The transmission of characters between parental organisms and their offspring is a complex and fascinating subject. The first person to carry out an analytical investigation of this matter was the monk Gregor Mendel in the 19th century, without knowing what a gene was.
Mendel became famous for his experiments with peas ( Pisum sativum ), in which he used parental individuals with certain traits and observed how these traits were transmitted to the offspring .
If you had used any other species for those same experiments, you may never have reached the same conclusions, since peas are a species with which it is easy to trace the transmission of characters.
The traits Mendel studied were the shape of the seed (round or rough), its color (green or yellow), the color of the flowers (white or purple), the color of the pod (green or yellow), and the shape of this (inflated or narrow).
The merit of his research was not only to use the analytical method to reach conclusions, but the first description of the rules that govern the transmission of characters between generations.
Thanks to his experiments, Mendel was able to establish three norms or Laws that are considered still in force today: Mendel’s laws:
- Mendel’s First Law : When two “purebred” individuals are crossed, all descendants are equal.
- Mendel’s Second Law : Certain individuals are capable of transmitting a character even if it is not manifested in them.
- Mendel’s Third Law : Each of the characters is transmitted independently.
Today it is known that the third principle is not always fulfilled , but Mendel had good reason to believe that it is. The characters he studied in peas were on different chromosomes, so their transmission was independent.
However, in cases where two genes are very close on the same chromosome, a phenomenon called “linkage ” can occur and the transmission of both to the offspring occurs jointly.
Genes: transmitters of characters to offspring
Today we know that characters are transmitted thanks to genes. A gene is, explained without going into much detail, a segment of DNA that codes for a protein.
[box type = »info» align = »» class = »» width = »»] A gene is, explained without going into much detail, a segment of DNA that encodes a protein. [/ box]
The characters that are expressed in an individual depend on their genes. Sometimes a trait depends on a single gene (as happened with Mendel’s peas) and sometimes it depends on several genes acting together.
Mendel was lucky in this too, as the traits he studied in peas were determined by a single gene . The term he used to refer to trait transfer mechanisms was not genes, but “traits.”
However, a question can jump us. If the traits of an individual depend on the genes of the individual and these are transmitted from parents to children, how can a trait not be present in one generation but in the next?
Mendel himself knew about this phenomenon, described in his second Law, but did not know the mechanisms by which it happened. Today we know that it occurs because there are some genes that are expressed more than others.
Dominant and recessive genes
A gene can have several alternative forms (called alleles), each with a different DNA sequence and expressing the same trait differently.
An example of this is found in the color of the eyes. In humans, the color of the eyes depends on several genes and each of these has several alleles. The color of a person’s eyes will depend on the combination of alleles of the genes involved in that trait.
Some of these alleles are expressed with greater “intensity” than others, masking their presence. For example, the allele for brown eyes masks the allele for blue eyes. Genes that are expressed with greater intensity are called dominant genes, while those that are not expressed in the presence of a dominant allele are considered recessive alleles or recessive genes.
[box type = »info» align = »» class = »» width = »»] When a gene is expressed with greater intensity it is called a dominant gene , while the one that is not expressed in the presence of a dominant allele is considered a recessive allele or recessive gene . [/ box]
There are different degrees of dominance . For example, if we have a gene with three alleles A, B and C, it may happen that B is dominant over C and A is dominant over B and C. Alleles B and C will never be expressed if A is present, and C it will not be expressed if B is found in the genome.
In living beings, the set of genes (called the genome) is divided into molecules called chromosomes. In bacteria, relatively simple organisms, there is only one chromosome. Human beings have 46. But it is not about 46 different chromosomes, we have 23 duplicated chromosomes .
Two of these are the sex chromosome pair, so called because they determine the sex of the person. These chromosomes are called XX in women and XY in men, from which it follows that the sex of the children depends on the father, since he is the only one who can contribute a Y chromosome to the offspring.
The genes found on a chromosome are also found in its twin chromosome , but on both chromosomes alleles of the same gene may be different.
An example of a recessive gene: the inheritance of baldness (androgenic alopecia)
A very visual example in humans is that of androgenic alopecia, determined by a single gene located on the X chromosome. This gene has two alleles, one that causes baldness (G allele) and the other that does not (A allele). Allele A is dominant over G.
For men, it is enough to have a G allele to remain bald, since since the gene is located on the X chromosome we cannot mask it with an A allele. On the other hand, even if a woman has a G allele, she will not be bald if she has another allele. A. Women need to have both G alleles to develop alopecia.
It should be clarified that this example has been simplified a lot, since in the case of baldness this may be due to other causes, and the G allele does not ensure the development of alopecia in men, but the chances are approximately 70% .
Dominance in Mendel’s experiments
Below is a schematic about the transmission of two genes and their expression in peas. Peas, like humans, have two copies of their genome, which are known as diploid organisms. Peas have 14 chromosomes, as opposed to 46 in humans.
In this experiment, carried out by Mendel, two individuals are crossed: a smooth yellow one with a rough green one.
- The first individual all has two alleles for the yellow trait (YY) and two alleles for the smooth trait (RR), which Mendel called “pure race.”
- The second individual is similar, with identical alleles but with green (yy) and rough (rr) features. In peas, the yellow allele is dominant over the green one and the smooth one over the rough one.
Thus, the offspring have a combination of alleles but only the yellow and smooth traits are expressed. However, if these individuals are crossed again in the descendants (the third generation of the experiment) there will be individuals who express the recessive traits, as seen in the diagram.
This is the classic proportion that follows the distribution of recessive traits encoded by recessive alleles in haploid organisms: 1/4 of individuals have a recessive trait and 1/16 have both recessive traits.
In humans there are several traits that follow the distribution that Mendel described in his peas, since they are encoded by a single gene. Some examples are albinism (which is a recessive trait), brachydactyly (dominant), having a sixth finger (dominant), a split chin (dominant), facial freckles (dominant) , attached earlobes (recessive), or the widow’s peak in hair (dominant).