Sex-linked inheritance means the passing of traits (characteristics) through the sex chromosomes (X and Y) from parents to their children.
It is called sex-linked because the genes are present on the X chromosome, Y chromosome, or both X and Y chromosomes.
There are 3 types:
X-linked inheritance – The gene is present only on the X chromosome.
Example: Haemophilia.
Y-linked inheritance – The gene is present only on the Y chromosome.
These traits are passed only from father to son, because only males have a Y chromosome.
XY-linked inheritance – The gene is present on both the X and Y chromosomes (usually in matching regions), so it can be inherited by both males and females.
In one line:
Sex-linked inheritance = Passing of genes located on the X or Y chromosomes from parents to their children.
Complete and Incomplete Sex Linkage
1. Complete Sex Linkage
Definition:
Complete sex linkage occurs when a gene is present on the non-homologous region (a region found only on one sex chromosome) of the X or Y chromosome.
Why is it called complete?
Crossing over does not occur in this region.
Therefore, the gene is passed on without being exchanged between the X and Y chromosomes.
The trait is inherited exactly as it is.
Examples of X-linked traits:
Haemophilia (blood does not clot properly)
Red-green colour blindness (difficulty distinguishing red and green colours)
Myopia (near-sightedness; difficulty seeing distant objects)
Ichthyosis (a skin disorder causing dry, scaly skin)
Examples of Y-linked traits:
Hypertrichosis (excessive hair growth, especially on the ears)
H-Y antigen gene (a gene involved in male development)
2. Incomplete Sex Linkage
Definition:
Incomplete sex linkage occurs when a gene is present on the homologous region (a matching region found on both the X and Y chromosomes).
Why is it called incomplete?
Crossing over occurs in this region during meiosis.
As a result, genes can be exchanged between the X and Y chromosomes.
Therefore, the traits do not always inherit together.
Examples of X-Y linked traits:
Total colour blindness
Nephritis (inflammation of the kidneys)
Retinitis pigmentosa (an inherited eye disease that gradually causes vision loss)
Difference Between Complete and Incomplete Sex Linkage
| Complete Sex Linkage | Incomplete Sex Linkage |
|---|---|
| Gene is present on the non-homologous region of the X or Y chromosome. | Gene is present on the homologous region of the X and Y chromosomes. |
| Crossing over does not occur. | Crossing over occurs. |
| Genes are inherited together. | Genes may separate due to crossing over. |
| Trait is passed unchanged. | Trait inheritance may change because of gene exchange. |
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