Human Genetics

Chromosomes: Structure and Organisation

Chromosomes are composed of a DNA molecule and proteins. During normal cell function they appear as long, stringy, decondensed structures. During the early stages of cell division they become visible as two strands joined at a constricted area called the centromere; the two strands exist because the DNA has already replicated, so one is an exact copy of the other.

Human somatic cells contain 46 chromosomes in 23 pairs. Until 1956 the number was incorrectly believed to be 48. There are two categories of chromosome:

• Autosomes (22 pairs): Carry genetic information governing all physical characteristics except primary sex determination.
• Sex chromosomes / Allosomes (1 pair): Females have two X chromosomes (XX); males have one X and one Y (XY). The Y chromosome directly determines maleness; the X functions more like an autosome because it influences many non-sex traits.

Each gamete (ovum or sperm) contains only 23 chromosomes, one of each pair, so that fertilisation restores the diploid number of 46. Abnormal numbers of autosomes are nearly always fatal at or soon after conception. Abnormal numbers of sex chromosomes are usually not fatal but may result in sterility and other consequences. Structural or numerical alterations of chromosomes collectively are called chromosomal aberrations, and the cluster of symptoms they produce is called a syndrome.

Classification of Chromosomes (Denver System)

Chromosomes are classified from Group A to G based on overall morphology, size, and centromere position:

• Group A (1-3): Large, approximately median centromeres.
• Group B (4-5): Large, submedian centromeres.
• Group C (6-12 and X): Medium, submedian centromeres.
• Group D (13-15): Medium acrocentric; chromosomes 13 and 14 have satellites on short arms.
• Group E (16-18): Short, approximately median or submedian centromeres.
• Group F (19-20): Short, approximately median centromeres.
• Group G (21, 22, Y): Very short acrocentric.

Euchromatin and Heterochromatin

Euchromatin is the loosely packed, gene-rich region of the chromosome. It is transcriptionally active and covers approximately 90% of the human genome. Active genes in euchromatin are transcribed to mRNA and subsequently encoded into functional proteins. Housekeeping genes are a form of euchromatin.

Heterochromatin is tightly packed, intensely stained, and transcriptionally inactive. Its compact structure prevents endonuclease damage and blocks access to gene expression proteins. It has two subtypes:

• Facultative heterochromatin: Contains inactive genes silenced through histone methylation or siRNA (RNAi); not a permanent feature of every cell nucleus.
• Constitutive heterochromatin: Contains structurally functional elements like telomeres and centromeres; a permanent feature of every cell nucleus and contains no coding genes.

Causes and Mechanisms of Structural Chromosomal Aberrations

Structural aberrations arise when chromosomes break and the broken ends rejoin wrongly, or when crossing over occurs between misaligned sequences. They may be balanced (no material gained or lost, carriers usually healthy) or unbalanced (net loss or gain, usually harmful).

• Radiation and chemicals: X-rays and gamma rays (Hiroshima and Nagasaki survivors show raised aberration rates), benzene, alkylating agents and some anticancer drugs, and some viruses break chromosomes.
• Inherited instability: recessive DNA-repair disorders such as Bloom syndrome, Fanconi anaemia and ataxia telangiectasia show many spontaneous breaks and a high cancer risk.
• Unequal crossing over: repeated sequences pair out of register in meiosis, leaving a deletion on one chromatid and a duplication on the other. The 17p12 duplication of Charcot-Marie-Tooth disease type 1A arises this way.

Mechanisms and consequences:

• Deletion: loss of a terminal or interstitial segment, as in cri-du-chat (5p-), Wolf-Hirschhorn syndrome (4p-), and microdeletions such as 22q11.2 (DiGeorge syndrome) and 7q11.23 (Williams syndrome).
• Inversion: a segment rejoins after turning through 180 degrees. Carriers are healthy, but at meiosis the inverted chromosome pairs with its homologue by forming a loop. Crossing over inside the loop of a pericentric inversion produces gametes with duplications and deletions, and inside a paracentric inversion it produces dicentric and acentric chromatids that are usually lost, so carriers have more miscarriages.
• Reciprocal translocation: exchange of segments between non-homologous chromosomes. Balanced carriers, about 1 in 500 people, form a cross-shaped group of four chromosomes at meiosis, and faulty segregation yields unbalanced gametes, causing infertility, recurrent miscarriage or affected children.
• Robertsonian translocation: two acrocentric chromosomes fuse at the centromere and lose their short arms. A healthy carrier of a 14 and 21 fusion has 45 chromosomes and a raised risk of a child with translocation Down syndrome.
• Cancer: acquired translocations activate genes. The Philadelphia chromosome (Peter Nowell and David Hungerford, 1960), shown by Janet Rowley (1973) to be a translocation between chromosomes 9 and 22, fuses the BCR and ABL genes and drives chronic myeloid leukaemia.

Karyotyping in the Diagnosis of Chromosomal Aberrations

A karyotype is the chromosome complement of a cell arranged in standard order by size, centromere position and banding pattern. Joe Hin Tjio and Albert Levan established the human number of 46 in 1956.

1. Cells are taken, usually blood lymphocytes, or amniotic cells, chorionic villi, bone marrow or skin fibroblasts.
2. Lymphocytes are stimulated with phytohaemagglutinin and cultured for about 72 hours at 37°C.
3. Colchicine arrests division at metaphase, when chromosomes are most condensed.
4. A hypotonic solution swells the cells, which are fixed in methanol and acetic acid and spread on slides.
5. Chromosomes are G-banded (trypsin and Giemsa), imaged, paired and arranged in groups A to G.
6. The result is written in standard notation, for example 47,XX,+21 (Down syndrome) or 46,XY,del(5)(p15) (cri-du-chat).

