The term ‘lethal’ in genetics is applied to those changes in the genome of an organism that produces effects severe enough to cause death. The fully dominant lethal allele kills the individual both in in its homozygous and heterozygous conditions. The individuals with dominant lethal allele die before they can produce progeny. Therefore, the mutant dominant lethal allele is removed from the population in the same.
Lethal alleles in plants
In snapdragons three types of plants occur
Green plants (green due to the presence of chlorophyll)
Yellowish green plants ( due to the presence of carotenoids)
White plants ( due to the absence of chlorophyll)
Homozygous green plants have genotype CC, cc are white plants which do not have chlorophyll. The yellowish green plants also called golden or auria plants, have the genotype Cc.
When two auria plants are crossed, albino plant dies at lacks chlorophyll , the ratio is modified into 2:1, the homozygous recessive genotype being lethal.
Similar lethal gene controlling the amount of chlorophyll is found in Zea Mays. The recessive allele g controls the chlorophyll content and exhibits its effect in homozygous and heterozygous condition (gg) combination is lethal and so the ratio is modified to 2:1.
Lethal alleles in animals
“Yellow” lethal in mice – In mice, an incompletely dominant allele Y for yellow coat has been found lethal in homozygous condition. This condition of yellow lethal of mice was first reported by Cue’not.
(yy) genotype produced black coat color in mice. When homozygous black mouse was crossed with another black mouse, only black mice were produced. Cue’not then crossed a black homozygous mouse (yy) with a yellow mouse (Yy) and a ratio of 1:1 was obtained by him, proving that yellow mouse was heterozygous.
When a yellow mouse was crossed with another yellow mouse, Cue’not found that the yellow and black mice appeared in the ratio of 2:1, the homozygous dominant allele was resulted in the death of the bearer before birth therefore was never seen among the phenotype.
Lethal genes in Drosophila
In Drosophila certain recessive lethal genes like curly wings (Cy), plum eyes (Pm), and stubbles (Sb) influence the viability of the flies when present in homozygous condition.
Several sublethal genes too have been reported in Drosophila. Genes which handicap an individual but do not destroy their possessor are called sublethal genes, that reduce the viability of flies.
Lethal genes in Man:
a) Congenital Ichthyosis: It is an example of homozygous recessive lethal gene in man. Children affected with this disease are born with crusted leathery skin with deep fissures. These fissures lead to bleeding, infection and death.
b) Amaurotic idiocy: This too is like Congenital Ichthyosis, is caused by a recessive allele in homozygous condition. Bearers of this genotype are affected in juvenile stage itself, they lose their eyesight between four and seven years of age. This is accompanied by mental degeneration and finally death before adolescence.
c) Sickle cell anemia: In certain African Tribes, the presence of codominant gene in Hb1S present in chromosome 11 homozygous condition (codominant with HbA ) cause a disease called Sickle cell anemia which leads to death. In the presence of Hb1S Hb1S, the hemoglobin formed not being the right type (the hemoglobin found in sickle celled individuals has valine, instead of glutamic acid as one of its 287 amino acids) leads to sickle shaped RBC instead of normal biconcave oval appearance. This inturn affects the oxygen carrying capacity of hemoglobin; and impedes blood circulation as well.
However, the heterozygous individuals, Hb1S Hb1A manifest none of these symptoms and are outwardly indistinguishable from the normal homozygotes (Hb1A Hb1A )
d) Huntington’s Chorea: So far, all the lethal effects, that discussed are attributable to recessive genes. But there is the peculiar case of Huntington’s Chorea which manifests by the working of a dominant gene. Huntington’s Chorea leads to degeneration of cerebral cortex, corpus stratum, especially the caudate nucleus. The onset of the disease is in the middle age marked by occurrence of rapid, jerky, ceaseless involuntary movements, poor memory, irritability and violence are some of the other characteristics of this disease.
The gene responsible for this disease lies on chromosome 4 and finally causes death of the individual.