(a) Diandric sex linked or X linked traits : Genes for these characters are located on non-homologous segment of X chromosome. Alleles of these genes do not occur on Y chromosome. Genes of such characters are transferred from father to his daughter and from his daughter to her sons in F2 generation. This is known as Cris-cross inheritance. As the genes for most sex linked characters are located in X chromosome, they are called X-linked characters e.g. colour blindness and haemophilia in man and eye colour in Drosophila.

Sex linked inheritance in Drosophila : Drosophila melanogaster has XX and XY sex chromosomes in the female and male respectively. Its eye colour is sex linked.

Allele of the eye colour gene is located in the X chromosome, and there is no corresponding allele in the Y chromosome. The male expresses a sex-linked recessive trait even if it has a single gene for it, whereas the female expresses such a trait only if it has two genes for it. The normal eye colour is red and is dominant over the mutant white eye colour. The following crosses illustrate the inheritance of X-linked eye colour in Drosophila.

(i) Red-eyed female x White-eyed male : If a homozygous red-eyed female fly is mated with a hemizygous (having a single allele for a trait) white-eyed male fly, all the F1 flies, irrespective of their sex, are red eyed. When the red-eyed male and female flies of F1 are intercrossed (equivalent to self pollination in peas), the F2 flies are in the ratio of 2 red-eyed females to 1 red-eyed male to 1 white-eyed male. Thus, the red-eyed and white-eyed flies are in the ratio of 3 : 1 in F2 generation (Mendelian monohybrid ratio).
If XR represents a gene for red eye and Xr that for white eye colour, the above cross may be diagramed as follows. The above cross shows that a recessive X-linked trait follows criss-cross inheritance, i.e., transmission from the father to the grandsons through the daughters. The latter are called carriers because they have a trait but do not express it.

Colour blindness and Haemophila are the two main sex linked or X - linked disease are found in man.

: Person unable to distinguish certain colours are called colour blind. Several types of colour blindness are known but the most common one is �red-green colour blindness�. It has been described by HORNER (1876).
The red blindness is called protanopia and the green blindness deutoranopia. X-chromosome possesses a normal gene which control the formation of colour sensitive cells in the retina. Its recessive allele fails to do its job properly and results in colour blindness. These alleles are present in X chromosome is evidenced by the following results.

# The above results could easily be explained with the assumption that colour vision is sex linked character and its gene is present on X-chromosome, Y-chromosome lacks its allele. Always male receives its X-chromosome from mother (through ovum) and Y-chromosome from father (through sperm), whereas the female receives one X-chromosome from each parent (through ovum and sperm). From the above result following conclusions may be drawn.
(1) Colour blindness is more common in males than in females.
(2) Two recessive genes are needed for the expression of colour blindness in female, whereas only one gene gains expression in male.
(3) Males are never carriers.
(4) Colour blind women always have colour blind fathers and always produce colour blind sons.
(5) Colourblind women produce colour blind daughters only when their husbands are colour blind.
(6) Women with normal colour vision, whose fathers are colour blind, produce normal and colour blind sons in approximately equal proportion.

In haemophilia the blood fails to clot when exposed to air and even a small skin injury results in continuous bleeding and can lead to death from loss of blood.
It is also called bleeder�s disease, first studied by John Cotto in 1803. The most famous pedigree of haemophilia was discovered by Haldane in the royal families of Europe. The pedigree started from Queen Victoria in the last century. In a patient of haemophilia blood is deficient due to lack necessary substrate, thromboplastin. It is of two types.
(a) Haemophilia-A : Characterized by lack of antihaemophilic globulin (Factor VIII). About four-fifths of the cases of haemophilia are of this type.
(b) Haemophilia-B : �Christmas disease� (after the family in which it was first described in detail) results from a defect in Plasma Thromboplastic Component (PTC or Factor IX).
Like colour blindness, haemophilia is a well known disorder which is sex-linked recessive condition. The recessive X-linked gene for haemophilia shows characteristic Criss-cross inheritance like the gene for colour blindness. Its single gene in man results in disease haemophilia, whereas a woman needs two such genes for the same.