Introduction
Mendel's laws are the set of basic rules on the transmission by genetic inheritance of the characteristics of parent organisms to their offspring. They are the basis of genetics.
The laws derive from the work on plant breeding by Gregor Mendel, an Austrian Augustinian monk, published in 1865 and 1866, though it was long ignored until its rediscovery in 1900.
The history of science finds in the Mendelian heritage a milestone in the evolution of biology, only comparable to Newton's laws in the development of physics. Such appreciation is based on the fact that Mendel was the first to formulate with total precision a new theory of inheritance, expressed in what would be called "Mendel's laws", which confronted the little required theory of inheritance, by mixing blood. . This theory brought to biological studies the basic notions of modern genetics.
However, it was not only his theoretical work that gave Mendel his scientific stature; No less remarkable have been the epistemological and methodological aspects of his research. The recognition of the importance of rigorous and systematic experimentation and the expression of observational results in quantitative form through the use of statistics revealed a novel epistemological position for biology. For this reason, Mendel is usually conceived as the paradigm of the scientist who, based on a meticulous and unprejudiced observation, manages to inductively infer his laws, which constitute the foundations of genetics. In this way, Mendel's work has been integrated into the teaching of biology: in the texts, the Mendelian theory appears constituted by the famous three laws, conceived as inductive generalizations from the data collected from experimentation.
HISTORY
The theory of inheritance by mixture assumed that characters are transmitted from parents to children through body fluids that, once mixed, cannot be separated, so that the descendants will have characters that are the mixture of the characters of the parents. This theory, called pangenesis, was based on facts such as the crossing of red-flowered plants with white-flowered plants produce pink-flowered plants. Pangenesis was advocated by Anaxagoras, Democritus, and the Hippocratic treatises, and, with some modifications, by Charles Darwin himself.
Mendel owned a personal copy of Charles Darwin's On the Origin of Species and was influenced by it. Between the years 1856 and 1863, Gregor Mendel cultivated and tested some 28,000 plants of the species Pisum sativum (pea). His experiments led him to conceive of two generalizations that would later be known as Mendel's Laws, Laws of Inheritance, or Mendelian Inheritance. The conclusions are described in his paper titled Experiments on Plant Hybridization (the original German version of which is called Versuche über Pflanzenhybriden), which was read before the Brno Natural History Society on February 8 and March 8, 1865, and later published. published in 1866
Mendel sent his work to the Swiss botanist Karl Wilhelm von Nägeli, one of the greatest authorities of the time in the field of biology. It was he who suggested that she perform his series of experiments on various species of the genus Hieracium. Mendel was unable to replicate his results, since after his death in 1903 it was
discovered that a special type of parthenogenesis occurred in Hieracium, causing deviations from the expected Mendelian ratios. From his experiment with Hieracium, Mendel possibly came to think that his laws could only be applied to certain types of species and, therefore, he moved away from science and dedicated himself to the administration of the monastery of which he he was a monk. . He died in 1884, completely ignored by the scientific world
In 1900, however, Mendel's work was rediscovered by three European scientists, the Dutch Hugo de Vries, the German Carl Correns, and the Austrian Erich von Tschermak, separately, and unaware of Mendel's work they came to the same conclusions. that he. De Vries was the first to publish on the laws, and Correns, having read his article and searched the published bibliography, in which he found Mendel's forgotten article, declared that Mendel had gone ahead and that de Vries's work it was not original. In fact, the idea that the factors were physical particles would not catch on until the early twentieth century. It seems more likely that Mendel interpreted hereditary factors in terms of neo-Aristotelian philosophy, interpreting recessive traits as potentialities and dominant traits as actualities.
EXPERIMENTS
Mendel published his experiments with peas in 1865 and 1866. The choice of Pisum sativum gave him many advantages as a model organism: its low cost, short generation time, high offspring rate, several varieties within the same species with easily identifiable characters ( color, shape and size, among others)
Pisum sativum is an autogamous plant, that is, it self-fertilizes. Mendel avoided it by emasculating it (removing the anthers). Thus he was able to exclusively cross the desired varieties. He also bagged the flowers to protect the hybrids from uncontrolled pollen during flowering. He carried out a control experiment by crossing over two successive generations by selfing to obtain inbred lines for each character.
Mendel carried out the same series of crosses in all of his experiments. He crossed two different varieties or inbred lines with respect to one or more characters. As a result he obtained the first filial generation (F1), in which he observed the phenotypic uniformity of the hybrids. Subsequently, the selfing of the F1 hybrids gave rise to the second filial generation (F2), and so on. He also made reciprocal crosses, that is, he alternated the phenotypes of the parental plants:
♀P1 x ♂P2
♀P2 x ♂P1
(being P the parental generation and the subscripts 1 and 2 the different phenotypes of this).
