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Agents responsible for hereditary transmission:
It all starts with the joining of the egg and the sperm, they form an egg (or zygote), which is the first cell that constitutes the individual. This cell will divide into two, which in turn will be subdivided until the organism is generated. This process is called mitosis.
Chromosomes:
Chromosomes are chemically made up of DNA - deoxyribonucleic acid - and proteins. The human being has 23 pairs of chromosomes, 22 of which are common among the sexes - female and male. Chromosome 23, called the sex chromosome, differs between the two sexes: - XY in men - XX in women.
23 pairs of humans' chromosomes, being the last one the sexual chromosome.
DNA
Structure: Each DNA molecule to a twisted rope ladder or two helix-wrapped chains - a double helix. The structure of DNA is universal throughout the living world and the same in each species, thus, it is the number and sequence of nucleotides that define the characteristics of the living being. Components: The “stair steps” are made up of 4 nitrogenous substances: o Adenine (A); o Thymine (T); o Cytosine (C); o Guanine (G). These are always linked two by two:
- Adenine -> Thymine (T);
- Cytosine -> Guanina (G). The sides of the stairs are made up of chains of a sugar - deoxyribose -, which alternates with phosphate groups - phosphoric acid. The nucleotide forms the base unit of DNA, which is formed by long chains of nucleotides. Nucleotide is a group formed by deoxyribose, a molecule of phosphoric acid and a nitrogenous base. The structure of DNA is universal throughout the living world and the same in each species, so it is the number and sequence of nucleotides that define the characteristics of each living being.
Meiosis is a process of cell division that occurs during fertilization (gamete foundation). The number of chromosomes is reduced to 23, thus ensuring the 46 chromosomes characteristic of the human species. If this division did not take place, the chromosomes would double in relation to the previous one, which would cause a change in the characteristics of the species. During meiosis, 2 successive divisions of the nucleus occur: o Division 1; o Division 2. Fig. 1. Meiosis process. (NOTE: Division 1 = Meiosis 1; Division 2 = Meiosis 2.). Meiosis is very important for living beings that reproduce sexually, because it contributes to genetic variability. The random character of the encounter of gametes during fertilization reinforces the genetic diversity resulting from meiosis and explains the great variety of beings within the same species.
Genetic variability
Genetic variability is the set of genetic variations that exist between members of the population. This is caused by the random separation of homologous chromosomes, which halves the number of chromosomes (thus contributing to increased variability). In humans, the gamete may have 23 chromosomes, all of paternal, maternal origin or any other combination of the many that can occur during the distribution of parents by gametes. If the chromosomes were genetically identical, this random separation would have no effect, but since the parents are genetically distinct in many ways, there will be gametes with combinations of genes very different from those that occur in either parent. Regarding the color of the eyes and hair, genes could be formed whose information for blue eyes and dark hair did not exist in either parent.
Genetic and Epigenetic Influences on Behaviour
Specific and individual heredity
Specific heredity corresponds to the genetic information responsible for the characteristics common to all individuals of the same species, determined the physical constitution and some behaviours. The migration of birds, the way in which mating takes place, care for the young and the construction of nests are some examples of specific heredity. Among human beings there is a set of common characteristics that define us as human. As examples: the constitution of the face, the constitution of the hands, the structure of the skeleton, the brain, etc. these and other common characters constitute our specific heredity. Individual heredity corresponds to the genetic information responsible for the individual's characteristics and which distinguishes him from all other members of his species. It is what makes you unique.
Genotype and phenotype
The genotype corresponds to the collection of genes that the individual is endowed with at the time of its conception and that results from the set of genes from the mother or father. The genotype is the genetic constitution of an individual, the set of inherited genetic determinations that may or may not be expressed according to the characteristics of the environment in which it develops. The genotype is, therefore, the genetic blueprint of an organism, it is the set of characters as defined by genes. The phenotype designates the appearance of the individual, that is, the set of observable characteristics - anatomical, morphological, physiological - that result from the interaction between the genotype and the environment where development occurs. The phenotype is, therefore, a set of individual characters of genetic origin that have received modifications due to the relationship with the environment. Corresponds to the update of the genotype. The appearance of one is provided by the genotype, that is to say, hereditary heritage and the environment, which includes all conditions: food, socioeconomic, socio-cultural, climatic, etc. Thus, the person is the result of a history in which hereditary and environmental factors are intertwined. The complexity of who we are derives from the inherited potential and the effects of the environment. The genetic potentials that the individual has at birth are developed by the interaction with the environment, which starts from conception, in its intrauterine genesis. Effectively, from the embryo that the effects of the environment are felt: in the embryonic development, potentialities appear that were not part of the fertilized cell and that develop from influences of the prenatal environment. This interaction - heredity and environment - can be good or bad: it can allow the harmonic development of the genetic potential, but it can also negatively influence the process of expression and development of that potential.
