The most mysterious and intriguing aspect of life is why we are what we are. Why is each human being unique? Why are characteristics passed on from parents to children? In this chapter, we will try to clear up some of these questions. In the conception of a human being, two sexual cells (haploid cells) –a sperm and an ovule- join in order to generate a single cell (diploid cells). All of the information necessary to create a new individual is contained inside of it, like for example, eye color, size and artistic and intellectual capabilities. This information stored in the genes is coded in the deoxyribonucleic acid molecule (DNA) and is inherited from the parents, as well as what is not transmitted from them (variation).Specifically, inheritance corresponds to the similar characteristics that are transmitted from parents to children, and variation is the non-similar characteristics. These last ones are genetically transferred and affected by the environment. Therefore, inheritance and variation make up the base of genetics and has come to be an important science. In fact, in the XX and XXI century this science has developed, and today, it has enabled not only the understanding of why some characteristics are inherited, but it has also allowed us to determine the origin and how to develop treatments for a variety of serious human illnesses. The very creation of human beings has even been attempted, activity that has had its share of controversy.
A bit of history
Until the end of the XIX century, the way the physical characteristics were passed on from parents to children was uncertain. Said doubt was also repeated in plants and the rest of the animal species. However, the work of Gregor Mendel and the later contributions of molecular genetics helped solve these questions. This way, the way inheritance mechanisms worked was able to be understood. Mendel’s research took place because he was interested in understanding the way the characteristics were transmitted, meaning, the characteristics of living beings, especially plants, from one generation to the next. His experiments consisted of crossing pure strains of pea plants. A pure strain is based on plants that, upon crossing with others of the same strain, spawn identical plants. Mendel obtained seven pairs of pure strains: seed shape (smooth or wrinkled), seed color (yellow or green), flower color (purple or white), shape of the legume (smooth or strangled), color of the mature legume (green or yellowish), position of the flowers (axial or terminal) and the size of the plants (normal or tiny). This way, he began to conduct hundreds of experiments and to obtain a lot of data. He registered said data on charts and submitted them to probability analyses. This way, Mendel synthesized his results into the conclusions that today are known as the Mendelian laws of inheritance.
Mendel’s laws
– First law: it proposes that when crossing two homozygote parental individuals (P) (one dominant and one recessive) for the same characteristic, its offspring (F1) will be uniform, meaning, all of the F1 individuals will be identical to the dominant homozygote. For example, for the seed color characteristic, yellow is dominant and green is recessive and the resulting F1 will be yellow.
– Second law: it comes from the results obtained with the crosses using F1 individuals. As green reappeared in the offspring (F2), it was deduced that the seed color characteristic is represented by variables or alleles that are yellow (dominant color) and green (recessive color).
– Third law: it proposes the alleles of different characteristics are transmitted independently from offspring. This was proven in an experiment in which Mendel analyzed the heritage of two characteristics simultaneously (seed color and shape). He took the yellow and smooth ones for dominant alleles and for recessives he chose the color green and a wrinkly texture. Then, he crossed pure plants for both characteristics and obtained F1, which only had dominant alleles (yellow and smooth). The crossing of F1 individuals spawned F2 individuals with yellow and green seeds at a 3:1 ratio. This showed independent combinations of alleles.
Mateo de Toro y Zambrano convocó a cabildo abierto