In human beings a dominant X - linked gene is necessary for the formation of colour sensitive cells, the cones. The recessive form of this gene is incapable of producing colour sensitive cone cells. Homozygous recessive females (XCXC) and hemizygous recessive males (X'Y) are unable to distinguish red and green colour. The inheritance of colour blindness can be studied in the following two types of marriages.
Marriage between colour blind man and normal visioned woman:
(i) A marriage between a colour blind man and a normal visioned woman will produce normal visioned male and female individuals in F1 generation but the females are carriers.
(ii) The marriage between a F1 normal visioned carrier woman and a normal visioned male will produce one normal visioned female, one carrier female, one normal visioned male and one colour blind male' in F2 generation ..
(iii) Colour blind trait is inherited from the male parent to his grandson. through carrier daughter, which is an example of criss-cross pattern of inheritance.

Rh incompatability has great significance in childbirth. If a woman is Rh negative and the man is Rh positive, the foetus may be Rh positive having inherited the factor from its father. The Rh negative mother becomes sensitized by carrying Rh-positive foetus within her body. Due to damage of blood vessels, during childbirth, the mother's immune system recognizes the Rh antigens and gets sensitized. The sensitized mother produces Rh antibodies. The antibodies are IgG type which are small and can cross placenta and enter the foetal circulation. By the time the mother gets sensitized and produce anti 'D' antibodies, the child is delivered.
Usually, no effects are associated with exposure of the mother to Rh positive antigen during· the first childbirth, subsequent Rh positive children carried by the same mother may be exposed to antibodies produced by the mother against Rh antigen, which are carried across the placenta into the foetal blood circulation. This causes haemolysis of foetal RBCs resulting in haemolytic jaundice and anaemia. This condition is known as Erythroblastosis foetalis or Haemolytic disease of the newborn (HDN).

The Mendelian disorders in human beings are
(a) Thalassemia
(b) Sickle cell anaemia
(c) Huntington chorea
(d) Phenylketonuria
(e) Albinism
(a) Thalassemia
(i) Thalassemia is an autosomal recessive disorder. It is caused by gene mutation resulting in excessive destruction of RBC's due to the formation of abnormal haemoglobin molecules.
Normally haemoglobin is composed of four polypeptide chains, two alpha and two beta globin chains.
(ii) Thalassemia patients have defects in either the alpha or beta globin chain causing the production of abnormal haemoglobin molecules resulting in anaemia.
Classified based on heaemoglobin
Alpha Thalassemia:
(i) Mutation or deletion of one or more of the four alpha gene alleles.
(ii) It is controlled by two closely linked genes HBAI and HBA2 on chromosome 16.
Beta Thalassemia:
(i) Production of beta globin chain is affected
(ii) It is controlled by a single gene (HBB) on chromosome 11.
(iii) It's is also known as Cocley's anemia.
(iv) It increases the alpha chain production and damages the membranes of RBC.
Huntington's chorea:
(i) It is inherited as an autosomal dominant lethal gene in man.
(ii) It is characterized by involuntary jerking of the body and progressive degeneration of the nervous system, accompanied by gradual mental and physical deterioration.
(iii) The patients with this disease usually die between the age of 35 and 40.

Mitotic or meiotic non-disjunction of sex chromosomes causes allosomal abnormalities. Several sex chromosomal abnormalities have been detected. Eg. Klinefelter's syndrome and Turner's syndrome.
i) Klinefelter's Syndrome (XXY Males)
This genetic disorder is due to the presence of an additional copy of the X chromosome resulting in a karyotype of 47,XXY. Persons with this syndrome have 47 chromosomes (44AA+XXY). They are usually sterile males, tall, obese, with long limbs, high pitched voice, under developed genitalia and have feeble breast (gynecomastia) development.
(ii) Turner's Syndrome (XO Females)
This genetic disorder is due to the loss of a X chromosome resulting in a karyotype of 45, X. Persons with this syndrome have 45 chromosomes (44 auto somes and one X chromosome) (44AA+XO) and are sterile females. Low stature, webbed neck, under developed breast, rudimentary gonads lack of menstrual cycle during puberty, are the main symptoms of this syndrome.

In human beings a dominant X - linked gene is necessary for the formation of colour sensitive cells, the cones. The recessive form of this gene is incapable of producing colour sensitive cone cells. Homozygous recessive females (X^cX^c) and hemizygous recessive males (X^cY) are unable to distinguish red and green colour. The inheritance of colour blindness can be studied in the following two types of marriages.
(i) Marriage between colour blind man and normal visioned woman:
A marriage between a colour blind man and a normal visioned woman will produce normal visioned male and female individuals in Fl generation but the females are carriers. The marriage between a Fl normal visioned carrier woman and a normal visioned male will produce one normal visioned female, one carrier female, one normal visioned male and one colour blind male. Colour blind trait is inherited from the male parent to his grandson through carrier daughter, which is an example of criss-cross pattern of inheritance.

(ii) Marriage between normal visioned man and colour blind woman:
a) If a colour blind woman (X^CX^C) marries a normal visioned male (X^Cy), all F₁ sons will be colourblind and daughters will be normal visioned but are carriers.
b) Marriage between F₁ carrier female with a colour blind male will produce normal visioned carrier daughter, colourblind daughter, normal visioned son and a colourblind son in the F₂ generation.