We call lethal alleles those that determine the death of their bearer. These genes were discovered by the fact that they provided a change in expected values at each cross according to Mendelian frequencies .
Lethal Alleles: The lethal alleles can be dominant, acting even in single doses, or recessive, expressing themselves only in homozygosity . It is worth noting that dominant lethal alleles are rarely observed, since people with these genes are generally unable to reproduce before dying.
→ Discovery of lethal alleles
Lethal alleles were discovered by Lucien Cuenot (1866-1951) in 1904 after studies with crosses in mice. When the biologist crossed gray-coated mice with each other, all the offspring that were born had gray fur, but when crossing yellow-coated individuals, 2/3 of yellow mice and 1/3 of gray mice were born, which did not obey the proportion of 3:1 expected.
In view of these results, Cuenot came to the conclusion that, in homozygosity, the allele that determined the color yellow was lethal. Therefore, all yellow mice were obligatorily heterozygous and, to prove this theory, he crossed one of these individuals with the gray mouse, obtaining 50% of the yellow individuals and 50% of the gray ones. From this, it was possible to conclude that mice with genotype homozygous for yellow did not survive, which explained the inadequate proportion.
→ Complete and semi-lethal lethal alleles
We call complete lethal those genes that kill before the individuals who have them reach reproductive age. One example is cystic fibrosis , a disease that affects the exocrine glands , which produce thick secretions that prevent the body from functioning properly. This thick substance can harm, for example, the respiratory system, which has difficulty in bringing oxygen to the pulmonary alveoli. In addition, thick and sticky secretions make it difficult for the digestive system to function.
Semi – lethal genes are those genes that allow the carrier to survive beyond their reproductive age. As an example, we have Achondroplasia, a genetic syndrome that directly affects ossification. Patients with this problem have short stature, shortened limbs, short hands and separation between the third and fourth fingers, among other characteristics. This syndrome is determined by a dominant gene, which causes death when in double dose.