In addition, he carried out backcrosses, which consist of crossing the hybrids of the first filial generation (F1) by the two parents used, in the two possible directions:
♀F1 x ♂P2 and ♀P2 x ♂F1 (reciprocal crosses)
♀F1 x ♂P1 and ♀P1 x ♂F1 (reciprocal crosses)
Experiments showed that:
Inheritance is transmitted by particulate elements (thus refuting the inheritance of mixtures).
They follow simple statistical rules, summarized in their two principles
MENDEL'S LAWS
Mendel's three laws explain and predict how the physical characteristics (phenotype) of a new individual will be. They have often been described as "laws to explain the transmission of characters" (genetic inheritance) to offspring. From this point of view, of character transmission, strictly speaking, it would not be appropriate to consider Mendel's first law (Law of uniformity). It is a widespread error to suppose that the uniformity of the hybrids that Mendel observed in his experiments is a law of transmission, but dominance has nothing to do with transmission, but with the expression of the genotype. So this Mendelian observation is sometimes not considered to be Mendel's law. Thus, there are three Mendelian laws that explain the characteristics of the offspring of two individuals, but there are only two Mendelian laws of transmission: the law of segregation of independent characters (2. law, which, if the law of uniformity, is described as 1. Law) and the law of independent inheritance of characters (3rd law, sometimes described as 2. Law).
Mendel's 1st Law: Principle of Uniformity
It establishes that, if two inbred lines are crossed for a certain trait, the descendants of the first generation will all be equal to each other, phenotypically and genotypically, and phenotypically equal to one of the parents (of dominant genotype), regardless of the direction of the crossing. . Expressed with capital letters for the dominant ones (A = yellow) and lowercase for the recessive ones (a = green), it would be represented as follows: AA x aa = Aa, Aa, Aa, Aa. In a nutshell, there are factors for each character which separate when gametes are formed and come back together when fertilization occurs.
Mendel's 2st law: Principles of segregation
This law states that during the formation of gametes, each allele of a pair separates from the other member to determine the genetic makeup of the daughter gamete. It is very common to represent the possibilities of hybridization using a Punnett square.
Mendel obtained this law by crossing different varieties of heterozygous individuals (diploids with two allelic variants of the same gene: Aa) and he was able to observe in his experiments that he obtained many peas with yellow skin characteristics and others (less) with green skin characteristics, he verified that the ratio was 3/4 yellow and 1/4 green (3:1). Aa x Aa = AA, Aa, Aa, aa
According to the current interpretation, the two alleles, which code for each characteristic, segregate during the production of gametes by meiotic cell division. This means that each gamete will contain only one allele for each gene. This allows combining the maternal and paternal alleles in the offspring, ensuring variation.
For each trait, an organism inherits two alleles, one from each parent. This means that in somatic cells, one allele comes from the mother and one from the father. These can be homozygous or heterozygous.
In Mendel's own words:
"It is now clear that hybrids form seeds having one or the other of the two differential characters, and of these half develop back into the hybrid form, while the other half produce plants which remain constant and receive the dominant or dominant character. . recessive in the same number." gregor mendel
Mendel's 3st law: Sometimes it is described as the 2-law, in case of considering only two laws (criterion based on the fact that Mendel only studied the transmission of hereditary factors and not their dominance/expression). Mendel concluded that different traits are inherited independently of each other, there is no relationship between them, therefore the inheritance pattern of one trait will not affect the inheritance pattern of another. It is only fulfilled in those genes that are not linked (that is, they are on different chromosomes) or that are in widely separated regions of the same chromosome. In this case the offspring follows the proportions. Representing it with letters, from parents with two characteristics AALL and aall (where each letter represents a characteristic and dominance by uppercase or lowercase), by interbreeding of pure breeds (1 Law), applied to two traits, the following gametes would result: Al x aL = AL, Al, aL, al.
By exchanging between these four gametes, the ratio AALL, AALl, AAlL, AAll, AaLL, AaLl, AalL, Aall, aALL, aALL, aAlL, aAll, aaLL, aaLl, aalL, aall is obtained.
As a conclusion we have: 9 with "A" and "L" dominant, 3 with "a" and "L", 3 with "A" and "l" and 1 with recessive genes "aall". In Mendel's own words:
"There is no doubt, therefore, that to all the characters involved in the experiments the principle is applied that the offspring of hybrids in which several different essential characters are combined, present the terms of a series of combinations, resulting from the meeting of the developmental series of each pair of differential characters" gregor mendel
Mendelian inheritance patterns
Mendel described two types of "factors" (genes) according to their phenotypic expression in offspring, dominant and recessive, but there is another factor to take into account in dioecious organisms and it is the fact that female individuals have two X chromosomes (XX) while males have one X and one Y chromosome (XY), with which four ways or "patterns" are formed according to which a simple mutation can be transmitted:
- Dominant gene located on an autosome (autosomal dominant inheritance).
