NCERT Solutions for Class 12 Biology Chapter 4: Principles of Inheritance and Variation

Gregor Johann Mendel, an Austrian monk, carried out extensive hybridisation experiments over seven years (1856–1863) on the garden pea, Pisum sativum, and from this work derived the fundamental principles of heredity that we now call Mendel’s Laws of Inheritance. His choice of the pea plant as his experimental organism was one of the most crucial factors in his eventual success, at a time when the science of statistics and genetics did not yet exist. The specific advantages of the pea plant are:

1. Availability of clear-cut, easily distinguishable contrasting characters: the pea plant shows several traits that occur in only two sharply distinct, easily scoreable forms with no intermediate or overlapping states — for example, tall or dwarf stem, round or wrinkled seeds, yellow or green seed colour, violet or white flower colour, inflated or constricted pods, and axial or terminal flower position. This meant Mendel could unambiguously classify every single plant in his large experimental population, which was essential for reliable counting and ratio calculation.
2. Naturally bisexual and self-pollinating flowers: pea flowers contain both male (stamens) and female (pistil) parts, and under natural conditions they self-pollinate because the petals enclose the reproductive structures. This allowed Mendel to easily obtain and maintain pure-breeding (true-breeding, homozygous) parental lines simply by letting the plants self-pollinate generation after generation.
3. Ease of artificial cross-pollination: despite being naturally self-pollinating, the flowers are large enough that Mendel could easily emasculate them (remove the anthers before they matured, to prevent self-pollination) and then dust pollen from a chosen second plant onto the stigma by hand, achieving controlled cross-pollination. Such artificial crosses reliably produced fertile hybrid offspring, which was essential to carry the experiment through multiple generations.
4. Short life cycle with abundant offspring: the pea plant completes its full life cycle — from seed to flowering to seed production — within a single growing season, and each plant produces a large number of seeds/offspring per cross. This gave Mendel enough data, generation after generation, to apply rigorous mathematical and statistical analysis to his results, something no one before him had systematically done in the study of heredity.
5. Ease of cultivation and maintenance: the pea is a hardy annual plant that is simple and inexpensive to grow in large numbers in an ordinary garden, requiring no special equipment or conditions — practical considerations that made large-scale, multi-year experimentation feasible for Mendel.

Conclusion: the pea plant’s sharply contrasting heritable traits, its natural self-pollination (giving ready-made pure lines), the ease of artificial cross-pollination into fertile hybrids, its short life cycle, and the large numbers of offspring it produced together made it the ideal experimental organism for Mendel’s pioneering, statistically-grounded study of inheritance.

(a) Dominant allele vs Recessive allele

In a pair of contrasting alleles for the same gene, one — the dominant allele — determines the phenotype whenever it is present, while the other — the recessive allele — is expressed only when the dominant allele is absent. This relationship is central to Mendel’s Law of Dominance.

(b) Homozygous vs Heterozygous

These terms describe the genetic constitution (genotype) of an individual at a particular locus, and directly determine what kinds of gametes that individual can produce.

(c) Monohybrid cross vs Dihybrid cross

These describe crosses that track different numbers of traits simultaneously, and correspondingly give different characteristic ratios in the F₂ generation.

Conclusion: a dominant allele expresses itself and masks the recessive in every genotype it appears in, while the recessive is hidden except when homozygous; a homozygote carries two identical alleles and breeds true, a heterozygote carries two different alleles and produces two gamete types; and a monohybrid cross tracks one gene to a 3:1 F₂ ratio, while a dihybrid cross tracks two genes to a 9:3:3:1 F₂ ratio.

The number of genetically distinct types of gametes an organism can form depends directly on the number of heterozygous gene loci it carries, because of two of Mendel’s core principles working together: the Law of Segregation (the two alleles at each locus separate from each other during gamete formation, so each gamete gets only one) and the Law of Independent Assortment (the alleles of different genes, located on different chromosome pairs, sort into gametes independently of one another, generating every possible combination).

Combining these two laws, the general formula for the number of types of gametes produced by an organism heterozygous at n loci is:

Number of gamete types = 2ⁿ

This arises because at each heterozygous locus there are exactly 2 choices of which allele goes into a given gamete, and since the loci assort independently, the total number of combinations is the product of the choices at each locus — i.e., 2 × 2 × 2 × … (n times) = 2ⁿ.