Diagnostic uses: numerical aberrations and mosaicism, visible structural rearrangements, balanced carriers in couples with recurrent miscarriage, prenatal diagnosis, disorders of sex development, and leukaemias. Limits: banding misses changes smaller than about 5 to 10 million base pairs. Fluorescence in situ hybridisation (FISH) detects microdeletions such as 22q11.2, and chromosomal microarrays now detect small gains and losses genome-wide, though only karyotyping reveals balanced rearrangements.

Cell Division: Mitosis and Meiosis

Mitosis

Mitosis, first described by Flemming in 1879, is somatic cell division producing two genetically identical daughter cells, each with the same chromosome number and DNA content as the parent cell. It is essential for growth, development, tissue repair, and asexual reproduction.

The process proceeds through four sequential phases:

• Prophase: Chromosomes condense; the mitotic spindle begins to form from centrosomes; the nucleolus disappears; in late prophase (prometaphase) the nuclear envelope breaks down and spindle microtubules begin capturing chromosomes.
• Metaphase: All chromosomes align at the metaphase plate, each attached to microtubules from opposite spindle poles at its kinetochores. The spindle checkpoint ensures correct attachment before division proceeds.
• Anaphase: The protein glue holding sister chromatids together is broken down; chromatids separate and are pulled to opposite poles by microtubule motor proteins; the cell elongates.
• Telophase and Cytokinesis: Two new nuclei form around each chromosome set; nuclear membranes and nucleoli reappear; chromosomes decondense. In animal cells, cytokinesis occurs by constriction of an actin band (cleavage furrow); in plant cells, a cell plate forms between the two daughter nuclei.

Meiosis

Meiosis is the specialised cell division used to produce gametes (sperm and eggs). It reduces the chromosome number by half, from diploid (2n = 46) to haploid (n = 23), and introduces genetic variation through crossing over. When two haploid gametes fuse at fertilisation, the diploid number is restored.

Meiosis proceeds through two sequential rounds of division:

Meiosis I (Reduction Division):

• Prophase I: Chromosomes condense; homologous chromosomes pair and align precisely along their full length. Crossing over occurs: homologues exchange segments of DNA at points called chiasmata, assisted by the synaptonemal complex. This is the key source of genetic recombination.
• Metaphase I: Homologous pairs (bivalents) line up at the metaphase plate; each chromosome attaches to microtubules from only one pole; the orientation of each pair is random (independent assortment).
• Anaphase I: Homologous chromosomes are pulled to opposite poles; sister chromatids remain attached.
• Telophase I and Cytokinesis: Two haploid daughter cells form, each containing one chromosome from each homologous pair (still as joined sister chromatids).

Meiosis II (Division without DNA replication):

• Each of the two cells from Meiosis I divides again through prophase II, metaphase II, anaphase II, and telophase II.
• Sister chromatids are separated in anaphase II.
• The final result is four haploid cells, each with a unique genetic composition.

Mendelian Inheritance

Gregor Mendel's experiments with pea plants established the foundational principles of heredity. His four postulates are:

• Hereditary characteristics are controlled by particulate unit factors (genes) that exist in pairs in diploid organisms.
• When an individual has two different unit factors (alleles) for a characteristic, only one is expressed in the phenotype: the expressed allele is dominant, the unexpressed allele is recessive.
• Law of Segregation: During gamete formation, paired unit factors separate randomly, so each sex cell receives one allele with equal probability. This reflects chromosome segregation during meiosis.
• Law of Independent Assortment: During gamete formation, segregating pairs of unit factors assort independently of each other: the segregation of one chromosome pair does not influence another. This holds when genes are on different chromosomes.

Mendelian Traits in Humans

Mendelian traits (also called discrete traits) are controlled by alleles at a single genetic locus. The most comprehensive listing, Online Mendelian Inheritance in Man (OMIM), currently includes more than 20,000 human characteristics inherited on Mendelian principles. Most Mendelian traits are biochemical rather than visibly morphological, and many genetic disorders result from harmful alleles inherited in Mendelian fashion.

ABO Blood Group: A Classic Mendelian Example

The ABO blood group system is governed by three alleles (A, B, and O) at the ABO locus on chromosome 9. These alleles code for antigens on the surface of red blood cells:

• Only antigen A present: blood type A.
• Only antigen B present: blood type B.
• Both antigens present: blood type AB.
• Neither antigen present: blood type O.

The O allele is recessive to both A and B. Blood type A results from genotype AA or AO; blood type B from BB or BO. Blood type AB is a case of codominance: both A and B alleles are expressed simultaneously since neither is dominant to the other. Blood type O requires the homozygous genotype OO.

Modes of Inheritance

Autosomal Dominant Inheritance

The responsible gene is on an autosome and expresses the trait even in a single copy (heterozygous state). Characteristic features:

• The trait can appear in either sex.
• The trait does not skip generations.
• An affected person always has at least one affected parent.
• Normal children of affected individuals do not transmit the trait.

Examples: Anonychia (absent nails), chin fissure, mid-digital hair, dimples, Marfan syndrome.

