动物遗传学,实验动物 z-bo 2020-10-13 332

　　1. Basic concepts of genetics

　　(one) chromosome

　　The filaments in the cell nucleus during the division phase, which are observed under an optical microscope, are called chromosomes. The number and morphology of chromosomes vary from species to species. Genes are arranged on the chromosomes, and the chromosomes formed by the combination of DNA, histones and non-histone proteins are the main components of the cell nucleus. The content of DNA in the nucleus of different kinds of organisms varies greatly; while somatic cells from different tissues of the same organism have the same DNA content in the nucleus; the content of DNA in the nucleus of sex cells is only half of that in the nucleus of somatic cells. Each individual cell of an animal has two sets of the same chromosomes called diploid, which is represented by 2n. For example, the somatic cell of a mouse is 2n=40, and the sex cell is n=20. One pair of chromosomes is related to sex and is called sex chromosome, and the rest are called autosomes. In autochromosomes, every two chromosomes of the same shape and size form a pair, which is called a homologous chromosome. One of the homologous chromosomes comes from the father and the other comes from the mother. The chromosome numbers of commonly used experimental animals are shown in Table 7-1.

　　(2) Genes, alleles and gene mutations

　　1. Gene Gene is a specific segment of DNA molecules on chromosomes in a cell, and is the basic unit for transmitting genetic information. In animal somatic cells, only 1% to 2% of genes are active and can transcribe mRNA. These genes are present in a loose state; the rest of the genes are combined with histones to become inactive high-level helices. Different cells have different active and inactive gene profiles, which is the basis of cell differentiation.

　　2. Alleles Genes are arranged in a straight line on the chromosome, and each gene has its own specific locus. Genes occupying the same locus on homologous chromosomes are called alleles. There are two or more variants of genes in a locus in a population, which are called multiple alleles. Many genes have multiple alleles, which makes biological populations have great variability. This is very beneficial to the biological population. Diversified individuals can easily adapt to different environmental conditions. For example, gene loci that control immunological traits such as blood type are multiple alleles.

　　3. Gene mutations Each animal gets two sets of genes from their parents, one of which is from the father and the other from the mother. There are thousands of genes in each set, and they can be stably passed on to the next generation. But in the process of its transmission, a certain gene may mutate accidentally, that is, gene mutation (mutation). The so-called gene mutation is the change of the base on the long chain of the DNA molecule or the mutation of the genetic material at a certain locus on the chromosome. Mutations that occur under natural conditions are called natural mutations or spontaneous mutations, and those induced by artificial methods are called induced mutations. Mutations are universal, that is, genes that affect various traits can be mutated. Mutations are reversible. This rule is very important in experimental zoology. When we use mutant animals, if the conservation work is not good, the mutant species may return to the wild type.

　　(three) traits

　　Character refers to the morphological, physiological, biochemical, or psychological characteristics that can be observed in any organism. The different traits are the result of the interaction of the genes involved in the expression process with the environment. The non-continuous variation of hair color, such as black and brown, hairy and no hair, is called qualitative character. Traits that can be clearly distinguished are called qualitative characters, while those that are continuous but cannot be clearly graded in weight, length, and life span. Quantitative character. Since the phenotype of a quantitative trait is a continuous observation value obtained by measurement, each observation value (phenotype) cannot directly correspond to the genotype, so statistics such as population average or standard deviation must be used for analysis when selecting.

　　(4) Genotype frequency and gene frequency

　　These two concepts are the category of population genetics. Here, try to examine the pair of alleles A and a at a gene locus on an autosome. There are three possible genotypes, namely A/A, A/a, and a/a. The ratio of the three genotypes can be used as an indicator of population genetic composition. In a population composed of N individuals, the number of individuals with A/A, A/a and a/a genotypes is n1, n2, n3 (n1+n2+n3=N), then the A/A in the population The proportions (P, Q, R) of A/a and a/a genotypes are:

　　A／A= P=n1／N

　　A／a type Q=u2/N

　　A／a type R=n3/N

　　P, Q, R are called genotype frequency (genotype frequency). Secondly, judging from the ratio of A and a genes in the population, the population is composed of N individuals, so the corresponding gene locus is 2N. Furthermore, individuals with genotype A/A have two A genes, and individuals with A/a type have one A gene, so the proportion of A genes in the population (p) is 3/4.

　　(5) Mendel's law of inheritance and its extension

　　Mendel (Mendel, 1865) used 7 pairs of relative traits such as flower color (white flower and purple flower) and seed shape (full circle and shrunken) of pea as indicators, and crossed experiments to derive Mendel's law of inheritance. The genetic law of experimental animals still follows this law. Now take mice as an example.