Applying this to the given problem: the organism is heterozygous at n = 4 loci (for example, a genotype such as AaBbCcDd). Substituting into the formula:

2⁴ = 2 × 2 × 2 × 2 = 16 types of gametes

These 16 gamete types would represent every possible combination of one allele from each of the four gene pairs — for instance ABCD, ABCd, ABcD, ABcd, AbCD, AbCd, AbcD, Abcd, aBCD, aBCd, aBcD, aBcd, abCD, abCd, abcD, and abcd.

Conclusion: applying the formula 2ⁿ with n = 4 heterozygous loci, the diploid organism can produce 2⁴ = 16 genetically distinct types of gametes.

Mendel’s Law of Dominance is one of the three foundational principles of inheritance he derived from his pea-plant experiments. It states, in essence, three related ideas: (i) characters (traits) are controlled by discrete, particulate hereditary units, which we now call genes (Mendel called them “factors”); (ii) these factors occur in pairs in an organism, one inherited from each parent; and (iii) in a heterozygous pair carrying two dissimilar factors (alleles), one factor dominates and fully expresses itself (the dominant allele), while the other remains hidden or suppressed (the recessive allele). This law directly explains two otherwise puzzling observations: why only one parental character shows up in the F₁ generation, and why the “missing” character mysteriously reappears in the F₂ generation.

The clearest demonstration of this law is Mendel’s classic monohybrid cross between a pure-breeding tall (TT) pea plant and a pure-breeding dwarf (tt) pea plant, tracking a single character (stem height) across two generations:

F₂ Punnett square, from self-pollinating the F₁ (Tt × Tt)

• Result in F₁: every single plant in the F₁ generation is tall, even though each one is genetically heterozygous (Tt), carrying one T factor from the tall parent and one t factor from the dwarf parent. The dwarf character has been completely masked — this directly demonstrates that T (tallness) is dominant over t (dwarfness).
• Result in F₂: when the F₁ plants are allowed to self-pollinate, the F₂ offspring appear in a very specific and reproducible ratio: a phenotypic ratio of 3 tall : 1 dwarf, corresponding to a genotypic ratio of 1 TT : 2 Tt : 1 tt.
• Significance of the reappearing dwarf trait: the fact that roughly one-quarter of the F₂ plants are dwarf proves conclusively that the recessive factor (t) had not been lost, altered, or “blended away” in the F₁ hybrid — it had simply been present but unexpressed, hidden behind the dominant T factor, exactly as the Law of Dominance predicts.

Conclusion: in this monohybrid cross, all F₁ plants are tall because the dominant T factor masks the recessive t factor in the heterozygote, and the F₂ generation segregates into a 3 tall : 1 dwarf phenotypic ratio (1:2:1 genotypic ratio), together demonstrating the Law of Dominance.

A test cross is defined as a cross between an individual that displays the dominant phenotype but whose genotype is unknown (it could be either homozygous or heterozygous) and an individual that is homozygous recessive for the trait in question. Its purpose is to determine, by examining the phenotypes of the resulting offspring, whether the tested individual is genetically pure (homozygous) or a hybrid (heterozygous) — a distinction that cannot be told simply by looking at the individual, since both genotypes produce an identical dominant phenotype.

Design of the test cross: take a tall pea plant of unknown genotype (written as T_, since we don’t yet know the second allele) and cross it with a dwarf plant, whose genotype must necessarily be homozygous recessive (tt), since dwarfness only shows up when both alleles are recessive. Two outcomes are possible, depending on the true genotype of the tall parent:

Case 1 — if the tall plant is homozygous dominant (TT):

Case 2 — if the tall plant is heterozygous (Tt):

Interpretation of the results — this is the entire logic of why a test cross works:

• Since the recessive homozygous parent (tt) contributes only one kind of gamete (t) to every offspring, the phenotype of each offspring is determined entirely by which allele it received from the unknown tall parent — the tt parent’s gamete acts like a “neutral background” against which the tall parent’s own gametes can be directly read off.
• If all the offspring turn out tall, this means the unknown parent could only have contributed the T allele to every offspring — which is only possible if the unknown parent is homozygous (TT).
• If the offspring appear in a 1 tall : 1 dwarf ratio, this means the unknown parent must have contributed T to half its gametes and t to the other half — which is only possible if the unknown parent is heterozygous (Tt).