Autosomal Recessive Inheritance

The gene must be in the homozygous state (double dose) for the trait to manifest. Heterozygotes are called carriers and appear phenotypically normal. Characteristic features:

• The trait can appear in either sex.
• The trait can skip generations.
• Parents of an affected individual are usually heterozygous carriers or have the trait.
• Consanguinity among parents increases the risk of the disorder appearing.

Examples: Albinism, alkaptonuria, sickle cell anaemia (in terms of its disease state), phenylketonuria.

X-linked Inheritance

Genes located on the X chromosome show characteristic inheritance patterns. X-linked recessive traits affect males more frequently because males have only one X chromosome (hemizygous): a single copy of a recessive allele on the X is sufficient to express the trait. Females need two copies to be affected but can be carriers. X-linked dominant traits affect females more frequently as they have two X chromosomes, both of which can carry the allele.

Classic example: Haemophilia in European royal families, traced to a carrier mutation in Queen Victoria, spread across more than twenty royal family members within a century.

Exceptions to Mendelian Inheritance

Codominance

Both alleles in a heterozygote are fully expressed, each producing its own phenotype. Neither allele is dominant over the other. The ABO AB blood type is the standard human example.

Incomplete Dominance

The dominant allele does not completely suppress the recessive allele. Heterozygotes display an intermediate phenotype: a blend of the two parental phenotypes. Example: carriers of the sickle cell allele (one normal, one sickle) produce both normal and sickled red blood cells, intermediate expression. In plants, red × white crosses can produce pink offspring.

Polygenic (Multifactorial) Inheritance

Many traits are determined by more than one gene (polygenes). These genes have equal influence and their alleles have an additive effect. Polygenic traits exhibit incomplete dominance rather than the clear dominant/recessive patterns of single-gene traits. Environmental factors can also influence polygenic traits. Polygenic traits show a bell-shaped (normal) distribution in a population: most individuals fall in the average range, with extremes at either end. Human examples include skin colour (at least 3 genes; amount of melanin determines darkness), eye colour (up to 16 genes; determined by melanin in the iris; key genes on chromosome 15: OCA2 and HERC2), height, hair colour, body shape, and weight.

Genetic Linkage

Genes that are physically close together on the same chromosome tend to be inherited together: they do not assort independently. This is the chief exception to Mendel's Law of Independent Assortment. The closer two genes are on a chromosome, the lower the chance of recombination between them through crossing over, and the more likely they are to be inherited together. Genes on different chromosomes assort independently. Types of linkage:

• Complete linkage: No crossing over occurs; only parental combinations appear (example: Drosophila male).
• Incomplete linkage: Some crossing over occurs; recombinant types appear alongside parental types.
• Coupling (cis) linkage: Dominant genes linked with each other (or recessive with recessive).
• Repulsion (trans) linkage: Dominant genes linked with recessive genes.
• Autosomal linkage: Linked genes on autosomes.
• Allosomal (X-linked) linkage: Linked genes on sex chromosomes.

The HLA (Human Leukocyte Antigen) region on chromosome 6 carries the major histocompatibility complex. Loci A, B, C, and D are tightly linked and nearly always inherited together as a haplotype.

Pleiotropy

One gene influences two or more seemingly unrelated phenotypic traits. A mutation in a pleiotropic gene can simultaneously affect multiple traits. Examples:

• Phenylketonuria (PKU): A defect in a single gene on chromosome 12 (coding for phenylalanine hydroxylase) causes dangerous accumulation of phenylalanine, affecting the nervous system, skin pigmentation, and other systems.
• Marfan syndrome: A single gene mutation reducing fibrillin protein production causes unusually tall stature, thin fingers and toes, lens dislocation, and aortic abnormalities: seemingly unrelated traits all tracing to one gene.

Multiple Alleles and Genetic Polymorphism

A gene locus may have more than two alternative alleles (multiple alleles). Polymorphism is the occurrence of more than one form (morph) in a population such that the rarest cannot be maintained by recurrent mutation alone. For a trait to be classified as polymorphic, it must occur at a frequency greater than 1% in the population; below this threshold it is assumed to arise from mutation alone, with no selection maintaining it.

Genetic polymorphism is expressed at multiple levels:

• Cell surface (antigenic): Blood group antigens (ABO, Rh) on red blood cells; HLA antigens on tissue cells (extraordinarily polymorphic: no two individuals except identical twins share the same HLA system).
• Chromosomal (functional): Chromosomal rearrangements including insertional elements, transposons, deletions, inversions, and translocations.
• Gene products (protein): Protein variants identified by electrophoresis and amino acid sequencing. Haemoglobin sequencing has revealed hundreds of variants. Isoenzymes (e.g., lactic dehydrogenase, which appears in 5 forms within a single individual) are another example.

Balanced Polymorphism and Heterozygote Advantage

Genetic polymorphisms are balanced when natural selection favours heterozygotes over both homozygotes. A stable equilibrium is maintained: both alleles persist at substantial frequencies. The classic human example is the sickle cell allele of the β-globin gene:

• Homozygous for sickle cell allele (SS): develop sickle cell disease, usually fatal without intensive medical care.
• Heterozygous (HbS/HbA): red blood cells are functional and resistant to the malaria parasite, these individuals survive malaria better than those with only the normal allele.
• Homozygous normal (HbA/HbA): susceptible to malaria.

In malaria-endemic regions, the sickle cell allele is maintained by balancing selection because heterozygotes have a reproductive advantage. This is called heterozygote advantage.

