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Lethals or lethal genes or lethal alleles are alleles causing the end of an entity which carries it. Basically, lethal genes are lethal to the organism carrying it, lethal meaning (here) death. 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 is unaffected. They are a pair of identical alleles that ultimately result in the death of an entity. 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 are transmitted 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 themselves 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

This was a brief on lethal genes. Discover other interesting write-ups for your NEET preparation, at BYJU’S.

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Frequently Asked Questions

Why are lethal dominant genes rare?

Lethal dominant genes are rare because the organism dies and the alleles are not passed onto other generations.

What is a semi lethal gene?

A semi lethal gene is the one that affects only some of the individuals carrying the allele, not all are affected.

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