Lethal genes cause death of the organism at embryonic stage shortly after birth or at a later age. The mutation in critical genes essential for growth and development. It may be recessive dominant or condition dependent based on genetic expression.
L Cuenot 1905 discovered.
ALS C9ORF72 gene mutation. Chromosome 9
Huntington disease. Chromosome 4.
Recessive. may be silent or mild inconvenience when heterozygous but lethal when homozygous.
Sickle cell anaemia
Conditional lethal. Genes that are fatal in some environments.
G6PD deficiency or favism. when a person consumes fava beans they developed hemolytic anaemia and possibly die.
Gametic lethal. that lead to in viability of gametes and made them incapable of fertilization.
Sub lethal genes.
do not cause death of organism but physiological metabolic or mental impairment.
Haemophilia impaired blood clotting and coagulation. X-linked.
Natural selection favours a gene.
Some persist due to selective advantage like G6PD and sickle cell anaemia advantage against malaria. Dhobzansky.
Muller argues. Modern medical practices are increasing there propagation like phenylketonuria PKU disease.