Haemoglobin in Health and Disease

Haemoglobin, the oxygen-carrying protein of red blood cells, is a tetramer of two α-like and two β-like globin chains, each holding an iron-containing haem group that binds one oxygen molecule. The α-globin gene cluster lies on chromosome 16 and the β-globin cluster on chromosome 11.

• Foetal haemoglobin (HbF, α2γ2) predominates before birth. Its higher oxygen affinity lets the foetus draw oxygen from the mother's blood. Embryonic haemoglobins precede it.
• Adult haemoglobins: HbA (α2β2), about 97 per cent, and HbA2 (α2δ2), about 2 to 3 per cent. A raised HbA2 is the standard laboratory marker of β-thalassaemia trait.

Haemoglobinopathies, the commonest single-gene disorders in the world, fall into two groups:

• Structural variants change one amino acid. HbS (valine for glutamic acid at position 6 of the β chain) polymerises at low oxygen, so red cells sickle, block small vessels and break down, causing painful crises, anaemia and organ damage in homozygotes. HbC (position 6, lysine) is common in West Africa. HbE (position 26) is mild alone but severe when inherited with β-thalassaemia. HbD Punjab (position 121) occurs in north-west India.
• Thalassaemias reduce the output of a globin chain. α-thalassaemia usually results from gene deletions: loss of three of the four α genes causes HbH disease and loss of all four causes fatal hydrops foetalis. β-thalassaemia usually results from point mutations. β-thalassaemia major (Cooley's anaemia, described by Thomas Cooley in 1925) causes severe anaemia from infancy and requires lifelong transfusion and iron chelation.

Malaria and selection: J.B.S. Haldane suggested in 1949 that thalassaemia carriers resist malaria, and A.C. Allison (1954) showed the same for the sickle-cell trait. These alleles are therefore common in present or former malarial regions as balanced polymorphisms.

India: H. Lehmann and M. Cutbush reported the sickle-cell trait among tribal groups of the Nilgiri hills in 1952. HbS is concentrated in the tribal belt of central, western and southern India, with carrier rates above 20 per cent in some communities. HbE is common in Assam and other north-eastern states, and β-thalassaemia carriers form about 3 to 4 per cent of the population, with higher frequencies in certain communities. The National Sickle Cell Anaemia Elimination Mission, launched in 2023 at Shahdol, Madhya Pradesh, aims to eliminate sickle cell disease as a public health problem by 2047 through screening about seven crore people up to 40 years of age in tribal areas.

Rh Blood Group System

The Rh blood group system was discovered by Karl Landsteiner and A.S. Wiener in 1940, using rhesus monkey blood in the initial tests. Unlike ABO (which involves one chromosome pair with one set of alleles), the Rh system was described by Fisher and Race as three linked pairs of alleles (C/c, D/d, E/e) on chromosome 1. Molecular work has since shown two adjacent genes, RHD and RHCE, and there is no 'd' antigen: d only marks the absence of D. The most important is the D antigen (RhD), which causes the most severe immune reaction. People with at least one RHD gene (DD or Dd in the old notation) are Rh positive (about 85% of the US population); most Rh-negative people (dd) lack the RHD gene altogether.

Rh incompatibility during pregnancy is medically significant. When an Rh-negative mother carries an Rh-positive foetus, foetal blood entering the mother's circulation (usually during delivery) stimulates her immune system to produce anti-Rh antibodies. In subsequent pregnancies with Rh-positive foetuses, these antibodies cross the placenta and attack the foetal red blood cells, causing erythroblastosis foetalis (haemolytic disease of the newborn), which can be fatal. Prevention is by vaccinating the mother with Rh immunoglobulin after delivery of the first Rh-positive child to destroy foetal blood cells before the immune system produces antibodies.

The highest frequency of Rh negativity is among the Basques of the Pyrenees (25-35%) and the Imazighen (Berbers) of Africa and Bedouins of the Sinai Peninsula (18-30%).

Pedigree Analysis

Pedigree analysis is the method used to study inheritance in humans, where controlled crosses are impossible. It examines the presence or absence of a trait across generations within a family, tracking its relationship among parents, offspring, and siblings. Pedigrees are used to determine the mode of inheritance of a disease or trait, predict the probability of occurrence among offspring, and provide a basis for genetic counselling. The technique is also valuable for species with long generation times.

Twin Method and Heritability Estimation

Francis Galton proposed in 1875 that twins could separate the effects of "nature and nurture". The twin method compares monozygotic (MZ) twins, who share virtually all their genes, with dizygotic (DZ) twins, who share about half, assuming that both types share their family environment to the same degree.

• Zygosity: judged from the placental membranes (a single chorion means MZ twins), physical similarity, blood groups and dermatoglyphics, and now with certainty from DNA markers.
• Concordance: for qualitative traits, the percentage of pairs in which both twins are affected. For schizophrenia MZ concordance is roughly 45 to 50 per cent against about 15 per cent in DZ pairs: a genetic contribution, but MZ concordance well below 100 per cent shows that environment matters too.
• Correlation: for quantitative traits such as stature, MZ and DZ correlations are compared. MZ twins reared apart, as in the Minnesota study begun in 1979, strengthen the design.

Heritability is the proportion of phenotypic variance in a population that is due to genetic variance. Broad-sense heritability (H² = VG/VP) includes all genetic variance. Narrow-sense heritability (h² = VA/VP) includes only additive variance and predicts the response to selection (R = h²S, the breeder's equation).

