Genetic inheritance

1. Molecular and chromosomal basis

Genetic inheritance provides continuity between generations while permitting variation. In cellular organisms, DNA stores hereditary information in its nucleotide sequence. A chromosome consists of a long DNA molecule associated with proteins. Genes occupy positions called loci on chromosomes. Homologous chromosomes carry corresponding loci, although their alleles may differ. A diploid individual generally possesses two alleles at an autosomal locus, one inherited from each parent.

Genotype refers to an organism’s genetic constitution, either overall or at a particular locus. Phenotype refers to observable or measurable characteristics resulting from genotype, environment and their interactions. An individual with identical alleles at a locus is homozygous; one with different alleles is heterozygous. A dominant allele determines the relevant phenotype in a heterozygote under complete dominance, while the recessive phenotype ordinarily appears when both alleles are recessive.

In humans, 22 chromosome pairs are autosomes and one pair comprises sex chromosomes. The usual complements are XX in females and XY in males, although variations occur. DNA replication copies genetic material before cell division. Mitosis generally preserves chromosome number, whereas meiosis produces haploid gametes. Fertilisation restores diploidy. Crossing over between non-sister chromatids of homologous chromosomes during prophase I of meiosis generates new combinations of alleles.

2. Mendelian principles and their limits

Gregor Mendel studied contrasting characters in garden pea, Pisum sativum, using controlled crosses and numerical analysis. His work was presented in 1865 and published in 1866. True-breeding parental lines produced uniform first-generation offspring for the traits he examined. Reappearance of a recessive trait in the second generation showed that hereditary factors remained discrete rather than blending permanently.

The law of segregation states that the two alleles of a gene separate during gamete formation, so each gamete receives one. In a cross between two heterozygotes, Aa × Aa, the expected genotype ratio is 1 AA : 2 Aa : 1 aa. Under complete dominance, the phenotype ratio is 3 dominant : 1 recessive. These are probabilities across many offspring, not compulsory outcomes within every family.

The law of independent assortment states that different gene pairs assort independently under appropriate conditions. A dihybrid cross can produce a 9:3:3:1 phenotype ratio when the genes assort independently and each exhibits complete dominance. Closely located genes on the same chromosome are linked and tend to be inherited together. Crossing over can break linkage; recombination frequency helps estimate relative distances between loci.

A test cross pairs an individual showing a dominant phenotype with a homozygous recessive individual. Its offspring help distinguish homozygous dominant from heterozygous genotypes. Mendelian laws describe transmission of alleles; their familiar phenotype ratios require additional assumptions.

How sexual reproduction transmits and reshuffles genetic information

1. DNA replicates before meiosis.
2. Homologous chromosomes pair and exchange segments through crossing over.
3. Homologous chromosomes separate during meiosis I.
4. Sister chromatids separate during meiosis II, producing haploid products.
5. Gametes unite during fertilisation, restoring the diploid chromosome number.
6. Gene expression and environmental influences together shape the offspring’s phenotype.

3. Patterns beyond simple dominance

In incomplete dominance, the heterozygote has an intermediate phenotype: crossing red- and white-flowered snapdragons can yield pink offspring. Alleles themselves do not blend. In codominance, both alleles are expressed distinctly. The human ABO system illustrates codominance and multiple alleles: Iᴬ and Iᴮ are codominant, and each is dominant over i. A population contains three common alleles, but an individual ordinarily carries only two.

Polygenic inheritance occurs when several genes contribute to a character, often producing continuous variation. Height is influenced by many genes and by nutrition, health and other environmental conditions. Pleiotropy means that one gene influences multiple phenotypic features. Epistasis occurs when variation at one gene modifies or masks the expression of another gene. These concepts explain why most biological characteristics do not follow a simple dominant–recessive model.

Mitochondria possess their own DNA. In humans, mitochondrial inheritance is ordinarily maternal: both sons and daughters may inherit a pathogenic variant, but transmission to the next generation is through daughters. Heteroplasmy, the coexistence of different mitochondrial DNA variants within a cell or individual, helps explain variation in disease severity.

Epigenetic mechanisms alter gene activity without changing the underlying DNA sequence. Genomic imprinting produces parent-of-origin-dependent expression of certain genes. However, not every epigenetic change is transmitted across generations, and ordinary acquired characteristics should not be assumed to become inherited.

Comparison of major inheritance patterns
Pattern	Typical clue	Example	Important qualification
Autosomal dominant	Often seen in successive generations	Huntington disease	A heterozygous affected parent and a parent without the variant give a 50% transmission risk per pregnancy.
Autosomal recessive	Unaffected carriers can have affected children	Sickle-cell disease	Both sexes can be affected; carrier status is not equivalent to disease.
X-linked recessive	More frequent expression in males; no father-to-son transmission	Haemophilia A	Females can also be affected under particular genetic circumstances.
Mitochondrial	Maternal transmission	Leber hereditary optic neuropathy	Inheritance of a variant does not necessarily imply identical symptoms or severity.
Polygenic or multifactorial	Several genes contribute; environmental effects may be important	Human height	Simple Mendelian phenotype ratios generally do not apply.

