Question:

In a cross between a male and female, both heterozygous for sickle cell anaemia gene, what percentage of the progeny will be diseased ?

Updated On: Nov 13, 2025

100%
50%
75%
25%

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The Correct Option is D

Solution and Explanation

To determine what percentage of the progeny will be diseased in a cross between two heterozygous parents for sickle cell anaemia, let's break down the problem using genetic concepts.

Sickle cell anaemia is caused by a recessive allele represented as \(s\). The normal allele is represented as \(S\). A heterozygous individual has one copy of the normal allele and one copy of the sickle cell allele, represented as \(Ss\).

Using a Punnett square, we can predict the possible genotypes of the offspring from two heterozygous parents (\(Ss \times Ss\)):

	S	s
S	SS	Ss
s	Ss	ss

From the Punnett square, the possible genotypes for the offspring are as follows:

\(SS\): 1/4 or 25% (Normal phenotype)
\(Ss\): 2/4 or 50% (Carrier, but not diseased)
\(ss\): 1/4 or 25% (Diseased phenotype)

Therefore, the percentage of the progeny that will be diseased, showing symptoms of sickle cell anaemia, is \(25\%\). This corresponds with option \(25\%\), which is the correct answer.

To summarize, only individuals with the \(ss\) genotype will exhibit symptoms of sickle cell anemia. Hence, \(25\%\) of the offspring are expected to be diseased.

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Concepts Used:

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.