Phenotype of the lethal allele when expressed is fatal to an organism. Lethal alleles take place when a mutation to a usual allele disrupts the function of an essential gene. Lethal alleles can be embryonic or postnatal (after child birth).. Postnatal lethal alleles results in abnormalities in the children that proves them fatal early on in development. Embryonic lethal alleles results in the death of the fetus. Lethal alleles can be either recessive, dominant and can be either autosomal , sex linked. Lethal alleles which are dominant are rapidly removed from the population, because they normally cause individual to die before the individual is able to have children. Lethal alleles which are recessive result in death of the individual only in the homozygous recessive state.

Examples of diseases caused by recessive lethal alleles in humans

- Brachydactyly - A genetic state in which the fingers are unusually short in heterozygotic condition. But, this condition is lethal during early years to homozygous recessive individuals due to major skeletal defects
- Sickle Cell Anemia - A genetic state that is often fatal in the homozygous recessive condition. Concerned gene codes for the manufacture of hemoglobin, which is an O2 transport protein found in red blood cells.
- Tay-Sachs Disease - A genetic state that is fatal to every homozygous recessive members by the age of 4. Not curable Metabolic Disorder which causes brain deterioration.
- Cystic Fibrosis - A genetic state that is fatal to every homozygous recessive person by age 30. Not curable disorder in which sticky mucus accumulates in the lungs giving rise to constant and risky respiratory infections.
- Congenital Ichthyosis: It is an instance of homozygous recessive fatal gene in individuals. Children with this disease are born with crusted leathery skin with deep splits. These splits lead to bleeding, infection and death.

Examples of diseases caused by dominant lethal allele in humans

- Huntington's Disease - A genetic state caused by a dominant lethal allele. This dominant allele is still persists in the population because the disease does not have an effect on peoples until after reproductive period of life (40-50 years of age) Symptoms include jerking and depression and also results in brain degeneration and death within 5 years of arrival.

Lethal alleles in animals

- An incompletely dominant allele (Y) for yellow coat in mice has been found fatal in homozygous dominant condition. Genotype (yy) generated black coat colour in mice. When the black mouse which is homozygous is crossed with another black mouse, just black mice were generated. A cross between a black homozygous mouse(yy) with a yellow mouse (Yy) produced a ratio of 1:1 which, establish that yellow mouse is heterozygous. When a yellow mouse is crossed with a yellow mouse, it was discovered that the yellow and black mice occurred in the ratio of 2:1, the homozygous dominant allele (YY) resulted in the death of the individual before it was born.

- In chickens the incompletely dominant gene (cp) in heterozygous state causes the legs to be short and stunted (creeper condition).If two creeper chickens are crossed produces a viable ratio of 2:1 (2 creepers to 1 normal). The homozygous creepers have such a great abnormalities that they die.

- In cattle, a lethal gene in recessive homozygous state (aa) results in the birth of amputated calves that die shortly after the birth.

Lethal alleles in plants

- Snapdragon plants come about in 3 types, green plants ( due to the occurrence of chlorophyll), yellowish green plants ( due to the occurrence of carotenoids) and white plants ( due to the lack of chlorophyll). Green plants which are homozygous have genotype (CC), white plants have (cc) which does not have chlorophyll and the yellowish green plants also called auria or golden plants, have the genotype (Cc). When two yellowish green plants are crossed, albino plant dies because of the lack of chlorophyll .The ratio is altered to 2:1, because of the homozygous recessive genotype that is fatal.

- The gene regulating the amount of chlorophyll in maize( Zea mays) is the recessive allele (g) which exhibits a lethal result in homozygous (gg) and the heterozygous condition (Gg) has phenotype similar to homozygous condition for dominant gene (GG).It alters the phenotypic ratio into 2:1.

The hint to lethal alleles is a 2:1 phenotypic ratio. This ratio looks different than the typical 3:1 ratio observed, implying the variations of the Mendelian genetics.

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