Mendel is very serious about the experiment. His experimental thinking is meticulous and careful, and his selection of materials is also very careful. It is no accident that he chose peas as the main research object, because peas have many advantages as materials for genetic research: first, peas are strictly self-pollinating plants, and pollination is completed before flowering, avoiding the chaos caused by natural hybridization; Secondly, peas have a short growth period and are easy to cultivate; There are also advantages of peas, such as large flowers, easy manual operation and many variations of peas.
Mendel repeatedly studied the genetic variation of seven related traits of pea, and analyzed the experimental results by mathematical statistics. He found that: after the hybridization between red pea and white pea, the first generation all bloomed red flowers; About 3/4 of the second generation hybrids were produced by self-crossing between the first generation hybrid and the second generation hybrid of 1/4 safflower and white flower. That is to say, the ratio of the number of red flowers to the number of white flowers in the second generation of hybrid peas is 3: 1.
When Mendel explained the genetic behavior of this trait, he thought that after the hybridization between red pea and white pea, the first generation of hybrid offspring all bloomed red flowers, indicating that the trait of red pea was hereditary and dominant. Although the white flower of pea also exists in pea flower, it is recessive, so it is called recessive; In the second generation, peas with red flowers accounted for 3/4, of which 1/4 contained only red flowers, 1/2 contained both red flowers and white flowers, and 1/4 contained white flowers. This experiment proves that all the different characters of hybrid plants will be included in their first generation offspring, but there are dominant and recessive differences; After the second generation, the different characters of these plants will gradually separate through certain laws and return to their original state. This is the famous separation of mendelian law.
At the same time, Mendel also studied the inheritance and variation behavior of cotyledon color, seed roundness and plant height. He found that if two pairs of traits, such as flower color and plant height, were investigated at the same time, the separation of traits did not interfere with each other. In the second generation hybrid, the ratio of safflower tall stalk, safflower short stalk, white flower tall stalk and white flower short stalk is close to 9∶3∶3∶ 1 (tall stalk is dominant character), which is the famous Mendel independent distribution law in plant genetics.
Mendel's work reveals two basic laws of biological inheritance-separation phenomenon and free combination law, which are collectively called Mendel's law by later generations.
Mendel believes that every characteristic of plants is transmitted by genetic factors. Heredity is not a specific trait, but a genetic factor, because there are no specific traits such as safflower and white flower in sex cells. He also believes that genetic factors exist in pairs in somatic cells, but in single cells and particles. After hybridization, the particles remain independent and do not fuse with each other. When hybridization produces gametes (that is, sex cells), different genetic factors are separated. And assign it to different gametes to complete the inheritance to the next generation. This is Mendel's concept of granular genetic factor.
Mendel's law and the concept of genetic factors are the basic laws of plant inheritance, which also laid the foundation of biological inheritance theory and opened the curtain of modern genetic research.
By the beginning of the 20th century, with the efforts of many scientists, genetics had made new progress, among which American scholar Morgan made the greatest contribution. Morgan, his students and their research team took Drosophila as experimental materials, made a lot of genetic and cytological studies on its inheritance and variation, and put forward the theory of chromosome inheritance.
Most people raise pigs, dogs, cats, chickens, ducks and so on. Have you ever heard of someone who actually likes to keep flies and keeps thousands of them? People with this hobby really exist. They are Morgan and his research team. However, Morgan and his disciples raised a special kind of fly-fruit fly. Drosophila has small body, low feeding cost and fast reproduction. At the temperature of 25 degrees Celsius, fruit flies can reproduce one generation in 12 days, and a female fruit fly can produce thousands of offspring at a time, which is why Morgan raises fruit flies. In fact, fruit flies have many advantages as genetic experimental materials, which Morgan did not think of at that time.
The male Drosophila used by Morgan for experiments at that time had black body, purple eyes, broken wings and other traits, which were recessive and could be truly inherited. Their wild-type fruit flies are gray bodies, red eyes and long wings, all of which are dominant traits. The first generation hybrids obtained by crossing male Drosophila melanogaster with recessive traits and wild Drosophila melanogaster with dominant traits all showed gray body, red eyes and long wings. According to Mendel's law of free combination, the offspring of hybrid male Drosophila and recessive female Drosophila should show equal sixteen different combinations. But in fact, there are only two combinations of their descendants: they are exactly like their ancestors, either black with purple eyes and broken wings, or gray with red eyes and long wings, and there are no other types.
How to explain this phenomenon? Scientists have encountered another big problem. However, the more difficult it is, the more it can stimulate the interest of scientists. For this reason, many scientists have studied and put forward various hypotheses, but only Morgan has given a successful explanation for this phenomenon.
Morgan first assumes that the genes of these three traits are all located on one chromosome, so the genes on different chromosomes are distributed according to the law of free combination, but the genes on the same chromosome cannot be combined freely. Morgan called this phenomenon a chain reaction. Later, scientists conducted many experiments and finally proved the existence of chain phenomenon. Scientists call linked genes linkage groups, and they find that the number of linkage groups and single chromosomes in many organisms is always consistent with each other. For example, the haploid number of Drosophila melanogaster is 4, and Drosophila has exactly 4 linkage groups. Maize has 10 pairs of chromosomes, and more than 400 genes studied in maize also belong to 10 linkage group. The pea used in Mendel's experiment has seven pairs of chromosomes. Interestingly, Mendel used seven pairs of genes with relative traits only on seven pairs of chromosomes, so it showed the law of free combination.
Later, Morgan found in the experiment that male black-bellied Drosophila melanogaster with two recessive traits, corpus luteum and white eyes, mated with female black-bellied Drosophila melanogaster with gray body and red eyes with dominant traits, and gave birth to offspring with dominant traits; Then, by backcrossing the first generation female Drosophila melanogaster with the male Drosophila melanogaster with two recessive genetic traits of corpus luteum and supercilious look, four kinds of grandchildren were obtained. Among the four individuals, two are the same as their ancestors, either corpus luteum, white eyes, gray body and red eyes, accounting for 99% of the total number of grandchildren. This shows that most of the traits that parents combine together are still combined in the hybrid offspring. Surprisingly, there are two new types of grandchildren: one is corpus luteum red eye, and the other is gray body white eye, accounting for 1% of the total number of offspring. Morgan believes that these two genes must be located on the same chromosome, so most (99%) offspring are still linked together. However, a few individuals (1%) exchanged between the two genes during gamete formation, resulting in new combinations. He called this phenomenon exchange.
Morgan and later scholars did a lot of experiments, which proved that the new individuals of 1% were not acquired by accident, but had certain regularity. In their experiments, they found that there is indeed a certain exchange between different types of genes, and the exchange rate is actually different, which is the famous chain exchange law in genetics.
Unexpectedly, we seemingly ordinary peas and a little annoying fruit flies have become the carriers for human beings to discover the laws of biological inheritance. It seems that peas and fruit flies have made great contributions to the development of genetics.