Does the hybrid population 7: 1 conform to Mendelian inheritance?

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The formation and development of any subject is always closely related to the outstanding figures who were keen on this scientific research at that time, and the formation and development of genetics is no exception. Mendel is an outstanding founder of genetics. He revealed two basic laws of genetics-separation phenomenon and free combination law.

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catalogue

Introduction to 1 Mendel

2 separation of mendelian law

3 Mendel's Law of Free Combination

4 the significance of Mendel's genetic law in theory and practice

Mendel's Law of Inheritance-Introduction of Mendel

1822 was born in a poor peasant family in Heisendorf, Austria. His father is good at gardening. Under the direct influence of his father, Mendel loved gardening since childhood. 1843, after graduating from high school, he was admitted to the School of Philosophy of Olmertz University for further study, but he was forced to drop out of school because of his poor family. 1843 10 month, forced by life, he stepped into a monastery in the Austrian city of Bloom and became a monk. From 185 1 to 1853, Mendel studied in Vienna University for four semesters, systematically studying botany, zoology, physics and chemistry. At the same time, he received good scientific research training, which laid a solid theoretical foundation for his later scientific research on plant hybridization. 1854, Mendel returned to his hometown and continued to work in the monastery. In his spare time, he started 12 years of plant hybridization experiments.

Among a large number of plant hybridization experiments conducted by Mendel, pea hybridization experiment is the best. After eight years' unremitting efforts (1856- 1864), the paper "Plant Hybridization Experiment" was finally published in 1865, which put forward the argument that genetic unit is genetic factor (called gene in modern genetics) and revealed two basic laws of genetics-separation phenomenon and the law of free combination. The discovery and presentation of these two important laws laid a solid foundation for the birth and development of genetics, which is also an important scientific research achievement that Mendel will leave behind.

Although Mendel's immortal thesis has been published, it is a pity that his creative thought, which is different from his predecessors, is too advanced for his time, so that his scientific papers have not attracted the attention of his biological colleagues for 35 years. It was not until 1900 that his discovery was confirmed by three European botanists of different nationalities in their pea hybridization experiments that he was paid attention to and recognized, and the study of genetics developed rapidly from then on.

Mendel's Law of Inheritance-separation of mendelian law

Pea has some stable and easily distinguishable characters, which meet Mendel's experimental requirements. The so-called traits refer to the shape, structure, physiology and health of organisms.

Culture and other characteristics of the floorboard. In the hybridization experiment, Mendel devoted himself to studying the genetic laws of seven pairs of related traits. The so-called relative character refers to different types of expression of the same character of the same organism. For example, the color of peas is divided into red flowers and white flowers, and the shape of seeds is divided into round grains and wrinkled grains. In order to facilitate the analysis and research, he first studied the transmission of only one pair of relative traits, and then observed the transmission of multiple pairs of relative traits together. This analysis method is an important reason for Mendel's success.

Dominant traits and recessive traits

As we all know, the eye-catching title of Mendel's paper is "plant hybridization test", so his method is mainly "hybridization test method". He cross-pollinated their different plants with purebred high-stem peas and short-stem peas as parents (parents are denoted by P). Fig. 2-4 is a schematic diagram of cross pollination of high-stem pea and short-stem pea. The results showed that the first generation plants (referred to as "offspring", F 1) obtained by crossing tall stems and short stems as female parents, tall stems as male parents, and short stems as female parents (that is, both orthogonal and cross) showed tall stems. That is to say, as far as this pair of relative traits is concerned, the trait of F 1 can only show the trait of one parent-tall stem, while the trait of the other parent-short stem is not shown at all in F 1.

For another example, when improved varieties of red peas and white peas are crossed, F 1 plants are all red peas, regardless of cross or backcross. Because of this, Mendel called the traits that F 1 can express, such as tall stems and safflower, as dominant traits, while the traits that F 1 cannot express, such as short stems and white flowers, as recessive traits. Mendel got the same result on the other five pairs of relative traits of peas, that is, they all have dominant traits and recessive traits that are easy to distinguish.

