Having thoroughly investigated this question, the British historian Robert Albee asked himself, was Mendel a Mendelian? In other words, Albee believes that much of what is attributed to Mendel in modern biology textbooks might surprise this founder of genetics.
To test Albee's findings, let's first figure out why Mendel began researching pea plants in the late 1850s. If we understand this, we will also understand that he least of all hoped to discover the laws of heredity. In fact, Mendel devoted most of his life in science to theories that are considered absolutely dead-end today.
Let's start with the title of Mendel's most famous article, Experiments on Plant Hybridization. Note that the title does not mention the laws of transmission of hereditary properties or the mechanism of heredity, just as there is no mention of peas, with which he experimented. The word "hybridization" is often found in the writings of Mendel, while the word "heredity" is unlikely to be found, and it says a lot. After carefully reading the introduction to the article, we will find out what Mendel himself thought about his work. Here he did not hide anything and openly said that he was presenting the results of a "detailed experiment", the purpose of which was to discover "a generally applicable law governing the formation and development of hybrids." At the end of the work, he repeats this idea again. And not a word about the fact that he discovered the statistical laws of the transmission of heredity. Instead, he claims that he succeeded in shedding light on the theory of a botanist named Görtner, and his, Mendel's, results contradict the opinions of those naturalists who disputed the resistance of plant species and believed in the continuous evolution of the plant world. There is only one difficulty for us in this - to understand what all this means!
A brief excursion into the botany of the 18th and 19th centuries makes it possible to clarify the meaning of his statement. In the 60s of the XIX century, Mendel was actively engaged in a problem that became key for the entire community of botanists of that time. It was first formulated by the famous Swedish naturalist Karl Linnaeus, who proposed a classification of organisms that scientists still use today.
In the middle of the 18th century, Linnaeus already doubted that all kinds of animals after the act of Creation remain in an unchanged state, as insisted on by religious orthodoxy. His doubts were reinforced by the incredible variety of exotic forms of flora and fauna that travelers brought to Europe. The number and variety of new plants and animals soon confused all the classifications that existed in Europe. And, since Linnaeus set out to put some order here, he could not help but admire the abundance of living forms in nature. Soon, he had thoughts that had never crossed his mind before. Did God really create the living world of the Earth in a short period of Creation? Or maybe all the existing diversity arose from a much smaller number of primitive forms?
Gradually, Linnaeus became an adherent of the evolutionary theory. However, the evolutionary mechanism he proposed was not like Darwinism. Linnaeus did not take into account the influence of the external environment or the manifestation of random variations. His interest was limited only to the study of the botanical phenomenon of the crossing of various species. Since this clearly led to the emergence of new forms of plants, he began to argue that after several generations, hybrids can gradually develop into completely new species. Over the next century, the idea of so-called interspecific hybridization dominated the minds of many scientists. At various times, countries such as Holland, France and Prussia even established monetary bonuses for work in this area. But the researchers not only failed to confirm Linnaeus's ideas, but even to stabilize hybrid forms. Over and over again in the new generation, they either returned to their paternal forms, or, ceasing to bear fruit, died out.
Despite everything, plant breeding by hybridization has forever remained an area of science in which hope remains unrelenting. For almost the entire 19th century, there were botanists who believed in the possibility of breeding resistant hybrids that would become new species. For example, when Mendel was at the University of Vienna, a botanist named Franz Unger convinced him that hybridization could be the source of new species. Since we have no reason to doubt the truth of Mendel's religious feelings, it is not surprising that he began to conduct appropriate research. The fact is that the variability observed in the process of hybridization was explained by the then scientists not by the action of the blind forces of Darwinian evolution, but by God's providence. After all, what better way to demonstrate the greatness of the Creator than endowing initially humble plants with the ability to transform almost indefinitely?
Thus, Mendel's experiments on plant hybridization were quite in line with the then botanical research. Mendel was most interested in hybrids not because it was the most visual way to demonstrate the dynamics of the transfer of hereditary properties, but because it allowed him to check the validity of Linnaeus's reasoning. Mendel was convinced that hybridization made possible the "constant evolution of vegetation," and the goal of his experiments was to grow hybrids generation after generation to see if they could become a new species. That is why he constantly rejected those hybrids obtained from pure seeds that turned out to be infertile or simply grew poorly. His work from 1865 is a detailed account of attempts at new plant species. The proof of Linnaeus's correctness seemed so important to Mendel that he even significantly distorted some of the views of one of his predecessors.