Conclusion: a test cross — crossing an individual of unknown but dominant phenotype with a homozygous recessive — reveals the unknown genotype: an all-dominant F₁ indicates the tested parent is homozygous, while a 1:1 ratio of dominant to recessive offspring indicates it is heterozygous.

Consider a single gene locus with a dominant allele A and a recessive allele a. The question specifies a homozygous female, but a homozygous individual can be homozygous in two possible ways — either homozygous dominant (AA) or homozygous recessive (aa) — so a complete answer must work out both possibilities using a Punnett square, crossed against the given heterozygous male (Aa).

Case 1 — Homozygous DOMINANT female (AA) × Heterozygous male (Aa):

The female (AA) can contribute only the A allele to every gamete, so every single offspring receives at least one A allele — resulting in genotypes 2 AA : 2 Aa. Since A is dominant, all four offspring (100%) display the dominant phenotype, with none showing the recessive trait.

Case 2 — Homozygous RECESSIVE female (aa) × Heterozygous male (Aa):

Here the female (aa) contributes only the a allele to every gamete, while the male contributes A to half his sperm and a to the other half — resulting in genotypes 2 Aa : 2 aa. This gives a phenotypic distribution of 2 dominant (Aa) : 2 recessive (aa), i.e. a 1 : 1 ratio.

This exercise nicely illustrates that the outcome of a cross depends entirely on the specific genotypes involved, not merely on the general description “homozygous × heterozygous” — which is precisely the logic exploited in a test cross (see the previous question).

Conclusion: when the homozygous female is dominant (AA), crossing her with a heterozygous male (Aa) gives 100% dominant-phenotype offspring; but when the homozygous female is recessive (aa), the same cross gives a 1 : 1 ratio of dominant to recessive offspring — showing that both possibilities must be considered for a complete answer.

This is a dihybrid cross involving two independently-assorting genes — stem height (T/t) and seed colour (Y/y). Rather than constructing a large 4×4 or 16-outcome Punnett square, the most efficient and reliable method is the product rule (forked-line / branching method): since the two genes assort completely independently of each other (Law of Independent Assortment), we can analyse each gene separately as its own simple monohybrid cross, obtain the probability of each individual trait, and then multiply the probabilities together to get the combined probability.

Step 1 — Analyse the height gene alone: Tt (tall) × Tt (tall) is a standard monohybrid cross, giving the familiar ratio 3 tall : 1 dwarf. So, P(tall) = 3/4 and P(dwarf) = 1/4.

Step 2 — Analyse the seed-colour gene alone: Yy (yellow) × yy (green) is a test-cross-type monohybrid cross, giving the ratio 1 yellow : 1 green. So, P(yellow) = 1/2 and P(green) = 1/2.

Step 3 — Multiply the relevant probabilities for each required combination:

For completeness, the full phenotypic distribution of this cross across all four possible combinations works out to: tall-yellow = 3/8, tall-green = 3/8, dwarf-yellow = 1/8, dwarf-green = 1/8 — i.e., an overall ratio of tall-yellow : tall-green : dwarf-yellow : dwarf-green = 3 : 3 : 1 : 1.

Conclusion: using the product rule, (a) the proportion of tall and green offspring is 3/4 × 1/2 = 3/8, and (b) the proportion of dwarf and green offspring is 1/4 × 1/2 = 1/8.

To answer this properly, we must first contrast the unlinked (independently assorting) case with the linked case, since the whole point of the question is to show how linkage changes the expected outcome.

If the two genes were unlinked (located on different chromosome pairs, or far apart on the same chromosome), a dihybrid cross between two double heterozygotes (RrYy × RrYy) would obey the Law of Independent Assortment and produce the classical Mendelian ratio of 9 : 3 : 3 : 1 in the offspring — this is the “default” expectation taught with Mendel’s dihybrid cross.