Separation phenomenon and separation ratio

In the above Mendel hybridization experiment, because only one relative trait-dominant trait was shown in the hybrid F 1, did the other relative trait-recessive trait disappear? Can you show it? With such doubts, Mendel continued his hybridization experiment.

Mendel personally pollinated the tall pea of F 1, and then sowed F2 pea seeds in the second year to obtain hybrid F2 pea plants. Results There are two types: one is high pea (dominant trait) and the other is short pea (recessive trait), that is, two different manifestations of a pair of relative traits-high pea and short pea. Mendel's suspicion was lifted, and this phenomenon was called separation. Moreover, Mendel also found from the statistics of tall and short-stemmed peas in F2: among 1 0,064 peas, there are 787 tall and 277 short-stemmed peas, and the ratio between them is about 3∶ 1.

Mendel used the same experimental method to carry out self-pollination with red pea F 1. There are also two types of pea plants in hybrid F2: one is red pea (dominant trait) and the other is white pea (recessive trait). The statistical results showed that among 929 peas, there were 705 red peas and 224 white peas, and the ratio of them was also close to 3∶ 1.

Mendel also did the same hybridization test for five other pairs of relative traits, and the results were the same.

We summarize the results of Mendel's hybridization experiment, and at least three points are worth noting:

All the plants of (1)F 1 showed only one parent's trait (dominant trait), while the other parent's trait was temporarily concealed and not shown (recessive trait).

(2) In F2, the relative traits of hybrid parents-dominant traits and recessive traits-appear again, which is the phenomenon of trait segregation. It can be seen that the recessive trait did not disappear in F 1, but was temporarily concealed and failed to show.

(3) In F2 population, the number of dominant traits and the number of recessive traits often showed a certain segregation ratio, which was about 3∶ 1.

Explanation of character separation phenomenon

Mendel was surprised by three remarkable regularity phenomena reflected in the above seven pea hybridization experiments. In fact, he has realized that this is definitely not some accidental coincidence, but a universal genetic law, but he is still puzzled by the trait segregation ratio of 3∶ 1. After some creative thinking, I finally realized that I put forward the hypothesis of separation of genetic factors, and its main contents can be summarized as follows:

(1) The inheritance of biological characters is determined by genetic factors (genetic factors are later called genes).

(2) Genetic factors exist in pairs in somatic cells, in which one member comes from the male parent and the other member comes from the female parent, which are brought in by sperm cells respectively. In the process of gametophyte formation, pairs of genetic factors are separated from each other and enter a gamete respectively. In this way, each gamete contains only one member of the paired genetic factors, which may come from the male parent or the female parent.

(3) In somatic cells of hybrid F 1, the members of the two genetic factors are different, and they are in a state of independence and non-interference, but their roles in trait development are obviously different, that is, one plays a decisive role in the other, so there are dominant factors and recessive factors, and then there are dominant and recessive traits.

(4) The hybrid F 1 produced the same number of different types of gametes, and the combination of male and female gametes was random, that is, the combination opportunities of different types of female gametes and male gametes were equal.

In order to better prove the separation phenomenon, a pair of genetic factors are used to illustrate Mendel's pea hybridization experiment and its hypothesis, as shown in Figure 2-5. We use the capital letter D to represent the dominant genetic factor that determines the tall pea, and the lowercase letter D represents the recessive genetic factor of the short pea. Genetic factors exist in pairs in somatic cells of organisms. Therefore, there are a pair of dominant genetic factors dd that determine the high stem traits in the somatic cells of purebred high stem peas, and a pair of recessive genetic factors DD that determine the short stem traits in the somatic cells of purebred short stem peas. In the somatic cells of F 1 produced by hybridization, D and D combine to form D D. Because D (tall stem) is dominant to D (short stem), all plants in F 1 are tall-stem peas. When F 1 undergoes meiosis, its paired genetic factors D and D have to be separated from each other, resulting in two different types of gametes. One is a gamete with a genetic factor of D, and the other is a gamete with a genetic factor of D. The two gametes are equal in number, each accounting for 1/2. Therefore, the combination of the above two kinds of male and female gametes produces three combinations of dd, Dd and DD, and the ratio between them is close to 1: 2: 1, while the personality performance is close to 3 (height): 1 (height).

