Allele Definition - What Is The Meaning Of Allele?

Introduction to Allele

An allele is a variant form of the gene that occurs at a particular locus on the chromosome. Individuals have two alleles for every gene they inherit from their parents. The combination of these alleles decides the genotype of the organism and can result in affecting the phenotype.

This broadening of the term allele doesn't only define the genetic variation but also allows the descriptions of the forms that may be presented in a gene and in what ways the existence of one variety will provide alternative apparent traits in organisms. For example, discussing the blood type in the ABO blood group, several alleles determine the blood type of a person thus providing a reason for how variations can be a basis for differences in phenotype.

Formal Definition of Allele: An allele refers to one of the alternative forms of a gene that may cause different phenotypic effects. The presence of varied alleles at a locus is an important reason for genetic diversity in populations.

What is An Allele?

In biology, when we ask what allele is, we describe their role in genetics as building blocks of heredity. Different alleles could have dominant as well as recessive effects on the trait manifested in offspring. For example, it was due to the two different types of alleles for purple-flowering plants and white-flowering plants in the pea plant study of Gregor Mendel.

Examples Of Allele

• One of the best examples of an allele is the ABO blood group system, which occurs in six common varieties.

• In the case of the ABO blood group system, it has six common alleles that combine to determine an individual's blood type.

• There are also brown eyes, so this illustrates how many variants affect a single trait.

Types Of Alleles

Numerous alleles at every locus are often present in a species or population of organisms, among different individuals. The allele frequency (polymorphism) present or the population's fraction of heterozygotes are two ways to measure allelic variability at a locus. Alleles can be classified based on effects on phenotypes:

• Dominant Alleles: They are expressed in the phenotype even if only one copy is present-for example, brown eyes.

• Recessive Alleles: Two copies must be present to be expressed. For example, blue eyes.

Dominant or Recessive Phenotype-Causing Alleles

The two homozygous phenotypes that the heterozygote most closely resembles will determine whether the genotypic interaction between both the pair of alleles at a locus is dominant or recessive. The allele displayed is the one that results in the "dominant" phenotype, and the other allele is referred to as "recessive" when the heterozygote cannot be distinguished from any of the homozygotes. Different loci exhibit different degrees and patterns of dominance. Gregor Mendel was the first to formally describe this kind of interaction. Co-dominance & polygenic inheritance are used to represent the phenotypes because many features defy this straightforward categorization.

An allele that contributes to the usual phenotypic feature as found in "wild" species of organisms, like fruit flies, is referred to as a "wild type" allele (Drosophila melanogaster). Whereas, the mutant alleles that arise from mutation in the wild type. They are generally recessive, uncommon, and frequently harmful phenotypes. On the other hand "wild type" alleles were thought to be dominant (overpowering), common, and normal phenotypes.

Genetic Interaction Between Alleles

The interaction of different alleles of a locus can result in other phenotypic effects. Co-dominance is a condition wherein both contribute equally toward the phenotype; the blending of traits is displayed in incomplete dominance.

Allele Frequency & Hardy-Weinberg Principle

Predicting the frequencies of the appropriate genotypes from the frequencies of alleles in such a diploid population (see Hardy–Weinberg principle). Two alleles make up a straightforward model:

where q seems to be the frequency of an alternative allele and p seems to be the frequency with one allele, which must add up to one. Following that, 2pq is the percentage of heterozygotes, q2 is the percentage of people who are homozygous for such alternative allele, and p2 is the percentage of the population that is homozygous for the first allele. If the first allele was dominant towards the second, then p2 + 2pq represents the population's proportion showing the dominant phenotype, while q2 represents the population proportion showing the recessive phenotype.

Three alleles:

The number of potential genotypes (G) with such several alleles (a) at a diploid locus is indicated by the expression:

Genetic Diseases Caused by Alleles

When a pair of grey alleles for just a single gene trait is inherited by one individual, a variety of genetic diseases might result. Galactosemia, Phenylketonuria (PKU), Tay-Sachs disease, cystic fibrosis and albinism are examples of recessive genetic diseases. Other ailments are also caused by recessive alleles, but since the X chromosome is the location of the gene locus and males only have one copy (i.e., these are hemizygous), these are more common among males than in females. Red-green colour blindness & fragile X syndrome are two examples.