Defending the correctness of his hypothesis that hybrids can develop into new species, Mendel argued that Max Vihura, who was the world authority on willows, also believed that willow hybrids "spread in the same way as pure species." However, when Robert Albee turned to Vihura's original work, it was found that it says the opposite: willow hybrids do not retain their properties in subsequent generations. And although Mendel attributed to Vihura belief in Linnaeus's hypothesis, he actually seriously doubted its validity.
Unfortunately for Mendel, no matter how hard he tried, his hybrids also showed a return to the original properties of the parental forms. Modern genetics answers the question of why this happens. The naturalist priest got involved in an unequal struggle against the dominance and recessiveness of gene pairs. Mendel's experiments have shown convincingly that no hybrid line can create only hybrids.
This, of course, was a depressing result for a scientist who wanted to prove the opposite, namely that hybrids can give new species. Mendel by nature was a closed, taciturn, closed person, but in his articles here and there disappointment is still visible. This is especially felt in his most famous work "Experiments on Plant Hybridization", published in 1865. In the final part, he tried to bypass the unpleasant data. Declaring that his experiments could not be regarded as decisive, he awkwardly started talking about the fact that the results obtained are not entirely clear and cannot be considered as absolute. Despite everything, while writing the article, he did not stop believing in the possibility of creating "permanent hybrids". Understanding this fact makes us look differently at Mendel's famous speech to the Society for the Study of Natural Sciences in 1865.
Lauren Ainsley, who acknowledged his character's exceptional conviction, described the event as follows:
The enthusiastic speech of this blue-eyed priest, who presented his research, as the surviving protocols of the society show, did not provoke any discussion ... No one asked a single question, no one had a heart beat faster. In a small auditorium, one of the most outstanding discoveries of the 19th century was presented by a professional teacher who presented a huge amount of evidence. But there was no soul there that would understand him.
If you read Albee's work, Mendel's articles immediately appear in a different light. And if you consider that Mendel appeared in the monastery twenty years before the publication of his works and devoted about a decade to the experiments, then, it is likely that many who attended his lecture could know what he was striving for. Removing the huge superstructure of presentism, we see that in 1865 Mendel reported his complete failure. His quite pragmatic efforts to stabilize hybrids for use by local farmers did not lead to anything, and he left out a very interesting statistic, which he could not explain. So it was a complete failure, and the silence of his listeners was most likely quiet sympathy.
Since 1856, Gregor Mendel conducted experiments with peas in the monastery garden.
In their experiments on crossing peas Gregor Mendel showed that hereditary traits are transmitted by discrete particles (which are now called genes).
To evaluate this conclusion, one must take into account that, in the spirit of that time, heredity was considered continuous, not discrete, as a result of which, as it was believed, the ancestors' traits were "averaged" in descendants.
In 1865, he made a report on his experiments in the Brunnian (now the city of Brno in the Czech Republic) Society of Naturalists. At the meeting, he was not asked a single question. A year later, Mendel's article "Experiments on plant hybrids" was published in the writings of this society. The volume was sent to 120 university libraries. In addition, the author of the article ordered an additional 40 individual prints of his work, almost all of which he sent to botanists known to him. There were no responses either ...
Probably, the scientist himself lost faith in his experiments, because he conducted a series of new experiments on crossing the hawk (a plant of the Aster family) and then on crossing the varieties of bees. The results he had earlier obtained on peas were not confirmed (modern geneticists have figured out the reasons for this failure). And in 1868 Gregor Mendel was elected abbot of the monastery and never returned to biological research.
“Mendel’s discovery of the basic principles of genetics was ignored for thirty-five years after it was not only presented at a meeting of the scientific society, but even its results were published. According to R. Fischer, each subsequent generation tends to notice in Mendel's original article only what it expects to find in it, ignoring everything else. Mendel's contemporaries saw in this article only a repetition of the then well-known hybridization experiments. The next generation understood the importance of his findings regarding the mechanism of heredity, but could not fully appreciate them, since these findings seemed to contradict the especially hotly debated theory of evolution at the time. Let me add, by the way, that the famous statistician Fischer double-checked the results. Mendel and stated that when processed with modern statistical methods, the findings of the father of genetics show a clear bias in favor of the expected results. "
Incredible, but true: a person is able to control his genes. We have already achieved so much in the field of genetics:
- we know how all the signs of an organism are determined;
- cloning has become a reality;
- changing genes has become common in certain sciences.