- Recessive gene located on an autosome (autosomal recessive inheritance).
- Dominant gene located on the X chromosome (X-linked dominant inheritance).
- Recessive gene located on the X chromosome (X-linked recessive inheritance).
Phenomena that alter Mendelian segregation
sex-linked inheritance
It is the inheritance related to the pair of sex chromosomes. The X chromosome carries numerous genes, but the Y chromosome carries only a few and most of them related to maleness. The X chromosome is common to both sexes, but only the male has a Y chromosome.
Sex-influenced and sex-limited inheritance
In inheritances limited to sex, mutations of genes with autosomal chromosomes whose expression only takes place in organs of the male or female reproductive system may be involved. An example is the transverse vaginal septum congenital defect, of autosomal recessive inheritance, or the deficiency of 5 α reductase that converts testosterone into dihydrotestosterone that acts in the differentiation of the male external genitalia, so its absence simulates female genitalia when the child is born.
A mutation may be influenced by sex, this may be due to the effect of endocrine metabolism that differentiates males and females. For example, in humans baldness is due to the effect of a gene that is expressed as autosomal dominant, however in a family with the segregation of this gene only men suffer from baldness and women will have thinner hair after menopause . Another example may be the deficiency of the enzyme 21 hydroxylase, which is involved in the metabolism of glucocorticoids. When this enzyme is absent, the synthesis of glucocorticoids shifts towards the formation of testosterone and this hormone is involved in the embryogenesis of the external genitalia of the male, so its abnormal presence in the development of a female fetus produces masculinization of the male. female genitalia, while in the case of a male fetus, it only increases the development of the male ones. An abnormality of this type will allow a clinical diagnosis to be suspected more quickly in a girl, based on examination of the newborn's genitalia, than in a boy.
Y chromosome gene structure
Because they have only one X chromosome, the words "heterozygous" or "homozygous" cannot be applied to male individuals for genes located on this chromosome and absent on the Y chromosome. Whether they are genes that express the dominant character or recessive, if they are located on the X chromosome, males will always express it and the individual carrying it is called homozygous.
From the above it follows that, since females have only one type of sexual chromosome, the X, their gametes will always have the 22+X chromosome set, while males can carry an X, giving rise to a female individual (XX) , or a Y, with which a male individual (XY) would originate. Because of this it is said that women are homogametic (all their gametes have the same constitution) and that men are heterogametic (they have gametes 22+X and 22+Y).
X-chromosome gene dose compensation system
In insects, as has been seen in Drosophila, the existence of a gene that acts as a dose compensator was discovered. When it is present in a single dose (as occurs in males), it activates the expression of genes on the X chromosome. In mammals, no gene with equivalent function has been found.
lionization
Lyonization or inactivation of the X chromosome occurs because, unlike the Y chromosome, the X has a large number of active genes that code for important products such as coagulation factor VIII. It could be thought, therefore, that if females have two X's they must have twice the products or enzymes whose genes are on that chromosome in relation to males, however, this is not the case.
It has been observed in mammals that in the somatic cells of the female sex (44+XX), only one of the two X chromosomes is active. The other remains inactive and appears in interphase cells as a dense, strongly colored body, which becomes inactivated and attaches to the nuclear envelope at the periphery of the nucleus, called the Barr body. The inactivation of the X chromosome takes place in the morula state, around the third day after fertilization, and is completed, in the mass of internal cells that will give rise to the embryo, at the end of the first week of embryonic development. The selection of the X chromosome to be inactivated is a generally random phenomenon, taking into account that at fertilization each X chromosome has a maternal and paternal origin, in some cells the maternal X (Xm) will be inactivated and in others the paternal X (Xp ). Once one of the two X chromosomes is inactivated, the descending cells will maintain the same inactive X chromosome, originating an active cell clone (Xm) or (Xp). That is, at the beginning of the inactivation, this is random, first any of the two X is randomly inactivated, whether it is the one inherited from the mother or the father; but once it occurs, the same X chromosome that was inactivated in the first cell of the clone is maintained and the cells that derive from it during the growth and development process will keep the same X chromosome inactivated from now on.
The inactivation (turning off) of the X chromosome is determined by the XIST gene. This gene is involved in inactivation-specific transcription that works by a preferential methylation mechanism, this means that if there is no structural alteration in the two X chromosomes of the female genome, inactivation must occur randomly, but if there is any alteration with great compromise in the function of one of the two X chromosomes, there would be a non-completely random activation. The XIST gene locus is located at Xq13.3.