The development of the new individual is therefore limited to the increase in the size of the miniature being (homunculus), to the amplification of the pre-existing structures in the egg. At the end of the 17th century, two distinct orientations were defended: some considered that the future being was already in miniature in the sperm; others, that the new being existed preformed in the egg. Inside would be a small preformed man, who integrated a homunculus into his own sex cells, to infinity. In the 18th century, the Swiss naturalist Charles Bonnet developed this concept by reaffirming that embryonic development was nothing more than the expansion of the characteristics of the homunculus. Preformism accentuates the genetic dimension of development, disregarding the effect of the environment. The development, therefore, only of the hereditary component. One could speak of hereditary determinism.
Epigenesis
In 1759, preformism is denied by Caspar Friedrich Wolff, who presents a new conception: the egg is a disorganized structure, and the embryo is differentiated by the effect of external forces. This conception - Theory of epigenesis or epigenitism - denies the existence of preformed structures in the egg and that develop later. Development is the result of gradual progress in growth, differentiation and modification. It is from a simple and homogeneous state that the embryo progressively develops. In this process there are potentialities that were not present in the original fertilized egg, but that develop from environmental influences. These notions lead to the concepts of genotype and phenotype. The term epigenesis, which, etymologically, means “what is added to the genome”, refers to everything that is not determined by genetic heritage. Corresponds to transmissible and reversible modifications of gene expressions, but which do not amplify changes in DNA sequences. An example that reflects this reality is that of true or homozygous twins who have the same genetic load, which means that all their characteristics are absolutely identical, including the nervous system. What is verified however is that the number of nerve cells is different and the connections between neurons also differ, that is, there is an individual variability that escapes genetic determinism. This example shows the importance of non-hereditary factors in the individual's construction process. In most species, the construction of the organism and the specification of its activities does not depend only on its genetic heritage. Also, behaviours do not depend only on genes or on the influence of the environment, on learning: they result from the confrontation of the embryo, carrier of the genes, with the biological influence of all kinds found during its growth, its embryonic development that new potentials emerge that were not included in the original fertilized cell, but that develop from influences of the prenatal environment and the interaction of the cells. The concept of epigenesis designates precisely what, in the construction of the organism and its behaviour, does not depend only on genes or on learning. Epigenesis therefore prolongs the effects of genetics without being determined by it.
Phylogenesis and Ontogenesis
When we talk about human development, we can refer to the development of the species or the development of the individual. In the first case, we refer to phylogenesis; in the second, onto ontogenesis. These two concepts are related, as can be seen when analysing its etymological origin. Phylogenesis is the set of evolutionary processes of living beings from the most elementary to the most complex; it is the set of biological transformation processes that explain the appearance of species and their differentiation. Phylogenesis is the evolutionary history of a species, of a specific group of organisms. Ontogenesis refers to the individual's development from fertilization to adulthood: it begins with embryogenesis, continues in postnatal development until adulthood. There are authors who extend the concept of ontogenesis to the life cycle of the individual, defining it as the process in which the individual changes in the course of his life, from conception to death.