　　1. A pair of relative traits and the law of separation. Each generation of mice that produce only colored strains is mated with albino (Albino, white fur and red eyes) mice, and the first filial generation (F1) mice produced are all colored. The colored traits displayed by F1 are called dominant traits, and the albino traits lacking in F1 are called recessive traits. This is Mendel's law of dominance (low 0f dominance). Secondly, the colored traits and albino traits each account for a certain proportion (3:1) in the second generation of hybrids (F2) produced by F1 mating with each other, which is called the 10w of segregation. This phenomenon can be explained by the gene locus (allele) that governs the presence or absence of pigment in the fur. For example, write C as the gene supporting color and c as the gene that determines albino, the relationship between the law of explicitness and the law of separation

　　2. Two or more pairs of relative traits and the law of free combination As mentioned above, traits controlled by a pair of genes in a dominant and recessive relationship are inherited according to the law of separation. And how are two or more pairs of traits inherited? Two pairs of relative traits, such as colored and hairy mice, when mating with albino and hairless mice, F1 are all colored and hairy mice. F2 from F1 mating with each other is colored and hairy, colored and hairless, albino hairy and albino hairless mice each account for a certain proportion, namely 9:3:3:1. At this time, if you only look at colored and albino, or hairy and hairless, they are separated by 3:1. That is to say, colored and albino traits have no effect on the hairy and glabrous traits, while the hairy and glabrous traits have no effect on the colored and albino traits. These alleles are independent of each other and each play a genetic role, which is called the law of free combination. For alleles with a complete recessive relationship, when there are n pairs of alleles inherited independently from each other, the theoretical separation ratio of F2 produced by their mating is an expansion of (3+1)n.

　　3. The dominant-recessive relationship between incomplete dominant and co-dominant alleles is not obvious. The phenotype of heterozygous (Aa) is between the homozygous dominant and homozygous recessive phenotypes, rather than homozygous The dominant is the same. The inheritance of this trait is called incomplete dominance, semi dominance, and non dominance. The mouse W gene (dominant spotting) is equivalent to this. White spots appear on the fur of heterozygotes (W/+), and the fur of homozygotes (W/W) is white. Different phenotypes are called codominance when they have traits of both parents. It should be emphasized that the traits of heterogeneous proteins or isoenzymes and certain immunological traits that are seen on the electrophoresis diagram in genetic monitoring rarely have a genetic recessive relationship, and they are generally co-dominant.

　　4. Sex-linked heredity The genes on the sex chromosomes are inherited differently from autosomal ones. An example is the sparse-fur gene (Sparse-fur, Spf). When a homozygous hair-poor female mouse mates with a normal male mouse, the F1 female mouse is normal but the male mouse is hair-poor. This is because the hair-poor gene is located on the X chromosome. The F1 female mouse has an X chromosome from both parents, so the inherited gene is +/Spf, and its phenotype is normal. However, the chromosome combination of the male mouse is XY, that is, the male mouse only inherits one X chromosome from the parental generation, and there is no Spf gene on the X chromosome on the Y chromosome, so the genotype is Spf/Y (hemizygous), and the phenotype is For poor hair. This phenomenon is called sex-linked inheritance.

　　2. Experimental animal species and strains

　　(One) kind

　　species (species) is the most basic unit of biological classification. In experimental zoology, species refers to animals of the same species that can mate with each other and whose offspring have the ability to reproduce.

　　(two) varieties

　　is used to refer to domestic animals and laboratory animals as a classification unit below species, and is not a classification of animal taxonomy. Breed (stock, hreed) generally refers to an animal population that has some traits that are easy to recognize and people need, and can be basically inherited stably. Generally used in closed groups, such as New Zealand white rabbits, big-eared white rabbits, Wistar rats, KM mice, etc.

　　(Three) strain

　　In experimental zoology, animals that are highly genetically homozygous are called strain animals. Usually refers to inbred lines and mutant lines. For example, C57BL/6 is a strain of inbred animals, and nude mice are strains with mutant genes (nu/nu).

　　Three, genetic classification

　　From the perspective of genetics, laboratory animals are animals with a clear genetic background and strict genetic control. According to different genetic characteristics, the experimental animals are divided into inbred lines, closed groups, hybrid groups, etc. Different experiments require the use of experimental animals of different breeds and strains. In addition, it can be divided into the same genotype and different genotypes according to the gene type. The same genotype includes inbred lines and hybrids (F1), and different genotypes include closed groups and mutant lines.