What are Lethal Genes? – Definition
Genes which result in viability reduction of individual or become a cause for death of individuals carrying them are called as lethal genes.
Phenomenon of action of lethal genes is called as lethality.
Some lethal genes cause death of zygote or the early embryonic stage while some express their effect in later stages of development.
Individuals carrying dominant lethal will die. Even though the dominant lethal is eliminated from the genotype of population previously carrying it, the recessive lethals are still carried in heterozygous condition. The recessive lethal in heterozygous condition reduce the viability and when they occur in homozygous condition produce lethal effect.
Usually lethal genes are dominant with respect to the phenotype they control and recessive for their lethal actions.
Example of Lethal Genes
In case of mice (By Cuenot)
In a cross of a yellow mouse with another yellow, yellow and brown mice are obtained in 2:1 ratio. True breeding yellow mice never obtained. In 1917, Stiegseder reported that 1/4th of the offspring die in embryonic condition in such crosses.
These premature dead forms are dominant homozygous.
According to Cuenot gene Y has multiple effects. It controls the yellow color of the body and affects to individual viability. It means the gene is dominant for body color and recessive with respect to lethality. The living yellow forms are heterozygous and somehow manage to survive.
In case of maize, albinism is example of lethal factor.
The lethal factor in heterozygous condition has no lethal effect but in homozygous condition it makes plant to die. Lethal factor in homozygous condition produces albino plant, which is unable to synthesize food due to lack of chlorophyll. Thus, the lethal factor modifies normal ratio from 3:1 to 2:1.
For other types of gene interactions go to the page Types of Gene Interactions.