Lethal alleles
alleles that cause an organism to die when present in homozygous condition are called as lethal alleles. Mutations resulting
in a gene product that is non-functional can sometimes be tolerated in the heterozygous state; that is, one wild-type allele may
produce enough of the essential product for the organism to survive. However, such a mutation behaves as a recessive lethal allele,
and homozygous recessive individuals will not survive. In some cases, the allele responsible for a lethal effect when present in
homozygous state may also result in a distinctive mutant phenotype when it is present heterozygously. Such an allele behave as a
recessive lethal, but is dominant with respect to the phenotype.
Example: a well known example of such lethals is from mice, given by Cuenot. He found that the yellow mice never breed
true. Whenever the yellow mice were crossed with yellow mice, always yellow and brown mice were obtained in the ratio of
2 : 1. a cross between brown and brown mice always produced brown offspring. a cross between brown and yellow mice always
produced yellow and brown in equal proportions. Cuenot suggested that gene y has a multiple effect.
- It controls yellow body colour and has a dominant effect.
- It affects viability and acts as a recessive lethal.
In 1917, Stiegleder concluded that yellow mice are heterozygous.
The homozygous yellow (1/4th of the total offspring) die in the
embryonic condition. When these unborn ones are added to the 2 : 1
ratio of yellow and brown, these form typical 3 : 1 ratio.
In instances where one copy of the wild-type gene is not sufficient for
normal development, even the heterozygote will not survive. In this
case, the mutation behave as a dominant lethal allele, because its
presence somehow overrides the expression of the wild-type product,
or the amount of wild-type product is simply insufficient to support
its essential function.
Example: In humans, a disorder called Huntington disease is due
to a dominant allele, H. The onset of the disease in heterozygotes
(Hh) is delayed, usually well into adulthood, typically at about age
- affected individuals then undergo gradual nervous and motor
degeneration and eventually succumb to the disorder a number of
years later.
Dominant lethal alleles are very rare. for them to exist in a population,
the affected individual must reproduce before it dies. If all affected
individuals die before reaching the reproductive age, the mutant allele
will not be passed to future generations and will disappear from the
population unless it arises again as a result of a new mutation.
intergenic interaction
In intergenic or nonallelic interactions, two or more independent
genes present on same or different chromosomes interact to produce
a different expression, e.g., epistasis, duplicate genes, complementary
genes, supplementary genes, inhibitory genes, etc.
complementary genes
Complementary genes may be defined as, two or more dominant genes present on separate gene loci (non-allelic pair), which
interact to produce a particular phenotypic trait, but neither of them produces the phenotypic trait in the absence of other. There is
complementation between two genes implying that both genes are necessary for the production of a particular phenotype.
Characteristics of complementary genes are:
- they are non-allelic genes.
- they are present on separate gene loci.
- together in dominant form they produce a specific phenotypic character.
- absence of either of the two will not produce specific phenotypic character as these genes work in collaboration.