Question

(a) Why are thalassemia and haemophilia categorized as Mendelian disorders? Write the symptoms of these diseases. Explain their pattern of inheritance in humans.
(b) Write the genotypes of the normal parents producing a haemophilic son. [5]

Answer 1

(a) Thalassemia and Haemophilia are categorised as Mendelian disorders because they occur by mutation in a single gene and their mode of inheritance follows the principles of Mendelian genetics. [1]

Symptoms of Thalassaemia

• Anaemia[0.5]

Symptoms of Haemophilia

A haemophilic patient suffers from non-stop bleeding even on a simple cut, which may lead to death. [0.5]

Pattern of Inheritance of Thalassaemia [1]
It is an autosome -linked recessive blood disease
Pair of alleles $H{b}^A$ and $H{b}^T$ controls the expression of this disease.
Conditions for thalassaemia :
$H{b}^A$ and $H{b}^A$ : Normal
$H{b}^A$ and $H{b}^T$ : Carrier
$H{b}^T$ and $H{b}^T$ : Diseased
Let us assume that both father and mother are the carriers $\left(H{b}^{A}H{b}^T\right)$ of beta thalassaemia.
Parents: $H{b}^{A}H{b}^T$ $H{b}^{A}H{b}^T$
(Father) $\times$ (Mother)
Offspring:
$H{b}^{A}H{b}^A$ $H{b}^{A}H{b}^T$ $H{b}^{A}H{b}^T$ $H{b}^{T}H{b}^T$
$\begin{array}{cccc}\text{Normal}& \text{Carrier child}& \text{Carrier child}& \text{Child with}\\ & \text{with}& \text{with}& \text{severe}\\ & \text{thalassaemia}& \text{thalassaemia}& \text{thalassaemia}\\ & \text{trait}& \text{trait}& \end{array}$

Pattern of Inheritance of Haemophilia : [1]
Haemophilia is an X-linked recessive genetic disorder.
Conditions for haemophilia :
XY;XX : Normal
$X^{h}Y$ : Haemophilic male
$X^{h}X$ : Carrier female
$X^h{X}^h$ : Haemophilic female
Let us assume that a carrier female $\left(X^{h}X\right)$ is married to a normal male.

(b) When a normal male marries a carrier female (she is considered normal as she contains the mutant gene on one of her X chromosomes), they can produce a haemophilic son. So, the genotype of the parents would be XY and $X^{h}X$. [1]