The biological inheritance It is the process by which the offspring of a cell or an organism receive the genetic endowment of their parents. The study of genes and how they are transmitted from one generation to another is the field of study of the genetics of heredity..
Heredity can be defined as the similarity between parents and their descendants, but this term also encompasses the differences inherent in the genetic variation that occurs during reproduction.
Biological inheritance is one of the most significant attributes of living cells and, therefore, of multicellular organisms, as it means the ability to transmit properties and characteristics determined by successive generations, collaborating with adaptive mechanisms and natural selection..
Many authors are of the opinion that man has become familiar with the basic concepts of biological inheritance for thousands of years, when the processes of domestication of plants and animals began and, selecting the characteristics that he considered most important, he made crosses to obtain offspring that will retain these characteristics.
However, our current understanding of the physical and molecular foundations of heredity is relatively recent, as these were not elucidated until the early 20th century, when the scientific community came up with the chromosomal theory of heredity..
Despite the above, the principles of the heritability of traits or characters were analyzed in some depth some years before by Gregorio Mendel, who is currently considered the "father of inheritance.".
It is important to mention that both before and after the formulation of this chromosomal theory of inheritance, many other discoveries were of transcendental importance for our modern understanding of the genetics of living beings. These could include the isolation and characterization of nucleic acids, the discovery and study of chromosomes, among others..
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The basic foundations of inheritance were initially proposed by the Austrian monk Gregory Mendel (1822-1884) in the year 1856.
Mendel understood from his experiments with pea plant crosses that both the similarities and differences between parents and their descendants could be explained by a mechanical transmission of discrete hereditary units, that is, of genes.
This laid the foundations for the understanding of modern genetics, since it was known that the reason for the heritability of traits was due to the physical transfer of genes on chromosomes through reproduction (sexual or asexual)..
According to the results he obtained, Mendel formulated the "laws of inheritance" that are in force today for some characters:
- Law of segregation: it maintains that genes have "alternative" forms known as alleles and that directly influence the phenotype (visible characteristics).
- Law of independent inheritance: implies that the inheritance of one trait is independent of that of another, although this is not entirely true for many characteristics.
Thomas Morgan in 1908, studying the genetics of the fruit fly, Drosophila melanogaster, showed that the inheritance of phenotypes occurred as previously described by Mendel. In addition, it was he who elucidated that the units of heredity (genes) are transmitted through chromosomes.
Thus, and thanks to the participation of many other researchers, today we know that biological inheritance occurs thanks to the transmission of the hereditary units contained in the chromosomes, either in the autosomal, sexual or plastid chromosomes (in eukaryotes).
The expression or appearance of a given phenotype depends on the characteristics of the genotype (dominance and recessivity).
To understand how the mechanisms of biological inheritance work, it is necessary to understand some fundamental concepts.
Although not all inherited traits are due to the sequence of nucleotides that make up the DNA of each organism, when we refer to biological inheritance, we are generally talking about the transmission of the information contained in the genes.
A gene is defined, then, as the fundamental physical unit of inheritance and it is a defined DNA sequence, which contains all the information necessary for the expression of some trait or characteristic in a living being..
Genes can have more than one form, the product of small variations in the sequence that characterizes them; alternative forms of the same gene are known as alleles.
Alleles are usually defined according to the phenotypic characteristics they confer and, in a population, it is common to find multiple alleles for the same gene.
A mammal, for example, inherits two alleles of each gene from its parents, one from its mother and one from its father. That allele of the genotype that is always expressed in the phenotype is called the dominant allele, while the one that remains “silent” (of which no phenotypic characteristics are observed) is known as the recessive allele..
In some cases it may occur that both alleles are expressed, generating the appearance of an intermediate phenotype, which is why these are called codominant alleles together..
When an individual inherits two identical alleles from their parents, be they both dominant or recessive, this is known, genetically speaking, as homozygous for these alleles. When, on the other hand, an individual inherits from his parents two different alleles, one dominant and the other recessive, this is called heterozygous for these alleles..
