The study of the basic laws of heredity and variability of organisms is one of the most complex, but very promising tasks facing modern science. In this article we will consider both the basic theoretical concepts and postulates of science, and we will figure out how to solve problems in genetics.
The relevance of the study of the laws of heredity
Two of the most important branches of modern science - medicine and selection - are developing thanks to the research of geneticists. The biological discipline itself, the name of which was proposed in 1906 by the English scientist W. Betson, is not so much theoretical as practical. Anyone who decides to seriously understand the mechanism of inheritance of various traits (for example, such as eye color, hair, blood type) will first have to study the laws of heredity and variability, as well as find out how to solve problems in human genetics. It is this issue that we will deal with.
Key concepts and terms
Each industry has a specific, only inherent to it, set of basic definitions. If we are talking about a science that studies the processes of transmission of hereditary traits, by the latter we will understand the following terms: gene, genotype, phenotype, parental individuals, hybrids, gametes, and so on. We will meet each of them when we study the rules that explain to us how to solve the problems of biology for genetics. But in the beginning we will study the hybridological method. After all, it is he who underlies genetic research. It was proposed by Czech natural scientist G. Mendel in the 19th century.
How are traits inherited?
The regularities of the transfer of the body's properties were discovered by Mendel thanks to the experiments that he conducted with a well-known plant - sowing peas. The hybridological method is the crossing of two units that differ from each other by one pair of traits (monohybrid crossing). If the experiment involves organisms that have several pairs of alternative (opposite) characters, then they talk about polyhybrid crosses. The scientist proposed the following form of recording the hybridization of two pea plants, which differ in the color of the seeds. And - yellow paint, and - green.
In this record, F1 are hybrids of the first (I) generation. They are all absolutely uniform (identical), as they contain the dominant gene A, which controls the yellow color of the seeds. The above record corresponds to the first law of Mendel (The rule of uniformity of hybrids F1). His knowledge explains to students how to solve genetics problems. Grade 9 has a biology program in which the hybridological method of genetic research is studied in detail. It also considers the second (II) Mendel rule, called the law of splitting. According to him, F2 hybrids obtained by crossing two first-generation hybrids with each other show splitting in the ratio of phenotype 3 to 1, and genotype 1 to 2 and 1.
Using the above formulas, you will understand how to solve genetics problems without errors, if in their conditions you can apply the first or already known II Mendel’s law, given that crossing occurs when one of the genes is completely dominant .
The law of the independent combination of states of signs
If the parental individuals are distinguished by two pairs of alternative characters, for example, the color of the seeds and their shape, in plants such as seed peas, then during the genetic crossing, you need to use the Pinnet lattice.
Absolutely all hybrids, which are the first generation, obey the rule of uniformity of Mendel. That is, they are yellow, with a smooth surface. Continuing to interbreed plants from F1, we get second-generation hybrids. To find out how to solve problems in genetics, grade 10 in biology classes uses a record of dihybrid crosses, using the 9: 3: 3: 1 phenotype cleavage formula. Provided that the genes are located in different pairs, you can use the third Mendel postulate - the law of independent combinations of states of characters.
How are blood groups inherited?
The transmission mechanism of such a sign as a person’s blood group does not correspond to the patterns that we examined earlier. That is, he does not obey Mendel’s first and second law. This is due to the fact that such a sign as a blood group, according to Landsteiner, is controlled by three alleles of gene I: A, B and 0. Accordingly, the genotypes will be as follows:
- The first group is 00.
- The second is AA or A0.
- The third group is BB or B0.
- The fourth is AB.
Gene 0 is a recessive allele to genes A and B. And the fourth group is the result of coding (the mutual presence of genes A and B). It is this rule that must be taken into account in order to know how to solve genetics problems for blood groups. But that is not all. To establish the genotypes of children by blood group born from parents with its various groups, we will use the table below.
Morgan's theory of heredity
Let us return to the section of our article “The Law of Independent Combination of Character States”, in which we examined how to solve genetics problems. The hybrid cross, as well as the third law of Mendel, to which it obeys, is applicable for allelic genes located in the homologous chromosomes of each pair.
