What is sex linked inheritance? Explain with example of colour blindness and haemophilia.
There are two types of chromosomes in the diploid cells. Autosomes bearing genes for somatic characters and sex chromosomes bearing genes for sex. Sex chromosomes also carry some genes of non-sexual characters such as colour blindness, haemophilia. Such genes are always associated with sex chromosomes called sex linked genes. Human has many more genes on the X chromosomes than on the Y chromosome. There are many more X linked characters than Y linked. In female it is homogametic with two XX chromosomes, while in male it is heterogametic with one X chromosomes. The male is hemizygous because it has half the number of X chromosomes. Genes on the X or Y chromosome are called sex linked genes and their mode of inheritance is called sex linked inheritance.
X linked genes
Genes located on X chromosomes are called X-linked. Such genes do not have alleles on Y chromosomes. In man about 300 genes are X-linked genes.
Diagynic
The sex-linked genes are present on the non-homologous part of X chromosome and are passed on from father to grandson through daughter of F1 generation.
Diandric
The mother transfers traits to her granddaughter through her son.
Hologynic Character is passed directly from mother to daughter. Holandric or Y linked gene. Only few genes are located on Y chromosome. They are Y linked genes. These characters are inherited directly from father to the son but never to the daughter. Ex. Hypertrichosis of the ear in man, TDF testis determining factor, H-Y antigen.
TDF (testis determining factor): The Y chromosome carries a gene sry that coded for a protein called testis determining factor. TDF is required for the development and differentiation of the testis and its system and in absence lead to the development of ovaries.
Pseudoautosomal genes (XY linked): Genes located on homologous parts of both X and Y chromosomes. Ex. Total colour blindness.
Characteristic of sex-linked inheritance: It is a criss cross inheritance as the father passes its sex linked character to his daughter who in turn passes it to the grandson. Daughter do not express the recessive character but act as carrier in the heterozygous condition. Female homozygous for recessive trait express the character. Any recessive gene borne by X chromosome of male is expressed as Y chromosome has no allele to counteract.
Colour blindness
John Dalton discovered colour blindness in 1792. Colour blindness is a sex-linked recessive disorder. Those who suffer from red, green blindness cannot distinguish between red and green colour. The gene for this defect is located on X chromosome. It was study by Horner.in female colour blindness appears only when the sex chromosome carries the recessive gene XcXc. In human male the defect appears in the presence of a single recessive gene XcY because Y chromosome of male does not carry any gene for colour vision. As a result, colour blindness is more common in males as compared to female.
Female – Normal XX
Carrier – XXc
Colour blindness - XcXc
Male – Normal – XY
Colour blindness - XcY
Examples:
A Colour-blind women marries a Normal man: A colour blind women for (XcXc) marries a normal man XY. The colour-blind women produce two types of ova. One with Xc and other with Xc. normal male also produces sperms of two types. One with X and other with Y chromosome.
Colour blind women- XcXc – Xc Xc (ova)
normal man - XY - X Y (sperm)
X Y
Xc XcX
Career daughter XcY
Colour blind son
Xc XXc
career daughter XcY
Colour blind son son
A career women marries a Normal man:
A carrier woman for colour blineness (XXc) marries a normal man. The careerists produce two types of ova. One with X and other with Xc. the normal male also produces sperms of two types. One with X and other with Y chromosome.
Career women- XXc – X Xc (ova)
Normal man - XY - X Y (sperm)
X Y
Xc XXc
Career daughter XcY
Colour blind son
X XX
Normal daughter XY Normal son
A colour blind man marries a normal women:
If a colour-blind man XcY marries a normal woman XX all the daughters are careers as they receive one Xc from their father whereas all sons are normal as they receive X from their mother.
Normal women- XX – X X (ova)
Colour blind man XcY - Xc Y (sperm)
Xc Y
X XXc
career daughter XY Normal son
X XXc
Career daughter XY Normal son
A Colour-blind women marries a colour blind man:
A colour-blind women for (XcXc) marries a colour blind man XcY. The colour-blind women produce two type of ova. One with Xc and other with Xc. normal male also produces sperms of two types. One with Xc and other with Y chromosome.
