Lethals or lethal genes or lethal alleles are alleles causing the end of an entity which carries it. Typically it is the consequence of gene-mutation that is required to grow and develop. In 1905, French geneticist Lucien Cuénot was the first person to discover Lethal genes when he was carrying out research on inheritance of the coat color in mice. As per Cuenot, color yellow was dominant over brown presided over by the single gene “Y”. Hence, he found that under homozygous conditions, mice could never be yellow in color.
The nature of lethal genes can be dominant, recessive or even conditional, depending on the participating genes. Death of an entity can be caused at any time, the most common occurrence is in the early stages of development.
Types of Lethal genes
Classification of lethal genes are as follows:
1. Recessive lethals
These are commonly found as most lethal are recessive. Their expression is in homozygous conditions only, hence survival of heterozygous ones are unaffected. They are a pair of identical alleles that results in the death of an entity ultimately. Despite the fact that recessive lethals can code for recessive or dominant characteristics, they turn fatal only under homozygous conditions. Example of recessive lethals is as seen in the case of a Manx cat.
2. Dominant lethals
These are the alleles whose presence is required in one copy in an entity for them to turn fatal. These are not so frequently found as they cause the death of an entity before they transmit to their offspring. A good example of dominant lethal alleles that is seen in humans is the rare – Huntington’s disease. It is a neurodegenerative disorder which results in death ultimately. One more example in humans is epiloia genes.
3. Conditional lethals
These alleles turn deadly only when there is an external environmental aspect involved. For instance, Favism is a conditional lethal gene. This sex-inherited condition causes the carrier to develop hemolytic anemia upon consuming fava beans.
4. Balanced lethals
In a self permanent stock, the balancing effect between two different lethals is the balanced lethal system. The lethal genes that are associated in the repulsion stage of linkage are balanced lethals. They sustain in the repulsion phase as a result of tight linkage. The recessive alleles in the repulsive stage of one gene and the dominant allele of the other gene are found to be present in the same chromosome. This lethal system sustains genes associated closely with the lethal genes in a permanent heterozygous state. Such lethals are observed in Drosophila, mice, etc.
5. Gametic lethals
Gametic lethals are the ones that make the gametes incompetent to fertilize. Meiotic drive is often used to describe these types of lethals. This drive is a series of events which causes the production of unequal numbers by a heterozygote of functional gametes.
Classification of Genes Based on Effect of Survivability
Genes can be grouped into 5 classes based on their effect on survivability as listed below:
- Lethal genes
- Vital genes
- Sub-vital genes
- Sub-lethal genes
- Super-vital genes
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