In the mid-20th century, the American geneticist T. Morgan proved that most traits are controlled by genes that are located on the same chromosome. They are linearly arranged and form clutch groups. And their number is exactly the haploid set of chromosomes. In the process of meiosis, which leads to the formation of gametes, not individual genes, as Mendel believed, enter the sex cells, but their entire complexes, called adhesion groups by Morgan.
Crossingover
During prophase I (it is also called the first division of meiosis) between the internal chromatids of homologous chromosomes, an exchange of sites (lukus) occurs. This phenomenon is called crossing over. It underlies hereditary variation. Crossover is especially important for the study of biology related to the study of human hereditary diseases. Applying the postulates set forth in Morgan's chromosome theory of heredity, we will define an algorithm that answers the question of how to solve problems in genetics.
Gender-linked inheritance cases are a special case of gene transfer that are located on the same chromosome. The distance that exists between genes in linkage groups is expressed as a percentage - morganides. And the adherence between these genes is directly proportional to the distance. Therefore, crossing over most often occurs between genes that are located far from each other. Consider the phenomenon of linked inheritance in more detail. But in the beginning we recall what elements of heredity are responsible for the sexual characteristics of organisms.
Sex chromosomes
In the human karyotype, they have a specific structure: in females, they are represented by two identical X chromosomes, and in males, in addition to the X chromosome, there is also the U variant, which differs both in form and in the set of genes. This means that it is not homologous to the X chromosome. Such hereditary human diseases as hemophilia and color blindness arise as a result of the “breakdown” of individual genes in the X chromosome. For example, from the marriage of a carrier of hemophilia with a healthy man, the birth of such offspring is possible.
The above course of genetic crossing confirms the linkage of the gene that controls blood coagulation with the sex X chromosome. This scientific information is used to teach students techniques that determine how to solve genetics problems. Grade 11 has a biology program in which sections such as genetics, medicine, and human genetics are examined in detail. They allow students to study human hereditary diseases and to know the reasons why they arise.
Gene interaction
The transmission of hereditary traits is a rather complicated process. The schemes given earlier become understandable only if students have a basic minimum of knowledge. It is necessary because it provides mechanisms that provide an answer to the question of how to learn how to solve problems in biology. Genetics studies forms of gene interaction. This is polymerization, epistasis, complementarity. Let's talk about them in more detail.
An example of human inheritance of hearing is an illustration of this type of interaction as complementarity. Hearing is controlled by two pairs of different genes. The first is responsible for the normal development of the cochlea of the inner ear, and the second is for the functioning of the auditory nerve. In the marriage of deaf parents, each of which is a recessive homozygote for each of the two pairs of genes, children with normal hearing are born. In their genotype, both dominant genes are present that control the normal development of the auditory apparatus.
Pleiotropy
This is an interesting case of gene interaction, in which the phenotypic manifestation of several characters at once depends on a single gene present in the genotype. For example, in western Pakistan, human populations of some representatives have been discovered. They lack sweat glands in certain areas of the body. At the same time, such people were diagnosed with the absence of some molars. They could not form in the process of ontogenesis.
In animals, for example, Karakul sheep, there is a dominant W gene that controls both the color of the fur and the normal development of the stomach. Consider how the W gene is inherited when two heterozygous individuals are crossed. It turns out that in their offspring ¼ lambs with the WW genotype die due to abnormalities in the development of the stomach. Moreover, ½ (having gray fur) are heterozygous and viable, and ¼ are individuals with black fur and normal stomach development (their WW genotype).
Genotype - a holistic system
The multiple action of genes, polyhybrid crossbreeding, and the phenomenon of linked inheritance are indisputable evidence of the fact that the set of genes in our body is a complete system, although it is represented by individual alleles of genes. They can be inherited according to the laws of Mendel, independently or by loci, linked obediently to the tenets of Morgan's theory. Considering the rules responsible for how to solve problems in genetics, we were convinced that the phenotype of any organism is formed under the influence of both allelic and non-allelic genes that affect the development of one or more traits.