Lethal Gene Action | Anthropology Optional for UPSC PDF Download

Introduction

• Lethal genes refer to certain genetic factors or genes that can be harmful or fatal to an organism when present. These genes can cause the organism's death during its early developmental stages, either in a homozygous dominant or homozygous recessive condition.
• In 1905, French geneticist L. Cuenot discovered the concept of lethal genes while studying the inheritance of body color in mice. He found that the "yellow" body color was dominant over the normal "brown" color and was controlled by a single gene called "Y". However, he also observed that yellow mice could never be obtained in a homozygous condition.
• When Cuenot crossed two yellow-coated mice, he found that the offspring had a segregation of yellow and brown body colors in a 2:1 ratio, instead of the expected 3:1 ratio. The brown individuals were pure and homozygous, while the yellow individuals were heterozygous. This suggested that the dominant allele for yellow body color is lethal when present in a homozygous condition.

• (Inheritance of yellow coated colour in mice. Mice with yellow coated colour are always heterozygous.) 
• Similarly, in 1907, E. Baur observed a lethal gene in Snapdragon (Antirrhinum) plants, characterized by variegated leaves. He found that when the "golden" variety of the plant was self-fertilized, it produced offspring with golden and green leaves in a 2:1 ratio instead of the expected 3:1 ratio. The golden plants were heterozygous, while the green plants were recessive homozygous and bred true.

Types of Lethal genes

• Recessive lethal alleles: These alleles are quite common, as most lethal genes are recessive. They are only expressed in homozygous conditions, meaning that heterozygous individuals are not affected. Recessive lethal alleles result in the death of an organism when present in pairs. Though these alleles can code for either recessive or dominant traits, they only become fatal in homozygous situations. An example of a recessive lethal allele is seen in the Manx cat.
• Dominant lethal alleles: These alleles cause the death of an organism when present in a single copy. They are less common than recessive lethal alleles, as they often result in the death of an organism before the gene can be passed on to offspring. Huntington's disease, a rare neurodegenerative disorder that ultimately results in death, is an example of a dominant lethal allele in humans. Another example is epiloia genes.
• Conditional lethal alleles: These alleles become deadly only when an external environmental factor is involved. For example, Favism is a sex-linked conditional lethal gene. This condition causes carriers to develop hemolytic anemia if they consume fava beans.
• Balanced lethal alleles: In a self-perpetuating population, the balanced lethal system is the balancing effect between two different lethal genes. Lethal alleles that are present in the repulsion phase of linkage are considered balanced lethals. They maintain themselves in the repulsion phase due to tight linkage. Recessive alleles in the repulsive phase of one gene and the dominant allele of another gene are found on the same chromosome. This lethal system maintains genes closely associated with the lethal genes in a permanently heterozygous state. Such lethals are observed in organisms like Drosophila and mice.
• Gametic lethal alleles: These alleles render gametes unable to fertilize. The term "meiotic drive" is often used to describe this type of lethal allele. Meiotic drive refers to a series of events that cause a heterozygote to produce an unequal number of functional gametes.

On the basis of effect of survivality the genes may be grouped in to five classes:

• Vital genes: Vital genes are those that do not affect the survival of the individuals carrying them. This means that the presence of these genes is not necessary for the individual's survival. When an organism has wild-type alleles for all its genes, these are considered vital genes. In other words, the organism's survival is not influenced by these genes, whether they are present in a homozygous or heterozygous condition.
• Lethal genes: Lethal genes cause the death of an organism when present in the appropriate genotype. These genes can be dominant or recessive, and their presence usually results in a decrease in the overall fitness of the population.
• Sub-lethal or semi-lethal genes: Sub-lethal or semi-lethal genes do not necessarily cause death in the organisms carrying them in the appropriate genotype. Instead, they may result in the death of up to 90% of these individuals, with less than 10% surviving. Some Xantha mutants in various plants are examples of sub-lethal or semi-lethal genes when present in a homozygous state.
• Sub-vital genes: Sub-vital genes are mutant genes that reduce the viability of individuals carrying them in the appropriate genotype compared to normal individuals. Most mutant genes are sub-vital in their effect, causing the death of less than 90% of the individuals carrying them. Examples of sub-vital genes include some virdis mutants of barley and miniature wings in Drosophila.
• Super-vital genes: Super-vital genes are mutant genes that increase the survival of individuals carrying them in the appropriate genotype compared to those with the wild-type allele. These genes are beneficial, as they protect the individuals carrying them against various diseases, thus increasing their chances of survival. They also provide resistance or tolerance to different environmental pressures or strains, such as high and low temperatures, low and high light intensity, drought, salinity, and alkalinity.

Conclusion

Lethal genes are genetic factors that can be harmful or fatal to an organism when present in specific conditions. French geneticist L. Cuenot first discovered the concept of lethal genes while studying the inheritance of body color in mice. Lethal genes can be classified into different types, including recessive, dominant, conditional, balanced, and gametic lethal alleles. Furthermore, genes can be grouped into five classes based on their effect on survivability: vital, lethal, sub-lethal or semi-lethal, sub-vital, and super-vital genes. Understanding lethal genes and their effects on organisms is crucial for studying genetic inheritance and the role of genes in survival, adaptation, and evolution.