How did this become possible and what does the future hold for us? This book will briefly and clearly tell you about the history of genetics, about scientists and their discoveries.
Stay on top of scientific discoveries - in just an hour!
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2.1. The beginning of genetics. Gregor Mendel: great discoveries, but unnoticed
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2.1. The beginning of genetics. Gregor Mendel: great discoveries, but unnoticed
So, by the end of the XIX century. Scientists were more than ever close to discovering all the secrets of heredity: almost all elements of the cell were isolated and described, the connection of chromosomes with the transmission of traits from parents to offspring was assumed. But the patterns in the manifestation of certain traits were still not visible. At least officially. An interesting historical incident: when August Weismann, Walter Flemming and Heinrich Waldeyer conducted their research and tried to find answers to questions related to heredity, the Augustinian monk Gregor Mendel in the city of Brunn (at that time the Austrian Empire; now - the city of Brno, Czech Republic) for a long time already deduced the main rules for the inheritance of various characters, using mathematical methods to establish patterns. But his discoveries, which became a bridge from the hypotheses of the 19th century. to modern genetics, during the life of the researcher were not considered and evaluated ... However, first things first.
Gregor Mendel was born in 1822 in Moravia, came from a poor peasant family and was baptized with the name Johann. From early childhood, the boy showed an aptitude for learning and an interest in science, but due to the difficult financial situation of the family, he could not complete his education in his youth and in 1843 he took monastic vows as a monk in the Augustinian monastery of St. Thomas, taking the monastic name Gregor. Here he got the opportunity to study biology, which he passionately loved. It would seem like a strange occupation for a monk. No wonder: the Augustinians paid special attention to education and enlightenment - primarily, of course, religious, but the monastery in Brunn kept pace with the times. There was a magnificent library, laboratories, extensive collections of scientific instruments and, most importantly, beautiful gardens and greenhouses, in which Mendel spent most of his time. Having become interested in issues of heredity, he turned to the works of his predecessors. Paying tribute to their works, Gregor Mendel rightly noted that they did not find any patterns in crossing and manifestation of certain traits in hybrids.
Is there any general law at all that determines what kind of flowers will be in hybrid roses or sweet peas? Is it possible to predict what color the kittens will be from the cat and the cat, differing in color and structure of the coat? Finally, is it possible to calculate mathematically in what generation and with what frequency this or that feature will manifest itself?
For the experiments, Gregor Mendel, following the example of Thomas Andrew Knight, chose the most common garden, or seed peas (Pisum sativum). It is a self-pollinating plant: under normal conditions, pollen from the stamens of a flower is transferred to the pistil of the same flower (as opposed to cross-pollination, in which pollen must be transferred from one plant to another).
In genetics, self-pollinated plants are those in which pollination occurs between different flowers of the same specimen.
The researcher believed that such a feature would ensure the purity of the experiment, because during self-pollination, seeds and fruits receive certain characteristics from only one plant. Therefore, pollinating peas artificially, transferring pollen from one specimen to another, it is possible to reduce the number of unforeseen accidents and purposefully use only those plants that interest us as experimental ones. In addition, peas have a set of diverse and well-recognizable traits: seed color, pod shape, stem height. Mutually pollinating peas with sharply differing traits, Mendel intended, having received hybrid samples, to deduce the patterns of inheritance. He began by distributing the plants of his choice according to the following criteria:
By the length (height) of the stem: tall or undersized;
By arrangement of flowers: along the stem or mainly at its top;
By the color of the pods (yellow or green);
By the shape of the seeds (smooth or wrinkled);
According to the color of the seeds (yellow or green) and so on.
Then there were eight years of experiments, several tens of thousands of original plants and hybrids, complex calculations and statistical tables. Gregor Mendel crossed plants with very different traits: for example, he chose parents, one of whom had smooth seeds, and the other had wrinkled seeds.
