Genetic Disorders as Adaptation: Heterozygote Advantage and Evolutionary Medicine

Some genetic disorders are common because the gene that causes them was once an advantage. A person with two copies of the gene suffers the disorder, but a carrier with one copy may be protected against an infectious disease. Sickle-cell anaemia, beta-thalassaemia and G6PD deficiency persist at high frequencies in malarial regions for this reason, and similar explanations have been proposed for cystic fibrosis and Tay-Sachs disease. This is heterozygote advantage, the main way in which a "disorder" can be an evolutionary adaptation.

This note explains the classification of genetic disorders, single-gene, polygenic and multifactorial disease, the HapMap and common variants, the evolution of disorders as adaptations, and the ethical issues of genetics and health, for the UPSC Anthropology Optional. It relates to Paper I, topic 9.3 (polymorphism and selection) and the topics on genetic disorders and human adaptability in the Anthropology Optional syllabus.

Key points at a glance

• Classification: monogenic, polygenic and multifactorial disorders; also chromosomal disorders, mitochondrial disorders and somatic disorders such as cancer.
• Monogenic disorders are individually rare and follow Mendelian inheritance.
• Common diseases such as diabetes, heart disease and most cancers are multifactorial: many genes of small effect plus environment.
• HapMap: a catalogue of the blocks of linked variants (haplotypes) in the human genome, used to find genes for common diseases.
• Disorder as adaptation: carriers of some recessive disease genes are protected against infection, so the genes are kept by balancing selection.
• Best-proven case: sickle-cell trait and malaria. Proposed cases include cystic fibrosis and cholera, and Tay-Sachs and tuberculosis.
• Other evolutionary explanations: mismatch with a changed environment, such as the "thrifty genotype", and genes that help early in life but harm later.
• Ethics: privacy, discrimination, consent and the risk of genetic determinism.

Classification of genetic disorders

Genetic disorders can be classified in several ways (Ronald Trent). For the question of evolution, the most useful division is by the number of genes involved and the role of the environment.

Patterns of single-gene inheritance

The recessive disorders are the ones that can become common through heterozygote advantage, because the allele is carried, unseen, by healthy heterozygotes.

Monogenic disorders

A monogenic disorder is caused by a mutation in one or both copies of a single gene. Huntington's disease, a dominant disorder that appears in adult life, made clear how directly a single gene can decide health. A person who inherits the expanded form of the gene will, with very few exceptions, develop the disease.

• Predictive power. Because the link between gene and disease is direct, a genetic test can say a great deal about a person's future health. This makes the protection of genetic information especially important for these disorders.
• Not the same as untreatable. A genetic cause does not mean nothing can be done. Phenylketonuria is prevented from causing damage by diet. Thalassaemia major is managed by transfusion and iron-chelation, and can be cured by bone marrow transplantation. Gene therapies for sickle-cell disease and thalassaemia have been approved in recent years.
• Rarity. Each monogenic disorder is rare, though together they affect a substantial number of people. Several thousand human genes are now known in which mutations cause a single-gene disorder.

The science writer Matt Ridley, often quoted on this subject, argued that apart from rare and serious genetic conditions, "the impact of genes upon our lives is a gradual, partial, blended sort of thing". We are not either tall or dwarf like Mendel's peas, but somewhere in between.

Polygenic disorders and the haplotype map

Most medical conditions with a genetic component involve either many genes (polygenic) or genes interacting with the environment (multifactorial). These are much harder to understand than single-gene disorders.

The early successes of gene mapping were with single-gene disorders. As more genes were mapped, the rate at which new disease links were found slowed sharply, suggesting that most of the simple links had been made. Finding the genes behind common diseases needed a new approach.

Haplotypes and the HapMap

• Human similarity. Any two people share about 99.9 per cent of their DNA sequence. The remaining 0.1 per cent includes the variants that affect disease risk.
• SNPs. The commonest variants are single nucleotide polymorphisms, differences at single bases.
• Haplotypes. SNPs that lie close together on a chromosome tend to be inherited together as blocks. The particular combination of alleles in a block is a haplotype.
• Tag SNPs. A block may contain many SNPs, but a few are enough to identify which haplotype a person carries.
• The HapMap. After the Human Genome Project, the International HapMap Project, begun in 2002, catalogued these blocks and the SNPs that identify them in populations of Africa, Asia and Europe. Francis Collins, then head of the United States National Human Genome Research Institute, described it as a reference to the genetic variations of most importance to health and disease.

