What are the classifications of genes?

In the early 1960s, F. Jacob and J. Mono discovered regulatory genes. Genes are divided into structural genes and regulatory genes, focusing on the function of protein encoded by these genes: all genes encoding protein such as enzyme protein, hemoglobin, collagen or crystallin are called structural genes; Any gene encoding protein that inhibits or activates the transcription of structural genes is called a regulatory gene. But from the original function of genes, they all encode protein. According to the original function (that is, the product of the gene), the gene can be divided into:

(1) The gene encoding protein. Includes structural genes encoding enzymes and structural proteins and regulatory genes encoding repressors or activators acting on the structural genes. ② Genes without translation products. Transcribed into RNA, it will not be translated into protein's transfer ribonucleic acid (tRNA) gene and ribosomal nucleic acid (rRNA) gene. ③ Non-transcribed DNA fragments. Such as promoter region, operator gene, etc. The former is the site where RNA polymerase begins to bind to DNA during transcription; The latter is the site where the repressor or activator protein binds to DNA. It has been found that there are many mutants in Drosophila that affect the development process. The genes that control the temporal-spatial relationship include time sequence genes, model genes, selection genes and so on.

The action time of each gene in organisms is often different, and some genes are transcribed before replication, which is called early genes; Some genes are transcribed after replication and are called late genes. When a gene mutates and changes several seemingly unrelated traits at the same time, the gene is called pleiotropic.

The number of genes in different organisms varies greatly. It has been proved that the RNA phage MS2 has only three genes, while each cell of mammals has at least 1 10,000 genes. But most of them are repetitive sequences. In non-repetitive sequences, it is estimated that there are no more than 65,438+10,000 genes encoding peptide chains. In addition to simple repetitive genes, there are many genes with similar structures and functions, which are often closely linked to form so-called gene complexes or gene families.

allelic genes

Genes located in the same position of a pair of homologous chromosomes control different forms of a trait. Different alleles can lead to changes in genetic characteristics, such as hair color or blood type. Alleles can be divided into different categories by controlling the dominant and recessive relationships and genetic effects of relative traits. In an individual, one form of allele (dominant) can be expressed much more than other forms (recessive). An allele is another version of the same gene. For example, there is more than one "version" of the gene that controls tongue rolling, which explains why some people can roll their tongues and some people can't. Defective gene versions are associated with certain diseases, such as cystic fibrosis. It is worth noting that each chromosome has a pair of "copies": one from the father and the other from the mother. In this way, each of our approximately 30,000 genes has two copies. These two copies may be the same (same allele) or different. This is what chromosomes look like when cells divide. If we compare the gene bands in the same part of two chromosomes (male and female), we will see that some gene bands are the same, which means that the two alleles are the same; However, some gene bands are different, indicating that these two "versions" (that is, alleles) are different.

Pseudoalleles: Genes with similar phenotypic effects, closely related functions and closely linked positions on chromosomes. They are like alleles, but they are not alleles.

Due to the discovery of quasi-allele phenomenon, the traditional concept of gene is more complicated. In the early discovery, the Morgan School was particularly surprised that adjacent genes generally seemed to have nothing to do with each other in function and went their own way. Genes that affect eye color, wing vein formation, bristle formation, body color, etc. May all be adjacent to each other. "Genes" with very similar functions are generally just alleles of a single gene. If genes are exchange units, alleles will never recombine. In fact, Morgan's students failed in their early attempts to find the allele exchange at the white eye locus, and later learned that this was mainly due to the small number of experimental samples. Oliver succeeded first, and found evidence of unequal allele exchange in the diamond locus of common Drosophila. The frequency of heterozygotes with two different alleles (Izg/Izp) spliced together by marker genes is about 0.2%. The recombination of marker genes proved that there was an exchange between "alleles".

The exchange between very close genes can only be observed in a very large number of test samples. They are called quasi-alleles because their normal behavior seems to be alleles. They are not only functionally similar to real alleles, but also can produce mutant phenotypes after transposition. They exist not only in fruit flies, but also in corn, especially in some microorganisms. There have been many explanations for this problem in molecular genetics, but the gene regulation of eukaryotes is still unknown, so it has not been fully understood.

The discovery of position effect has a far-reaching impact. Dubzhansky made the following conclusion in a review article: "Chromosome is not only a mechanical aggregate of genes, but also a unit of higher structural level ... The nature of chromosome is determined by the nature of genes as its structural unit; However, chromosome is a harmonious system, which not only reflects the history of biology, but also is the decisive factor of this history. "

Some people are not satisfied with this gentle revision of the gene "beading concept". Since the rise of Mendelianism, some biologists have cited evidence that seems to be enough to oppose the particle theory of genes. The position effect is just in their favor. As a result, Goldschmidt became their most eloquent spokesman. He proposed "modern gene theory" to replace particle theory. According to his new theory, there is no localized gene, only "a certain molecular pattern on a certain segment of chromosome, and any change of this pattern (the position effect in the broadest sense) will change the role of chromosome components, thus showing itself as a mutant." Chromosome as a whole is a molecular "field". Traditionally, the so-called genes are discrete or even overlapping areas in this field. Mutation is the recombination of chromosome field. This field theory contradicts a large number of facts in genetics and cannot be recognized, but the fact that an experienced and famous geneticist like Goldschmidt put forward this theory so seriously shows how unstable the genetic theory is. Many theoretical articles published from 1930 to 1950 also reflect this point. Multi-allele: If a gene has multiple allele forms, this phenomenon is called multi-allele. There are only two different alleles in any diploid individual.

In complete dominance, homozygotes and heterozygotes in dominant genes have the same phenotype. The phenotype of incomplete dominant heterozygote is the intermediate state between dominant homozygote and recessive homozygote. This is because one gene in the heterozygote has no effect, while the other gene has dose effect. The phenotype of complete dominant heterozygote is the phenotype of dominant and recessive homozygote. This is due to the expression of a pair of alleles in heterozygotes.

For example, genes IA, IB and I that determine the four blood types of human ABO blood group system, each person can only have any two of these three alleles.