First of all, he drew attention to the fact that in the first generation, hybrids showed in one or another part of them the characteristics of only one parent. When crossing a plant with yellow seeds and a plant with green seeds, the hybrid did not have yellow-green or variegated seeds - their color was completely inherited from one parent. Thus, Mendel enriched the lexicon of future geneticists with important terms: the traits that manifested themselves in the first hybrid generation, he called dominant; and those that faded into the background and were not reflected in the first generation of hybrids were recessive.
He achieved interesting results when crossing tall and stunted pea plants. The offspring in the first generation were entirely tall. But when these plants self-pollinated and gave seeds, the next generation was already divided in this way: one low plant for three tall ones. The appearance of subsequent generations and the ratio of tall and low specimens could also be mathematically predicted. The same ratio was observed in combinations of other features.
Most modern geneticists are convinced that Gregor Mendel anticipated the concept of a gene. Only many years later, the gene will receive a definition - a piece of DNA responsible for heredity. But let's not get ahead of ourselves: we have yet to talk about DNA. And Mendel did not use the concept of "gene", this term will appear much later. He wrote about "factors" or "inclinations", arguing that one or another trait (color, size, shape) of a plant is determined by two factors, one of which is contained in the male, and the other - in the female reproductive cell. The researcher called the plants, which appeared as a result of the fusion of cells carrying the same "inclinations", constant (later they would be called homozygous).
To simplify the work, Gregor Mendel designated dominant characters in a pair of plants in capital letters (A, B, C), and recessive ones in lowercase (a, b, c). Consequently, when describing hybrids, it was possible to draw up simple formulas that clearly demonstrate the combination of traits and their "manifestation". Mendel was well served by the fact that for some time he was fond of mathematics and taught it at school. His penchant for systematization and confident handling of digital and letter designations helped him to do something that was not available to researchers before him: to identify and describe the patterns of heredity. These patterns are now known as Mendel's laws. Let's take a closer look at them.
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First and the second hybrid generations in Mendel's experiments with short and tall peas
1. The law of uniformity of hybrids of the first generation (aka the law of dominance of traits) says that when two constant (or, as they would say now, homozygous) plants are crossed, the entire first generation of hybrids will be completely similar to one of the parents - dominant traits will come to the fore. True, there are cases of incomplete dominance: when the dominant trait cannot completely suppress the weaker, recessive one. Remember, earlier we described the assumption of a number of scientists of the XVIII-XIX centuries, who argued that, according to the logic of things, a hybrid should always be something between the parent specimens? In some cases, this is possible, for example, in some types of flowers when crossing plants with red and white flowers in the first generation of hybrids, the flowers will be pink. That is, the dominant red color of the petals could not completely suppress the recessive white. There may be other particular features in the law of uniformity, but our task is to give the reader the most general information about genetics and its history.
2. The law of splitting traits: if you cross between hybrids of the first generation, then in the second generation the traits of both parental forms will appear in a certain ratio.
3. The law of independent inheritance of traits: if two individuals are crossed that differ from each other in two pairs of traits, the factors and associated traits will be inherited and combined independently of each other. Thus, Mendel crossed peas with smooth yellow grains and peas with wrinkled green grains. At the same time, the yellow color and smoothness of the grains were the dominant features. The first generation of hybrids was completely represented by plants with dominant traits - peas had yellow smooth grains. After self-pollination of the hybrids, new plants were obtained: nine had yellow smooth grains, three had yellow wrinkled grains, three had green smooth grains, and one plant had green wrinkled grains.
Of course, Mendel's laws were subsequently refined in accordance with new scientific data. For example, it became known that if more than one gene is responsible for a particular trait of a plant or organism, then the forms of inheritance will be more complex and complex. Nevertheless, Gregor Mendel was a pioneer in the field of inheritance laws, and in his honor the doctrine of heredity was later named Mendelism.
Why was his research not recognized during his lifetime? It is known that in 1865 Gregor Mendel made a presentation at the Society of Naturalists and published an article "Experiments on Plant Hybridization", which did not gain much success in the scientific community. Most likely, the discoveries of the Brunnian monk did not develop primarily because he himself soon became disillusioned with their results. Mendel set about crossing some plant species that initially had features in their methods of reproduction. Thus, the patterns that he deduced while working with peas have not been confirmed - an unpleasant result of almost a dozen years of hard work! Gregor Mendel soon became abbot, and his new responsibilities forced him to completely abandon biological research. His works were remembered only at the beginning of the 20th century, when several scientists "discovered" Mendel's laws and confirmed his developments. The Augustinian biologist himself died in 1884, long before the triumphant return of his ideas to the scientific community ...
