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.

Types of Lethal Genes:

I. lethal genes are classified on the basis of time of death as

  • Absolute lethal : Individual die at gametic stage : 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.
  • Sub-lethal: die before reprodutive stage. Ex: epiloia gene
  • Semi-lethal : die at post reproductive stage. Ex: Huntington’s disease

II. Lethel genes may be classified based nature

  • 1. Recessive lethals
  • 2. Dominant lethals
  • 3. Incomplete dominant lethals
  • 4. Conditional 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. other examples of human diseases caused by recessive lethal alleles include cystic fibrosis, sickle-cell anemia, and achondroplasia. Achondroplasia is an autosomal dominant bone disorder that causes dwarfism. While the inheritance of one achondroplasia allele can cause the disease, the inheritance of two recessive lethal alleles is fatal.

2. Dominant lethal:

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. other example of a disease caused by a dominant lethal allele is Huntington’s disease, a neurological disorder in humans, which reduces life expectancy. Because the onset of Huntington’s disease is slow, individuals carrying the allele can pass it on to their offspring. This allows the allele to be maintained in the population. Dominant traits can also be maintained in the population through recurrent mutations or if the penetrance of the gene is less than 100%.

3.Incomplete dominant lethals : neither of the two alleles are completely dominant over the other.

  1. Sickle cell anemia is caused due to the faulty gene HbS.
  2. The normal or healthy gene is HbA.
  3. When an individual is a heterozygote (HbS/ HbA) for the alleles, he/she is said to be a carrier.
  4. A carrier shows mild anemia as their RBCs become sickle shaped in oxygen deficiency.
  5. He/she is said to have sickle cell trait and are normal in normal conditions.
  6. However, an individual homozygous for HbS gene dies of fatal anemia.
  7. Sickle cell anemia is lethal in homozygous condition and produces sickle cell trait in carriers who are heterozygotes.
  8. Carriers are said to suffer from a milder form of anemia.
  9. Carriers produce sickle shaped RBCs as well as normal RBCs.

Hence, Sickle cell anemia is an example of incomplete dominance since neither of the two alleles are completely dominant over the other and the heterozygote is an intermediate.

4. Conditional lethal:

The lethal genes require a definite or specific condition for their lethal action are said as conditional lethals. Only If a woman is Rh negative and her husband has Rh positive then the fetus is Rh positive, it leads to erythroblastosis fetalis from second pregnancy onwards.

Join Telegram