Mendel, Gregor - Bohemian naturalist (Heinzendorf, Silesia, 1822-Brno, Moravia, 1884). Having become an Augustinian friar, he entered the Brno convent in 1843; later 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 in the convent garden to long practical experiments on pea hybridization. After careful and patient observation of the results, he was led to state with clarity and mathematical accuracy the important laws that go by the name of Mendel's laws. Equally valid for the plant world as for the animal world, 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 noted all his observations, the results of which were presented in a brief memoir to the Brno Naturar History Society in 1865. At the time, the publication was not appreciated in all its importance and did not arouse that 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 the Netherlands, C. Currens in Germany, E. von Tschermak in Austria; but in the meantime the study of biology had made great progress, the times had changed and the discovery immediately had a great repercussion.
The first law, or law of dominance, is also more properly called the law of hybrid uniformity. Mendel took two pea plants (which he called capostipiti) both of pure breed, one with yellow seeds, the other green and used the pollen of one to fertilize the other. From this cross derives a first generation of peas of hybrid plants, that is, no longer of pure breed; all the plants produced peas with yellow seeds, none showed the green seed character. The yellow character, in other words, dominated the green; in other words, the yellow was dominant, the green, masked, recessive. There is also a particular case, when there is an 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 an explanation of the brilliant and brilliant phenomena; he assumed that together with the gametes, factors were transmitted which were responsible for the development of the characters; he thought that in each organism a given character is regulated by two factors, one transmitted by the mother and one by the father, and that these two factors are equal in the pure-bred individuals, different in the hybrids and that finally in the gametes there is always only one factor . Mendel pointed out 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 e.g. with AA the pea which bears the dominant yellow character, with aa that carries the recessive green character. The hybrid, which receives A from one parent and from the other will be Aa.
It may be pointed out here that from the appearance of an individual one cannot always know whether it belongs to a pure race or whether it is a hybrid; instead, it is necessary to examine its behavior at intersections and cross-references. In fact, the pure and hybrid yellow peas seem to be identical; it is known, however, that their genetic composition is different, one being AA and the other Aa. While crossing between them yellow peas of pure race (AA) you will always have only peas with yellow seeds, crossing between them yellow peas or semi-yellow but hybrid (Aa) you will see appear in their descent also plants with green seeds. The yellow peas Aa, although identical, are genotypically different, that is in their genetic composition. Other important laws of Mendel are: the law of segregation or disjunction of characters and the law of the independence of characters.
At the time of Mendel the phenomena of mitosis and meiosis were not yet clarified, 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 organism (diploid) the factors are present in pairs, and only in the gametes (haploid) there is a single factor. And where they are present in pairs they can be either equal or different.
When two equal factors (be they dominant or recessive, GG or gg) have merged in the zygote, the resulting individual is said to be homozygous for that character, while the one in which two different factors have converged (Gg) is called heterozygous .
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.
Alleles for a certain character can also be more than two. We will therefore talk about dialelelic and polyalelic characters, or, respectively, of dimorphism and genetic polymorphism .
By convention, the generations of the experimental cross are indicated with the symbols P, F1 and F2, which mean respectively:
P = parental generation;
F1 = first branch generation;
F2 = second branch generation.
In the Mendelian cross, yellow X green gives all yellow; any two of these, crossed each other, give a green every three yellow. The yellows and greens of the P generation are all homozygous (as ascertained with a long selection). Each of them gives always equal gametes, so their sons are equally equal, all heterozygous. Since yellow is dominant over green, heterozygotes are all yellow (F1).
However, by crossing two of these heterozygotes together, we see that everyone 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 particular cases), for which zygotes of the four possible types are formed with equal probability in F2: GG = homozygote, yellow; Gg = heterozygote, yellow; gG = heterozygote, yellow; gg = homozygote, green.
Yellow and green are therefore in a ratio of 3: 1 in F2, as the yellow manifests itself as long as it is present, while the green manifests itself only in the absence of yellow.
To better understand the phenomenon from the point of view of molecular biology, it is sufficient to hypothesize that a given 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 any of the two homologous chromosomes that carry 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 genotype.
The external manifestation of the organism of genetic characters (what we see), more or less modified by environmental influences, is called phenotype . The set of only genetic characters, which may or may not be manifested in the phenotype, is called genotype .
The yellow peas of F2 have an equal phenotype but a variable genotype. In fact, they are for 2/3 heterozygotes (carriers of the recessive character) and for 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 that are the subject of the 1st and 2nd Mendel's laws respectively. All this refers to the crossing between individuals that differ in a single pair of alleles, for a single genetic character.
If you make any other such crossing, the Mendelian pattern repeats itself; for example, 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 double homozygotes, that is varieties that differ in more than one character (for example GGLL, yellow and smooth, with ggll, greens and regoses), we see that in F1 all will be heterozygous with both dominant, phenotyped characters, but in F2 will have 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 two by two in the zygotes).
It is evident that two characters that were together in the first generation segregate independently of each other in the third. Each pair of homologous chromosomes segregates, independently of the other, in meiosis. And this is what establishes Mendel's 3rd law.
Let us now see, as a whole, a formulation of Mendel's three laws :
1a: law of dominance. Given a couple 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. The cross between F1 hybrids gives three dominants for each recessive. The phenotypic ratio is therefore 3: 1, while the genotype is 1: 2: 1 (25% dominant homozygotes, 50% heterozygotes, 25% recessive homozygotes).
When crossing individuals that differ in more than one pair of alleles, each pair segregates in 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 always happens in the great principles of biology, the general character of these laws does not mean that they have no exceptions.
Indeed, the possible exceptions are so many that today it is customary to divide genetics into Mendelian and Neo -endelian, including in the latter all the phenomena that do not fall within the Mendelian laws.
While, however, the first exceptions made doubts about the validity of Mendel's discoveries, it was possible to later show that his laws are of general scope, but the phenomena that underlie them are combined with a great variety of other phenomena that modulate it otherwise the expression.
CONTINUE: Predict your child's blood type "
Edited by: Lorenzo Boscariol
