Question:

Multiple alleles are present

Updated On: May 1, 2024

at the same locus of the chromosome
on non-sister chromatids
on different chromosomes
at different loci on the same chromosome.

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The Correct Option is A

Solution and Explanation

Multiple alleles refer to the presence of more than two alternate forms of a gene that should be accommodated on two loci on a homologous pair. This is facilitated by the presence of more than one allele on the same locus of a chromosome provided one allele is present at a locus at a time.

Important Notes on the Concept:

In humans, the condition of multiple allelism is best illustrated by the ABO blood group system.
In the ABO blood group system in humans, gene I i.e. isohemagglutinin exists in three allelic forms. They are IA, IB, and IO.
IA and IB are codominant and they are responsible for type A and type B antigens, respectively, on the surface of the cell; erythrocytes.
IO is a recessive allele and does not produce antigens.
Even if there are more than two alleles present in the population, the individual comprising the population would possess only two such alleles.
In the case of ABO blood group system, inheritance of IA and IB alleles results in having a blood type AB.
The formula to find the number of genotypes for multiple allelism: (n/2)(n+1) where n is the number of alleles.

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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.