• Twin formulas: Holzinger's index (1929), H = (CMZ - CDZ)/(100 - CDZ) for concordances in per cent, or (rMZ - rDZ)/(1 - rDZ) for correlations. Falconer's formula, h² = 2(rMZ - rDZ).
• Parent-offspring regression: the slope of offspring values on the mid-parent value estimates h², the slope on one parent h²/2.
• Sibling and half-sibling correlations: full sibs share half and half-sibs a quarter of additive variance, though full sibs also share an environment.
• Adoption studies: resemblance to biological compared with adoptive parents.
• Genomic estimates: heritability from genome-wide SNP similarity among unrelated people, usually lower than twin estimates (the "missing heritability" problem).

Dermatoglyphic total ridge count has a heritability of about 0.9, and stature about 0.8 in well-nourished populations. Cautions: heritability belongs to a population at a time, not to an individual, and falls when environments vary more, as among poorly nourished groups. A highly heritable trait can still change, as the secular trend in stature shows. Heritability within groups says nothing about differences between groups, as Richard Lewontin (1970) showed with seeds grown in rich and poor soil. MZ twins may also share more similar environments than DZ twins, which inflates estimates.

Genetic Counselling: Steps, Pedigree Analysis and Ethics

Genetic counselling helps a person or family understand a genetic disorder, its chance of occurring or recurring, and the options available. Since Sheldon Reed coined the term (1947), its guiding principle has been non-directiveness: the counsellor informs and supports, and the family decides.

1. Referral and history: an affected child, a family history of disease, consanguinity, recurrent miscarriage, advanced maternal age or an abnormal prenatal screening result.
2. Diagnosis: clinical examination, karyotype, biochemical and DNA tests. All later advice depends on an accurate diagnosis.
3. Pedigree: at least three generations drawn with standard symbols, to establish the mode of inheritance.
4. Risk estimation: Mendelian risks for single-gene disorders (50 per cent for each child of a parent with an autosomal dominant disorder, 25 per cent for each child of two carriers of a recessive disorder, half the sons of a carrier mother for an X-linked recessive disorder), empirical risks from family studies for multifactorial disorders such as cleft lip, and age-related risks for chromosomal disorders.
5. Options: carrier testing of relatives, prenatal diagnosis (chorionic villus sampling at about 10 to 13 weeks, amniocentesis from about 15 weeks), preimplantation genetic testing with in vitro fertilisation, donor gametes, adoption, or accepting the risk.
6. Decision and follow-up: the family's own decision, psychological support, and help in informing relatives at risk.

Pedigree analysis in counselling reveals the mode of inheritance, identifies obligate carriers (such as the mother of two sons with haemophilia), shows consanguinity, reduced penetrance and variable expressivity, and points to a new mutation when an affected child has unaffected parents. It also identifies relatives for cascade carrier testing. Its limits are small families, incomplete or wrong information, misattributed paternity and late-onset disorders.

Serogenetics and cytogenetics in reproductive biology: Rh typing of mothers and anti-D immunoglobulin prevent haemolytic disease of the newborn. Cytogenetics finds balanced translocations in couples with recurrent miscarriage, sex chromosome anomalies and Y-chromosome microdeletions in infertility, and chromosomal disorders in prenatal samples. Maternal serum screening and cell-free foetal DNA testing identify high-risk pregnancies before invasive tests.

Ethical issues in reproductive technology:

• Sex selection: banned in India by the Pre-Conception and Pre-Natal Diagnostic Techniques (Prohibition of Sex Selection) Act, 1994, amended in 2003.
• Eugenics and disability: selective termination may express a devaluation of disabled lives.
• Consent, privacy and confidentiality, including whether relatives must be told of their risk.
• Embryo testing and editing: the Assisted Reproductive Technology (Regulation) Act, 2021, permits preimplantation testing only for known genetic diseases. The birth of gene-edited babies in China in 2018 showed the dangers of heritable genome editing.
• Exploitation and access: commercial surrogacy, now banned by the Surrogacy (Regulation) Act, 2021, and unequal access to costly technologies.

In India the main application is carrier screening and counselling for β-thalassaemia and sickle cell disease in high-frequency communities and tribal areas.

Lethal Genes

Many genes are essential for survival. Alleles that make such genes non-functional may be impossible to recover in living, homozygous organisms: these are lethal alleles. First discovered by Lucien Cuénot in 1905 while studying coat colour in mice (the yellow agouti allele), where homozygous yellow mice were never obtained because homozygosity was lethal in the embryo. Types:

• Recessive lethal: Fatal only in the homozygous state; heterozygotes may display a diseased phenotype or appear normal carriers.
• Dominant lethal: Fatal even in the heterozygous state (one copy is sufficient to kill).

Population Genetics: Evolutionary Forces

Population genetics studies the distribution and change of allele frequencies in populations over time. A Mendelian population is a group of interbreeding individuals sharing a common gene pool. The gene pool is the total set of gene copies for all genes in a population; greater diversity in the gene pool means better ability to adapt to environmental change through natural selection.

Hardy-Weinberg Law

Formulated independently in 1908 by Godfrey Hardy (English mathematician) and Wilhelm Weinberg (German obstetrician), the Hardy-Weinberg law of equilibrium states that in a population meeting all ideal conditions, allele and genotype frequencies remain constant from generation to generation after one generation of random mating.

