Sex Linked Inheritance: Colour Blindness & others | Additional Study Material for NEET PDF Download

SEX LINKAGE

When the genes are present on sex-chromosome is termed as sex linked gene and such phenomenon is known as sex-linkage. Two - types of sex linkage :

1. 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

2. Y- linkage - The genes of somatic characters are located on Y- chromosome. The inheritance of such type of character is only through the males. Such type of character is called Holandric character. These characters
found only in male.

e.g. (1) Gene which forms TDF /sry-gene                   
(3) Webbed toes
(2) Hypertrichosis (excessive hair on ear pinna.)       
(4) Porcupine skin

Gene which is located on differential region of Y - chromosome is known as Holandric gene.

Example of X-Sex linkage :-
 

[i] Eye colour in Drosophila :- Eye colour in Drosophila is controlled by a X–linked gene.

If a red eyed colour gene is represented as '+' and white eyed colour represented as 'w', then on basis of this different type of genotypes are found in Drosophila.

Gene for red eye is dominant (+) and white colour of eye is recessive (w)

Homozygous red eyed female = X⁺X⁺
Heterozygous red eyed female = X⁺X^w
Homozygous white eyed female = X^wX^w
Hemizygous red eyed male = X⁺Y
Hemizygous white eyed male = X^wY

It is clear by above different types of genotype that female either homozygous or heterozygous for eye colour. But, for the male eye colour, it is always hemizygous.

[ii] Haemophilia :-

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.

X^h X = Carrier female
X^hX^h = Affected female
X^hY = Affected male.

But, X^hX^h type of female dies during embryo stage because in homozygous condition, this gene becomes lethal and causes death.

Haemophilia -A → due to lack of factor -VIII (Anti-haemophilic globulin AHG)

Haemophilia B or Christmas disease - due to lack of factor - IX (Plasma thromboplastin component)

Haemophilia - C (Antosomal disorder) → due to lack of factor - XI (Plasma Thromboplastin antecedent)

[iii] Colour Blindness :- The inheritance of colour-blindness is alike as haemophilia, but it is not a lethal disease so it is found in male and female.(discovered by Horner)

Three types of colour blindness are-
 [a] Protanopia :- It is for red colour.
[b] Deuteranopia :- It is for green colour
[c] Tritanopia :- For blue colour blindness. Colour blindness is checked by ishihara - chart.

Other examples of X - sex linkage
[iv] Diabetes insipidus (recessive).
[v] Duchenne muscular dystrophy (recessive).
[vi] Fragile xsyndrome(recessive).
[vii] Pesudoricketes (Dominant)
[viii] Defective enamel of teeth (Dominant)

Examples of X-Y linkage
(i) Xeroderma pigmentosum
(ii) Epidermolysis bullosa 

Types of Inheritance of sex linked characters :-
 1. Criss cross inheritance (Morgan) :- In criss-cross inheritance male or female parent transfer a X- linked character to grandson or grand daughter through the offspring of opposite sex.

(a) Diagenic (Diagynic) :- Inheritance in which characters are inherited from father to the daughter and from daughter to grandson.

Father → daughter → grand son.

(b) Diandric :- Inheritance in which characters are inherited from mother to the son and from son to grand daughter.

Mother → Son → Grand-daughter.

(2) Non criss-cross inheritance : In this inheritance male or female parent transfer sex linked character to grand son or grand daughter through the offspring of same sex.
(a) Hologenic (Hologynic) :- Mother → Daughter → Grand-daughter (female to female)
(b) Holandric :- Father → Son → Grand-son (male to male)

Sex-Limited Character :- These characters are present in one sex and absent in another sex. But their 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.

For example - genes of beard-moustache express their effects only in the presence of male hormone - testosterone.

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 is found in male due to effect of this gene, but baldness is absent in female with this genotype.