The genes of prokaryotic and eukaryotic organisms are found on structures known as chromosomes. In prokaryotes, cells that do not have a nucleus, there is normally only one chromosome; it is made up of DNA associated with some proteins, is circular in shape and highly coiled.
Eukaryotes, cells that have a nucleus, have one or more chromosomes, depending on the species, and these are made up of a material called chromatin. The chromatin of each chromosome consists of a DNA molecule closely associated with four types of proteins called histones, which facilitate its compaction within the nucleus..
In eukaryotes there is more than one type of chromosome. There are nuclear, mitochondrial, and chloroplastids (restricted to photosynthetic organisms). Nuclear chromosomes are autosomal and sexual (those that specify sex).
Ploidy in the entire number of chromosome “sets” that a cell has. Human beings, for example, like all mammals and many animals, have two sets of chromosomes, one from the father and the other from the mother, therefore, their cells are said to be diploid..
Those individuals and / or cells that have only one set of chromosomes are known as haploids, while organisms that have more than two sets of chromosomes are, together, polyploids (triploids, tetraploids, hexaploids and so on).
In organisms with sexual reproduction this occurs thanks to the fusion of two specialized cells, produced by two different individuals: a "male" and a "female". These cells are the gametes; males produce sperm (animals) or microspores (plants) and females produce eggs or egg cells.
Normally, the chromosomal load of sex cells (gametes) is haploid, that is, these cells are produced through cell division that involves a reduction in the number of chromosomes.
In a living organism, the genotype describes the set of genes (with their respective alleles) that code for some specific trait or characteristic and that are distinguished from others by a certain function or sequence. So, the genotype represents the allelic constitution of an individual.
Although it is generally said that the genotype is what gives rise to the phenotype, it is important to bear in mind that there are many environmental and epigenetic factors that can condition the appearance of a phenotype.
Phenotype literally means "the form shown." Many texts define it as the set of visible characteristics of an organism that result both from the expression of its genes (genotype) and from its interaction with the environment that surrounds it..
Most phenotypes are the product of the concerted action of more than one gene and the same gene can participate in the establishment of more than one specific phenotype.
The transmission of genes from parents to their descendants occurs in the cell cycle through mitosis and meiosis.
In prokaryotes, where reproduction occurs mainly by bipartition or binary fission, genes are transferred from one cell to its descendants via the exact copy of its chromosome, which is secreted into the daughter cell before division occurs..
Multicellular eukaryotes, which are generally characterized by having two types of cells (somatic and sexual), have two mechanisms of transmission or gene inheritance.
Somatic cells, as well as simpler unicellular organisms, when they divide they do so by mitosis, which means that they duplicate their chromosomal content and secrete the copies towards their daughter cells. These copies are generally exact, so the characteristics of the new cells are equivalent to those of the "progenitors".
Sex cells play a fundamental role in the sexual reproduction of multicellular organisms. These cells, also known as gametes, are produced by meiosis, which involves cell division with a concomitant reduction in chromosomal load (they are haploid cells).
Through sexual reproduction, two individuals pass their genes from one generation to the next by fusing their haploid gametic cells and forming a new diploid individual that shares characteristics of both parents..
The classic examples of biological inheritance that are used in teaching texts to explain this process are based on different "model" organisms..
The fruit fly (D. melanogaster) for example, one of the study models for animals used by Thomas Morgan, usually has red eyes. However, during his experiments, Morgan found a white-eyed male and crossed it with a red-eyed female..
In view of the fact that the “red eyes” phenotype is dominant in this species, the result of the first generation of this cross consisted of individuals with red eyes. Subsequent crosses between the individuals of the first generation (F1) resulted in an F2 generation in which both phenotypes appeared..
Given that white eyes only appeared in F2 in a proportion close to 50% of males, Morgan assumed that it was a "sex chromosome-linked inheritance".
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