Microsoft User Microsoft User 2 0 2010-04-12T15:04:00Z 2010-04-12T15:04:00Z 3 407 2323 Belief Technology Forum 19 5 2725 11.9999 Clean 7.8 Pounds 0 2 False False False MicrosoftInternetExplorer4 Non-allelic Interaction
Based on the nature of non-allelic interactions, they can be categorized as follows:
① Complementary genes. Several non-allelic genes only express a certain trait when present simultaneously. A mutation in any one of these genes leads to the same mutant phenotype, and such genes are called complementary genes.
② Epistatic dominant genes. When two non-allelic genes affecting the same trait interact, the gene that determines the trait's expression is called an epistatic dominant gene or simply an epistatic gene.
③ Additive genes. For the phenotype of the same trait, each of several non-allelic genes has only a partial effect. These genes are referred to as additive genes or polygenes. In additive genes, each gene has only a small phenotypic effect, hence they are also known as minor-effect genes. Relative to minor-effect genes, genes determining a trait by themselves are called major-effect genes.
④ Modifier genes. Genes that either possess or lack any phenotypic effects but influence the expression level of another gene when both are present together. If the modifier gene itself has the same phenotypic effect, it does not differ from additive genes.
⑤ Suppressor genes. When one gene mutates and causes the phenotypic effect of another mutated gene to disappear, restoring the wild-type phenotype, the former gene is called the suppressor gene of the latter. If the suppressor gene itself has a phenotypic effect, there is no distinction between it and an epistatic dominant gene.
⑥ Regulator genes. A gene that exerts a repressive or activating effect on another gene or multiple genes is called a regulator gene. Regulator genes function by controlling the transcription of regulated structural genes. Repressive regulator genes differ from suppressor genes because suppressor genes act on mutant genes and are themselves mutant genes, whereas regulator genes act on wild-type genes and are themselves wild-type genes.
⑦ Minor-effect polygenes. The number of genes affecting the same trait is so large that their types cannot be clearly distinguished in hybrid offspring. These genes are collectively referred to as minor-effect polygenes or simply polygenes.
⑧ Background genotype. Theoretically, the action of any gene is influenced by other genes within the same cell. Except for the few genes under study, the rest constitute what is called the background genotype or residual genotype.
Allelic Interaction
In 1932, H.J. Muller summarized the relationship between mutant alleles and wild-type alleles into five categories: amorphic genes, hypomorphic genes, hypermorphic genes, neomorphic genes, and antimorphic genes.
① Amorphic genes. Mutant alleles that cannot produce the wild-type phenotype and have completely lost activity.
② Hypomorphic genes. Mutant alleles whose phenotypic effects are qualitatively similar to the wild type but quantitatively inferior.
③ Hypermorphic genes. Mutant alleles with phenotypic effects exceeding those of the wild-type allele.
④ Neomorphic genes. Mutant alleles producing new traits not found in the wild-type allele.
⑤ Antimorphic genes. Mutant alleles whose actions oppose those of the wild-type allele.
⑥ Mosaic dominance. For a given trait, one allele affects one part of the body, while the other allele affects another part. In heterozygotes, both parts are affected, a phenomenon known as mosaic dominance.
Gene and Environmental Factor Interaction
The manifestation of gene action cannot be separated from the influence of internal and external environments. Among individuals with a specific gene, the percentage of individuals expressing the gene’s trait is called penetrance. In individuals with a specific gene who exhibit the trait, the degree of its expression is called expressivity. Both penetrance and expressivity are influenced by internal and external environments.
Internal Environment refers to biological sex, age conditions, and background genotypes.
① Sex. The influence of sex on gene action is essentially the influence of sex hormones on gene action. Since sex hormones are controlled by genes, these are fundamentally results of gene interaction.
② Age. Different genes in humans display their phenotypes at varying ages.
③ Background genotype. Through selection, the penetrance and expressivity of a particular genetic trait in animal and plant strains can be altered, indicating that some gene actions are often influenced by a series of modifier genes or background genotypes.
Differences caused by background genotype can be minimized in three cases: pure lines obtained through high levels of inbreeding, monozygotic twins, and asexual reproduction lines (including asexual reproduction lines of certain higher plants, microbial asexual reproduction lines, and cell strains of higher animals). Using these systems as experimental models allows for a clearer demonstration of environmental factor influences and more precise explanations of gene action. The application of twin studies in human genetics and pure line organisms in genetics and many biological studies are based on this principle.
External Environment:
① Temperature. Temperature-sensitive mutants only show mutant characteristics at certain temperatures. For general mutations, temperature also has varying degrees of influence on gene action.
② Nutrition. The yellow fat in rabbits is determined by the homozygous state of the y gene and the presence of lutein in food. If the food lacks lutein, the fat of yy homozygotes will not turn yellow. The action of the y gene is clearly related to the assimilation of lutein.
Evolution
In terms of DNA content in cells, generally, the lower the organism on the evolutionary scale, the less DNA content, and the higher the organism, the greater the DNA content. Regarding the quantity and variety of genes, generally, the lower the organism, the fewer genes, and the higher the organism, the more genes. The increase in DNA content and gene numbers is closely related to the gradual perfection of physiological functions.
Initially, genes were abstract symbols, later confirmed as functional units occupying a specific position on chromosomes. The separation of genes in the lactose operon of E. coli and the realization of transcription in vitro further demonstrated that genes are physical entities. Now, genes can be modified in test tubes (see recombinant DNA technology) and even artificially synthesized. Research on gene structure, function, recombination, mutation, regulation of gene expression, and gene interaction remains the central focus of genetic studies.
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