Key Concepts in Genetics: Gene, Locus, and Allele
Key Concepts in Genetics
The Gene
A gene is organized in a linear sequence of nucleotides in the molecule of DNA (or RNA in the case of some viruses), which contains the information necessary for the synthesis of a macromolecule with specific cellular function, usually proteins, but also mRNA, rRNA, and tRNA.
The Locus
In biology, a locus is a fixed position on a chromosome, the position of a gene or a genetic marker. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a genome is called a genetic map, and the process of determining the locus of a given biological nature is called gene mapping.
Alleles and Genetic Dominance
Allele Definition
An allele is each of the alternative forms that a gene can have, differing in their sequence, which can manifest in specific changes in the function of that gene. As the majority of mammals are diploid, they have two alleles for each gene, one from the father and one from the mother. Each pair of alleles is located in the same locus or location on the chromosome.
Genetic Dominance
Genetic dominance describes the relationship between different forms (alleles) of a gene at a particular physical location (locus) on a chromosome. Animals and plants usually have two sets of chromosomes (diploid), inherited from one parent, meaning they have two alleles at each locus.
- If the two alleles of the same locus are identical, the individual is said to be homozygous.
- If the two alleles are different, the individual is heterozygous.
The genetic organization of an organism, both in terms of a locus and over all genes collectively, is called its genotype. The genotype of an organism affects, directly or indirectly, the manifestation of traits at the molecular, physical, and behavioral levels, which is collectively called the phenotype.
Recessive Alleles
In genetics, the term applied to the recessive gene refers to a member of an allelic pair unable to be expressed when the dominant allele is present. For this allele to be observed in the phenotype of the organism, it must have two copies, one coming from the mother and one from the father.
Homozygous and Heterozygous States
Homozygous
It is said that a cell is homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell or organism in question is called homozygous. This term comes from the Greek homozygous (homo = same, zygote = egg).
Heterozygous
Heterozygous refers to an individual in which the two chromosomes of a pair are in the same situation, carrying two different genes (e.g., one normal gene and one disease gene).
Genotype and Phenotype
Phenotype
In biology and health sciences, the phenotypic expression of genotypes in a given environment is called the phenotype. Phenotypic traits include both physical and behavioral characteristics. Importantly, the phenotype cannot be defined merely as the “visible manifestation” of the genotype, because sometimes the characteristics studied are not visible in the individual, such as the presence of an enzyme. A phenotype is any observable characteristic or trait of an organism, such as its morphology, development, biochemical properties, physiology, and behavior.
Genotype
The genotype is the content of an individual’s genome specified in the form of DNA. Along with environmental variation that influences the individual, it encodes the individual’s phenotype. Alternatively, the genotype can be defined as the set of genes of an organism, and the phenotype as the set of traits of an organism. Therefore, scientists and doctors sometimes talk, for example, about the (geno)type of cancer, particularly separating the patient’s disease. Although a random mutation can change the codons for different amino acids (changing the sequence encoding a gene that does not necessarily alter the phenotype), the entire genetic endowment is called the genotype.