Two important concepts for the development of genetics in the early twentieth century were those of phenotype and genotype, created by the Danish researcher Wilhelm L. Johannsen (1857 - 1912).
The term “phenotype” (from Greek pheno, evident, brilliant, and typos, characteristic) is used to designate the characteristics presented by an individual, whether morphological, physiological and behavioral. Also part of the phenotype are microscopic and biochemical characteristics, which require special tests for their identification.
Among the visible phenotypic characteristics we can mention the color of a flower, the color of a person's eyes, the texture of the hair, the color of an animal's fur, etc. The blood type and amino acid sequence of a protein are phenotypic characteristics revealed only by special tests.
An individual's phenotype undergoes change over time. For example, as we get older our body changes. Environmental factors can also change the phenotype: If we are exposed to sunlight, our skin will darken.
The term “genotype” (from Greek genos, originate, stem, and typos, characteristic) refers to the genetic constitution of the individual, that is, to the genes he has. We are referring to the genotype when we say, for example, that a pea plant is dominant homozygote (VV) or heterozygote (Vv) relative to the color of the seed.
Phenotype: genotype and environment in interaction
The phenotype results from the interaction of genotype with the environment. Consider, for example, two people who have the same types of alleles for skin pigmentation; If one of them sunbathes more often than the other, their skin tones, phenotype, are different.
An interesting example of interaction between genotype and environment in phenotype production is the reaction of Himalayan rabbits to temperature. At low temperatures, hairs grow black and at high temperatures they grow white. The normal coat of these rabbits is white, less on the ends of the body (snout, ear, tail and paws), which, because they lose more heat and have a lower temperature, develop black coat.
Determining the genotype
While an individual's phenotype can be directly observed, even through instruments, the genotype has to be inferred by observing the phenotype, analyzing its parents, children and other relatives, or by sequencing the individual's genome. , ie reading what is in the genes. The sequencing technique is not widely used due to its high cost and the need for specialized equipment. For this reason the observation of the phenotype and analysis of relatives is still the most used resource to know the genotype.
When an individual presents the phenotype conditioned by the recessive allele, it is concluded that he is homozygous for the allele in question. For example, a green pea seed is always homozygous vv. An individual who presents the phenotype conditioned by the dominant allele may be homozygous or heterozygous. A yellow pea seed, for example, may have genotype VV or Vv. In this case, the genotype of the individual can only be determined by analysis of his parents and descendants.
If the dominant phenotype individual is a father's son with a recessive phenotype, he will certainly be heterozygous because he inherited a recessive allele from his father. However, if both parents have a dominant phenotype, nothing can be said. It will be necessary to analyze the offspring of the individual under study: if any child exhibits the recessive phenotype, this indicates that he is heterozygous.
This crossing is made with a homozygous individual recessive to the factor to be studied, which is easily identified by its phenotype and another of known or unknown genotype. For example, if we cross an unknown male with a recessive female we can determine if the male is bearer of that recessive character or is pure. If he is pure, all his children will be like him, if he is a bearer, 25% will be white, etc. This explanation is very basic, as it usually takes a little more than this single intersection.
The limitation of these crosses is that they do not allow the identification of carriers of multiple alleles for the same trait, that is, in some cases there may be more than two alleles for the same gene and the effect of their combination will vary. In addition, we may be crossing a factor to which the test male or female are not carriers but other alleles.