Fundamentals of Genetics

      DNA is the molecular basis of heredity. Each DNA molecule is a double strand of nucleotides (polynucleotide) joined in the middle to form a double helix.

       Genetics (from the ancient Greek γενετικός genetikos - to give rise to and γένεσις genesis - to originate) is a branch of biology; the science of genes, heredity and variation in organisms. The fact that living beings inherit the traits of their parents was used in prehistoric times to improve the productivity of cereal crops and animals through selective breeding. However, modern genetics, which seeks to understand the process of inheritance, really only began with the work of Gregor Mendel in the mid-nineteenth century. Even though he did not know the physical basis of heredity, Mendel observed that organisms inherit traits through special discrete units of heredity, which today we call genes.

      Genes are stretches of DNA - a molecule made up of a chain of four different types of nucleotides - whose sequence is the actual genetic information that organisms inherit. In the vast majority of cases, DNA is present in the form of double bases, with nucleotides on each base complementing each other. Each base acts as a template for the creation of a new partner base - a physical method for copying genes that can be inherited.

       The nucleotide sequences in genes are translated by cells to produce a chain of amino acids and synthesize proteins - the order of the amino acids in a protein corresponds to the order of the nucleotides in the gene. This relationship between the nucleotide sequence and the amino acid sequence is called the genetic code. The amino acids in a protein determine how they fold into a three-dimensional shape; this structure, in turn, is responsible for the function of the protein. Proteins perform almost all the functions necessary for cellular life. Changes in the sequence of bases in the part of the DNA molecule that carries genetic information (genes) can alter the sequence of amino acids in a protein, changing its shape and function, which often leads to a negative impact on the functioning of the cell and the body as a whole. Moreover, in most cases, only one nucleotide is changed (single nucleotide polymorphism). This often causes a specific genetic disease that can be inherited.

      Although genetic features play an extremely important role in the emergence, development, functioning and behavior of organisms, the final result (a set of characteristics and special properties of an organism) is a combination of genetic factors and the conditions in which the organism develops. For example, heredity plays a significant role in the formation of a trait such as human height, but nutrition and other conditions (e.g., physical activity, specific exercises, etc.) can significantly modulate this trait depending on the conditions.

Gregor Mendel's work and classical genetics

      The foundations of modern genetics were laid by Gregor Johann Mendel, a German-Czech Augustinian monk and scientist who studied the nature of inheritance of traits in plants. In his work "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), published in 1865 by the Naturforschender Verein (Society for the Study of Nature) in Brno (modern Czech Republic, then part of the Austrian Empire), Mendel traced the inheritance of certain traits in peas and correctly described them mathematically. Although the described type of inheritance can be observed for only a few traits, Mendel's work suggests that heredity is discrete and permanent, not acquired, and that the inheritance pattern of many traits can be explained and described using simple mathematical rules and proportions.

     The extraordinary importance of Mendel's work was not widely understood until the 1890s, when, after his death, other scientists working on similar problems again paid attention to his research. In 1905, William Batson, a supporter of Mendel's work, proposed the name of a new scientific discipline - Genetics (The adjective genetic, derived from the Greek word genesis-"γένεσις", origin, and the latter from the word gennō-γεννώ, "to give life, to give birth", precedes the noun and was first used in biology in 1860). Batson popularized the use of the word genetics to describe the science of inheritance in his opening speech at the Third International Conference on Plant Hybridization in London, England, in 1906.

     After returning to the results of Mendel's research, scientists tried to determine which molecules in the cell were responsible for heredity. In 1910, Thomas Gantt Morgan argued that genes are located on chromosomes, based on the observation of linked inheritance in Drosophila. In 1913, his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on a chromosome.

Features of inheritance

Discrete inheritance and Mendel's laws

      At a fundamental level, inheritance in organisms occurs through certain discrete traits that are uniquely determined by genes. This property was first discovered by Gregor Mendel, who studied the segregation of hereditary traits using peas as an example. In his experiments, where he studied flower color, Mendel noticed that the flowers of each pea were either purple or white - and never observed the presence of an intermediate color. This difference, the presence of different variants of the same gene, is called alleles.