Therefore, Mendel's hypothesis of genetic factors gives a scientific and satisfactory explanation to the similar results obtained from pea hybridization experiments.

Genotype and phenotype We can see that in the genetic analysis of the above-mentioned pair of genetic factors, what is inherited and what is finally displayed is not exactly the same thing. For example, when the genetic structure is Dd type, it is a long-stemmed pea, and when the genetic structure is DD type, it is also a long-stemmed pea. In this way, the physical and physiological characteristics of biological individuals are called phenotypes, such as tall and short stems, red flowers and white flowers; However, the genetic basis of an individual or a trait is called genotype. For example, there are two genotypes of tall pea, Dd and DD, while there is only one genotype of short pea. Individuals developed from zygotes combined with gametes of the same genetic factor are called homozygotes, such as dd and DD plants; Individuals developed from zygotes formed by the combination of gametes of different genetic factors are called heterozygotes, such as Dd.

Genotype is the genetic material structure within an individual, so the genotype of an individual largely determines its phenotype. For example, pea plants with dominant genetic factor D (dd and Dd) all show tall stems, while pea plants without dominant genetic factor Dd all show short stems. It can be seen that genotype is the internal factor of trait expression, while phenotype is the expression form of genotype.

From the above analysis, we can also know that the phenotype is the same, but the genotype is not necessarily the same. For example, the phenotype of Dd and Dd are both tall stems, but the genotypes are different, and the next generation is different: the next generation of DD is tall stems, while the next generation of DD is separated-both tall stems and short stems.

Separation phenomenon's verification

As mentioned earlier, Mendel's explanation of the phenomenon of separation is only based on a hypothesis, and he himself is well aware of this. Hypothesis, after all, is only a hypothesis and cannot be used to replace truth. It is far from enough to make this hypothesis rise to scientific truth, and it needs to be verified by experiments. The following introduces the first verification method designed by Mendel, which is also the intersection method he uses the most.

Test crossing is to cross the first generation hybrid seeds with recessive types to determine the genotype of F 1. According to Mendel's explanation of segregation, F 1(Dd) of hybrid seeds will certainly produce two kinds of gametes with genetic factors of D and D, and the number is equal; However, recessive type (dd) can only produce one kind of gamete with recessive genetic factor D, which will not cover the role of genetic factor in F 1. Therefore, the offspring produced by test crossing should be half height (dd) and half height (Dd), that is, the ratio of the two traits is 1: 1.

Mendel crossed the first generation of tall pea (dd) with short pea (Dd) and got 64 offspring * * * plants, including 30 tall peas and 34 short peas, that is, the character segregation ratio was close to1:1.The experimental results were in line with the pre-hypothesis. The segregation ratio of other pairs of related traits is about 1∶ 1 without exception.

Mendel's experimental results eloquently prove that his hypothesis of genetic factor separation is correct and completely scientific.

The essence of separation phenomenon

The hypothesis of genetic factor separation put forward by Mendel has been fully verified by a series of experiments such as his own crossover experiment, and it has also been confirmed by countless experiments in later generations. Now it has been recognized by the world and is honored as separation of mendelian law. So, what is the essence of separation of mendelian law?

This can be summarized in one sentence, that is, in the process of meiosis, the paired genetic factors that determine a certain trait are separated from each other and do not interfere with each other, so that there is only one paired genetic factor in the gamete, thus producing two gametes with the same number and passing them on to the offspring independently. This is separation of mendelian law.

Mendel's law of heredity-Mendel's law of free combination

After Mendel revealed the genetic law of a pair of relative traits crossing controlled by a pair of genetic factors (or a pair of alleles)-separation phenomenon, the quick-thinking scientist successively conducted two, three or more genetic experiments of relative traits crossing, and then discovered the second important genetic law, namely the law of free combination, also known as the law of independent distribution. This paper only introduces the hybridization experiment of two pairs of relative traits he carried out.

