Question:

A colour blind man marries a woman with normal sight who has no history of colour blindness in her family. What is the probability of their grandson being colour blind ?

Updated On: Apr 14, 2025
  • Nil
  • 0.25
  • 0.5
  • 1
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The Correct Option is B

Solution and Explanation

Colorblindness is X-linked recessive disorder. A man requires only one copy of defective gene to be colorblind. Whereas a woman requires two copies of defective genes to be colorblind. When a colorblind (X^c Y) man marries a normal woman (XX), all of their sons will be normal (XY) and all of their daughters will be carrier ($X^ c X)$. Hence, in the next generation, all the children of all of their son will be normal. There will be nil probability of being colorblind. In the next generation of their daughter, all the daughters will be either normal or carrier. Whereas 50% of the sons will be colorblind acquiring defective gene ($X^c$ ) from mother. Hence, considering the grandsons of both son and daughter, there are 25% chances or 0.25 probability of grandson being colorblind.
Thus, the correct answer is option B.
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Principles of Inheritance and Variation - Mutation

A Mutation is a change in the sequence of our DNA base pairs caused by numerous environmental stimuli such as UV light or mistakes during DNA replication. Germline mutations take place in the eggs and sperm and can be passed onto offspring, whereas somatic mutations take place in body cells and are not passed on.

Types of Mutations

There are three types of mutations, which are as follows:

Silent mutation

It refers to any change in DNA sequence that has no effect on the amino acid sequence in a protein or the functions that a protein performs. There is no phenotypic indication that a mutation has occurred.

Nonsense mutation

When there is a change in the sequence of base pairs due to a point mutation, that results in a stop codon. This leads to a protein that is either shortened or non-functional.

Missense mutation

A missense mutation occurs when a point mutation causes a change in the codon, which then codes for another amino acid.

The mutation is caused by the following factors:

Internal Causes

When DNA copies incorrectly, the majority of mutations occur. Evolution occurs as a result of all of these mutations. DNA makes a copy of itself during cell division. When a copy of DNA isn't flawless, it's called a mutation since it differs somewhat from the original DNA.

External Causes

When certain chemicals or radiations are used to break down DNA, it causes the DNA to break down. The thymine dimers are broken by UV radiation, resulting in altered DNA.