Mendel, Gregor - Bohemian naturalist (Heinzendorf, Silesia, 1822-Brno, Moravia, 1884). Having become an Augustinian friar, he entered the convent of Brno in 1843; subsequently he completed his scientific studies at the University of Vienna. From 1854 he taught physics and natural sciences in Brno. Between 1857 and 1868 he devoted himself to long practical experiments on the hybridization of peas in the convent garden. After careful and patient observation of the results, he was led to state with clarity and mathematical exactness the important laws that go by the name of Mendel's laws. Equally valid for the plant world as for the animal one, these laws constituted the starting point for the creation of a new branch of biological sciences: genetics. For nine years, analyzing the results of hundreds and hundreds of artificial pollinations, cultivating and examining some 12,000 plants, Mendel patiently recorded all his observations, the results of which were presented in a short memoir to the Brno Natural History Society in 1865. At the time, the publication was not appreciated in all its importance and did not arouse the "interest it deserved. Ignored by scholars for more than thirty" years, the laws were rediscovered in 1900 simultaneously and independently by three botanists: H. de Vries in Holland, C. Currens in Germany, E. von Tschermak in Austria; but in the meantime the study of biology had made great progress, times had changed and the discovery immediately had a great impact.
The first law, or the law of dominance, is also more properly called the law of the uniformity of hybrids. Mendel took two pea plants (which he called progenitors) both of pure breed, one with yellow seeds, the other green, and used the pollen of one to fertilize the other. From this cross a first generation of peas of hybrid plants, that is no longer pure bred, derived; all plants produced yellow-seeded peas, none exhibited the green-seeded character. The yellow font, in other words, dominated the green; that is, yellow was dominant, green, masked, recessive. There is also a particular case, when there is incomplete dominance and the first generation shows an intermediate character between the paternal and the maternal; but even in this case the hybrids will be equal to each other. Mendel gave a brilliant and ingenious explanation of the phenomena; he assumed that together with the gametes, factors responsible for the development of characters were transmitted; he thought that in every organism or a given character is regulated by two factors, one transmitted by the mother and one by the father, and that these two factors are the same in purebred individuals, different in hybrids and that finally only one factor is contained in gametes . Mendel indicated the two factors of the antagonistic characters with letters of the alphabet, uppercase for the dominant, lowercase for the recessive; and since each parent has a couple of factors he indicated for example with AA the pea that carries the dominant yellow character, with aa the one that carries the green recessive character. The hybrid, which receives A from one parent and from the other will be Aa.
Here it can be pointed out that from the appearance of an individual it is not always possible to know whether it belongs to a pure breed or whether it is a hybrid; instead, it is necessary to examine its behavior in interbreeding and re-crossing. In fact, purebred yellow peas and hybrid peas are apparently identical; however, it is known that their genetic composition is different, one being AA and the other Aa. While crossing between them pure-bred yellow peas (AA) you will always and only have yellow-seeded peas, crossing yellow or semi-yellow but hybrid peas (Aa) with each other you will see also green-seeded plants appear in their descendants. The yellow peas Aa, although identical, are different genotypically, that is, in their genetic composition. Other important Mendel's laws are: the law of segregation or disjunction of characters and the law of independence of characters.
At Mendel's time the phenomena of mitosis and meiosis were not yet clear, but today we know that in meiosis the gametes receive only one chromosome of each pair and that exclusively with fertilization these chromosomes return to mate at random.
If we think (for temporary simplification) that a certain factor is localized on a single pair of chromosomes, we see that in the eukaryotic (diploid) organism the factors are present in pairs, and only in the gametes (haploid) is there a single factor. where they are present in pairs they can be either the same or different.
When two equal factors (whether dominant or recessive, GG or gg) have merged into the zygote, the resulting individual is said to be homozygous for that character, while heterozygous is one in which two different factors have merged (Gg).
The alternative factors that determine the character in the individual are called alleles. In our case G and g are respectively the dominant allele and the recessive allele for the color character of peas.
The alleles for a certain character can be even more than two. We will therefore speak of diallelic and poliallelic characters, or, respectively, of genetic dimorphism and polymorphism.
By convention, the generations of the experimental cross are indicated with the symbols P, F1 and F2, which respectively mean:
P = parental generation;
F1 = first filial generation;
F2 = second generation filial.