4. Human disorders and pedigree reasoning

Pedigrees trace traits through families and help identify likely modes of inheritance. Autosomal dominant conditions can appear in successive generations and affect either sex. Autosomal recessive conditions may appear in children of unaffected carrier parents. For two carriers of the same autosomal recessive condition, each pregnancy has a 25% probability of an affected child, 50% of a carrier and 25% of a child without the pathogenic allele.

Haemophilia A and red–green colour vision deficiency commonly show X-linked recessive inheritance. Males are more frequently affected because they usually have only one X chromosome. A father does not transmit his X chromosome to a son. A carrier mother and an unaffected father have, under the standard model, a 50% probability of an affected child among sons and a 50% probability of a carrier among daughters.

Sickle-cell disease and beta-thalassaemia are important autosomal recessive disorders in India. Chromosomal conditions are different from single-gene disorders: Down syndrome usually results from trisomy 21, often following nondisjunction. Turner syndrome commonly involves monosomy X, while Klinefelter syndrome commonly involves a 47,XXY complement. Such conditions need not be inherited from an affected parent.

Mutation creates new genetic variants; recombination rearranges existing variants. Germline variants can pass to offspring, whereas somatic mutations are generally not inherited through sexual reproduction. Reduced penetrance, variable expressivity and new mutations can complicate pedigree interpretation.

5. Applications, public health and ethical safeguards

Inheritance principles support crop improvement, livestock breeding, diagnosis and genetic counselling. Selective breeding combines desirable variants, while molecular markers help track relevant genomic regions. In medicine, carrier screening estimates reproductive risks and diagnostic testing can identify particular pathogenic variants. A negative result reduces only the risks covered by that test; it does not guarantee absence of every genetic disorder.

Genetic counselling should explain probabilities, test limitations and reproductive options without coercion. Screening identifies people who may need further assessment; it is not automatically a definitive diagnosis. Risks should be communicated for each pregnancy rather than as predictions about the exact composition of a family. Genetic susceptibility also differs from certainty, especially for multifactorial conditions such as type 2 diabetes.

India’s National Sickle Cell Anaemia Elimination Mission, launched in 2023, combines awareness, screening and care, particularly in affected tribal populations. Genetic information requires informed consent, confidentiality and protection against stigma. The Pre-Conception and Pre-Natal Diagnostic Techniques (Prohibition of Sex Selection) Act, 1994, regulates relevant diagnostic practices and prohibits sex selection. Permitted medical diagnosis must not be confused with unlawful prenatal sex determination. Somatic gene therapy targets the treated individual, whereas heritable germline editing raises intergenerational safety and ethical concerns.

Real-world case studies

India: sickle-cell screening and counselling

The National Sickle Cell Anaemia Elimination Mission seeks elimination of sickle-cell disease as a public-health problem by 2047. Its initial screening target covered seven crore people aged 0–40 years in affected tribal and other populations across 17 states during 2023–24 to 2025–26. This is a programme target, not a statement of achievement. Screening must distinguish sickle-cell trait from disease and connect individuals with confirmatory testing, counselling and appropriate care without stigmatising carriers.

United Kingdom: mitochondrial donation

The United Kingdom introduced regulations permitting mitochondrial donation in 2015 under oversight of the Human Fertilisation and Embryology Authority. These techniques aim to reduce transmission of serious mitochondrial DNA disease by combining the intended parents’ nuclear genetic material with healthy donor mitochondria. They do not replace all of the mother’s genes or prevent every inherited disorder. The example highlights the distinction between nuclear and mitochondrial inheritance.

Previous year questions

No source-linked PYQ is available here yet. Use the practice questions below.

Practice questions

Practice MCQ 1

With reference to Mendelian inheritance, consider the following statements: 1. A dominant allele must be more frequent than a recessive allele in a population. 2. The alleles of a gene segregate during gamete formation. 3. Closely linked genes always assort independently. Which of the statements given above is/are correct?

A. 1 only
B. 2 only
C. 2 and 3 only
D. 1, 2 and 3

Practice MCQ 2

A person with blood group AB and a person with blood group O have children. Under the standard ABO inheritance model, which blood groups are possible among their children?

A. A and B only
B. AB and O only
C. A, B and AB only
D. A, B, AB and O

Practice MCQ 3

Consider the following statements: 1. Human mitochondrial DNA is ordinarily maternally inherited. 2. Somatic mutations are normally transmitted to offspring through gametes. 3. A chromosomal disorder may occur in a child whose parents do not have that disorder. Which of the statements given above are correct?

A. 1 and 2 only
B. 2 and 3 only
C. 1 and 3 only
D. 1, 2 and 3

Mains practice · Explain why genetic inheritance cannot be understood solely through dominant and recessive traits. Discuss the public-health and ethical implications of genetic screening in India. Answer in 250 words.

Introduce Mendelian segregation and distinguish allele transmission from phenotype expression.
Explain codominance, polygenic inheritance, linkage and mitochondrial inheritance with examples.
Identify environmental influences, penetrance and variable expressivity.
Discuss carrier screening, confirmatory diagnosis and non-directive counselling using sickle-cell disease.
Address informed consent, confidentiality, stigma and equitable access to treatment.
Distinguish legitimate genetic diagnosis from prohibited sex selection under the PCPNDT Act.