Unique differencs between some common human genetic disorders?

Sickle cell anaemia:
This is an autosome linked recessive trait that is transmitted from the parent to the offspring when both the partners are carriers for the gene (heterozygous). It is a metabolic disorder characterized by a decreased oxygen transport in the blood. The decrease is due to the presence of defective haemoglobin (HbS) in place of normal haemoglobin (Hb). HbS has low affinity for oxygen and the RBC becomes sickle shaped with pointed edges. This is due to mutation in the autosomal genes which are responsible for the production haemoglobin. This mutation is characterized by a change in a single amino acid in the sequence. This defect arises from the substitution of glutamine by valine at the sixth position in chain of globin molecule of haemoglobin. The heterozygous individuals are unaffected but they are carriers of the disease as there is 50% chance of transmitting the mutant gene to the progeny.

Down’s syndrome:
Duplications are usually lethal. Duplication of chromosome 21 in humans results in Trisomy-21 and Down syndrome. In these individuals an additional copy of chromosome 21 is present. This disorder was first reported in the year 1866 by Langdon Down. Down syndrome features are mental retardation, short statured with small round head, furrowed tongue.

Hemophilia:
This is an example of sex linked recessive disease. This disease shows its transmission from carrier female to some of the male progeny. For the clotting of blood a cascade of proteins are involved. Of these a single protein is affected in hemophilia. Due to this in an affected individual a simple cut will result in non stop bleeding. The heterozygous female (carrier) may transmit the disease to sons. The family pedigree of Queen Victoria shows a number of haemophilic descendents as she was a carrier of the disease.