The passing of characters expressed by sex determining chromosomes or allo- chromosome from parental generation to progeny’s generation is sex linked inheritance. Examples : muscular dystrophy, color blindness, Hemophilia ( Royal’s disease ,
Following are the properties of this type of inheritance :
1. These type of gene are mostly present in X- chromosome.
2. So Y- chromosome has no such role in this type of generation so, called holandric gene.
3. Genes responsible for this type of heritance are usually recessive in nature.
4. So , female( XX), when have only one out of 2 chromosome of defect or linked character, phenotypically they don’t have defect but acts as carrier.
5. Whereas ,male (XY) doesn’t get chance to be carrier as with one defective X- chromosome only phenotypic expression is possible unlike in females, as X and Y are non –homologous so Y is incapable to mask expression of X.
6. They also show Criss-cross inheritance which means the sex linked characters in one sex , gets transferred to own sex progeny through opposite sex . For instance character inheritance in grand pa to his grandson only via his daughter.
Some examples of Criss cross inheritance :
1. Eye colour in Drosophila
This example was experimentally done by T.H Morgan.
When normal red eyed female was crossed with white eyed male.
In f1 generation we got 50 percentage of carrier red eyed female and rest normal red eyed male.
This clearly infers that sex linked trait i.e white eyed gene is inherit from grandfather Drosophila to his daughter Drosophila as carrier genotype of white eye
In f2 generation we get
25% , normal red eyed female , 25 % carrier but red eyed female 25 % normal red eye male and 25 % white eyed male .
So, the inference from cross is clear that the gene expressed in 25% of white eyed male in F2 generation had come all way from grand pa through his daughter as carrier not from his son as they are all normal red eyed due to their mother’s normal genes.
Cross between white eyed female and red eyed male
From the cross above, we get 50 % red eyed female and 50% white eyed male .
The grand mother’s gene of white eyed is inherited to her son only while daughter become only carrier .
Now, further white eyed male from above cross was used and crossed with normal red eyed female,
In f2 generation , we get
25 % white eyed male , 25 %normal eyed male , 25 %carrier red eyed female and 25% white eyed female.
So, here also, gene linked with grandma’s has been inherit to her granddaughter only through her infected sons.
2. Colour blindness in Human
Red green colour blindness i.e. Daltonism also showed criss cross inheritance in humans.
Here is the cross between colour blind male and normal female
From the cross above, like in our first example we can infer the reason for grandson in F2 generation to be colour blind is the gene of his grandfather inherit through grandson’s mother only.
cross between colour blind female and normal male.
Here we got colour blind granddaughter only because the gene of colour blind grandma got inherited to her granddaughter infecting her son only.
3. Hemophilia (Bleeder’s disease)
Cross between hemophilic male and normal female :
then, the carrier daughters are crossed with normal males.
We got only females as carrier of disease while male children are found completely Ok. Had the hemophilia causing gene would be linked to Y chromosome, male child won’t be safe from hemophilia.
Then, progeny i.e. infected female is crossed with normal male :.
So, here we get 25 % of total progeny as male get hemophilia all due to carrier female gene that came all along from hemophilic grandpa.
The female hemophilic ones doesn’t get born because these gene turns out to be fatal and cause natural abortion.
History of this disease ( JUST FOR EXTRA NOT IN CURRICULUM)
Hemophilia has been called a "royal disease". This is because the hemophilia gene was passed from Queen Victoria, who became Queen of England in 1837, to the ruling families of Russia, Spain, and Germany. Queen Victoria's gene for hemophilia was caused by spontaneous mutation. Of her children, one son, Leopold, had hemophilia, and two daughters, Alice and Beatrice, were carriers. Beatrice's daughter married into the Spanish royal family. She passed the gene to the male heir to the Spanish throne.
Queen Victoria's other daughter, Alice, had a carrier daughter, Alix. Alix became Empress Alexandra at her marriage to Russia's Czar Nicholas in 1894. Their son, born in 1904 and named Alexis, inherited hemophilia from his mother. The young man Alexis was treated for bleeds by the mysterious Rasputin, known as a holy man with the power to heal. The fascinating story of this royal family is told in the book Nicholas and Alexandra by Robert Massie (the father of a son with hemophilia).
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