Lethal alleles: The genes that kill

Mutations that occur in living things are totally random, and sometimes genetic varieties emerge that can lead to the death of the carrier before birth or, if he survives, before reaching sexual maturity. These genes leading to carrier death are known as lethal alleles.

For example, in a plant species there is the gene Ç, dominant, responsible for the green coloration of the leaves. The recessive allele ç conditions the absence of leaf coloration, so the recessive homozygote cc still dies in the young phase of the plant, because it needs the green pigment to produce energy through photosynthesis.

The heterozygote is a healthy plant, but not as efficient in capturing solar energy, because of its light green color on its leaves. Thus, crossing two heterozygous plants with light green leaves will result in the proportion 2:1 phenotypes among descendants, rather than the proportion of 3:1 that would be expected if it were a classic case of monohybridism (crossing between two heterozygous individuals for a single gene). In the case of plants the recessive homozygote dies shortly after germination, which leads to a 2: 1 ratio.

P Plant with light green leaves

Ç ç


Plant with light green leaves



Dark green


Light green


Light green



F1 = Phenotype: 2/3 Light Green

1/3 Dark Green

Genotype: 2/3 Cc

1/3 CC

This curious case of lethal genes was discovered in 1904 by the French geneticist Cuénot, who was surprised that the 3: 1 ratio was not obeyed. Therefore, it concluded that it was a case of recessive gene that acted as lethal when in double dose.

In humans, some lethal genes cause the death of the fetus. This is the case of genes for achondroplasia, for example. It is an anomaly caused by the dominant gene that, in double dose, causes the death of the fetus, but in single dose causes a type of dwarfism, among other changes.

There is lethal genes in man that manifest after birth, some in childhood and some in adulthood. In childhood, for example, we have the causes of cystic fibrosis and Duchenne muscular dystrophy (anomaly that causes degeneration of the myelin sheath in the nerves). Among those who express themselves late in the life of the carrier are the causes of Huntington's disease, in which there is the deterioration of nervous tissue, with cells losing mainly in one part of the brain, causing memory loss, involuntary movements and emotional imbalance.