Unit 3: Chromosomal Variations and Mutations - Subjective Questions

Deletion is a type of structural chromosomal aberration in which a segment of a chromosome is lost, resulting in the absence of certain genes.

Types of Deletion:

• Terminal Deletion: Loss of a segment from the end (tip) of a chromosome. A single break occurs and the acentric fragment (lacking a centromere) is lost during cell division.
• Interstitial (Intercalary) Deletion: Loss of an internal segment. Two breaks occur within the chromosome, the intermediate segment is deleted, and the broken ends rejoin.

Phenotypic Consequences:

• Loss of genes may be lethal if essential genes are deleted, especially in the homozygous condition.
• Pseudodominance: A recessive allele on the intact homolog gets expressed because its dominant counterpart is deleted.
• Cri-du-chat syndrome: Caused by deletion of the short arm of chromosome 5 (5p−), characterized by a cat-like cry, microcephaly, and intellectual disability.
• Deletions disrupt gene balance and can affect development, viability, and fertility.

During meiosis, a deletion loop is formed when the normal homolog pairs with the deleted chromosome to allow maximum gene alignment.

Duplication is a structural aberration in which a segment of the chromosome is present in more than one copy (repeated), leading to an increase in gene dosage.

Types of Duplication:

• Tandem Duplication: The repeated segment lies adjacent to the original in the same order (e.g., ABCD → ABCBCD).
• Reverse (Inverted) Tandem Duplication: The repeated segment lies adjacent but in reverse orientation (e.g., ABCD → ABCCB D).
• Displaced Duplication: The duplicated segment is located away from the original, either on the same chromosome or a different position.

Evolutionary Significance:

• Duplication provides raw material for evolution; the extra copy of a gene is free to mutate and acquire new functions without harming the organism (gene divergence).
• Origin of gene families, e.g., hemoglobin genes, are believed to have arisen by duplication and divergence.
• Bar eye phenotype in Drosophila is a classic example resulting from tandem duplication in the X chromosome (region 16A). Increased dosage reduces the number of eye facets.
• Contributes to genome size increase and creation of novel genetic variability.

During meiosis, a duplication loop (buckle) is formed to allow proper pairing of homologous regions.

Inversion is a structural aberration in which a segment of a chromosome breaks, rotates through 180°, and rejoins, reversing the gene order without changing gene content.

Types of Inversion:

Meiotic Consequences:

• Heterozygotes form an inversion loop during pairing to align homologous regions.
• Crossing over within the loop leads to abnormal chromatids:
  • Paracentric: results in a dicentric bridge and an acentric fragment, both leading to inviable gametes.
  • Pericentric: results in chromatids with duplications and deletions.
• Inversions act as crossover suppressors because recombinant products are non-viable; hence balanced gene combinations are preserved.

Significance: Inversions maintain favourable gene complexes (supergenes) and play a role in speciation and evolution.

Translocation is a structural chromosomal aberration in which a segment of a chromosome is transferred to a non-homologous chromosome.

Types of Translocation:

• Simple (Non-reciprocal) Translocation: A segment from one chromosome is transferred to another without exchange in return.
• Reciprocal (Mutual) Translocation: Two non-homologous chromosomes exchange segments.
• Robertsonian Translocation (Centric Fusion): Two acrocentric chromosomes fuse at the centromere to form a single large chromosome; the small fragments are usually lost. Important in familial Down syndrome.

Genetic Consequences of Reciprocal Translocation:

• Heterozygotes form a cross-shaped (cruciform) configuration during meiotic pairing involving four chromosomes.
• Segregation patterns:
  • Alternate segregation → produces balanced (viable) gametes.
  • Adjacent-1 and Adjacent-2 segregation → produces unbalanced gametes with duplications and deletions, leading to semi-sterility.
• Causes position effect, where gene expression changes due to a new chromosomal location.
• May result in pseudolinkage, where genes on different chromosomes appear linked.

Clinical example: Chronic Myeloid Leukemia (CML) is associated with the Philadelphia chromosome, a reciprocal translocation between chromosomes 9 and 22, t(9;22).

Non-disjunction is the failure of homologous chromosomes (in Meiosis I) or sister chromatids (in Meiosis II or mitosis) to separate properly during cell division. This results in gametes with an abnormal number of chromosomes.

Mechanism:

• Meiosis I non-disjunction: Homologous chromosomes fail to separate → both members go to the same pole → gametes with and .
• Meiosis II non-disjunction: Sister chromatids fail to separate → produces some normal and some abnormal gametes.

Fusion of an abnormal gamete with a normal gamete leads to aneuploidy — a condition where the chromosome number differs from the normal diploid () by one or a few chromosomes.

Types of Aneuploidy:

• Nullisomy (): Loss of a complete pair of homologous chromosomes; usually lethal.
• Monosomy (): Loss of one chromosome (e.g., Turner syndrome, 45,X).
• Trisomy (): Presence of one extra chromosome (e.g., Down syndrome, Trisomy 21).
• Tetrasomy (): Two extra copies of a chromosome.
• Double Monosomy () and Double Trisomy (): Involving two different chromosome pairs.

Significance: Aneuploidy causes gene dosage imbalance leading to developmental disorders, reduced viability, and sterility.

Down syndrome is the most common autosomal aneuploidy, caused by the presence of an extra copy of chromosome 21.

