Why are thalassemia and haemophilia categorised as Mendelian disorders? Write the symptoms of these diseases. Explain their pattern of inheritance in humans.

(N/A) Thalassemia and haemophilia are categorised as Mendelian disorders because these disorders are caused by an alteration or mutation in a single gene. They are transmitted to offspring following Mendelian principles of inheritance.
Symptoms and patterns of inheritance are as follows:
$(a)$ Thalassemia: It is an autosomal-linked recessive blood disorder characterized by defects in the $\alpha$,$\beta$,or $\delta$ globin chains,resulting in abnormal $Hb$ molecules.
Symptom: Severe anaemia.
Inheritance: It follows an autosomal recessive pattern. Two mutant alleles (one from each parent) must be inherited for an individual to be affected (homozygous). Heterozygous individuals are carriers and may pass the mutant allele to their children.
$(b)$ Haemophilia: It is a sex-linked recessive disorder where the defective gene is located on the $X$-chromosome.
Symptom: Prolonged blood clotting time and excessive bleeding even from minor injuries.
Inheritance: Since the gene is on the $X$-chromosome,it is more common in males as they possess only a single $X$-chromosome (hemizygous). Females have two $X$-chromosomes,so they are rarely affected unless the father is haemophilic and the mother is at least a carrier.