Genetics is the science of inheritance and variability. It goes beyond the similarities between you and your ancestors, not only trying to unravel what traits you inherited but also trying to decide how likely you were to get your grandmother's eye color or blood type. As with any science, there are different types of genetics – the one that deals with the study of plants, animals, humans, microorganisms, and viruses, and you can also divide it by methods – molecular genetics, medical, sports, genetic engineering, and many others. Let's take a closer look at the terms and definitions of this versatile science.
Basic terms
Genetics emerged and began to develop a few centuries ago. The founder of genetics was Gregor Mendel, who in 1865 published his first work on the subject, Experiments on Plant Hybrids. It was then that the principles of inheritance of traits by organisms, still relevant today, were first identified and explained.
Heredity is the ability of organisms to pass their inborn traits and characteristics from generation to generation from parents to children. Simply put, heredity is your mother's ability to give you her eye color, curly or straight hair, and other traits.
Eyes can be a different shade of blue, green, or brown – it's a trait we're all born with, and under normal circumstances, it won't change. Variability is the occurrence of individual differences between individuals of the same species within a single trait. Having a particular eye color just for you, a predisposition to hereditary diseases, the shape of your nose – these are all examples of variation within a single trait. Variability is very important; it causes genetic heterogeneity in a population, increasing the chances of a species surviving in changing conditions.
Genes
And how do organisms manage to pass traits from one generation to the next? There is a unit of heredity – the gene. It was previously thought that a gene was a part of DNA that defined a single trait of an organism. However, with the development of molecular biology, it became clear that a gene is a section of a DNA molecule that codes for a single protein. The totality of all the genes in an organism is called a genotype.
All genes interact with each other in some way, some are capable of suppressing or enhancing others. The outward manifestation of a genotype is called a phenotype. However, the phenotype can be influenced not only by genotype but also by environmental factors. For example, if we plant tubers obtained from one potato plant, we cannot find two completely identical daughters among the grown ones. Despite the identical genotypes, they will differ in stem height, degree of root system development, number and size of leaves, tubers, and a number of other traits. Differences between these plants are due to the action of environmental factors such as peculiarities of the mechanical and chemical composition of the soil, its moisture, presence of pests, competition with other plants, etc.
Genes cannot be randomly scattered around the cell nucleus, so they are packed into chromosomes, special structures consisting of DNA and proteins. The set of all the chromosomes in a cell is called the karyotype. Each gene has its own place in the chromosome, called a locus.
Each cell contains one or more identical sets of chromosomes, denoted by the letter . The number of such sets stored in the cell nucleus is called ploidy. Most sexually reproducing organisms are diploid, i.e. they contain 2 identical sets of chromosomes (). However, there are also monoploid (), triploid (), tetraploid () etc. in nature.
2 chromosomes with the same set of genes, identical in size and shape in the karyotype of a diploid organism will be called homologous. Homologous chromosomes will have allelic genes at the same locus. These genes will be responsible for the development of one trait, such as blue or brown eye color, or curly or straight hair.
As you know from previous threads, when a cell divides, the chromosomes are capable of swapping homologous chromosome segments (this process is called crossing over – the main cause of genetic variability).
Alleles
Let's take a conditional gene that will be responsible for coloring the flower red. Let us call this gene A. Gene A is dominant, i.e. it contains a trait that will be expressed in the phenotype in any case, regardless of the presence of other genes in the genotype. Let's also take the allele gene a – the gene of the absence of red color. In the phenotype, gene a will be expressed as white coloring of the flower (absence of red). Gene a is recessive, i.e. its expression will be influenced by the presence of gene A. We will consider genes in a diploid organism, i.e. there will be 2 genes in each cell: AA, aa, or Aa.
If the genotype of the organism is AA, then the color of the flowers will be red. If the genotype of the organism is aa, then the colors will be white. And if both Aa genes enter the cell, the color of flowers will still be red, because the dominant gene will suppress the recessive one and will show itself.
In practice, it happens that we cannot look at each particular flower and study its genotype, but we can see its coloring (phenotype). That is, going the opposite way, having seen a bouquet of identical flowers differing only in color, we can say with certainty that all red flowers will have the AA or Aa gene, and all white flowers will have the aa gene, without conducting any additional genome studies.
If homologous chromosomes contain the same allelic genes (either dominant or recessive), the organism is called homozygous (AA or aa). If homologous chromosomes contain different alleles of the same gene, the organism is called heterozygous (Aa).
Since the recessive gene does not manifest itself in any way, many people can be carriers of certain hereditary diseases and not even be aware of it. For such a disease to occur, a person must get recessive copies of the gene from both parents. Cystic fibrosis or phenylketonuria are examples of hereditary diseases transmitted through the recessive allele.
Conclusion
Genes are stored in loci of chromosomes, the totality of which is the karyotype. The outward manifestation of the genotype in the environment can be called the phenotype. Chromosomes can exchange alleles as a result of crossing over. Organisms can be homozygous or heterozygous, depending on the purity of the alleles.