What the haplotype studies showed

Stacey Gabriel, Mark Daly and colleagues (2002) reported three findings:

1. The genome can be divided into haplotype blocks, on average roughly 11,000 to 22,000 bases long, each with only a few common haplotypes, typically three to five.
2. Similar blocks are found in populations of Africa, Asia and Europe, so the map would be useful for most people. Blocks are generally shorter in African populations, reflecting their greater age and diversity.
3. The common haplotypes capture most of the variation in a region, of the order of 90 per cent.

This allowed researchers to test for disease associations block by block instead of base by base.

Uses of the haplotype map

• Locating the regions and genes associated with common diseases such as diabetes, cancer, hypertension and Alzheimer's disease, through genome-wide association studies.
• Studying why people with the same disease respond differently to the same drug (pharmacogenomics).
• Studying differences in response to environmental exposures.
• Studying population history and molecular evolution.

The main lesson, in Mark Daly's words, was that complex diseases are not caused by single high-penetrance genes but by many modest risk factors common in populations. Each common variant raises risk only slightly.

Multifactorial disease and the environment

In a multifactorial disorder an inherited variant makes a person susceptible, but other factors, such as diet, stress, infection or pollution, decide whether the disease appears. Heart disease, hypertension, diabetes, schizophrenia, dementia and most cancers belong here.

Nature and nurture

• For much of the 20th century the emphasis in public health was on the environment. In the 1980s writers such as Steven Rose, Richard Lewontin and Leon Kamin warned against biological determinism.
• With the rapid growth of genomics, the emphasis swung towards genes, perhaps too far, towards "genetic exceptionalism".
• The current view is interactionist. A person is not the sum of separate traits each produced by a gene, but the outcome of genes, their interactions with one another, and their continuing interaction with the environment.

Ridley's conclusion is worth quoting: the more we learn about the genome, "the less fatalistic it will seem". Grey indeterminacy, variable causation and vague predisposition are the marks of the system, and simple determinism, whether genetic or environmental, is not supported.

Disease or protective trait?

Many genetic variants are described as diseases or disorders. In the past, some of these variants improved the chances of survival in particular environments. The usual pattern involves an autosomal recessive condition: the carrier, with one copy, has no disease and gains some protection, while the person with two copies has the disorder.

The genetic principle is heterozygote advantage. When the heterozygote is fitter than both homozygotes, selection keeps both alleles in the population at a stable frequency, a balanced polymorphism. The cost is that some children in every generation inherit two copies and are affected. This is the segregational load.

1. Sickle-cell anaemia and malaria

• The gene. A single base change in the beta-globin gene produces haemoglobin S.
• Distribution. Sub-Saharan Africa, the Mediterranean, the Middle East and India, especially among tribal populations of central, western and southern India.
• The protection. In carriers, red cells infected by the malaria parasite tend to sickle and are removed by the spleen before the parasite can multiply. Normal red cells go on providing an environment in which it grows.
• The evidence. A. C. Allison showed in 1954 that carriers are protected against severe falciparum malaria. The geographical distribution of HbS matches that of malaria.
• The conclusion. Where malaria is endemic, being a carrier is an advantage in evolutionary terms. This is the best-established case of a disease gene maintained by selection.

2. Beta-thalassaemia and malaria

• The gene. Mutations in the beta-globin gene reduce or abolish the production of beta-globin.
• The disorder. Carriers (thalassaemia minor) have mild anaemia without serious health problems. Those with two mutations (thalassaemia major) have severe anaemia, usually needing lifelong transfusion.
• Distribution. The Mediterranean, the Middle East, South and South-East Asia. In some populations as many as one person in ten is a carrier. In India carriers are concentrated in particular communities of the north-west, west and east.
• The protection. J. B. S. Haldane suggested in 1949 that thalassaemia carriers are protected against malaria. Carriers' red cells are small and pale and appear to be a poorer environment for the parasite. Studies of alpha-thalassaemia in Melanesia, where its frequency follows the intensity of malaria, provided strong support.

3. Tay-Sachs disease and tuberculosis (a proposed case)

• The disorder. A neurodegenerative disease caused by deficiency of the enzyme hexosaminidase A. In its infantile form, children usually die by about four or five years of age. Carriers have no symptoms.
• Distribution. Much more frequent among Ashkenazi Jews of Central and Eastern European origin, with a carrier frequency of about 1 in 30, than among Sephardic Jews or non-Jewish populations.
• The hypothesis. It has been suggested that carriers had some protection against tuberculosis when Jewish communities were confined to crowded urban quarters in past centuries.
• The debate. The evidence is limited. Several other disorders of the same metabolic pathway are also frequent in the same population, which some see as support for selection and others explain by founder effect and drift.

