Which of the following disorder is also called the Royal disease?

The correct option is Haemophilia.

Haemophilia is a rare, inherited bleeding disorder in which the blood does not clot normally because it lacks sufficient blood-clotting proteins (clotting factors). This condition is famously known as the "Royal disease" due to its prevalence and transmission through the royal families of Europe during the 19th and 20th centuries.

The history and genetic lineage of the disease are explained below:

1. Queen Victoria's Role as a Carrier:
The spread of the disease started with Queen Victoria of the United Kingdom, who ruled from 1837 to 1901. Queen Victoria was a carrier of the haemophilia gene (specifically Haemophilia B, which is a deficiency in clotting Factor IX). Since she was a carrier, she did not show severe symptoms of the disease herself but passed the defective gene to several of her children.

2. Transmission to Royal Families:
Queen Victoria had nine children, who married into royal families across the European continent, including Spain, Germany, and Russia:
- Her son, Prince Leopold, inherited the disease and died from a brain hemorrhage at the age of 30 after a minor fall.
- Her daughters, Princess Alice and Princess Beatrice, were carriers of the gene and passed it to the royal houses of Russia and Spain, respectively.

3. Impact on the Russian Royal Family (The Romanovs):
Princess Alix of Hesse (Queen Victoria's granddaughter, who became Tsarina Alexandra of Russia upon marrying Tsar Nicholas II) was a carrier. She passed the disease to her son, Tsarevich Alexei, the heir to the Russian throne. His frequent and painful bleeding episodes had significant political consequences, leading the royal family to rely heavily on the mystic healer Grigori Rasputin, which contributed to the destabilization of the imperial dynasty during the Russian Revolution.

4. Genetic Nature of Haemophilia:
Haemophilia is an X-linked recessive genetic disorder. The gene responsible for producing the clotting factor is located on the X chromosome. Since males have only one X chromosome (XY), inheriting a single defective X chromosome from their mother will cause them to have the disease. Females, having two X chromosomes (XX), must inherit two defective genes to manifest the disease, which is extremely rare; instead, they usually become asymptomatic carriers if they possess one defective gene.