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The honor of discovery quantitative patterns, accompanying the formation of hybrids, belongs to a Czech monk, an amateur botanist Johann Gregor Mendel(1822-1884). In his works, carried out from 1856 to 1863. were disclosed foundations of the laws of heredity. V 1865 g. he sends to the Society of Naturalists an article entitled "Experiments on plant hybrids".
G. Mendel for the first time clearly articulated the concept discrete inheritance("Gene" - 1903, Johansen). Mendel's fundamental law is the law of gamete purity.
1902 - W. Batson formulates the position that the same inclinations are homozygous, different ones are heterozygous.
But! Experimental research and theoretical analysis of the results of crosses, carried out by Mendel, outstripped the development of science by more than a quarter of a century.
At that time almost nothing was known about the material carriers of heredity, the mechanisms of storage and transmission of genetic information and the internal content of the fertilization process. Even speculative hypotheses about the nature of heredity (Charles Darwin and others) were formulated later.
This explains the fact that the work of G. Mendel did not receive any recognition at one time and remained unknown until rediscovery of Mendel's laws.
In 1900 - three botanists independently of each other -
K. Correns (Germany) (corn)
G. de Vries (Holland) (poppy, dope)
E. Cermak (Austria) (peas)
They discovered in their experiments the patterns discovered earlier by Mendel, and, having come across his work, published it again in 1901.
It was established (1902) that it was chromosomes carry hereditary information(W. Setton, T. Boveri). This marked the beginning of a new direction in genetics - the chromosome theory of heredity. In 1906 W. Batson introduced the concepts of "genetics", "genotype", "phenotype".
Substantiation of the chromosomal theory of heredity
In 1901 Thomas Ghent (Hunt) Morgan(1866-1945) first began to conduct experiments on animal models- the object of his research was the fruit fly - Drosophilamelanogaster. Front sight features:
Unpretentiousness (breeding on nutrient media at a temperature of 21-25C)
Fertility (for 1 year - 30 generations; one female - 1000 individuals; development cycle - 12 days: after 20 hours - egg, 4 days - larva, another 4 days - pupa);
Sexual dimorphism: females are larger, the abdomen is pointed; males are smaller, the abdomen is rounded, the last segment is black)
A wide range of features
Small dimensions (approx. 3 mm.)
1910 Y. - T. Morgan - Chromosomal theory of heredity:
Heredity is discrete in nature. A gene is a unit of heredity and life.
Chromosomes retain their structural and genetic individuality throughout ontogenesis.
In R! Homologous chromosomes are conjugated in pairs, and then diverge, falling into different germ cells.
In the somatic cells arising from the zygote, the set of chromosomes consists of 2 homologous groups (female, male).
Each chromosome plays a specific role. The genes are arranged linearly and form one linkage group.
1911 - the law of linked inheritance of traits (genes)(genes localized on one chromosome are inherited linked).
Thus, there are two important stages in the development of genetics:
1 - Mendel's discoveries based on hybridological studies - the establishment of quantitative patterns in the splitting of traits during crossing.
2 - proof that the carriers of hereditary factors are chromosomes. Morgan formulated and experimentally proved the position of linkage of genes in chromosomes.
Gregor Mendel was the first to come close to solving an ancient mystery. He was a monk at the Brunn Monastery (now Brno, Czech Republic) and in addition to teaching, he was engaged in experiments on crossing garden peas in his spare time. His paper on this topic, published in 1865, was not widely accepted. Despite the fact that six years earlier, the theory of natural selection had attracted close attention of the entire scientific world, the few researchers who read Mendel's article did not attach much importance to it and did not connect the facts stated in it with the theory of the origin of species. And only at the beginning of the 20th century, three biologists, conducting experiments on different organisms, received similar results, confirming the hypothesis of Mendel, who became famous posthumously as the founder of genetics.
Why did Mendel succeed in what most other researchers failed? First, he only examined simple, clearly identifiable traits, such as the color or shape of the seeds. It is not easy to isolate and identify simple traits that can be inherited. Traits such as plant height, as well as the intelligence or shape of a person's nose, depend on many factors, and it is very difficult to trace the laws of their inheritance. Outwardly noticeable and at the same time independent of others, signs are quite rare. In addition, Mendel observed the transmission of the trait over several generations. And perhaps most importantly, he wrote down the exact number individuals with this or that trait and carried out a statistical analysis of the data.