Observation on the phenomenon of hybridization test

Mendel still used peas as materials when he carried out two pairs of cross experiments of related traits. He chose two pairs of homozygotes with relative differences in traits as parents for hybridization. One parent is a yellow round seed, and the other parent is a green wrinkled seed. F 1 obtained by orthogonal or cross is yellow round seed. Therefore, the yellow of pea is dominant to green, and the round grain is dominant to wrinkled grain, so the pea of F 1 shows the yellow round grain character.

If the seeds of F 1 are sown, the plants will be self-pollinated, and there will be obvious morphological separation and free combination in F2. Among the 556 F2 seeds counted by * * *, there are four different expression types.

If the least number of 32 blue-wrinkled seeds is taken as the ratio of 1, then the digital ratio of the four phenotypes of F2 is about 9: 3: 3:1. As shown in Figure 2-7, the genetic experiment of two pairs of relative characters of pea seeds.

It can be seen from the results of the above-mentioned pea hybridization experiment that among the four types of F2, there are two original combinations of parents, namely yellow wrinkle grains and green wrinkle grains, and two new combinations, namely yellow wrinkle grains and green wrinkle grains, which are different from the types of parents, and the results show free combinations of different related characters.

Analysis of hybridization test results

Mendel found that if only one pair of relative traits is considered, the ratio of dominant traits to recessive traits in the hybrid offspring still conforms to the approximate ratio of 3∶ 1.

The actual situation of the separation ratio of the above traits fully shows that the inheritance of these two pairs of relative traits is controlled by two pairs of genetic factors respectively, and their transmission mode still conforms to the separation law.

In addition, it also shows that the separation of a pair of relative traits has nothing to do with the separation of another pair of relative traits, and they are genetically independent of each other.

If these two pairs of relative traits are considered together, the segregation ratio of this F2 phenotype should be the product of their respective F2 phenotypic segregation ratios (3∶ 1). This also shows that the two alleles controlling the relative traits of yellow, green, round and wrinkle can be separated from each other and combined freely.

Explanation of the phenomenon of free combination

So, how to explain the above genetic phenomenon? According to the results of the above hybridization experiments, Mendel put forward the theory that different pairs of genetic factors can be freely combined in gamete formation.

Because one of the parents originally selected is homozygous, its genotype is YYRR. Here, y stands for yellow and r stands for circle. Because they are all dominant, they are represented by capital letters. Another parent, green wrinkled pea, is also homozygous, and its genotype is yyrr, where Y stands for green and R stands for wrinkle. Because it is hidden, it is represented by lowercase letters.

Since both parents are homozygotes, they can only produce one type of gamete, namely:

YYRR-Year

When they hybridize, yr gamete combines with YR gamete, and the genotype of F 1 is YyRr, that is, they are all heterozygotes. Because of the recessive relationship between genes, the phenotypes of F 1 are all yellow round seeds. When the heterozygote F 1 forms a gamete, according to the separation phenomenon, that is, Y is separated from Y, and R is separated from R, and then one member of each pair of genes enters the next gamete, so that a random free combination will appear between the separated pair of gene members, namely:

(1) Y and r are combined into yr;

(2)Y and R are merged into YR; (3)y and R are merged into YR;

(4)y and R are merged into yr.

Because they have equal opportunities to combine with each other, the hybrid F 1(YyRr) can produce four different types and equal numbers of gametes. When the hybrid F 1 selfed, the four different types of male and female gametes were randomly combined, resulting in 9 genotype zygotes in F2 16 combinations. Due to the existence of dominant and recessive genes, these nine genotypes can only have four phenotypes, namely: yellow circle, yellow wrinkle, green circle and green wrinkle. As shown in Figure 2-8, the ratio between them is 9: 3: 3: 1.

