Allelic genes. Definition, types of interaction and impact on hereditary characteristics
Most people on the globe know that genes pass on the hereditary traits of parents to their offspring, and this applies not only to humans, but also to all living things on the planet. These microscopic structural units represent a segment of DNA that determines the sequence of polypeptides (chains of more than 20 amino acids that make up the DNA). The nature and methods of gene interaction are quite complex, and the slightest deviation from the norm can lead to genetic diseases. Let's try to deal with the essence of genes and the principles of their behavior.
What genes are called allelic
The concept of "allelicity", according to Greek terminology, implies reciprocity. It was introduced by Danish scholar Wilhelm Johansen in the early twentieth century.The term “gene”, as well as “genotype” and “phenotype” was coined by the same Johannsen. In addition, he discovered the important law of heredity "pure line".On the basis of numerous experiments with plant material, it was found that the same genes within the locus (the same part of the chromosome) can take various forms that have a direct impact on the variety of variations of any parental trait. Such genes were called alleles, or allelic. In creatures whose organism is diploid, that is, it has paired sets of chromosomes, allelic genes may be present in two identical or two different ones. In the first case, they talk about the homozygous type, in which the inherited traits are identical. In the second case, the type is heterozygous. Its hereditary characteristics differ, as copies of genes in chromosomes differ from one another.
The dominant principle of heredity
The human body is diploid. The cells of our body (somatic) include two allelic genes.Only gametes (germ cells) contain a single allele that determines the sexual trait.At the confluence of the male and female gametes, a zygote is obtained in which there is a double set of chromosomes, that is, 46, including 23 maternal and 23 paternal. Of these, 22 pairs are homologous (identical) and 1 is sexual. If she received the XX chromosome set, the female develops, and if XY, then the male. Each chromosome, as noted above, has 2 alleles. For convenience, they were divided into two types - dominant and recessive. The first is much stronger than the second. The hereditary information contained in them is prevalent. What traits the newborn inherits from its parents depends on whose allelic genes (father or mother) were dominant. This is the easiest way to interact with alleles.
Other types of inheritance
Each parent can be a carrier of homozygous and heterozygous genes according to the dominant or recessive characteristics. A child who has received dominant and recessive allelic genes from homozygous parents will inherit only dominant traits.Simply put, if a pair of dominant is dark hair, and recessive - light, all children will be born only with dark hair.In the case when one of the parents has a dominant heterozygous gene and the other is homozygous, their children will be born with a dominant and recessive trait of approximately 50 X 50. In our example, the couple may have both dark-haired children and blondes. If both parents have both a dominant and a recessive heterozygous gene, every fourth child will inherit their recessive traits, that is, they will be fair-haired. This rule of inheritance is very important, since there are many diseases transmitted through genes, and the carrier may be one of the parents. Such pathologies include dwarfism, hemochromatosis, hemophilia, and others.
How alleles are designated
In genetics, alleles are usually denoted by the first letters of the name of a gene, of which they are forms. The dominant allele is written with a capital letter. Alongside indicate the sequence number of the modified gene form. The word "allele" in Russian can be used both in the feminine and masculine.
Allele Interaction Types
The interaction of allelic genes can be divided into several types:
- Full domination. In this case, the dominant gene completely suppresses the recessive one.
- Incomplete dominance. Accordingly, the signs of a recessive gene are not fully suppressed dominantly. This can be traced by the example of two flowers, red and white, the color of which is determined by the homozygous state. Their heterozygous offspring will be pink or variegated.
- Overdominance. This option is that the signs of a heterozygous descendant will be stronger than those of homozygous ancestors. This type of dominance formed the basis of heterosis, which consists in superiority over parents (for example, in viability and all its components).
- Co-dominance. It consists in the appearance in the descendant of new signs that his parents did not have. The most vivid example of this is the inheritance in people of the blood group. For example, if mom and dad have blood type I (00), then in a couple all children will be born with the same indicators. If the mother is the carrier of group II (AO), and the father is II (VO), then the couple can have a child from any of the four groups.
What is allelic exclusion
It happens that in homogametic individuals that contain sex cells with the same set of chromosomes, one of them becomes little or completely inactive.Regarding people, this condition is observed in women, while, say, in butterflies, on the contrary, in males. With allelic exclusion, only one of the two chromosomes is expressed, and the second becomes the so-called Barr body, that is, an inactive unit twisted into a spiral. Such a structure is called mosaic. In medicine, this can be traced in B-lymphocytes, which can synthesize antibodies only to certain antigens. Each such lymphocyte chooses between the activity of either the paternal allele or the maternal one.
In nature, there is a widespread phenomenon when the same gene has not two, but more forms. In plants, this is manifested by a variety of strips on the leaves and petals, in animals - by various combinations of colors. In humans, a vivid example of multiple allelism is the child's inheritance of a blood group. Its system is designated ABO and is controlled by a single gene. Its locus is designated I, and allelic genes - IA, IB, IO. Combinations IO IO give the first blood group, IA IO and IA IA - the second, IB IO and IB IB - the third, and IA IB - the fourth. In addition, people are determined rhesus. Positive give a combination of 2 allelic genes with the sign "+" or 1+ and 1-. Negative rhesus give two allelic genes with the sign "-".The rhesus system is controlled by the CDE genes, and the D gene often causes Rh-conflict between the fetus and the mother, if she has Rh-negative blood, and the fetus has Rh-positive blood. In such cases, in order to successfully complete the second and subsequent pregnancies, the woman is given a special therapy.
Lethal allelic genes
Alleles whose carriers die due to genetic diseases caused by these genes are called lethal. In humans, they cause Huntington's disease. In addition to the lethal there is also the so-called semi-lethal. They can cause death, but only under certain conditions, for example, at high ambient temperatures. If these factors can be avoided, the semi-lethal genes do not cause death to individuals.