Colour blind women- XcXc – Xc Xc (ova)
normal man - XcY - Xc Y (sperm)
Xc Y
Xc XcXc
Colour blind daughter XcY
Colour blind son
Xc XcXc
Colour blind daughter XcY
Colour blind son
100% colour blind daughter
100 % colour blind son
A career woman marries a diseased man:
A carrier woman for colour blind (XXc) marries a diseased man XcY. The career women produce two types of ova. One with X and other with Xc. diseased male also produces sperms of two types. One with Xc and other with Y chromosome. Career women- XXc – X Xc (ova)
diseased man - XY - Xc Y (sperm)
Xc Y
Xc XcXc
Disease daughter XcY diseased son
X XXc
career daughter XY Normal son
Haemophilia
It is commonly called as bleeder’s disease. It is a sex-linked disease first studied by John Cotto. This trait is inherited in the British royal family and has been passed on to other royal housed across Europe through Queen Victoria. Due to absence of antihaemophilic globulin or plasma thromboplastin in the blood the patient bleeds for hours. As a result of continuous bleeding, the patient may die of blood loss. Haemophilia is caused by a sex-linked recessive gene h located in the X chromosome. The gene h fails to produce necessary factor for quick clotting. A female become haemophilic only when both the X chromosomes carry the gene h (XhXh). Such females die before birth. A female possessing only one allele for hemophilia XXh appear to be normal as the other X is dominant as there is normal h allele. Such female is called carriers. In case male the recessive gene h on X chromosome expresses itself as Y chromosome is devoid of any h allele. XhY.
Haemophilia A:
It is most common type haemophilia. It is characterized by the absence of antihaemophilic globulin.
Haemophilia B:
Results from defect in Plasma Thromboplastic component. (PTC)
Examples:
A career women marries a Normal man: A carrier women for haemophilia (XXh) marries a normal man. The career women produce two types of ova. One with X and other with Xh. the normal male also produces sperms of two types. One with X and other with Y chromosome.
Career women- XXh – X Xh (ova)
Normal man - XY - X Y (sperm)
X Y
Xh XXh
Career daughter XhY
Haemophilic son
X XX
Normal daughter XY Normal son
The daughter 50% are normal and other remaining.
50% are career. In son 50% are normal and rest 50% are haemophilic.
A haemophilic man marries a normal women: If a haemophillic man XhY marries a normal women XX all the daughters are careers as they receive one Xh from their father whereas all sons are normal as they receive X from their mother.
Normal women- XX – X X (ova)
Haemophilic man XhY - Xh Y (sperm)
Xh Y
X XXh XY
career daughter Normal son
X XXh
Career daughter XY Normal son
A career woman marries a Haemophilic man:
A carrier woman for haemophilia (XXh) marries a Haemophilic man XhY. The career women produce two types of ova. One with X and other with Xh. thehaemophilic male also produce sperms of two types. One with Xh and other with Y chromosome. Career women- XXh – X Xh (ova)
haemophilic man -XY - Xh Y (sperm)
Xh Y
Xh XhXh
Disease daughter(dies) XhY
Haemophilic son
X XXh
career daughter XY Normal son
1. Haemophilic daughter(dies)
2. Career daughter
3. Haemophilic son
4. Normal son
A Haemophilic woman marries a Normal man: A Haemophilic women for (XhXh) marries a normal man XY. The haemophilic women produce two types of ova. One with Xh and other with Xh. normal male also produces sperms of two types. One with X and other with Y chromosome.
haemophilic women- XhXh – Xh Xh (ova)
normal man - XY - X Y (sperm)
X Y
Xh XhX
Career daughter XhY
Haemophilic son
Xh XXh
career daughter XhY
Haemophilic son