(a) Haemophilia 

(i) It is a sex-linked recessive disorder. 

(ii) Patient continues to bleed even with a minor cut because of a defect in blood coagulation. 

(iii) The gene for haemophilia is located on X chromosome. 

(iv) More males suffer from haemophilia than females because in males single gene for the defect is able to express as males have only one X chromosome. 

(v) The defective alleles produce non-functional proteins which later form a nonfunctional cascade of proteins involved in blood clotting. 

(vi) Females suffer from this disease only in homozygous condition, i.e., Xc Xc . 

(vii) Queen Victoria was a carrier of this disease and produced haemophilic offsprings. 

(b) Sickle-cell anaemia 

(i) It is an autosome-linked recessive trait. 

(ii) The disease is controlled by a single pair of allele HbA and HbS . 

(iii) Only the homozygous individuals for HbS , i.e., HbS HbS show the diseased phenotype. 

(iv) The heterozygous individuals are carriers (HbAHbS ). 

(v) Due to point mutation, glutamic acid (Glu) is replaced by valine (Val) at the sixth position of β-globin chain of haemoglobin molecule. 

(vi) HbS behaves as normal haemoglobin except under oxygen stress where erythrocytes lose their circular shape and become sickle-shaped. As a result, the cells cannot pass through narrow capillaries. Blood capillaries are clogged and thus affect blood supply to different organs.