However, if the two loci are linked — meaning they are located physically close together on the same chromosome — the outcome is markedly different, for the following reasons:

1. The alleles do not assort independently: because the two genes are physically joined on the same chromosome, they tend to travel together into the same gamete as a single linked unit, rather than sorting independently as Mendel’s second law assumes.
2. Offspring are predominantly of the parental (non-recombinant) types: since the linked alleles mostly stay together, the majority of offspring resemble the original combinations present in the parents, rather than showing the full range of new combinations expected under independent assortment.
3. The 9:3:3:1 ratio is NOT obtained — it deviates markedly: instead, the observed ratio is skewed heavily in favour of the two parental-type classes, with the two “new-combination” (recombinant) classes being far rarer than 3/16 each.
4. Recombinants do still arise, but only through crossing over: occasionally, during meiosis, crossing over between homologous chromosomes can separate linked alleles, producing a small proportion of recombinant-type gametes and hence recombinant-type offspring.
5. The tightness of linkage determines the recombination frequency: the closer together the two genes lie on the chromosome, the less likely a crossover event is to occur between them, and so the fewer recombinants are seen; genes lying farther apart show correspondingly higher recombination frequencies.

This exact phenomenon — genes not assorting independently, but instead showing predominantly parental combinations with a lower-than-expected frequency of new combinations — was precisely what T.H. Morgan observed in his classic studies of Drosophila melanogaster, and it directly led to the development of the concepts of linkage and recombination in genetics, later used by Morgan and Alfred Sturtevant to construct genetic (linkage) maps of chromosomes.

Conclusion: when the two loci are linked, independent assortment does not operate as it does for unlinked genes; the offspring are predominantly of the parental (non-recombinant) combinations, with only a smaller proportion of recombinant types arising from crossing over, so the expected 9:3:3:1 dihybrid ratio is not obtained.

Thomas Hunt Morgan is widely regarded as one of the founders of modern experimental genetics. He chose the fruit fly, Drosophila melanogaster, as his experimental organism, and this choice proved as pivotal to his success as Mendel’s choice of the pea plant had been decades earlier — Drosophila could be grown easily on simple synthetic medium in the laboratory, completed its entire life cycle in about two weeks, produced a large number of offspring from a single mating, showed clearly visible hereditary variations under simple observation, and had males and females that were easy to distinguish and separate. Morgan’s major, specific contributions to genetics were:

1. Experimental proof of the chromosomal theory of inheritance: by carefully studying the pattern of inheritance of sex-linked genes (such as the eye-colour gene) in Drosophila, Morgan provided the first direct experimental evidence that genes are physically carried on chromosomes, confirming what Sutton and Boveri had proposed on theoretical grounds in 1902.
2. Discovery of the phenomenon of linkage: Morgan found that genes located close together on the same chromosome do not assort independently as Mendel’s second law predicts, but instead tend to be inherited together as a group, because their physical proximity on the chromosome keeps them from being separated during normal meiosis.
3. Discovery of recombination: he also showed that new, non-parental combinations of linked genes can still arise occasionally, through the process of crossing over between homologous chromosomes during meiosis, and — crucially — that genes located farther apart on a chromosome show a correspondingly higher frequency of recombination than genes located close together.
4. Pioneering genetic (linkage) mapping: working together with his student Alfred Sturtevant, Morgan used the measured recombination frequencies between different gene pairs as a proxy for their physical distance apart, allowing them to construct the first genetic linkage maps showing the relative order and spacing of genes along a chromosome — a technique that remains foundational to genetics and genome mapping even today.

Conclusion: T.H. Morgan, working on Drosophila melanogaster, gave experimental proof of the chromosomal theory of inheritance and discovered the phenomena of linkage and recombination, which — together with his student Sturtevant — he then used to pioneer the construction of genetic (linkage) maps of chromosomes.

Pedigree analysis is defined as the systematic study of the inheritance of a particular trait, abnormality, or disease across several successive generations of a family, presented visually as a family tree (pedigree chart) that uses a standardised set of symbols to represent individuals, their sex, whether they are affected or unaffected carriers, and their relationships (marriages, parent–offspring links, etc.). This method exists specifically because, unlike with pea plants or fruit flies, controlled experimental crosses cannot be performed on human beings for obvious ethical reasons — so geneticists instead study the naturally-occurring pattern of a trait as it has already been passed down through a family’s real history.

