The hair is essentially a structure composed of proteins: keratin and melanin are the main ones. Some strands of hair are blond, others are black or even other intermediate tones. The substance that makes the tonalities vary from one to another is the melanin. Regarding the structure, some hairs are smooth, others curly. The substance that defines the type of capillary structure is keratin.
The way in which keratin is deposited in the capillary cortex defines the structure that this hair will be. Thus, the hair structure is a genetically defined factor: if your parents, grandparents and other backgrounds have had smooth hair, you have great chances of having them smooth too. If, on the other hand, any of them have had curly hair, the likelihood that their hair will also grow curly increases, although this is not an absolute rule. In a recent research, scientists discovered that the gene responsible for hair format is trichohyalin and it is dominant: a smooth-haired parent and a curly-haired mother (or vice versa) will likely have a child or more children with curly hair.
Also on the role of keratin in curly hair, it is distributed in the capillary cortex ellipsoid (outside the ellipse). This means that there is a greater amount of keratin around the hair, the sides, and less keratin in the center of the hair structure. That is, curly hair is almost hollow in its inner layer.
This curious form of deposit of the keratin in the hair does not only influence the visual, leaving it more or less curly, but also influences its fragility. Yes, because as in the center of the structure there is little keratin, the hair is more prone to bankruptcy. Therefore, curly hair requires twice as much care compared to other capillary structures: frequent moisturizing, delicate handling, special shampoos with low sulfate, among others.
Chemistry care is a special warning: because of this more fragile structure of curly hair, the chemistry applied on them has a huge potential to cause damage. It is necessary to protect the hair in any way before any procedure. To make a smoothing, for example, it is necessary to change the distribution of keratin inside the cortex by opening the hair cuticle well for the chemistry to act internally to alter its structure. Finally, these cuticles should be properly closed. If at the time of the smoothing step some step, the damage may be irreversible to curly hair.