In the Mendelian cross, yellow X green gives all yellows; any two of the latter, crossed with each other, give one green for every three yellows. The yellows and greens of generation P are all homozygous (as ascertained with a long selection). they always give equal gametes, so their offspring are equally equal, all heterozygotes Since yellow is dominant over green, heterozygotes are all yellow (F1).
However, by crossing two of these heterozygotes, we see that each one can give one or the other type of gametes with equal probability. Also the union of the gametes in the zygotes has the same probability (except in special cases), so zygotes of the four possible types are formed with equal probability in F2: GG = homozygous, yellow; Gg = heterozygous, yellow; gG = heterozygous , yellow; gg = homozygous, green.
Yellow and green are therefore in a ratio of 3: 1 in F2, as yellow occurs anyway as long as it is present, while green occurs only in the absence of yellow.
To better understand the phenomenon from the point of view of molecular biology, it is sufficient to assume that a certain basic substance, green, is not modified by the enzyme produced by the g allele, while the G allele produces an enzyme that converts the green pigment into yellow pigment. If the G allele is not present on either of the two homologous chromosomes carrying that gene, the peas remain green.
The fact that yellow peas can be characterized by two different genetic structures, the homozygous GG and the heterozygous Gg, gives us the opportunity to define the phenotype and the genotype.
The external manifestation of the organism's genetic characteristics (what we see), more or less modified by environmental influences, is called phenotype. On the other hand, the set of genetic characteristics alone, which may or may not be manifested in the phenotype, is called genotype.
F2 yellow peas have the same phenotype but variable genotype. In fact, they are 2/3 heterozygotes (carriers of the recessive trait) and 1/3 homozygotes.
Instead, for example, in green peas the genotype and phenotype are mutually invariable.
As we will see, the appearance of only one of the parental characters in F1, and the appearance of both characters in a 3: 1 ratio in F2, are phenomena of a general nature which are the subject of Mendel's 1st and 2nd law respectively. All this refers to the crossing between individuals who differ for a single pair of alleles, for a single genetic character.
If any other such crossing is made, the Mendelian pattern is repeated; for example, by crossing peas with wrinkled seed and smooth seed, in which the smooth allele is dominant, we will have LL X 11 in P, all LI (heterozygous, smooth) in F1, and three smooth for each wrinkled in F2 (25% LL , 50% LI, 25% 11). But if we now cross homozygous doubles, that is varieties that differ by more than one character (for example GGLL, yellow and smooth, with ggll, green and regosi), we see that in F1 all will be heterozygous with both dominant characters, phenotyped, but in F2 there will be the four possible phenotypic combinations in a numerical ratio of 9: 3: 3: 1 which derives from the 16 possible genotypes corresponding to the possible combinations of the four types of gametes (taken in pairs to two in the zygotes).
It is evident that two characters that were together in the first generation independently segregate each other in the third. Each pair of homologous chromosomes segregates independently of the other in meiosis, and this is what Mendel's 3rd law establishes.
Let's now see, as a whole, a formulation of Mendel's three laws:
1a: law of dominance. Given a pair of alleles, if the offspring of a cross between the respective homozygotes has only one of the parental characters in the phenotype, this is called dominant and the other recessive.
2a: law of segregation. Crossing F1 hybrids gives three dominants for each recessive. The phenotypic ratio is therefore 3: 1, while the genotypic ratio is 1: 2: 1 (25% dominant homozygotes, 50% heterozygotes, 25% recessive homozygotes).
When crossing individuals who differ by more than one pair of alleles, each pair segregates into descendants, independently of the others, according to the 1st and 2nd law.
These three laws, although not properly formulated as such by Mendel, are recognized as the foundation of eukaryotic genetics. As is always the case in the great principles of biology, the general character of these laws does not mean that they have no exceptions.
Indeed, there are so many possible exceptions that today it is customary to divide genetics into Mendelian and Neomendelian, including in the latter all phenomena that do not fall within Mendelian laws.
While, however, the first exceptions cast doubt on the validity of Mendel's discoveries, it was subsequently possible to demonstrate that his laws are of general application, but the phenomena underlying them combine with a great variety of other phenomena that modulate them. otherwise the expression.
CONTINUES: Predict your child's blood group "
