Genes That Influence Human Survival — 2018 Paper I

Approach

• Demand of Question: Classify genes according to their effects on viability and survival of individuals.
• Structuring the Response: Arrange genes from lethal to survival-enhancing categories and illustrate their biological consequences.
• Key Dimensions to Cover: Lethal, semilethal, sub-vital, vital and super-vital genes, viability and natural selection.

Model Answer

Introduction: Genes can be classified according to their effect on the viability or survival of individuals carrying particular genotypes. Their effects range from death before reproduction to enhanced survival.

Major categories

1. Lethal genes: May cause death during embryonic, prenatal or postnatal life. Many lethal alleles persist when they are recessive and hidden in heterozygotes.
2. Semi-lethal genes: Considerably reduce viability, though some affected individuals survive.
3. Sub-vital genes: Reduce fitness without necessarily causing death, for example by increasing susceptibility to disease.
4. Super-vital genes: Increase fitness under particular ecological conditions.

A classic population-genetic example is the HbS allele: HbSS has severe fitness costs, while HbAS can provide protection against severe malaria in endemic environments.

Thus:

Gene × Environment → Differential survival → Natural selection

Conclusion: Genes affecting survival form a continuum from lethal to advantageous, and their evolutionary fate depends upon genotype, environment and reproductive fitness.