The law of recap
At the end of the 19th century, beginning of the 20th century, several researchers, including Ernest Haeckel, defended that the embryonic development of vertebrates would recapitulate the stages of evolution. In this way, the development of a fish would stop prematurely, while, for example, that of a bird would complete the later stages. Haeckel then formulated the law according to which “ontogenesis would recapitulate phylogenesis”, that is to say that, in the course of its development, the embryo would reproduce the stages of the evolution of the species' life. It is the law of ontophylogenetic recapitulation: the development of the embryo expresses, recapitulates, stages of development that corresponded to stages of evolution, of the phylogenic history of species. Ontogenesis would thus be determined by phylogenesis. In the first stage, all embryos have organs that resemble gills. In the second, the gills remain, disappearing in the third stage in animals that do not live in water. It came to be noted that such representations did not correspond either to the proportions of each embryo or to the stage of maturation, which varies from species to species. Haeckel manipulated these data to justify his theory, which turned out to be false. However, this theory guided the work and conceptions of the researchers, who resorted to the law of recapitulation to distinguish animals in “inferior” and “superior”. Among the defenders of this conception was Langdon Down, who published a work in which he presents a classification of the mentally disabled to which he applies the foundations of Haeckel's theory. Thus, for example, mental deficiencies correspond to a setback to stages of earlier embryonic phytogenic development.
The role of ontogeny
There is, however, a big difference between “fully” programmed living things and other animals that are partially programmed. In humans, this programming is the least significant, compared to other animals - the genetic program is open. And this difference distinguishes human beings. There is a basic programming in the biological field, but the human being is not determined by a system of instincts that define, at the outset, his development and his behaviour. Animals have specialized behaviour patterns: lions have claws that allow them to hunt and tear their prey, polar bears withstand extremely low temperatures. However, these specializations, which so often dazzle us for their effectiveness, determine limitations: they work only in ecological niches where animals are inserted. The claws do not allow opening a door, the expert swimmer does not walk. The absence of these hyperspecializations in humans is their advantage. With an organism less genetically prepared to face challenges to which animals respond with total effectiveness, man has the capacity to face unforeseen situations. Its “imperfection”, its unfinished nature allows it to adapt to changes, unforeseen situations. Prematurity and Neotenic Mankind is a biologically unfinished being. Its body takes much longer to reach full development than that of other species: just after birth, the duck swims behind its mother. Genetic programs allow behaviours that are effectively oriented towards survival and adaptation to the environment. The human being is a premature being, born unfinished. Its immaturity explains why human childhood is so long: it is the period of completion of the development process that took place in intrauterine life. The embryonic character of the baby becomes an advantage, because the long period of immaturity is essential for the survival and adaptation of the species. On the physical plane, the human being presents the biological unfinished which is called neotenic: developmental delay that causes the individual to develop more slowly, depending, for much longer, on adults, because it is necessary to teach him how to eat, walking, talking, etc. the brain development process is linked to oncogenic retardation, that is, to the prolongation of the period of childhood and adolescence. It is the developmental genes, of which we have already spoken, that make the human being a neotenic being, that is, an animal in which there is an extension of the juvenile morphology until adulthood. A reflection of neotenic is the fact that the adult still has traces of childhood and adolescence. The juvenile, plastic characteristics are maintained, therefore, in adult individuals who would present traces of neotenic: egg-shaped cranial box, juvenile facies, occipital cavity located at the base of the skull. In conclusion, the biological unfinished nature of the human being and his prematurity imply an extension of childhood and adolescence, a necessary condition for his adaptation and development process. This apparent lack will be an advantage in allowing the possibility of a greater capacity to learn in the context of their environment, their culture.
Advantages of human unfinished work
The concept of neoteny states that man is “a being open to the world”: his biological unfinished, his prematurity, explains the absence of a biological program as rigid as that which exists in other animals. Learning will fulfil the tasks that in animals are destined for heredity: the human being has to learn what heredity provides to other species. Its biological nature makes the process of adapting to the environment more flexible. It creates the need for man to create his own adaptation, culture, which he transmits from generation to generation. Human status is achieved only through learning. Human prematurity is therefore an advantage. The following text explains this idea. The most obvious conclusion is that, given our radical differences with chimpanzees, genetic endowment is no longer decisive in establishing the human condition. The more our flagrant discontinuity in the field of actions is demonstrated, it must come from the other elements not identifiable in the DNA. This conclusion is not a reason to increase or decrease the self- celebration of human successes, but it does serve to put the importance of genetic influence in perspective. The human being has basic programming - biological - but must self-program as a human. Sometimes this humanizing self-proclamation implies a certain symbolic "deprogramming" ... Even compared to its closest zoological relatives, it offers a feeling of openness of unfinishedness: in short, of extreme availability.
