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Home/Anthropology/Mains PYQ/2024/Paper I/Human Genetics & Human Biology
UPSC MainsAnthropology Optional Paper I 2024 10 Marks Model Answer Available

Lethal and Sublethal Genes — 2024 Paper I

Question · 2024 · Paper I · 10 Marks

Lethal and sublethal genes

Model Answer

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Introduction

Lethal and sublethal genes are better understood as alleles that reduce survival and reproductive fitness. A lethal allele causes death in a particular genotype, while a sublethal or semilethal allele reduces viability, health, longevity or fertility but does not always cause death.

Lethal Genes

Lethal genes interfere with vital biological processes such as embryonic development, nervous function, blood formation or organ survival. Their effect may appear before birth, in childhood or even after reproductive age.

Recessive Lethal Alleles

Recessive lethal alleles cause death only in the homozygous condition. Heterozygous individuals usually survive as carriers.

A classical example is the yellow coat colour in mice. Lucien Cuénot, in 1905, crossed two yellow mice and obtained a 2:1 ratio of yellow to non-yellow offspring instead of the expected 3:1 ratio. Later, Castle and Little showed that the missing homozygous yellow class died during embryonic development.

Yy × Yy → 1 YY : 2 Yy : 1 yy

  • YY — embryonic lethal
  • Yy — yellow coat
  • yy — non-yellow coat

Thus, the surviving offspring show a 2:1 ratio.

Human examples include Tay-Sachs disease, where affected children suffer progressive nerve damage and historically died in early childhood, and homozygous achondroplasia, where two copies of the FGFR3 mutation are generally perinatally lethal.

Dominant Lethal Alleles

Dominant lethal alleles cause death even in the heterozygous condition. Such alleles are usually removed quickly by natural selection if death occurs before reproduction.

However, some dominant lethal alleles persist because of late onset. Huntington disease is an important example. It usually appears after reproductive age, so affected individuals may pass the allele to their children before symptoms develop.

Conditional Lethal Alleles

Some alleles become lethal or severely harmful only under specific environmental conditions. For example, in G6PD deficiency, fava beans, infections or oxidant drugs may trigger severe haemolytic anaemia. This shows that lethality may depend on gene–environment interaction.

Sublethal or Semilethal Genes

Sublethal genes reduce survival, fitness or lifespan but do not necessarily cause death in all individuals. In a strict classical sense, semilethal genes may cause death in more than 50% but less than 100% of individuals carrying the genotype.

Examples include haemophilia, sickle-cell anaemia, β-thalassaemia major and cystic fibrosis. These conditions may seriously reduce survival in natural conditions, but modern medical care has improved survival in many cases.

Evolutionary and Anthropological Significance

Natural selection

Lethal and sublethal genes are important targets of natural selection. Natural selection tends to remove harmful alleles when they reduce survival or reproduction.

Purifying selection

Strongly harmful lethal alleles are gradually eliminated from the gene pool, especially when they act before reproductive age.

Genetic load

Hidden recessive lethal alleles may remain in heterozygous carriers. Their accumulation in a population contributes to genetic load.

Consanguinity and endogamy

Consanguineous marriage and endogamy increase homozygosity and therefore increase the chance of expression of recessive lethal and sublethal disorders.

Founder effect and genetic drift

Small and isolated populations may show higher frequency of some harmful alleles due to founder effect and genetic drift.

Heterozygote advantage

Some sublethal alleles persist because heterozygotes have selective advantage. J. B. S. Haldane linked haemoglobin disorders with malaria, and A. C. Allison demonstrated that sickle-cell trait gives protection against malaria. Thus, an allele harmful in homozygous condition may be useful in heterozygous condition.

Critical Observation

Lethality is not always absolute. A disorder that was once fatal in childhood may become manageable through diet, drugs, transfusion, surgery or gene-based therapy. Similarly, an allele may be harmful in one environment but advantageous in another. Therefore, lethal and sublethal effects depend on genotype, environment, healthcare and population structure.

Conclusion

Lethal and sublethal genes show how Mendelian inheritance operates under differential survival. Their frequency is shaped by mutation, natural selection, genetic drift, founder effect, mating pattern and ecological conditions. Hence, their study is important for medical genetics, genetic counselling, population genetics and biological anthropology.

2026 Distinguishing features of nuclear and mitochondrial DNA. 2026 How would you find out that a particular disease/disorder is inherited and, if so, in which manner? Elaborate with suitable examples. 2026 What do you understand by fertility and mortality? How are these influenced by biological and socio-ecological factors? 2026 In what manner is our understanding of serogenetics and cytogenetics useful in reproductive biology? What are the major ethical issues involved in reproductive technology?

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UPSC Anthropology Optional 2024 — Paper I

28 questions

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10 Marks150 words
Syllabus: Paper I > Economic Organization > Reciprocity and Exchange > Kula ring
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10 Marks150 words
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1(e) Hemoglobin in health and disease

10 Marks150 words
Syllabus: Paper I > Human Genetics > Genetic Polymorphism > Haemoglobinopathies; health and disease
2024Paper IHuman Genetics & Human BiologyModel answer available

5(c) Heritability and its estimation

10 Marks150 words
Syllabus: Paper I > Human Genetics > Quantitative Genetics > Heritability and its estimation
2024Paper IHuman Genetics & Human BiologyModel answer available

1(d) Lethal and sublethal genes

10 Marks150 words
Syllabus: Paper I > Human Genetics > Mendelian Genetics in Man > Lethal and sublethal genes
2024Paper IHuman Genetics & Human BiologyModel answer available

5(e) Single-gene mutation disorders in man

10 Marks150 words
Syllabus: Paper I > Human Genetics > Mutation and Genetic Disorders > Single-gene disorders
2024Paper IHuman Genetics & Human BiologyModel answer available