Detailed uses and applications of pedigree analysis:

1. Tracing the inheritance pattern of a specific trait: by examining which family members over multiple generations show a trait, abnormality or disease, one can reconstruct exactly how it has been passed from parents to offspring.
2. Determining whether a trait is dominant or recessive: the characteristic pattern of appearance across generations (e.g., skipping a generation, or appearing in every generation) allows geneticists to deduce the mode of inheritance.
3. Determining whether a trait is autosomal or sex-linked: for example, if a disorder such as haemophilia (an X-linked recessive condition) is traced through a pedigree, it characteristically shows transmission from an unaffected carrier mother to roughly half her sons, a pattern quite distinct from that of an autosomal trait.
4. Predicting the probability of a disorder in future offspring: once the mode of inheritance is established from the family pedigree, geneticists can calculate the statistical risk that a couple’s future children will inherit a particular disorder — this is the scientific basis of genetic counselling provided to prospective parents.
5. Supporting the study and prevention of hereditary diseases: at a broader level, pedigree data collected across many families contributes to the scientific understanding of how particular genetic diseases behave in human populations, informing public-health and prevention strategies.

Conclusion: pedigree analysis is the study of a trait’s inheritance across a family tree using standard symbols, and it is useful because it reveals whether a trait is dominant or recessive, autosomal or sex-linked, and allows the probability of a disorder in future offspring to be predicted — forming the scientific foundation of genetic counselling.

In human beings, the sex of an individual is decided by a purely chromosomal (genetic) mechanism, specifically the XX/XY type of sex determination. A normal human somatic cell contains 46 chromosomes, arranged as 23 pairs: of these, 22 pairs are autosomes — identical in number and general appearance in both males and females — while the remaining one pair is the sex chromosomes, which differ between the sexes and are what actually determines an individual’s sex.

• The female has two identical sex chromosomes, XX, and is therefore described as homogametic — because all the ova she produces are genetically identical with respect to the sex chromosome, every single one carrying an X chromosome.
• The male has two different sex chromosomes, XY, and is therefore described as heterogametic — he produces two distinct types of sperm in exactly equal numbers: 50% of sperm carry an X chromosome and the other 50% carry a Y chromosome.

The determining event happens at fertilisation:

• If an X-bearing sperm happens to fertilise the ovum (which always carries an X), the resulting zygote has the combination XX, and develops into a female (girl) child.
• If a Y-bearing sperm happens to fertilise the ovum instead, the resulting zygote has the combination XY, and develops into a male (boy) child.

Since the two types of sperm are produced by the father in exactly equal numbers, there is always a 50 : 50 (1 : 1) statistical probability of conceiving a boy or a girl in any given pregnancy. This chromosomal logic also settles a matter of considerable social importance: because the mother’s ovum always contributes an X, and it is entirely the random chance of which type of sperm from the father fertilises the egg that decides the outcome, it is scientifically accurate to say that the sex of the child is determined by the father, and never by the mother — a fact of direct relevance to countering the unscientific social practice of blaming women for the birth of daughters.

Conclusion: human sex determination follows the XX/XY chromosomal mechanism: the homogametic mother always contributes an X, while the heterogametic father’s sperm — 50% X-bearing and 50% Y-bearing — decides whether the zygote is XX (girl) or XY (boy), giving an equal 1:1 chance of either sex and confirming that the father, not the mother, determines the child’s sex.

The ABO blood group system in humans is a classic example of multiple alleles combined with co-dominance. The single gene I (for isoagglutinogen) has three allelic forms: I^A, I^B, and i. Both I^A and I^B are completely dominant over i, but when I^A and I^B occur together in the same individual, neither masks the other — instead, both express themselves simultaneously, a relationship called co-dominance.