     In the case of peas, which are a diploid species, each plant has two alleles of a given gene, where one allele is inherited from each parent. Many species, including humans, have this pattern of inheritance. Diploid organisms with two copies of the same allele of a particular gene are called homozygous, and organisms with two different alleles of a particular gene are called heterozygous.

     The set of alleles for a given organism is called its genotype, and the observed characteristic or trait of the organism is called its phenotype. When an organism is said to be heterozygous for a gene, one allele is often referred to as dominant because its qualities dominate the organism's phenotype, while other alleles are called recessive because their qualities may be absent and not observed. Some alleles do not have full dominance, but instead have partial dominance of an intermediate phenotype, or so-called codominance - both traits are dominant at the same time, and both traits are present in the phenotype.

     When a pair of organisms reproduce sexually, their offspring randomly inherit one of two alleles from each parent. The observation of discrete inheritance and segregation of alleles is commonly known as Mendel's first law, or the law of segregation.
 

Interaction of several genes

    Human height is a complex genetic trait. The results of a study conducted by Francis Galton in 1889 show a correlation between the height of offspring and the average height of their parents. However, the correlation is not absolute and there are significant deviations from genetic variability in offspring height, indicating that the environment is also an important factor in this trait.

    Organisms have thousands of genes, and during sexual reproduction, the range of these genes is largely independent, meaning that they are inherited randomly with no linkage between them. This means that the inheritance of alleles for the yellow or green color of peas has nothing to do with the inheritance of alleles for the white or purple color of flowers. This phenomenon, known as Mendel's Second Law, or the Law of Independent Inheritance (the law of trait splitting), means that alleles of different genes are mixed between parents to form offspring with different combinations. Some genes cannot be inherited separately because they have been shown to have a specific genetic linkage, as discussed later in the article.

    Often, different genes can interact in such a way that they affect the same characteristic trait. For example, in the spring vetch (Omphalodes verna), there is a gene with alleles that determine the color of the flower: blue or purple. However, another gene controls whether the flower has any color at all or is white. When a plant has two copies of this white allele, its flowers are white, regardless of whether the first gene had a blue or a purple allele. This interaction between genes is called epistasis - the activity of one gene is influenced by variations in other genes.

     Many traits are not discrete traits (e.g., purple or white flowers), but instead are continuous traits (e.g., human height and skin color). This set of traits is the result of many genes. The influence of these genes is the link between the different degrees of environmental influence on organisms. Heritability is the degree to which an organism's genes contribute to a set of characteristic traits. The measurement of trait heritability is relative - in a more variable environment, it has a greater impact on the overall change in characteristic traits. For example, in the United States, human height is a complex trait with an 89% probability of inheritance. However, in Nigeria, where people have greater disparities in access to good nutrition and health care, the probability of inheriting a trait such as height is only 62%.

Reproduction.

     When cells divide, their genome is completely copied, and each daughter cell inherits one complete set of genes. This process is called mitosis, the simplest form of reproduction and the basis for vegetative (asexual) reproduction. Vegetative reproduction can also occur in multicellular organisms, creating offspring that inherit the genome from a single parent. Offspring that are genetically identical to their parents are called clones.

     Eukaryotic organisms often use sexual reproduction to produce offspring that have mixed genetic material inherited from two different parents. The process of sexual reproduction varies (alternates) depending on the type, which contains one copy of the genome (haploid) and a double copy (diploid). Haploid cells are formed as a result of meiosis, and when genetic material fuses with another haploid cell to create a diploid cell with paired chromosomes (e.g., the fusion of an egg (haploid cell) and a sperm (haploid cell)), it results in a zygote. Diploid cells divide to form haploid cells, without reproducing their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Most animals and many plants are diploid organisms for most of their lives, with the haploid form being characteristic of only one cell, the gamete.

      Although they do not use haploid/diploid sexual reproduction, bacteria have many ways to acquire new genetic information (i.e., to be variable). Some bacteria can undergo conjugation by transferring a small circular fragment of DNA to another bacterium. Bacteria can also take foreign DNA fragments from the environment and integrate them into their genome, a phenomenon known as transformation. This process is also called horizontal gene transfer, which is the transfer of fragments of genetic information between unrelated organisms.