The ideal conditions required are:

• Large population size (no genetic drift).
• Random mating (no assortative mating or inbreeding).
• No new mutations.
• No natural selection (all genotypes equally fertile and viable).
• No gene flow (no migration in or out).

The Hardy-Weinberg equation is: if the frequency of allele A = p, and of allele a = q, then p + q = 1, and the expected genotype frequencies are p² (AA) + 2pq (Aa) + q² (aa) = 1. This is considered the cornerstone of population genetics because it provides a mathematical baseline against which actual populations can be compared. If observed genotype frequencies deviate from Hardy-Weinberg predictions, the population is evolving at that locus.

Factors Disturbing Hardy-Weinberg Equilibrium (Evolutionary Forces)

Mutation

Mutation is the only source of genuinely new genetic variation in a population: it is the only mechanism that can introduce alleles that were not previously present. During DNA replication, copying errors or environmental damage can alter the sequence of bases. Types of mutations:

• Point mutation (synonymous): Changes the DNA triplet but the altered codon still codes for the same amino acid: no change in the resulting protein.
• Point mutation (nonsynonymous): The altered codon codes for a different amino acid, potentially altering protein structure and function. Classic example: a mutation on chromosome 11 converts the GAG codon (glutamic acid) to GTG (valine) in the beta-globin gene, causing sickle cell anaemia.
• Frameshift mutation: Insertion or deletion of base pairs shifts the reading frame, disrupting all codons downstream: usually produces a non-functional protein.
• Transposable elements: Genes that can copy themselves to different positions in the genome, potentially disrupting other genes.
• Chromosomal mutations: Large rearrangements including translocations, inversions, duplications, deletions, trisomies (extra chromosome, e.g., Down syndrome: trisomy 21; Klinefelter syndrome: XXY in males), and monosomies (missing chromosome).

Mutations are classified as spontaneous (no known cause, generally DNA copying errors) or induced (caused by mutagens such as ionising radiation like X-rays or toxic chemicals). The human mutation rate is approximately one new potentially significant mutation every other person born. Mutations in gametes are the ones with evolutionary consequences.

Natural Selection

Natural selection is the non-random, differential survival and reproduction of individuals based on heritable traits. Those variations that increase an organism's chances of survival and reproduction are preserved and multiplied across generations. Selection may act through differences in survival, fertility, mating success, developmental rate, or any other life-cycle aspect. Three types:

• Stabilising selection: Eliminates extreme variants from both ends of the frequency distribution; maintains the existing average. The most common form of natural selection. Example: human birth weight, very low and very high birth weights both have higher infant mortality.
• Directional selection: Favours individuals at one end of the distribution, shifting the population mean in that direction over generations. Example: increase in human brain size through hominin evolution; industrial melanism (selection for darker moths on pollution-darkened trees).
• Diversifying (disruptive) selection: Favours individuals at both extremes simultaneously, creating two distinct phenotypic groups. Can lead to the formation of distinct populations.

Two notable case studies of natural selection in living human populations:

• CCR5-Δ32 mutation: A 32-base-pair deletion in the CCR5 gene on chromosome 3 confers partial resistance to HIV in heterozygotes and near-complete resistance to AIDS in homozygotes. The mutation occurs at 0-14% frequency in European populations but is absent elsewhere. Its initial rise was probably due to selection by another disease such as smallpox.
• Lactase persistence: Most mammals shut down lactase enzyme production after weaning. Populations engaged in dairy farming gained a nutritional advantage from being able to digest lactose throughout their lives. Natural selection has favoured the lactase persistence allele in dairy-farming populations: highest in northern Europe, moderate in southern Europe and the Middle East, very low in African and Asian populations.
• Skin colour: Melanin concentration in skin is inversely related to distance from the equator. Pigmented skin acts as a protective barrier against UV radiation, making darker skin advantageous in high-UV equatorial environments. Lighter skin at higher latitudes allows greater UV penetration for vitamin D synthesis.

Genetic Drift

Genetic drift is the random change in allele frequencies over time due to chance events rather than natural selection. It has the greatest effect in small populations. Unlike mutation, selection, and gene flow, genetic drift is entirely random: it can fix or eliminate alleles without regard to their adaptive value.

The classic example is the Dunkers, a small religious sect that emigrated from Germany to Pennsylvania in 1719 with only 28 members. Because the breeding population remained small and endogamous, blood type frequencies diverged dramatically from both German and American populations over generations: simply due to chance.

Two special forms of genetic drift:

• Founder effect: Occurs when a small group breaks off from a larger population and establishes a new, reproductively isolated population. The founders' gene pool is not representative of the parent population. As the founding population grows, the divergence is amplified. Example: Native Americans, descended from a small founding group of East Asian migrants around 12,000 ybp, show very high frequencies of type O blood (sometimes 100%), while East Asian populations have among the lowest frequencies globally.
• Genetic bottleneck: A large population undergoes a drastic reduction in size, then expands again. Alleles present only at low frequency may be lost entirely, and the rebounded population has greatly reduced genetic diversity. Example: the southern elephant seal was hunted to near-extinction; the recovered population now has low genetic diversity. Among the Pingelapese people of Micronesia, a typhoon in 1780 reduced the population to about 19 survivors, creating a severe bottleneck; as a result, 4-10% of the current population is born with a rare form of colour blindness (achromatopsia), compared to 1 in 20,000-50,000 in most populations.

