There is a striking phenomenon in the process of biological reproduction, that is, the relative stability of traits between generations of the same species. As trees are, so are fruits. This is a biological inheritance. There is also a striking phenomenon in the process of biological reproduction, that is, the traits of the same organism between generations or between different individuals in the same generation will not be exactly the same. For example, there are more or less differences in characters between the grains on the same rice ear and the grown plants; Even identical twins can't be exactly the same. This difference is a manifestation, that is, biological variation.
Heredity and variation are a pair of contradictions in life activities, which are both opposite and unified. Heredity is relative and conservative; And change is absolute and developing. Without heredity, it is impossible to maintain the relative stability of species; Without variation, it is impossible to form new species, and it is impossible to have such a colorful and varied biological world today.
Variation caused by changes in genetic material is hereditary; Variations due to changes in environmental conditions are generally only manifested in the contemporary era and cannot be inherited. In other words, variation can be divided into two categories: genetic variation and non-genetic variation. What needs to be emphasized here is that the division of these two types of variation is relative. Because under certain environmental conditions, through long-term directional influence and selection, the accumulation of quantitative change can be transformed into qualitative change, and genetic variation may be formed without genetic variation.
The inheritance of biological characters is bridged by germ cells. That is, after meiosis during gamete formation, when gametes form zygotes, the number and content of chromosomes in parent cells are restored. And DNA is only an important part of chromosome, so chromosome is the main carrier of DNA, and gene is a DNA fragment with genetic effect.
The change and development law of genetic material is directly related to the stability and instability of biological material movement. The stable transmission of genetic material makes organisms show heredity, which is related to the stable development of biological races; The unstable transmission of genetic material makes organisms show variation, which is related to the development and evolution of biological races. This fully embodies some important laws of the movement, change and development of biological substances (mainly nucleic acids and protein).
The main carrier of genetic material-chromosome
Chromosomes can maintain certain stability and continuity during cell mitosis, meiosis and fertilization. This is the earliest observed phenomenon that chromosomes are related to heredity. The main components of chromosomes are DNA and protein. Chromosome is the main carrier of genetic material, because most genetic material (DNA) is on chromosome. Mitochondria and chloroplasts also contain a small amount of DNA, so mitochondria and chloroplasts are called secondary carriers of genetic material.
In genetic research and breeding practice, biological traits can be divided into two categories: qualitative traits and quantitative traits according to the genetic variation law in the population (natural population or hybrid offspring population).
Those traits that are not easily influenced by environmental conditions and show discontinuous variation in a population are called quality traits, such as the shape of pea seeds (integrity and shrinkage), the color of cotyledons (yellow and green), the color of flowers (red and white) and so on. Quality traits are determined by one or several genes with great effects (called major genes), which are less affected by the environment, so they show discontinuous variation and can clearly classify individuals in the population. The color of peas, the sex of animals, and various blood type systems of human beings all belong to this kind of traits. In genetic research, quality traits are often used as marker traits because they are easy to track.
Traits that are easily influenced by environmental conditions and show continuous variation in a population are called quantitative traits, also known as quantitative traits. In biology, quantitative traits are more common and extensive than qualitative traits; Most agronomic traits of crops are quantitative traits, such as plant grain yield or nutritional yield, plant height, maturity, seed grain weight, protein and oil content, and even disease resistance and insect resistance.
Because of the discontinuous variation of quality traits, individuals can be clearly grouped and classified according to the differences of relative traits, and the proportional relationship of the number of individuals contained in each type can be found out, and the characteristics of each type can be described and explained in words.
Because of the continuous variation of quantitative traits among different individuals in natural population or separated population of hybrid offspring, it is impossible to clearly group and classify individuals by Mendel method, and it is also impossible to analyze quantitative traits by analyzing quality traits. Instead, it uses biostatistics to quantitatively describe the genetic variation of traits and study the genetic dynamics of traits.
However, the division of qualitative traits and quantitative traits is not absolute, such as:
For the same trait of the same crop, it may be different in the cross combination of different parent materials, such as the plant height of rice and wheat.
On the basis of the inheritance of major genes, there are also a group of minor genes-modification genes for some traits, such as the red (crimson or purple-black) and white of wheat and rice seed coat, which show the separation of a pair of genes in some hybrid combinations, while in other hybrid combinations, the grain color of F2 turns red to varying degrees and becomes continuous variation, that is, it shows the characteristics of quantitative trait variation.
In practical application, all traits susceptible to environmental conditions can be analyzed by studying quantitative traits.
Quantitative traits are generally susceptible to environmental conditions and variation, which cannot be inherited.
Due to the influence of environmental conditions, even two parents with the same genotype (P 1 and P2) and the first hybrid with the same genotype (F 1), each individual presents continuous variation, instead of a genotype with only one value; This variation among individuals in the same genotype population is caused by environmental conditions and cannot be inherited. For F2 population, there are both phenotypic variations caused by genotype differences among individuals caused by gene segregation and phenotypic differences of the same genotype caused by environmental conditions. The former variation is heritable, while the latter variation is not. The combination of these two variations makes the continuous variation of F2 population more extensive than its parents and F 1 generation, and the coefficient of variation of F2 generation is obviously greater than that of P 1, P2 and F 1. Therefore, it is very important to accurately estimate the heritable variation of quantitative traits caused by genotype differences and the heritable variation caused by environmental conditions to improve the efficiency of quantitative trait breeding.