When the genes of vegetative /somatic character are present on sex chromosome is termed as sex linked gene and such phenomenon is known as sex linkage. Two types of sex linkage is-
(A). X linkage
Genes of somatic characters are found on x chromosome, the inheritance of x linked character may be through the males and females. e.g Haemophilia, colour blindness.
The genes of somatic characters are located on Y-chromosome. The inheritance of such type of character only through the males, such type of character is called Holandric character these character only found in male. e.g. Gene which forms TDF, Hypertrichosis (excessive hair on ear pinna)
Gene which is located on differential region of Y chromosome is known as Holandric gene.
Example of X Sex linkage
Eye colour in drosophila:–
Eye colour in drosophila is controlled by a x linked gene.
Homozygous red eyed female = X+X+If a red eyed colour gene is represented as + and white eyed colour represented w then on basis of this different type of genotypes are found in drosophila. Gene for red eye dominant (+) and white colour of eye is recessive (w)
Heterozygous red eyed female = X+XW
Homozygous white eyed female = XWXW
Hemizygous red eyed male = X+Y
Hemizygous white eyed male = XwY
It is clear by above different types of genotype that female either homozygous or heterozygous for eye color. But for the male eye color it is always Hemizygous.
Haemophilia is also called “bleeder’s disease” and first discovered by john otto (1803) the gene of Haemophilia is recessive and x-linked lethal gene.
On the basis of x-linked following types of genotype are found,
XhX = Carrier female
XhY= Affected male.
But XhXh type of female dies during embryo stage because in homozygous condition this becomes lethal and cause death.
Haemophilia -A→ Due to lack of factor –VIII (Anithaemophilic globulin AHG)
Haemophilia B or Christmas disease → Due to lack of factor IX (plasma thromboplastin component)
Haemophilia-c→ Due to lack of factor XI (plasma thromboplastin antecedent).
Color Blindness :-
The ininheritance of colour blindness is alike as Haemophilia but it is not lethal disease of it is found is male and female (discovered by Horner).
Three types of color blindness are-
(a) Protanopia :-it is for red color.
(b) Deuteranopia:-it is green color
(c) Tritanopia:- For blue color . Color blindness is checked by ishihara chart.
Other examples of X sex linkage
(a). Diabetes insipidus (recessive)
(b). Duchene muscular dystrophy (recessive)
(c). Fragile x syndrome (recessive)
(d). Pseudo rickets (dominant)
(e). Defective enamel of teeth (dominant)
Type of inheritance of sex linked character
Criss cross inheritance (Morgan):-
In criss cross inheritance male or female parent transfer x linked character to grandson or granddaughter through the offspring of opposite sex.
(a) Diagenic:- inheritance in which characters are inherited from father to the daughter and form daughter to grandson.
Father → Daughter → Grandson.
(b). Diandric:- inheritance in which characters are inherited from mother to the son and from son to grand Daughter.
Mother→ Son→ Granddaughter.
Non criss croos inheritance :
In this inheritance male or female parent transfer sex linked character to grandson or granddaughter through the offspring of same sex.
(a). Hologenic:- Mother→Daughter→Granddaughter ) i.e female to female.
(b). Holandric:- Father →son→ grandson (male to male).
(B). Sex limited character:– These characters are present in one sex and absent in another sex but the genes are present in both the sexes and their expression is depend on sex hormone.
Example:-Secondary sexual characters→these genes located on the autosomes and these genes are present in both male and female but effect of these are depend upon presence or absence of sex hormones.
(c). Sex influenced characters:- Genes of these characters are also present on autosomes but they are influenced differently in male and female. In heterozygous condition their effect is different in both the sexes. Example;- Baldness:- gene of baldness is dominant(B).
Gene Bb shows partiality in male and female baldness in found in male due to effect of this gene but baldness is absent in female with this genotype.