Gene Flow (Migration)

Gene flow is the transfer of alleles across population boundaries through the movement and interbreeding of individuals. It is distinct from migration: migration is the movement of people, while gene flow only occurs when migrants interbreed with the receiving population. Gene flow generally reduces genetic differences between populations and increases genetic variation within populations that receive migrants.

Key characteristics:

• The less the physical distance between populations, the greater the chance of gene flow.
• Endogamous societies (inbreeding within the group) have relatively little genetic diversity; exogamous societies have relatively high genetic diversity.
• The B blood group allele shows a clinal decrease in frequency from East Asia to far western Europe, suggesting gene flow as Mongol populations migrated westward from AD 500 to 1500.
• Gene flow studies of African American populations using the Duffy blood group allele (largely absent in Africa but common in European populations) showed European admixture ranging from 4% in Charleston, SC to 26% in Detroit.

Non-random Mating: Inbreeding and Consanguinity

Hardy-Weinberg equilibrium assumes random mating. Deviations occur in two directions:

• Inbreeding: Mating between closely related individuals more frequently than expected by chance. Increases homozygosity at the expense of heterozygosity.
• Outbreeding: Mating between less closely related individuals than expected by chance.

Assortative mating is another deviation. Positive assortative mating: individuals choose mates resembling themselves in traits such as intelligence, height, skin colour, or musical talent. Negative assortative mating: individuals choose mates phenotypically dissimilar to themselves.

Consanguinity refers specifically to marriages between blood relatives sharing at least one common ancestor no more remote than a great-great grandparent. All human cultures have rules prohibiting sexual contact between close kin, the incest taboo, which is universal but culturally variable in scope. The most common form of consanguinity worldwide is cousin marriage. In south India, particularly Andhra Pradesh, uncle-niece unions and cross-cousin marriages are practised among certain castes. First-cousin marriages make up close to 10% of marriages in some parts of India. In many populations of Asia and Africa, 20-50% of all unions are consanguineous.

Reasons consanguineous marriages are culturally favoured: Sociological studies indicate they reinforce marital stability (shared social network before and after marriage), improve wives' relationships with in-laws, facilitate family property transmission, and strengthen family solidarity. Premarital negotiations are easier and less costly. In low-population or endogamous communities, mate choice may simply be limited.

Inbreeding depression: The primary genetic consequence of sustained inbreeding is an increase in homozygosity, which increases the chance of harmful recessive alleles being expressed. Animal breeders consistently observe that inbreeding reduces fertility, viability, and overall fitness: a phenomenon called inbreeding depression. Studies in Japan, France, Sweden, and the United States showed increased frequencies of infant mortality, physical diseases, and mental disorders among children of first-cousin mating. The haemophilia that spread through European royal families, originating in Queen Victoria's X-linked carrier status and transmitted through consanguineous unions, is a classic historical example.

However, some amount of inbreeding can be beneficial by allowing the expression of recessive genes with positive effects (the reason Charles Darwin, himself a first-cousin marriage product, is sometimes cited). The net effect depends on whether the homozygotes formed carry advantageous or disadvantageous alleles.

Consanguinity and health parameters: Compared to non-consanguineous unions, first-cousin marriages show slightly higher fertility rates, similar abortion rates, slightly higher stillbirth and infant mortality rates, and an estimated 2-3% additional frequency of birth defects. They increase the expression of autosomal recessive disorders because offspring inherit identical copies of recessive alleles from a shared ancestor. First cousins share approximately 12.5% of their genes; their offspring are expected to be homozygous at about 6.25% of gene loci.

In populations with high consanguinity rates and common haemoglobinopathies (thalassemia, sickle cell anaemia), community premarital screening programs for carrier detection are active in Jordan, Saudi Arabia, Iran, Iraq, Bahrain, and Turkey, with significant success in reducing the birth prevalence of these disorders.

Measuring Genetic Load and the Factors Influencing It

H.J. Muller drew attention to "our load of mutations" in 1950, and J.F. Crow (1958) gave the standard definition: L = (Wmax - Wmean)/Wmax, the proportion by which the mean fitness of a population falls below that of the fittest genotype. Load cannot be seen directly and is estimated in several ways.

• From mutation rates (Haldane-Muller principle): at equilibrium between mutation and selection, the load from a locus is about equal to the mutation rate for a recessive allele and about twice the mutation rate for a dominant one, whatever the severity of the mutation.
• From consanguineous marriages: Newton Morton, J.F. Crow and H.J. Muller (1956) compared survival among the children of parents with different inbreeding coefficients (F) using -ln S = A + BF, where S is the proportion surviving. A measures the damage expressed in a random-mating population (genetic and environmental), and B the hidden damage that would appear under complete inbreeding. B is expressed in lethal equivalents: a set of harmful genes that would on average cause one death if made homozygous, such as one lethal gene or two genes each with a 50 per cent chance of causing death. A high B/A ratio was taken to indicate mainly mutational load, a low one load maintained by heterozygote advantage.
• From disease frequencies: the incidence of genetic disorders, malformations, stillbirths and early deaths in the children of cousin and uncle-niece marriages compared with others, which in south Indian studies showed a modest excess.
• Crow's index of the opportunity for selection (1958): a related demographic measure, I = Im + If/Ps. Here Im = Pd/Ps, where Pd is the proportion dying before reproductive age and Ps the proportion surviving, and If is the variance in number of offspring divided by the square of the mean. It sets an upper limit to selection intensity and has been calculated for many Indian tribal and caste populations.