In classical experiments in genetics, two or more varieties are always used, two varieties, or lines, of the same biological species, differing from each other in such simple ways as the color of the flower of plants or the color of the fur of animals. Mendel started with clean lines peas, that is, from lines that, over several generations, crossed exclusively with each other and therefore constantly showed only one form of the trait. Such lines are said to be reproduce clean. During Mendel's experiment crossed among themselves individuals from different lines and received hybrids. At the same time, on the stigma of a plant with anthers removed from one line, he transferred plant pollen from another line. It was assumed that the traits of different parent plants in the hybrid offspring should be mixed with each other. In one experiment (Fig. 4.1), Mendel crossed a pure variety with yellow seeds and a pure variety with green seeds. In the record of the experiment, the cross means "crossed with ...", and the arrow points to the next generation.
It could be assumed that the hybrid generation would have yellow-green seeds or some yellow and some green. But only yellow seeds were formed. It would seem that the sign "green" has completely disappeared from the generation F 1(letter F generations are indicated, from the Latin word filius - son). Then Mendel planted the seeds from a generation F 1 and crossed the plants among themselves, thus obtaining the second generation F 2. It is interesting that the trait "green", which disappeared in the first hybrid generation, reappeared: in some plants from generation F 2 had yellow seeds, while others had green ones. Other experiments on crossing plants with different manifestations of the trait gave the same results. For example, when Mendel crossed a pure pea cultivar with purple flowers and a pure cultivar with white flowers, in the generation F 1 all plants turned out to have purple flowers, and in a generation F 2 some plants had purple flowers while others had white flowers.
Unlike his predecessors, Mendel decided to count the exact number of plants (or seeds) with this or that trait. By crossing plants according to the color of the seeds, he received in a generation F 2 6022 yellow seeds and 2001 green seeds. By crossing plants according to the color of the flowers, he received 705 purple flowers and 224 white ones. These figures still do not say anything, and in similar cases, Mendel's predecessors gave up and argued that nothing reasonable could be said about this. However, Mendel noticed that the ratio of these numbers was close to 3: 1, and this observation prompted him to a simple conclusion.
Mendel developed model- a hypothetical explanation of what happens when crossing. The value of a model depends on how well it explains facts and predicts experimental results. According to Mendel's model, in plants there are certain "factors" that determine the transmission of hereditary traits, and each plant has two factors for each trait - one from each parent. In addition, one of these factors may be dominant that is, strong and visible, and the other - recessive, or weak and invisible. The yellow color of the seeds should be dominant, and the green color should be recessive; purple is dominant over white. This property of "factors of heredity" is reflected in the recording of genetic experiments: an uppercase letter means a dominant trait, and a lowercase letter means a recessive one. For example, yellow can be denoted as Ү, and green as at. According to the modern point of view, "factors of heredity" are individual genes that determine the color or shape of the seeds, and we call the different forms of the gene alleles or allelomorphs (morph- the form, allelon- each other).
Rice. 4.1. Explanation of the results obtained by Mendel. Each plant has two copies of a gene that determines color, but transfers one of these copies to its gametes. The Y gene is dominant with respect to the y gene; therefore, the seeds of all plants of the F t generation with a set of Yy genes are yellow. In the next generation, four combinations of genes are possible, three of which produce yellow seeds and one- green
In fig. 4.1 shows the course of Mendel's experiments, and also shows the conclusions to which he came. A clean line of yellow-seeded peas must have two factors Y (YY), and a pure line of peas with green seeds are two factors u (ooh). Since both factors in parent plants are the same, we say that they homozygous or that these plants - homozygotes. Each of the parent plants gives the offspring one factor that determines the color of the seeds, so all plants of the generation F t have factors Yy. Their two color factors are different, so we say that they heterozygous or that these plants - heterozygotes. When heterozygous plants are crossed with each other, each produces two species of gametes, half of which carry the factor Y, and the other half is a factor at. Gametes are randomly combined and give four types of combinations: YY, Yy, yҮ or wow. Green seeds are formed only with the last combination, since both factors in it are recessive; other combinations produce yellow seeds. This explains the 3: 1 ratio observed by Mendel.
