Lethal Genes

[Recessive Lethals](#Recessive Lethals)

[Dominant Lethals](#Dominant Lethals)

[Conditional Lethals](#Conditional Lethals)

[Balanced Lethals](#Balanced Lethals)

[Gametic Lethals](#Gametic Lethals)

Classification of Genes Based on Effect of Survivability

Lethal genes, also known as lethal alleles, are alleles that cause the death of the organism carrying them. This is usually a result of gene-mutation that is necessary for growth and development. In 1905, French geneticist Lucien Cuénot was the first to discover Lethal genes when researching inheritance of coat color in mice. He found that when homozygous conditions were present, mice could never be yellow in color, as the yellow color was dominant over brown and was presided over by the single gene “Y”.

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

1. Recessive Lethal Alleles

Recessive Lethals are a pair of identical alleles that can code for either recessive or dominant characteristics, but are only fatal when in a homozygous condition. An example of this is seen in Manx cats, where the homozygous condition of the alleles results in the death of the entity. Despite this, heterozygous individuals are unaffected and can survive. These alleles are commonly found, as most lethals are recessive.

2. Dominant Lethals

An example of a dominant lethal allele in humans is Huntington’s Disease, a neurodegenerative disorder that ultimately results in death. Another example is epiloia genes. These alleles are not very common as they cause the death of an entity before they are passed on to their offspring. For an entity to be affected by these alleles, only one copy of the allele is required.

3. Conditional Lethals

The condition of Favism is an example of a conditional lethal gene, as it causes the carrier to develop hemolytic anemia upon consuming fava beans. This is an example of how external environmental aspects can turn alleles deadly.

4. Balanced Lethality

In a self permanent stock, the Balanced Lethal System is the balancing effect between two different lethals. This system is characterized by Recessive Alleles in the repulsive stage of one gene, and a Dominant Allele of the other gene, which are found to be present in the same chromosome. As a result of this tight linkage, the lethal genes associated in the repulsion stage are sustained in a permanent heterozygous state. This phenomenon has been observed in organisms such as Drosophila and mice.

5. Gametic Lethals

Gametic lethals are ones that render the gametes incapable of fertilization. This phenomenon is often referred to as Meiotic Drive, which is a series of events that results in the production of unequal numbers of functional gametes by a heterozygote.

Classification of Genes Based on Impact on Survivability

Genes can be grouped into 5 classes based on their effect on survivability, as follows:

Lethal Genes

Vital Genes

Sub-Vital Genes

Sub-lethal Genes

Super-vital Genes

Frequently Asked Questions

What are the Advantages of Lethal Dominant Genes Being Rare?

Lethal dominant genes are rare due to the fact that the organism carrying the alleles does not survive, thus not allowing for the alleles to be passed onto subsequent generations.

What is a Semi-Lethal Gene?

A semi-lethal gene is a gene that has a negative effect on an organism’s health, but does not necessarily cause death. Instead, semi-lethal genes can cause reduced fertility, reduced growth, and other negative effects.

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

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