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

Thalassemia and haemophilia are categorised as Mendelian disorders because these disorders are due to alteration in a single gene. Also they are transmitted to offsprings through Mendelian principles of inheritance.

Symptoms and pattern of inheritance are given below :

$(a)$ Thalassemia: It is an autosomal linked recessive blood disorder characterised by defects in $\alpha$, $\beta$ or $\delta$ chain resulting in abnormal Hb molecule.

Symptom : Anaemia

Inheritance : Two mutant alleles (one from each parent) must be inherited for an individual to be affected i.e. homozygous. Heterozygous are carriers and may pass the mutant allele to children.

$(b)$ Haemophilia : It is a sex linked recessive disorder whose gene is located on $X$-chromosome.

Symptom : Prolonged clotting time and external bleeding even in minor injury.

Inheritance : The gene is present on $X$-chromosome, so it is inherited by males as they have single $X$-chromosome. Affected males are said to be hemizygous. Females have two $X$chromosomes, thus possibility of them being affected is rare as the mother of such female has to be atleast carrier and father should be haemophilic.