Working out the parental genotypes, step by step:

• The child’s blood group is stated to be O. Blood group O has only one possible genotype: ii (since both alleles must be the recessive i for the O phenotype to appear — if even one I^A or I^B allele were present, the child would show group A, B or AB instead).
• Since the child’s genotype is ii, the child must have inherited exactly one i allele from each parent — one from the father and one from the mother.
• The father shows blood group A, which can genetically be either I^AI^A or I^Ai — but since we’ve established he must carry an i allele (to pass to the child), his genotype must specifically be I^Ai (he is a “carrier” of the recessive i allele despite showing the dominant A phenotype).
• Similarly, the mother shows blood group B, and by the same logic — she too must carry an i to pass to the child — her genotype must be I^Bi.

Working out the possible genotypes of other offspring — Cross: I^Ai (father) × I^Bi (mother):

This confirms that from parents of blood group A (genotype I^Ai) and blood group B (genotype I^Bi), all four possible blood groups — AB, A, B and O — can appear among their children, each with an equal 1 : 1 : 1 : 1 probability, which also explains why the observed child with blood group O is a perfectly normal and expected outcome of this particular parental cross.

Conclusion: working backward from the child’s blood group O (genotype ii), the father must be I^Ai and the mother I^Bi; their cross can produce children of blood group AB, A, B or O in an equal 1:1:1:1 ratio, so the observed group-O child is fully consistent with these parental genotypes.

Both of these terms describe important exceptions to Mendel’s simple Law of Dominance, in which a heterozygote does not behave like a simple dominant homozygote — yet the two phenomena are distinct from each other and are commonly confused.

(a) Co-dominance

• Definition: co-dominance is the phenomenon in which both alleles of a heterozygous pair express themselves fully, simultaneously and independently in the phenotype of the heterozygote, so that neither allele masks the other, and the resulting F₁ phenotype shows the characteristics of both parents together (rather than a blend, as in incomplete dominance).
• Example — the ABO blood group system in humans: when the two alleles I^A and I^B are inherited together (genotype I^AI^B), the red blood cell membrane displays both the A-type sugar antigen and the B-type sugar antigen simultaneously, producing the distinct blood group AB. Since both antigens are fully and separately expressed — with neither being suppressed by the other — this is a textbook case of co-dominance.

(b) Incomplete dominance

• Definition: incomplete dominance is the phenomenon in which neither allele of a heterozygous pair is completely dominant over the other, so that the heterozygote displays a phenotype that is intermediate (a blend) between the two homozygous parental phenotypes — effectively a “new,” third phenotype not seen in either pure-breeding parent.
• Example — flower colour in the dog flower (Antirrhinum majus) and the four-o’clock plant (Mirabilis jalapa): when a pure-breeding red-flowered plant (RR) is crossed with a pure-breeding white-flowered plant (rr), the resulting F₁ generation is not red, nor white, but uniformly pink (Rr) — a clear intermediate blend of the two parental colours. When these pink F₁ plants are self-pollinated, the F₂ generation segregates into 1 red (RR) : 2 pink (Rr) : 1 white (rr) — and notably, in incomplete dominance, the phenotypic ratio and the genotypic ratio come out identical (1:2:1), because each genotype now corresponds to its own visibly distinct phenotype (unlike in simple dominance, where TT and Tt look identical).

Conclusion: co-dominance and incomplete dominance both deviate from Mendel’s simple dominance rule: in co-dominance both alleles are fully and separately expressed together (as in AB blood group), whereas in incomplete dominance the heterozygote shows a blended, intermediate phenotype resembling neither parent exactly (as in pink Antirrhinum flowers).

Point mutation is defined as a type of gene-level mutation that results from a change involving only a single base pair (nucleotide pair) in the DNA sequence — this could be the substitution of one base for another at a particular position along the DNA molecule. Although this may sound like an extremely small, almost trivial change, because the genetic code is read in fixed triplets (codons), altering even one base within a codon can change which amino acid is specified at that position in the resulting protein — and depending on which amino acid is affected and where, this can range from having no noticeable effect at all to causing a serious, even life-altering, disease.

