Language
Country/Area
No result found
The mystery of genetic diversity: Do you know how many alleles each person has?
In genetics, allele, or allelomorph, refers to the variant form of nucleotide sequence at a specific site (or locus) in the DNA molecule. These alleles can differ at a single position due to single nucleotide polymorphisms (SNPs) and can include insertions and deletions that can be thousands of base pairs in length. While most alleles cause no significant changes in the function of the gene product, some alleles cause observable changes in phenotypic characteristics, such as different color or appearance.
A classic example is Gregor Mendel's discovery that the white and purple colors of pea flowers are caused by a gene with two alleles. Almost all multicellular organisms are diploid, that is, have two sets of chromosomes, at some point in their biological life cycle. For a given locus, the organism is homozygous for the alleles if the two chromosomes contain the same alleles, and heterozygous if the alleles are different.
Popular definitions of "allele" usually refer only to the different alleles in a gene. For example, ABO blood grouping is controlled by the ABO gene, which has six common alleles (variations). In population genetics, almost every human's ABO gene phenotype is some combination of these six alleles.
In many cases, the genotypic interaction between two alleles can be described as dominant or recessive, depending on which homozygote the heterozygote is more like. When a heterozygote is indistinguishable from a homozygote, the expressed allele is called the "dominant" allele and the other is the "recessive" allele. The degree and pattern of dominance vary among different loci. This type of interaction was first formally described by Mendel.
However, many traits do not fit this simple classification, and many phenotypes are explained by codominance and polygenic inheritance models. Sometimes the term "wild-type" allele is used to describe those alleles thought to contribute to the acquisition of a typical phenotype, such as in "wild" populations of Drosophila melanogaster.
Historically, "wild-type" alleles were thought to cause dominant, common, and normal phenotypes, while "mutant" alleles caused recessive, rare, and often deleterious phenotypes. However, it is now understood that most loci are highly polymorphic, possessing multiple alleles whose frequencies vary among different populations.
Populations or species often exist that contain multiple alleles at each locus. The frequency of a genotype is also related to the proportion of a particular allele. Certain gene variants, such as the null allele, have no normal function of the gene, perhaps because they are not expressed or the expressed protein is inactive.
For example, the human ABO blood group carbohydrate antigen locus is controlled by three alleles, IA, IB, and i, which determine transfusion compatibility. Each individual has 6 possible genotypes, and these genotypes produce 4 possible phenotypes: type A (produced by IAIA and IAi genotypes), type B (produced by IBIB and IBi genotypes), type AB (produced by genotypes IBIB and IBi) Produced by IIAIB genotype) and type O (produced by ii genotype). It is now known that each A, B, and O allele is actually a different set of multiple alleles.
In the identification of human ABO blood types, researchers found that the diversity of blood types is not only due to the three basic alleles. Each A, B, and O allele also has diversity caused by more than 70 different DNA sequences.
The frequency of genotypes can be used to predict the corresponding genotype frequencies, which can be understood based on the Hardy-Weinberg principle. When there are only two alleles at a locus, the genotype frequency can be expressed as a simple ratio.
If there are multiple alleles, the number of possible genotypes at a diploid locus can be expressed by a formula. So understanding the effects and potential risks of these different alleles is actually very complicated.
Some genetic diseases develop because an individual inherits two recessive alleles. Recessive genetic diseases include albinism, cystic fibrosis, etc. In addition, some diseases, such as Huntington's disease, require only one dominant allele to cause genetic problems.
Finally, although we currently focus mainly on the genetic expression of genes, epigenetic marks such as DNA methylation can also be inherited between specific genomic regions. This phenomenon is called epigenetic inheritance of transgenes and shows the complexity of genetic diversity.
So, we can’t help but wonder: among such diverse allelic variations, can you recognize the one that best represents your own characteristics?
