Allele definition - What is the meaning of Allele?

In genetics, an allele is among the most simple concepts that constitute the fundamentals of genetic variation and inheritance. An allele is described as one form of a gene at a given locus on a chromosome. This is one of the important topics to NEET aspirants, CBSE, and several other competitive exams as it encompasses the meanings of alleles, the definition of alleles, and several examples of alleles in biological settings. The purpose of learning principles of heredity involves knowing what an allele is in biology and its implications on traits and diseases.

Etymology

The term "allele" is a short form of allelomorph, which was coined by British geneticists William Bateson and Edith Rebecca Saunders to characterize variable variants of a gene found as various phenotypes in the early days of genetics. It comes from the Greek adjective allos (cognate with Latin alias), which means "other," and the Greek prefix o-, allele-, which means "mutual," "reciprocal," or "each other."

What Is An 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.

Definition Of Alleles

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.

Definition Of Allele

Definition of Allele in Formal Sense: 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 Allele In Biology?

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.

Example 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. Example blue eyes.

Dominant or Recessive Phenotype-Causing Alleles

The 2 homozygous phenotypes that the heterozygote most closely resembles will determine whether the genotypic interaction between both the pair of alleles at a locus are 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 is assumed to contribute to the usual phenotypic feature as found in "wild" species of organisms, like fruit flies, is frequently referred to as a "wild type" allele (Drosophila melanogaster). In contrast to "mutation" alleles that result in recessive, uncommon, and frequently harmful phenotypes, such "wild type" alleles were historically thought to lead to a dominant (overpowering - usually produced), common, and normal phenotype.

Genetic Interaction

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.

Frequency of genotypes

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 a number of alleles (a) at a diploid locus is indicated by the expression:

Genetic Diseases Ensuing Through Allelic Dominance

When a pair of gray alleles for just a single gene trait are 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.

These relationships make the concept of alleles so crucial in genetics studies.

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.

Tips, Tricks And Strategies To Prepare For Alleles:

Some tricks and tips to revise the topic of alleles:

What is an allele?

Allele is the other form of a gene at a given locus on the chromosome.

Mnemonic: Allele = Alternate form of a gene.

Types of alleles

Dominant alleles. It will be expressed even when a single copy will be there. And,

Recessive alleles. Two copies are required for its expression.

Mnemonic: Dominant = Dominator, Recessive = Hider.

Genetic Interaction

Co-dominance- When both alleles have an equal effect on the phenotype

Incomplete Dominance- A mix of the two traits

Mnemonic: Co-dominance = Both, Incomplete = Mix

Allele Frequency

Allele frequency allows you to estimate genetic diversity within populations.

The Hardy-Weinberg principle predicts genotype frequencies from allele frequencies.

Mnemonic: Frequency = Diversity, Hardy-Weinberg = Math

Genetic Diseases

Recessive disorders are caused by two recessive alleles; dominant disorders are caused by one dominant allele.

Mnemonic: Recessive = Double, Dominant = Single.

Also Read:

Weightage Of Alleles In Various Entrance Exams

The topic of alleles has a lot of weightage in various entrance exams, as shown in the table given below:

Types Of Questions Asked On The Topic Of Alleles In Different Exams

When the students sit to write exams on NEET, CBSE and others, they should expect the sorts of types of alleles. The following is the general table that describes the sort of questions.

Conclusion

An allele or an allelomorph is the most elementary term in genetics that refers to different forms of genes existing at certain loci of the chromosomes. One of the significant tools a student of biology working to prepare for competitive exams, knowing what it is, how to define it, and examples of 'allele' will ensure victories in academics with enhanced comprehension of heredity and biodiversity.