X inactivation determines genetic and clinical consequences:
- Dose compensation: equalizes the dose of gene products with the hemizygous for genes located on the X chromosome, determining similar protein concentrations in both sexes, for X-linked genes.
- Variations in the expression of mutations in heterozygous females: for example, presence of more or less severe symptoms in carrier females for hemophilia A or B, Duchenne muscular dystrophy, X-linked recessive retinal dystrophies.
- The female organs behave like mosaics. This phenomenon can manifest itself in areas in which one allele is manifested (from the mother's X) and other areas in which the other allele is manifested. It is observed in phenomena such as the color of the fur of some female felines, so that felines with three colors are female, and those with two colors are male;8 in X-linked recessive ocular albinism; or in the immunohistochemical test for the detection of dystrophin in females heterozygous for Duchenne muscular dystrophy
Penetrance of a specific gene or mutation
Penetrance is the word used to refer to the expression in terms of all or nothing within a population of individuals. If the mutation is expressed in less than 100% of the carrier or heterozygous individuals, it is said that the mutation has a reduced penetrance and that the apparently "healthy" individual for the character or disease being studied in the family can transmit the mutation to his or her family. offspring and these express the defect. The reduced penetrance appears to be the effect of the relationship between the mutation in question and other genes in the genome, with which it is interacting in one of the cells.
Expressivity of a specific gene or mutation
Expressivity is used to refer to the degree of severity that is manifested in the phenotype. In clinical terms, it is synonymous with severity. The expression of a gene also depends on its relationship with the rest of the genome, but also on the genome-environment relationship. To refer to these phenotypic gradations, the term variable expressivity of the gene or mutation is used.
Pleiotropic effect of a specific gene or mutation
The term pleiotropy or pleiotropic effect of a gene refers to all the phenotypic manifestations in different organs or systems that are explained by a simple mutation. A classic example to explain this word is Marfan Syndrome, whose mutation affects the FBN1 gene that encodes the protein fibrillin, this protein is found in connective tissue and explains the skeletal, ocular and cardiovascular manifestations that characterize the syndrome.
Genetic heterogeneity
This term applies both to mutations in genes located on different chromosomes that produce similar expression in the phenotype (non-allelic heterogeneity) and to mutations that affect different sites on the same gene (allelic heterogeneity). This category extraordinarily complicates the etiological study of developmental variants of genetic origin and constitutes a large and fundamental source of developmental genetic diversity.
New mutations with dominant expression
When a de novo mutation occurs that is expressed as dominant, that is, in a heterozygous genotype, it happens that parents who do not have the effect of the mutation may have an affected offspring. The absence of a family history, once phenomena such as reduced penetrance of the gene and minimal variations in expressivity are excluded, makes it difficult to establish a de novo mutation when the defect or disease has not been previously reported in the literature, with a specific type of inheritance.
Lethality effect on a specific genotype
Some mutations are so severely expressed that they produce lethality in a specific genotype. An example could be the effect of a double dose of a mutation that is expressed as dominant or the effect on a hemizygous genotype, as occurs in incontinence pigmenti, a dominant human disease linked to the X chromosome
BIBLIOGRAPHY:
Gregor Johann Mendel was a Catholic Augustinian friar and naturalist. He formulated, through the work he carried out with different varieties of peas and peas, the now called Mendel's laws that gave rise to genetic inheritance. The first works in genetics were carried out by Mendel.
Born: July 20, 1822, Hynčice, Vražné, Czechia
Died: January 6, 1884 Brno, Czechia
Education: Palacký University (1840–1843), University of Vienna
Parents: Anton Mendel, Rosine Mendel
Awards: Imperial Order of Franz Joseph
Sisters: Veronica Mendel, Theresia Mendel
Conclusions and comments
It is a widespread error to suppose that the uniformity of the hybrids that Mendel observed in his experiments is a law of transmission, since dominance has nothing to do with transmission, but with the expression of the genotype. So this Mendelian observation is not usually considered a law. The Mendelian laws of transmission are therefore two: the law of segregation of independent characters (1 law) and the law of independent inheritance of characters (2 law).
Going through the Wikipedia website, this fact can be observed, for example, in the English, French and Portuguese versions they consider that Mendel's Laws are two. On the other hand, in other versions such as the Catalan, German, Italian and Basque ones, they continue to consider the law of Uniformity as Mendel's first law, without being strictly a law of transmission of characters.
work presented by
natalia pereira
sofia centeno
sara monsalve
Maria Siado