Factors influencing genetic load:

• Mutation rate: raised by ionising radiation, chemical mutagens and older paternal age, which adds new point mutations.
• Inbreeding and endogamy: consanguinity raises homozygosity. Long-standing caste and tribal endogamy has produced strong founder effects in many Indian groups (Nakatsuka and colleagues, 2017), so particular recessive disorders become common in particular communities, for example butyrylcholinesterase deficiency, dangerous with some anaesthetics, among the Vysya of Andhra Pradesh.
• Heterozygote advantage: keeps harmful alleles common, producing segregational load, such as deaths from sickle cell disease where the trait protects against malaria.
• Relaxed selection and population structure: medical treatment lets people with genetic disorders reproduce, slowly raising allele frequencies, while gene flow and outbreeding lower the expressed load and drift in small groups can fix harmful alleles.

Key Genetic Concepts Summary

• Allele frequency: The proportion of a particular allele among all copies of that gene in the population. If all alleles at a locus are A, frequency = 1.0; if half are A and half are a, each has frequency 0.5.
• Genotype frequency: The fraction of individuals in a population with a specific genotype (e.g., AA, Aa, aa).
• Phenotype frequency: The fraction of individuals displaying a specific phenotype.
• Genetic load: The reduction in the average fitness of a population due to the presence of deleterious alleles maintained in the gene pool.
• Isolate: A group totally separated from other groups, reproductively unstratified, so all individuals share the same gene pool. Prolonged isolation causes increasing divergence in gene frequencies between the isolate and neighbouring populations.
• Mitochondrial DNA (mtDNA): Inherited exclusively through the maternal line; does not undergo recombination and thus acts genetically like a single gene. Used by Rebecca Cann to trace all modern human mitochondrial lineages to a common ancestor approximately 200,000 years ago in Africa: the "Mitochondrial Eve" hypothesis. Y-chromosome analysis, similarly non-recombining, independently supports African origin with a common paternal ancestor around 100,000 years ago.

Applications of DNA Technology and Genomic Research

The Human Genome Project (1990 to 2003) sequenced about 3 billion base pairs and found only about 20,000 protein-coding genes. The Telomere-to-Telomere consortium published the first complete, gapless human sequence in 2022.

• Diagnosis: PCR and sequencing diagnose single-gene disorders, detect carriers and allow prenatal and preimplantation diagnosis. Exome and whole-genome sequencing now identify the cause of many rare disorders.
• Therapy: recombinant human insulin (approved in 1982, the first recombinant drug), growth hormone, clotting factors and hepatitis B vaccine. Gene therapy began in 1990 with a child with ADA immunodeficiency, and CRISPR-based treatments for sickle cell disease and β-thalassaemia were approved in 2023.
• Pharmacogenomics: genotype guides choice and dose of drugs, for example testing for HLA-B*15:02, which predicts severe skin reactions to the anti-epileptic carbamazepine in many Asian populations.
• Forensics: DNA profiling (Alec Jeffreys, 1984) identifies offenders, disaster victims and parents in disputed paternity. Lalji Singh developed DNA fingerprinting in India at the Centre for Cellular and Molecular Biology, Hyderabad.
• Molecular anthropology: David Reich and colleagues (2009) showed that most Indian groups descend from a mixture of Ancestral North Indians and Ancestral South Indians. The genome of a Harappan woman from Rakhigarhi (Vasant Shinde and colleagues, 2019) showed a mix of ancestry related to ancient Iranians and to South Asian hunter-gatherers, with no steppe ancestry.
• Indian reference data: the Indian Genome Variation Consortium (2008) and the GenomeIndia project, which sequenced about 10,000 genomes from diverse communities, help locate disease variants common in particular endogamous groups.

Genome-wide Association Studies (GWAS)

A GWAS scans the whole genome for common variants associated with a disease or trait. Hundreds of thousands to millions of single nucleotide polymorphisms (SNPs) are typed on microarrays in thousands of cases and controls, or in people measured for a quantitative trait, and each SNP is tested for a difference in frequency. Because so many tests are made, only associations with p below 5 × 10⁻⁸ count as genome-wide significant, and findings must be replicated in independent samples.

• Landmarks: Robert Klein and colleagues (2005) linked the complement factor H gene to age-related macular degeneration. The Wellcome Trust Case Control Consortium (2007) studied seven common diseases in about 14,000 cases and 3,000 shared controls.
• Advances in understanding: GWAS have mapped thousands of loci for type 2 diabetes (TCF7L2 among the strongest), coronary disease, inflammatory bowel disease, schizophrenia, height and body mass. They revealed unexpected biology, such as the role of autophagy in Crohn's disease. They show that common diseases are highly polygenic, with many variants of small effect lying mostly in regulatory rather than coding DNA, and they underlie polygenic risk scores for identifying people at high risk.
• Limitations: associated SNPs usually tag nearby causal variants rather than cause disease, effects are small and explain only part of heritability, rare variants are missed, and population structure can create false associations. About four-fifths of participants have been of European ancestry, so results transfer poorly to South Asians and Africans, and studies of South Asians have found additional risk loci, for example for type 2 diabetes.

Applied human genetics now reaches diagnosis, treatment, law and the reconstruction of human history, and raises questions of privacy, consent and benefit-sharing with the communities, often indigenous, whose DNA is studied.