4. Cystic fibrosis and diarrhoeal disease (a proposed case)

• The gene. Mutations in the CFTR gene, identified by John Riordan and colleagues in 1989, disturb the movement of chloride (salt) across cell membranes.
• The disorder. Those with two mutations develop thick secretions affecting the lungs and pancreas. Severity varies with the mutation.
• Distribution. Most common in populations of European ancestry, where about 1 in 25 people is a carrier, but found in many groups.
• The hypothesis. Carriers lose less salt and water through the gut. When cholera and dysentery were widespread, they may have had a lower risk of dying from diarrhoea. A protective effect against typhoid has also been proposed.
• The debate. Experimental support exists, but the case is not settled. Carrier status can be found only by DNA testing, since carriers have no symptoms.

Other proposed adaptive disease genes

Heterozygote advantage at a glance

Other evolutionary explanations of disease

Heterozygote advantage is not the only way evolution helps explain disease. The field that asks these questions is called evolutionary or Darwinian medicine.

A high frequency of a disease gene therefore does not by itself prove that it was once useful. Selection, drift, founder effect and mismatch must all be considered.

Relevance to India

• The malaria belt. India has long been malarial, and the sickle-cell, thalassaemia, HbE and G6PD genes are all common in parts of the country. Their distribution among tribal and other communities is a living example of balanced polymorphism.
• Public health. Where malaria has been reduced, the carrier advantage has gone but the disease burden remains. India launched a National Sickle Cell Anaemia Elimination Mission in 2023, centred on screening and counselling in tribal areas, and many states run thalassaemia screening programmes.
• Treatment cautions. G6PD-deficient patients can suffer red-cell breakdown when given certain antimalarial drugs, which affects malaria treatment in tribal regions.
• Endogamy and consanguinity. Founder effects in endogamous communities, and consanguineous marriage in the south, raise the frequency of particular recessive disorders.
• Mismatch. The rapid rise of type 2 diabetes in India is often discussed in terms of thrifty genes and the thrifty phenotype.

For health programmes among tribal communities, see tribal India today.

Issues in genetics and health

Genomic research may change the practice of health care profoundly. Research alone is not enough; the ethical, legal and social issues it raises must be understood if genetic knowledge is to help patients and not be misused. The Human Genome Project set aside part of its budget for such studies from the start.

The World Health Organization has stressed the need to regulate genetic testing and to protect the privacy of patients so that no one suffers discrimination. For anthropologists these issues are especially sharp, because much research on genetic disorders has been carried out among tribal and minority communities.

Using this topic in a UPSC answer

• Classify genetic disorders in a table: monogenic, polygenic, multifactorial, chromosomal, somatic.
• Explain why single-gene disorders are rare and common diseases are multifactorial.
• State the principle of heterozygote advantage and balanced polymorphism.
• Give sickle cell and malaria in detail, with a genotype-fitness table.
• Add thalassaemia, G6PD, cystic fibrosis and Tay-Sachs, marking which are proven and which are proposed.
• Bring in mismatch and the thrifty genotype, with the Indian diabetes example.
• Conclude with the ethical issues and the danger of genetic determinism.

For presentation, see our guide to anthropology answer writing with diagrams, thinkers and case studies.

Frequently asked questions

How can a genetic disorder be an adaptation?

When carriers of one copy of a disease gene are protected against an infection, they survive and reproduce better than non-carriers. Selection then keeps the gene in the population, even though people with two copies have the disorder. This is heterozygote advantage.

What is heterozygote advantage?

Heterozygote advantage is the situation in which a person with two different alleles at a locus has higher fitness than a person with two copies of either allele. It maintains both alleles in the population.

Why is sickle-cell anaemia common in malarial regions?

Because carriers of the sickle-cell gene are protected against severe falciparum malaria. In malarial regions the protection given to carriers outweighs the loss of those with sickle-cell anaemia, so the gene stays common.

What is the difference between monogenic and multifactorial disorders?

A monogenic disorder is caused by a mutation in a single gene and follows Mendelian inheritance. A multifactorial disorder results from many genes of small effect acting together with environmental factors.

What is the HapMap?

The HapMap is a catalogue of the blocks of linked genetic variants, called haplotypes, in the human genome, and of the SNPs that identify them. It was produced to help find the genes involved in common diseases.

Does cystic fibrosis give carriers an advantage?

It has been proposed that carriers lose less fluid in cholera and other diarrhoeal diseases, which would have helped them survive epidemics. The idea has some experimental support but is not proven.

What is the thrifty genotype hypothesis?

Proposed by James Neel in 1962, it suggests that genes which helped people store energy during famines now promote obesity and type 2 diabetes in environments where food is plentiful.

What ethical issues arise from genetic research?

The main issues are informed consent, privacy of genetic information, discrimination by employers and insurers, accurate interpretation of results, regulation of testing, cultural sensitivity and the danger of genetic determinism.