Classic example — Sickle-cell anaemia:

• This well-known genetic disorder arises from a single base substitution in the gene coding for the β-globin chain of the haemoglobin molecule — the DNA codon GAG changes to GUG at the position corresponding to the sixth codon of the gene.
• As a direct consequence of this single-base change, the amino acid glutamic acid (Glu), which is normally present at the sixth position of the β-globin polypeptide chain, is replaced by the amino acid valine (Val).
• This seemingly minor substitution of just one amino acid out of the hundreds making up the haemoglobin molecule has a dramatic structural consequence: under conditions of low oxygen tension (such as in tissues after exercise, or at high altitude), the mutant haemoglobin molecules (called HbS) tend to polymerise into long fibres inside the red blood cell, physically distorting the cell’s shape from its normal flexible biconcave disc into a rigid, elongated, sickle-like (crescent) shape.
• These sickle-shaped cells are fragile, are destroyed more rapidly than normal cells (causing anaemia), and can physically block small blood vessels, causing painful and dangerous complications throughout the body.

Conclusion: point mutation is a change involving a single base pair of DNA, and sickle-cell anaemia — caused by the substitution GAG→GUG changing glutamic acid to valine at the sixth position of the β-globin chain — is the textbook example of its serious phenotypic consequences.

The chromosomal theory of inheritance — the foundational idea that Mendel’s abstract “factors” of heredity are physically located on chromosomes — was proposed independently and almost simultaneously by two scientists, Walter Sutton and Theodore Boveri, in 1902, working separately on different organisms but arriving at strikingly similar conclusions.

The basis of their reasoning was a careful comparison between two sets of observations that had, until then, been studied by two different groups of scientists working in isolation from each other: cytologists, who had been microscopically observing the behaviour of chromosomes during cell division (mitosis and meiosis), and geneticists working in the Mendelian tradition, who had been studying the mathematical patterns of inheritance of traits across generations. Sutton and Boveri noticed a remarkable parallel between the two:

• Chromosomes exist in pairs in diploid cells, just as Mendel’s factors (genes) exist in pairs.
• During meiosis, the two members of each chromosome pair segregate (separate) from each other and go into different gametes — exactly paralleling Mendel’s Law of Segregation, under which the two alleles of a gene pair separate during gamete formation.
• Different chromosome pairs assort independently of one another during meiosis — exactly paralleling Mendel’s Law of Independent Assortment.
• Fertilisation restores the paired condition of chromosomes in the zygote, just as it restores the paired condition of Mendelian factors.

From this exact, point-by-point parallel behaviour between chromosomes and Mendelian factors, Sutton and Boveri concluded that Mendel’s hereditary factors (genes) must be physically located on chromosomes, and that chromosomes are therefore the physical carriers of hereditary information passed from parents to offspring.

It is worth noting that at the time this was proposed, it remained a largely theoretical/observational inference based on parallel behaviour, rather than direct experimental proof. That crucial experimental proof came later, when Thomas Hunt Morgan demonstrated the chromosomal basis of inheritance directly, through his studies of sex-linked gene inheritance in Drosophila melanogaster.

Conclusion: the chromosomal theory of inheritance — that genes are located on chromosomes — was proposed independently by Walter Sutton and Theodore Boveri in 1902, based on the exact parallel between the behaviour of chromosomes during meiosis and the behaviour of Mendel’s hereditary factors, and it was later experimentally confirmed by T.H. Morgan.

Autosomal genetic disorders are hereditary disorders caused by defective genes or abnormal chromosomes located on the autosomes — the 22 pairs of chromosomes common to both sexes — as opposed to disorders linked to the sex chromosomes (X or Y). These autosomal disorders can be further sub-classified as either Mendelian (single-gene) disorders or chromosomal disorders (involving an abnormal number or structure of whole chromosomes). Two well-documented examples of each type, with their causes and symptoms, are described below:

Two further autosomal recessive disorders worth mentioning for completeness are phenylketonuria — where the affected individual lacks the enzyme needed to convert the amino acid phenylalanine into tyrosine, causing toxic accumulation and resultant mental retardation — and thalassemia, in which mutation or deletion of the genes controlling globin-chain synthesis leads to a reduced production rate of either the α or β globin chains of haemoglobin, resulting in anaemia.

Conclusion: sickle-cell anaemia (an autosomal recessive, single-gene disorder causing sickle-shaped red blood cells and anaemia) and Down’s syndrome (an autosomal chromosomal disorder caused by trisomy of chromosome 21, causing distinctive physical features and developmental delay) are two well-known autosomal genetic disorders, with the symptoms described in detail above.