When a person gets just one dominant allele, other illnesses like Huntington's disease can develop. Some genetic diseases are known to result from some specific allelic types:

• Recessive Disorders: This category of disorders includes cystic fibrosis and Tay-Sachs disease, where an individual possesses two recessive alleles.

• Dominant Diseases: Diseases like Huntington's are caused by the presence of just one dominant allele.

Idiomorphs vs Alleles

In more advanced genetic work, especially in mycology, the conventional usage has typically superseded ordinary words for sequences of the same locus that lack sequence homology with the term "idiomorph". Usage of the term idiomorph marks an important sophistication beyond any simple notion of alleles.

Allele NEET MCQs

Q1. Alleles are:

1. Different molecular forms of a gene

2. Heterozygotes

3. Different phenotype

4. True breeding homozygotes

Correct answer: 1) different molecular forms of a gene

Explanation:

Alleles are different gene variants or molecular forms that occur at the same location (locus) on a chromosome and are caused by mutations. Different characteristics or variances in an organism can be caused by alleles.

People who have two distinct alleles at a specific gene locus (such as Aa) are known as heterozygotes.
Various phenotypes Although alleles are not distinct phenotypes, phenotypes are the observable qualities or characteristics of an organism that arise from the expression of alleles.
Although this is not the definition of alleles, true-breeding homozygotes are organisms that have two identical alleles for a particular attribute (AA or aa, for example).

Hence, the correct answer is option 1) different molecular forms of a gene.

Q2. Which of the following represents the homozygous condition?

1. TTRr

2. TtRR

3. TTRR4y

4. TT

Correct answer: 4) TT

Explanation:

Homozygous - Mendel also proposed that in true breeding tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous, example: TT and tt. Mendel demonstrated that homozygous organisms consistently pass on the same trait to their offspring due to the identical nature of the alleles. These purebred traits form the foundation of Mendel's principles of inheritance, such as the law of segregation.

Hence, the correct answer is option is 4) TT.

Q3. A recessive allele is expressed in:

1. Heterozygous condition only

2. Homozygous condition only

3. F3 generation

4. Both homozygous and heterozygous conditions

Correct answer: 2) Homozygous condition only

Explanation:

Genotype and phenotype -TT and tt are called the genotypes of a plant while the descriptive terms tall and dwarf are phenotypes. The genotype refers to the genetic makeup of an organism, consisting of alleles inherited from its parents. The phenotype is the observable expression of the genotype, influenced by both genetic factors and environmental conditions.

Hence, the correct answer is option 2) Homozygous conditions only.

Also Read:

• MCQs on Mendelian Genetics

• Sex Determination in Drosophila

• Linkage and Recombination

FAQs on Allele

Q1. What are the types of alleles?

Alleles are alternate forms of a gene present at the same locus on homologous chromosomes. Based on their expression, they are mainly of two types:

• Dominant alleles - They can express even when a single copy of the gene is present i.e., heterozygous condition. For example, the allele for brown eyes in humans.
• Recessive alleles - They can only be expressed when both the copies are present i.e., homozygous condition. For example, blue eye color allele.

Q2. What is a wild type allele?

A wild type allele means the allele that is most commonly found in the natural population. It is considered as the normal version of the gene, without any alterations and variations. The wild type allele is generally better suited for the environment. Any variation from the wild types is known as mutant gene. For example, in fruit flies (Drosophila melanogaster), red eyes are wild types and white eyes are mutant types.

Q3. What is allele frequency?

Allele frequency is the presence of a specific allele in a population. In simple words, it is the occurrence of a particular allele among all the alleles present in a population. For example,if a population has 100 individuals with two alleles (A and a) and 120 of the total 200 alleles are A, the allele frequency of A is 0.6 (60%).

Q4. What does the Hardy–Weinberg principle state about alleles?

The Hardy–Weinberg principle states that the frequency of allele and genotypes in a population remains constant if the population is in genetic equilibrium. It possible if the population has no mutation, no migration, random mating, no natural selection, and large population size. Using this principle, genotype frequencies can be predicted from allele frequencies using the equation: p² + 2pq + q² = 1

where p and q are the frequencies of the two alleles. This principle forms the basis for studying population genetics.