human beings a
dominant X – linked gene is necessary for the formation of colour sensitive
cells, the cones. The recessive form of this gene is incapable of producing
colour sensitive cone cells. Homozygous recessive females (XcXc) and hemizygous
recessive males (XcY) are unable to distinguish red and green colour. The
inheritance of colour blindness can be studied in the following two types of
marriages.
A marriage between a
colour blind man and a normal visioned woman will produce normal visioned male
and female individuals in F 1 generation but the females are carriers.
The marriage between a F1 normal visioned carrier woman and a normal
visioned male will produce one normal visioned female, one carrier female, one
normal visioned male and one colour blind male. Colour blind trait is inherited
from the male parent to his grandson through carrier daughter, which is an
example of criss-cross pattern of inheritance (Fig. 4.7).
If a colour blind woman
(XcXc) marries a normal visioned male (X+Y), all F1 sons will be
colourblind and daughters will be normal visioned but are carriers.
Marriage between F1
carrier female with a colour blind male will produce normal visioned carrier
daughter, colourblind daughter, normal visioned son and a colourblind son in
the F2 generation (Fig. 4.8).
Genes in the
non-homologous region of the Y-chromosome are inherited directly from male to
male. In humans, the Y-linked or holandric genes for hypertrichosis (excessive
development of hairs on pinna of the ear) are transmitted directly from father
to son, because males inherit the Y chromosome from the father. Female inherits
only X chromosome from the father and are not affected.
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