This is the hypothesis of free combination of genetic factors put forward by Mendel at that time, which satisfactorily explains the experimental results he observed. In fact, this is also a universal and basic genetic law, which is the second genetic law discovered by Mendel-the law of free combination, which is also called the law of independent distribution.

Verification of the Law of Free Combination

Similar to separation phenomenon, we also need to verify the law of free combination from hypothesis to truth. In order to prove that two pairs of F 1 hybrids with related traits did produce four different gametes with the same number, Mendel also verified it by means of test crossing.

When the hybrid of F 1 crosses with the double recessive parents, because the double recessive parents can only produce one kind of gamete (yr) containing two recessive genes, the offspring produced by test crossing can not only show the types of hybrid gametes, but also reflect the proportion of all kinds of gametes. In other words, if the hybrid F 1 can produce four different types of offspring after test crossing with double recessive parents, and the proportion is equal, then it is proved that when the hybrid F 1 forms a gamete, its genes are combined with each other according to the law of free combination.

The actual test results, whether orthogonal or crossing, obtained four different types of offspring with similar numbers, and the ratio was 1: 1: 1, which was completely in line with the expected results. This proves that the male-female hybrid F 1 does produce four gametes with equal number when forming gametes, thus verifying the correctness of the law of free combination.

The essence of the law of free combination

According to the above, we can know that when two (or more) pairs of parents with relative traits cross, when F 1 produces gametes, at the same time of allele separation, non-alleles on non-homologous chromosomes show free combination, which is the essence of the law of free combination. That is to say, the separation and combination of one pair of alleles and another pair of alleles do not interfere with each other, and they are assigned to gametes independently.

Mendel's Genetic Law —— Significance of Mendel's Genetic Law in Theory and Practice

Mendel's separation phenomenon and the law of free combination are the most basic and important laws in genetics, and many genetic laws discovered later are based on them. They are like a beacon, illuminating the future of modern genetics.

Theoretical application

Theoretically, the law of free combination provides an important theoretical basis for explaining the biodiversity in nature. As we all know, there are many reasons for biological variation, but the free combination of genes is an important reason for the diversity of biological traits. For example, a pair of biological hybrids with 20 pairs of alleles (these 20 pairs of alleles are located on 20 pairs of homologous chromosomes), and F2 = 1048576 has 220 possible phenotypes. This can explain why there are so many kinds of creatures in the world now. Of course, the causes of biodiversity include gene mutation and chromosome variation, which will be discussed later.

Separation phenomenon can also help us better understand why close relatives can't get married. Because some genetic diseases are controlled by recessive genetic factors, these genetic diseases rarely appear under normal circumstances, but in the case of consanguineous marriage (such as cousin marriage), they may inherit the same disease-causing genes from their ancestors, thus greatly increasing the chances of offspring getting sick. Therefore, it is necessary to prohibit consanguineous marriage, which has been clearly stipulated in China's marriage law.

practical application

An important application of Mendel's genetic law in practice is plant cross breeding. In the practice of cross breeding, we can purposefully combine the excellent characters of two or more varieties, and then continue to purify and select through selfing, so as to get new varieties that meet the ideal requirements. For example, there are two varieties of tomatoes: one is resistant to yellow meat and the other is susceptible to red meat. Now it is necessary to cultivate a new variety with genetic stability, disease resistance and red meat. You can cross these two varieties of tomatoes, and a new variety with both disease resistance and red meat will appear in F2. Breeding it as a seed, after selection and cultivation, you can get the new tomato variety with genetic stability you need.

Related literature

Wanfang data Journal Paper Inheritance of Exogenous 1Dx5 Gene of Transgenic Wheat in Two Hybrid Combinations-Journal of Huazhong University of Science and Technology (Natural Science Edition) -2005 33 (12)

Wan Fang Data Journal Paper "Genes on Chromosomes"-Teaching Design of New Curriculum. Middle school -20 1 1 (9)

RAPD analysis data of interspecific hybridization F_ 1 generation of seabuckthorn in Wan Fang-Journal of Northeast Agricultural University -20 10 4 1 (1)

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