Question

(a) Why are colour blindness and thalassemia categorised as Mendelian disorders? Write the symptoms of these diseases seen in people suffering from them.
(b) About 8% of human male population suffers from colour blindness whereas only about 0.4% of human female population suffers from this disease. Write an explanation to show how it is possible. [5]

Answer 1

(a) Colour blindness and thalassemia are categorised as Mendelian disorders because these are caused by mutation in a single gene and their mode of inheritance follows the principles of Mendelian genetics. [1]

Symptoms of Thalassemia
(a) Growth problems — not putting on weight or growing in height
(b) Anaemia — red blood cell deficiency, leading to tiredness, weakness and shortness of breath
(c) Jaundice — yellowing of the skin and eyes
(d) Swollen abdomen — this is caused by an enlarged liver or spleen [1]

Symptoms of Colour Blindness
(a) Difficulty distinguishing between colours
(b) Inability to see shades or tones of the same colours
(c) Rapid eye movement (in rare cases) [1]

(b) Colour blindness is a X-linked recessive disorder. Males have higher chances of getting affected in comparison to females because males have only one X with Y chromosome while females have two X chromosomes (XX). Thus, for a female to get affected by colour blindness, she has to have the mutant gene on both the X chromosomes while males can be affected if they carry it on the single X chromosome.
Conditions for colour blindness :
XY ; XX : Normal
$X^{c}Y$ : Colour-blind male
$X^c{X}^c$ : Colour-blind female
$X^{c}X$ : Carrier female
Let us assume that a carrier female $\left(X^{c}X\right)$ is married to a normal male.
Parents $XY$ $X^{C}X$
(Male) $\times$ (female)
$X^{C}X$ $XX$ $X^{C}Y$ $XY$
Offspring Carrier Normal Colour-blind Normal
female female Male Male
From the given table, it can be concluded that females have very less probability of getting this disease as compared to males. Females will be colour-blind only when either both parents are affected or male is affected and female is carrier, while males can be colour-blind even if female is carrier and male is normal. [2]