Question

Why are colour blindness and thalassemia considered as Mendelian Disorders? Write symptoms of these diseases seen in people suffering from them.About 8% of the human male population suffers from colour blindness whereas only 0.4% of the human female population suffers from these diseases. Write an explanation to show how it's possible.

Answer 1

The diseases can be characterised into mendelian disorder if there is a family history of having these diseases and they generally occur from the birth of the individual. This is generally due to a faulty allele on the responsible gene.

Complete answer:
We all know that Mendel was responsible for giving the laws of inheritance which gave us an insight about how the traits are passed on from parents to their offspring. He said that every trait is represented by a pair of alleles which are present on the gene together. Now these traits can be dominant and recessive. Dominant meaning that a single allele of that gene can express the trait and the other being the recessive one where a pair of those alleles is at least required to express the trait.
Now these diseases like colour blindness and thalassemia are also governed by a pair of alleles and these diseases are carried down from the parents to the offspring. That is why they are termed as Mendellian disorders as they are following the rules of Mendel’s Laws of inheritance.
The symptoms of the colour blindness are not that much, it is just that the person who is suffering from this disease loses his ability to see certain colours depending upon the severity of the disease. Colour blindness is not a lethal disease whereas thalassemia is a lethal disease and its symptoms include deformities in bones, fatigue, delayed growth and development, and also it is characterised by low hemoglobin levels in the blood.
This colour blindness disease as we know is an X-linked recessive disorder meaning that it requires only one affected allele of the gene and it may result in a boy having the disease and female becoming either a carrier or affected.
So if an unaffected male is married to a carrier female having the allele responsible for colour blindness then there are around 25% chances that there children mostly the male one will be having the disease and one female will be the carrier. The other two children will be unaffected ones.
If an unaffected male is married to an affected female then the chances of their offspring having the disease will be like, all the males will be affected by the diseases and all the females will be the carrier of the disease.
If an affected male is married to an unaffected female then the chances of their offspring having the disease will be like, the female offspring will be carrier of disease and the males will be unaffected as the female parent is unaffected and not the carrier.
If the affected male is married to the carrier female then the chances of offspring having the disease will be like, one male will be affected and one female will be affected, the other male will be unaffected and female will be the carrier.
So it is quite clear that males have higher chances of getting affected due to the presence of only one X chromosome and a single affecting allele can cause the disease.

Note: The colour blindness of a person cannot be cured and the person has to live with it throughout its life but in the case of thalassemia it shows complication in the early ages itself and if the pearson is not administered to the cure then he might die due to low levels of hemoglobin.