People have always been interested in patterns of inheritance of characters. Why do children look like their parents? Is there a risk of transmission of hereditary diseases? These and many other questions remained under a veil of secrecy until the 19th century. It was then that Mendel managed to accumulate all the accumulated knowledge on this topic, as well as to establish specific patterns through complex analytical experiments.
Mendel's contribution to the development of genetics
The main patterns of inheritance of traits are the principles according to which certain characteristics are transmitted from parent organisms to offspring. Their discovery and a clear statement is the merit of Gregor Mendel, who conducted numerous experiments on this issue.
The main achievement of the scientist is proof of the discrete nature of hereditary factors. In other words, a specific gene is responsible for each trait. The first maps were built for corn and Drosophila. The latter is a classic object for conducting genetic experiments.
Mendel’s merits can hardly be overestimated, as Russian scientists are talking about. So, the famous geneticist Timofeev-Resovsky noted that Mendel was the first to conduct fundamental experiments and give an accurate description of the phenomena that previously existed at the level of hypotheses. Thus, he can be considered a pioneer of mathematical thinking in the field of biology and genetics.
Predecessors
It is worth noting that the laws of inheritance of characters according to Mendel were not formulated from scratch. His research was based on the research of his predecessors. It is worth noting the following scientists:
- J. Goss conducted experiments on peas, crossing plants with fruits of different colors. Thanks to these studies, the laws of uniformity of the first generation of hybrids, as well as incomplete dominance, were discovered. Mendel only specified and confirmed this hypothesis.
- Augustin Sarge is a plant grower who has chosen pumpkin crops for his experiments. He was the first to study hereditary traits not in aggregate, but separately. He belongs to the statement that when transferring certain characteristics they do not mix with each other. Thus, heredity is constant.
- Noden conducted research on various types of plants such as Datura. After analyzing the results, he found it necessary to talk about the presence of dominant features, which in most cases will prevail.
Thus, by the 19th century, such phenomena as dominance, uniformity of the first generation, and also the combinatorics of traits in subsequent hybrids were known. Nevertheless, universal laws were not developed. It is the analysis of the available information and the development of reliable research methods that are Mendel's main merit.
Mendel’s working methodology
The patterns of inheritance of characters according to Mendel were formulated as a result of fundamental research. The activities of the scientist were as follows:
- hereditary traits were not considered in aggregate, but separately;
- for analysis, only alternative characters were selected that represent a significant difference between the varieties (this is what made it possible to most clearly explain the patterns of the inheritance process);
- the research was fundamental (Mendel studied a large number of pea varieties that were both pure and hybrid, and then crossed the “offspring”), which allowed us to talk about the objectivity of the results;
- the use of accurate quantitative methods in the analysis of the obtained data (using knowledge in the field of probability theory, Mendel reduced the rate of random deviations).
The law of uniformity of hybrids
Considering the laws of inheritance of characters, it is worth paying special attention to the uniformity of the first generation hybrids. It was discovered through experience, during which the parental forms were crossed with one contrasting sign (shape, color, etc.).
Mendel decided to conduct an experiment on two varieties of peas - with red and white flowers. As a result, first-generation hybrids received purple inflorescences. Thus, there was reason to talk about the presence of dominant and recessive signs.
It is worth noting that this experience of Mendel was not the only one. For experiments, he used plants with other shades of inflorescences, with different fruit shapes, different stem heights, and other options. Empirically, he was able to prove that all first-order hybrids are uniform and characterized by a dominant trait.
Incomplete dominance
In the study of such a question as the patterns of inheritance of traits, experiments were carried out both on plants and on living organisms. Thus, it was possible to establish that far from always the signs are in a relationship of complete domination and suppression. So, for example, when crossing chickens of black and white color, it was possible to obtain gray offspring. It was the same with some plants, when varieties with purple and white flowers gave pink shades at the exit. Thus, the first principle can be adjusted, indicating that the first generation of hybrids will have the same characteristics, while they can be intermediate.
Cleavage of symptoms
Continuing to study the patterns of inheritance of characters, Mendel considered it necessary to cross two descendants of the first generation (heterozygous). As a result, offspring were obtained, part of which bore a dominant trait, and the other recessive. From this we can conclude that the secondary trait in the first generation of hybrids does not disappear at all, but only is suppressed and may well manifest itself in subsequent offspring.
Independent inheritance
Many questions cause patterns of inheritance of characters. Mendel’s experiments also touched individuals who differ from each other in several ways at once. For each individually, the previous patterns were observed. But now, considering the totality of signs, it was not possible to identify any patterns between their combinations. Thus, there is reason to talk about the independence of inheritance.
Gamete purity law
Some patterns of inheritance of traits established by Mendel were purely hypothetical. We are talking about the law of gamete purity, which consists in the fact that only one allele from the pair contained in the gene of the parent individual enters them.
At the time of Mendel, there was no technical means to confirm this hypothesis. Nevertheless, the scientist was able to formulate a general statement. Its essence is that in the process of the formation of hybrids, hereditary traits are preserved unchanged, and not mixed.
Essential conditions
Genetics is a science that studies the laws of inheritance of characters. Mendel made a significant contribution to its development by developing fundamental provisions on this issue. However, in order for them to be fulfilled, the following essential conditions must be observed:
- initial forms must be homozygous;
- alternative features;
- the same probability of the formation of different alleles in the hybrid;
- equal gamete viability;
- during fertilization, gametes are randomly combined;
- zygotes with different combinations of genes are equally viable;
- the number of individuals of the second generation should be sufficient to consider the results obtained logical;
- the manifestation of signs should not be dependent on the influence of external conditions.
It should be noted that most living organisms, including humans, correspond to these signs.
Patterns of inheritance of traits in humans
Despite the fact that initially genetic principles were studied using plants as an example, they are also valid for animals and humans. It is worth noting the following types of inheritance:
- Autosomal dominant - inheritance of dominant traits that are localized through autosomes. In this case, the phenotype can be both strongly pronounced and barely noticeable. With this type of inheritance, the probability of a child getting a pathological allele from a parent is 50%.
- Autosomal recessive - inheritance of secondary traits connected with autosomes. Diseases manifest themselves through homozygotes, both alleles being affected.
- The dominant X-linked type involves the transmission of dominant traits by determined genes. Moreover, in women, diseases are 2 times more common than in men.
- Recessive X-linked type - inheritance occurs on a weaker basis. The disease or its individual signs are always manifested in the offspring of the male sex, and in women - only in a homozygous state.
Basic concepts
In order to understand how the patterns of inheritance of Mendel’s traits and other genetic processes work, it is worth familiarizing yourself with the basic definitions and concepts. These include the following:
- A dominant trait is the predominant characteristic, which acts as the defining state of a gene and inhibits the development of recessive ones.
- A recessive trait is a characteristic that is inherited, but does not act as a determining one.
- A homozygote is a diploid individual or cell whose chromosomes contain identical cells of the indicated gene.
- Heterozygote is a diploid individual or cell that splits and has different alleles within the same gene.
- Allele is one of the alternative forms of the gene, which is located in a certain place of the chromosome and is characterized by a unique nucleotide sequence.
- An allele is a pair of genes that are located in the same areas of homologous chromosomes and control the development of certain traits.
- Non-allelic genes are located on different parts of the chromosomes and are responsible for the manifestation of various traits.
Conclusion
Mendel formulated and proved in practice the basic laws of inheritance of characters. Their description is given on the example of plants and slightly simplified. But in practice, it is true for all living organisms.