A gene is a piece of DNA that stores genetic information for the synthesis of a protein or its regulation. An allele is each different version of a gene for the same protein. For example, the ABO gene codes for the human blood group protein ABO that has at least three alleles: A, B, and O.
Genes determine the physical and biochemical characteristics that an individual will have. They are manifested through visible physical features (phenotype), such as the color of the skin or eyes, in addition to the functioning of the organism.
An allele is a specific variation in the DNA sequence of a gene that determines how genetic characteristics are to be expressed in the individual. For example, the androgen receptor gene has several alleles, one of which is associated with hair loss in men (baldness or androgenic alopecia).
| Gen | Allele | |
|---|---|---|
| Definition | It is a piece of DNA that contains the genetic information for the synthesis of a protein. | It is a possible variation of a gene in a species. |
| Types | Coding genes : with information to synthesize proteins. Non-coding genes : they act as regulators of protein synthesis. Constitutive genes : they are expressed continuously. Household genes : they maintain the basic functions of the cell. Facultative genes : only expressed under certain conditions. Inducible genes : they are expressed in response to certain stimuli. | Dominant alleles : it is expressed whenever it is present. Recessive alleles : it is expressed only when it occurs in identical pairs. Wild alleles : is the allele that is predominantly expressed in the majority of the population of a species Mutant alleles : any allele other than the wild one. |
| Quantity | In diploid eukaryotic cells they occur in pairs, one from the father and the other from the mother. | For each gene there can be multiple alleles. |
| Examples | There is a gene for eye color, blood type, hair type, and skin color. | Whether the eyes are blue or brown eyes; blood, type A or B; hair, curly or straight, and skin, light or dark. |
Index
What is a gene?
The gene is the functional unit of heredity . It consists of a fragment of DNA (deoxyribonucleic acid) that contains the genetic information for the synthesis of a protein. Proteins are the macromolecules that perform the main functions of a cell.
The word “gene” comes from the Greek root génos , which means ‘race’ or ‘origin’. Gregor Mendel (1822-1884) was the first to describe genes as “bundles of heredity.”
The set of genes of an individual is known as genome and determines the genotype or DNA sequence of the genome. The phenotype is represented by the visible and measurable characteristics and traits of the living being.
The position on the chromosome of a gene is called a locus. For example, the insulin gene (INS) is found on human chromosome 11 at the 11p15.5 locus, while the gene for melanocortin receptor 1 (MC1R), responsible for skin pigmentation, is found at the 16q24.3 locus.
Characteristics of genes
- It is made up of DNA.
- They are transmitted by inheritance from the parents.
- There are about 20,000 genes in humans.
- They are transcribed in the form of messenger RNA.
- They can undergo changes that produce a genetic mutation, giving rise to alleles.
- In eukaryotes, genes have coding sequences (exons) and non-coding sequences (introns).
Functioning of a gene
For the information of the gene to be expressed in the form of protein, there are sequences called promoters in DNA, which are the starting site of transcription. The enzyme in charge of “reading the gene” is RNA polymerase, which forms messenger RNA from the genetic sequence. The messenger RNA is then translated into proteins on ribosomes.
Basically, genes store the code that transmits the specific biological characteristics of each living being, from one generation to another.
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What is an allele?
An allele is any of the variations in DNA sequence that a gene can adopt. Each gene can have two or more alleles, the difference of which can range from a single nucleotide to the addition or removal of a piece of its sequence.
Each individual inherits one allele from the father and one from the mother, for the same gene, which can be the same or different. If the alleles are the same, it is a homozygous individual; when the alleles are different, the individual is heterozygous.
The word allele is an abbreviation of allelomorph, it is composed of the Greek word allelo- , which means ‘from one to another’, and – morphé , which means ‘form’.
Characteristics of an allele
- It can be caused by a simple substitution of a nucleotide, by insertion of nucleotides or by deletion of nucleotides.
- They occupy the same locus on the chromosomes.
- They can be “wild” when it is the predominantly expressed allele in a species, or “mutant” when it differs from the wild one.
- When it is dominant : it determines characteristics visible in the next generation.
- When it is recessive : it is present, but not manifested, in the set of visible features.
- If there are two recessive alleles inherited from the parents, the traits of these alleles are manifested.
Origin of alleles
Alleles originate by substitution, insertion or deletion of nucleotides in the DNA sequence of the gene. For example, the protein Na 1.7 (which is a sodium channel) is encoded by the SCN9A gene that can present different alleles:
| Gen | Change in gene sequence | Allele |
|---|---|---|
| SCN9A | A cytosine C is exchanged for a thymine T at position 829 | 829 C>T |
| SCN9A | A cytosine C is exchanged for an adenine A at position 984 | 984 C>A |
| SCN9A | A guanine G is exchanged for an adenine A at position 5067 | 5067 G>A |
| SCN9A | A thymine T is inserted at position 2076 | 2076_2077insT |
| SCN9A | Nucleotides 3703 to 3713 are removed | 3703_3713del |
Functioning of an allele
Alleles contain information about specific traits that each gene possesses. These features can be manifest (can be seen) or can be hidden (even if they are present). For example, in rabbits the color of the coat is determined by the C gene that has four alleles:
- Brown c + color : brown rabbits are considered the wild type, which is considered the “normal” phenotype.
- Albino color c : Rabbits homozygous for this allele do not produce pigment and have white fur and pink eyes.
- Chinchilla color c ch : these rabbits have gray color.
- Himalayan color c h : these rabbits are white but have dark fur on the legs, ears and nose.
When one of the alleles is expressed in the phenotype (manifests itself), it is called dominant , while the allele that is not expressed in an individual is called recessive . For example, any rabbit with the dominant allele c + will be brown ( c + c, c + c ch, or c + c h ). The recessive allele c will only be evident in homozygous cc rabbits .
Homozygous and heterozygous
Most diploid organisms have pairs of chromosomes inherited from both parents. Each of these chromosomes has an allele of a gene. If the two alleles located in the same place ( locus ) of their respective chromosome are the same , the cell or individual is homozygous . When the alleles located at the same locus on the chromosome are different , they are called heterozygous .
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