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In this article we will discuss about the meaning and types of lethal genes.
Meaning of Lethal Genes:
It has been observed that all genes or genetic factors are not useful to the organism. There are some genetic factors or genes, when present in any organism cause its death during early stage of development. They may even cause death of the individual either in homozygous dominant or homozygous recessive condition.
A French geneticist L. Cuenot (1905) reported on the inheritance of mouse body colour. He found that “yellow” body colour was dominant over normal “brown” colour and was governed by single gene “Y”. It was observed that yellow mice could never be obtained in homozygous condition.
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When yellow coated mice was crossed with another yellow coated mice, segregation for yellow and brown body colour was obtained in 2: 1 ratio. The brown individuals were pure and homozygous where as yellow individuals were heterozygous. These results may be explained on the assumption that the dominant allele for yellow body colour is lethal in homozygous condition.
(Inheritance of yellow coated colour in mice. Mice with yellow coated colour are always heterozygous.)
E. Baur (1907) observed lethal gene in Snapdragon (Antirrhinum) and found that it is characterized by variegated leaves. The “golden” variety on selfing gives rise to 2 types of offsprings, golden and green in the ratio of 2:1 instead of 3: 1. The golden ones are heterozygous and the green ones breed true being recessive homozygous.
Types of Lethal Genes:
Lethel genes may be classified in to the following groups:
1. Recessive lethals
2. Dominant lethals
3. Conditional lethals
4. Balanced lethals
5. Gametic lethals
1. Recessive lethal:
Most of the lethal genes are recessive lethals. It is expressed only when they are in homozygous condition. The survival of heterozygotes is not affected e.g., coat colour in mice. According to Cuenot, Castle and Little, the dominant allele Y is a recessive lethal and it causes death of homozygous YY embryos at an early stage of development.
2. Dominant lethal:
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There are some lethal genes which reduce viability even in heterozygotes, are said as dominant lethals. e.g., epiloia gene in human beings. This cause mental defects, abnormal skin growth and tumors in heterozygotes, therefore, they die before reaching adulthood. The dominant lethals may be produced in every generation through mutation.
3. Conditional lethal:
The lethal genes require a definite or specific condition for their lethal action are said as conditional lethals. Many mutants of barley, maize, Neurospora, Drosophila and many other organisms are termed as temperature sensitive mutations. Each of them needs a definite, generally high temperature to express their lethal action.
A chlorophyll mutant of barley allows normal chlorophyll development at a temperature of 19°C or above, but it develops albina or abnormal white seedlings at temperature below 8°C. Temperature is not only responsible to bring out conditional lethals. Some conditional lethals require light, nutrition etc.
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4. Balanced Lethals:
The balancing effect between two different lethals in self permanent stock is called balanced lethal system- Muller (1918). Lethal genes linked in repulsion phase of linkage are said as balanced lethals. They are maintained in repulsion phase due to tight linkage. Crossing over is very low. In repulsion phase, the recessive allele of one gene and the dominant allele of the other gene are present in the same chromosome.
Mating between individuals heterozygous for these balanced lethals will produce 4 types of zygotes. 1/4 will be homozygous for the recessive lethal and will not survive. Another 1/4 of the zygotes shall be homozygous for the other recessive lethal and will die.
The only progeny which will survive, will be the heterozygotes for the 2 recessive lethals. Therefore, a balanced lethal system maintains the genes closely linked to the lethal genes in a permanent heterozygous state. Balanced lethals are seen in mice, Oenothra, Drosophila etc.
(A balanced lethal system having 2 recessive lethal genes (l1 and l2 ). Only 2 of the 4 heterozygotes survive (l1L2/L1l2)
5. Gametic Lethals:
Some genes make the gametes incapable of fertilization. Such genes are said as gametic lethals. Some times the term ‘Meiotic drive’ is used to describe gametic lethals. Meiotic drive may be called a mechanism that leads to the production of unequal numbers of functional gametes by a heterozygote.
It has been found in certain males of Drosophila pseudoobscura, produce only half amount of sperm as compared to a normal males. When these males are mated to normal females, most of the progeny are females. It demonstrates that the sperm cells produced by these males contain the ‘X’ chromosome only and their sperms having ‘Y’ chromosome are non-functional.
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It may be clarified as follows:
On the basis of effect of survivality the genes may be grouped in to 5 classes:
1. Vital genes
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2. Lethal genes
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3. Sub-lethal genes
4. Sub-vital genes
5. Super-vital genes
1. Vital genes:
The genes which do not affect the survival of the individuals in which they are present are said as vital genes. It does not mean that these genes are necessary for the survival of the concerned individual. Wild type alleles of all the genes of an organism are said as vital genes. In other words, the survival of the organism is not influenced by the vital genes, whether may be present in homo or heterozygous condition.
2. Lethal genes:
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Has been described in preceding page.
3. Sub-lethal or semi-lethal genes:
Such genes do not lead the organism to the death that carry them in appropriate genotype. 90% of the individuals die, however, only less than 10% of the individuals survive. Some Xantha mutants of several plants are sub-lethal or semi-lethal in the homozygous state.
4. Sub-vital genes:
Most of the mutant genes reduce the viability of individuals having them in appropriate genotype as compared to that of normal individuals. Most of the mutant genes are sub-vital in their effect and kill less than 90% of the individuals which carry them. The examples are some virdis mutants of barley, miniature wings in Drosophila.
5. Super-vital genes:
Most of the mutant genes increase the survival of such individuals which carry them in appropriate genotype as compared to that of wild type allele. Such genes are called as super-vital genes.
Super-vital genes protect the individuals carrying them against the various disease thus increasing the chance of their survival. Likewise, the genes providing resistance or tolerance to different environmental pressure or strain like high and low temperature, low and high light intensity, drought, salinity, alkanity etc..
