The postmendelian inheritance It includes all the ideas and / or works that arose after the studies published by the religious naturalist Gregorio Mendel in 1865, and that explain genetic phenomena or behaviors different from those described by this.
Gregorio Mendel is considered the father of genetics, since by studying crosses between pea plants, he established the first laws of inheritance more than 100 years ago. The first law, called "Law of segregation", proposes that genes can exist in different or alternative forms.
These alternative forms are called "alleles" and they directly influence the phenotypic characteristics of a species. Alleles can determine hair color, eye color, hair type, etc..
In his first law, Mendel proposed that each phenotypic trait transmitted from parents to their children is controlled by two alleles, each coming from a parent (one from the "mother" and one from the "father").
According to this, when the parents contribute different alleles, the progeny results with a mixture, where one of the alleles is dominant (the one that is expressed in the phenotype) and the other is recessive (it is not seen, or is "latent")..
Mendel's second law is known as the “Law of independent inheritance”, since in his crosses Mendel observed that the pattern of inheritance that characterized the alleles that defined a trait did not influence the pattern of inheritance of the alleles that defined to a different trait.
Article index
Since Gregor Mendel proposed his famous laws more than 100 years ago, new mechanisms have been elucidated to explain the inheritance of characters, many of which could be said to contradict or differ considerably from those described by Mendel..
An example of the phenomena described by "modern" science, whose advances have only been possible thanks to the advancement of experimental strategies and research methods, are inheritance linked to sex, incomplete dominance and codominance, among others..
Sex-linked inheritance has to do with genes that are found only on the sex chromosomes. In humans, sex is determined by the X and Y chromosomes. Women have two copies of X (XX) and men have one of X and one of Y (XY)..
The genes found on each of these two chromosomes are those that contribute to those traits that are linked to sex. Genes on the X chromosome are present in men and women, but Y genes are only in men.
Sex-linked inheritance was observed and described for the first time by William Bateson, one of the first scientists to “rediscover” Mendel's work and to take advantage of them to carry out new experiments based on his conclusions.
Bateson made experimental crosses of hens with plumages of different colors, some with dark and smooth feathers and others with smooth feathers and with a striped pattern..
Through these crosses, he observed that, in the offspring, the resulting males always had plumage with a striped pattern..
Only in the third generation, when a striped male was crossed with a hen with dark and smooth feathers, some roosters with black plumage were obtained. This resulting proportion of the crosses defied the proportions proposed by Mendel.
After Bateson's work, Thomas Henry Morgan found a logical explanation for the strange results obtained by Bateson, attributing the difference in coloration in chickens to the fact that the striped pattern was encoded by a dominant allele found on the X chromosome..
In hens, unlike in humans, the heterogametic sex (XY) corresponds to the females, while the homogametic sex corresponds to the males (XX).
The allele studied by Bateson and Morgan, then, was expressed by all those males with striped plumage, but when a male inherited two X chromosomes with the allele corresponding to the black coloration, then males with dark plumage and no striped pattern appeared..
Incomplete dominance is when the phenotype observed in the offspring that results from the crossing of two homozygous individuals (one dominant and the other recessive) is an "intermediate" phenotype between the dominant and the recessive..
It has to do with the "inability" of the "dominant" allele to completely mask the expression of the recessive allele, which is why the resulting phenotype corresponds to a mixture of the traits encoded by both alleles, often described as a "dilute phenotype ”Of the dominant.
For example, in the Dondiego plants at night (Mirabilis jalapa) the typical color of wildflower petals is red. When a plant with red petals is crossed with a pure line plant with white petals, it is obtained that all the progeny are pink (pink) petals.
When two dominant homozygous individuals are crossed and a third phenotype is observed in the offspring that corresponds to the two characteristics determined by the two parental alleles, we speak of codominance.
In this case, continuing with an example similar to that of the flowers in the previous example, the "third phenotype" would not correspond to pink flowers, but to a plant with white flowers and red flowers on the same stem, since the two alleles "codominate ”, And neither masks the other.
Many times, however, codominance and incomplete dominance are difficult to distinguish phenotypically, since both produce a phenotype of an individual that seems to express two different alleles..
However, genotypically the individuals that present codominant alleles are different from those that present incomplete dominance, since in one case the two dominant alleles are expressed in the same quantity, and in the other the proportions are unequal..
A classic example of codominance is the type of blood group in humans, since the alleles that code for antigens A and B in human populations are codominant.
That is, when alleles A and B are found together in the cells of an individual, this individual has blood type AB.
They are recessive alleles for some character that, when found in homozygous form, are lethal for the individual. Individuals who present these alleles are not born, since they prevent the normal functioning of the molecular machinery and prevent the translation of genes.
Many places in the genome of an individual interact with other parts of the same genome, that is, the specific modification of an allele within the genome can directly condition the expression of other alleles within the cells of an individual.
These interactions are difficult to determine and are classified into two different types: genes that interact in the same genetic pathway and those that interact in different genetic pathways..
A classic and well-known example of genes that interact in different genetic pathways has to do with the genes that determine the skin color of corn snakes (Pantherophis guttatus), whose common color is a repeating pattern of black and orange spots.
This phenotype is produced by two different pigments that are under strict genetic control. A gene determines the orange color and it has two alleles: one O +, which codes for the pigment and another O-, which determines the total absence of the orange pigment..
Another different gene determines the presence or absence of the black pigment, the alleles of this gene are known as B +, the one that determines the presence of the black pigment and B-, which determines the absence of the pigment.
Both genes are not linked and independently determine the color of snakes, that is, a snake with the O + / B + genotype is a normal snake with an orange and black phenotype, but any genotypic variation will result in a different phenotype.
The classic example of these genes is the one that has been described for the “bellflower” flowers (of the Campanulaceae family), whose “wild” phenotype corresponds to plants with blue flowers..
The blue color of bluebell flowers is determined not by a gene, but by a metabolic pathway where several enzymes, each encoded by a different gene, participate in the synthesis of a pigment..
Thus, there are phenotypes characterized by being the result of the interaction of several genes, whose products participate in the same metabolic pathway and where the mutation or malfunction of any of these yields a phenotype, in this case, of white, depigmented flowers..
Yet No Comments