Karyotype:

• Classic (95%): 47,XX,+21 or 47,XY,+21 (free trisomy due to non-disjunction).
• Translocation type (~4%): Robertsonian translocation, often t(14;21).
• Mosaic type (~1%): Some cells normal, some trisomic, due to mitotic non-disjunction.

Cause:

• Primarily non-disjunction of chromosome 21 during meiosis, most often in the mother.
• Risk increases sharply with advanced maternal age (> 35 years).

Clinical Features:

• Intellectual disability (variable IQ).
• Flat facial profile, upward slanting eyes with epicanthal folds.
• Protruding tongue, small ears, short neck.
• Single transverse palmar crease (simian crease).
• Hypotonia (poor muscle tone).
• Congenital heart defects (e.g., septal defects).
• Increased susceptibility to leukemia and early-onset Alzheimer's disease.
• Short stature and characteristic broad hands.

Life expectancy has improved with medical care, but affected individuals typically have reduced fertility.

Both Patau syndrome (Trisomy 13) and Edward syndrome (Trisomy 18) are autosomal trisomies caused by non-disjunction, and both are associated with severe abnormalities and very short lifespan.

Common Features:

• Both arise mainly due to maternal meiotic non-disjunction and correlate with advanced maternal age.
• Both cause profound intellectual disability and multiple congenital anomalies.
• Both have extremely poor survival rates compared to Trisomy 21.

Turner syndrome is a sex-chromosome monosomy affecting females, characterized by the presence of only one X chromosome.

Karyotype: 45,X (i.e., ; total 45 chromosomes with a single X and no second sex chromosome). Mosaic forms (45,X/46,XX) also exist.

Cause:

• Non-disjunction of sex chromosomes during gametogenesis (in either parent).
• Loss of one sex chromosome during early mitotic divisions (leading to mosaicism).

Clinical Features:

• Phenotypically female but with underdeveloped secondary sexual characters.
• Short stature (characteristic feature).
• Webbed neck and low posterior hairline.
• Broad shield-like chest with widely spaced nipples.
• Rudimentary/streak ovaries → underdeveloped gonads.
• Primary amenorrhoea (absence of menstruation).
• Coarctation of the aorta and other cardiac defects.
• Normal or near-normal intelligence, but may have spatial learning difficulties.

Reason for Sterility:

• The ovaries are rudimentary (streak gonads) and lack functional germ cells, so no ova are produced and the individual cannot reproduce. Two X chromosomes are required for normal ovarian development; a single X leads to accelerated loss of oocytes.

Klinefelter syndrome is a sex-chromosome trisomy affecting males, characterized by the presence of an extra X chromosome.

Karyotype: 47,XXY (most common). Variants include 48,XXXY and 49,XXXXY with more severe features.

Cause:

• Non-disjunction of sex chromosomes during meiosis in either parent (maternal or paternal).
• Correlated with advanced maternal age in some cases.

Phenotypic Features:

• Phenotypically male but with feminized characteristics.
• Tall stature with long limbs.
• Small, underdeveloped testes and reduced testosterone.
• Gynaecomastia (breast development).
• Sparse body hair and reduced facial hair.
• Sterility due to defective spermatogenesis (azoospermia).
• Mild learning difficulties may be present.

Barr Bodies:

• A Barr body is the condensed, inactivated X chromosome seen in the interphase nucleus.
• Number of Barr bodies = (Number of X chromosomes − 1).
• In a normal female (XX): 1 Barr body; normal male (XY): 0.
• In Klinefelter (XXY): 1 Barr body is present (unusual for a male), which is diagnostic.
• In 48,XXXY: 2 Barr bodies are observed.

Super Females (Triple X Syndrome / Metafemales):

• Karyotype: 47,XXX — presence of an extra X chromosome in females.
• Cause: Non-disjunction of X chromosomes during meiosis.
• Phenotypic Features:
  • Individuals are phenotypically normal females, often fertile.
  • May show tall stature, mild learning difficulties, and delayed development in some cases.
  • Menstrual irregularities and reduced fertility possible in some.
• Barr Bodies: Two Barr bodies present (X − 1 = 3 − 1 = 2).
• The term "super female" is a misnomer, as these individuals are usually normal rather than "super."

Lyon Hypothesis (X-Chromosome Inactivation):

Proposed by Mary Lyon (1961) to explain dosage compensation in mammals.

Main Points:

• In females, only one X chromosome remains active; the other X is randomly inactivated and condenses into a Barr body.
• Inactivation is random (either maternal or paternal X) and occurs early in embryonic development.
• Once an X is inactivated in a cell, all daughter cells inherit the same inactivation (clonal).
• This ensures equal gene expression (dosage compensation) between males (XY) and females (XX).

Example: The mosaic coat colour of calico/tortoiseshell cats results from random X-inactivation of coat-colour genes.

Polyploidy is the condition in which an organism possesses more than two complete sets of chromosomes (e.g., , , ). It is a type of euploidy involving whole genome multiplication.

Types based on origin:

Autopolyploidy:

• Multiple sets of the same genome.
• Produces larger cells and organs (gigas effect).
• Example: Triploid (3n) seedless watermelons and bananas.

Allopolyploidy (Amphidiploidy):

• Arises from interspecific hybridization followed by chromosome doubling to restore fertility.
• Example: Bread wheat (hexaploid, 6n = 42) and Raphanobrassica (radish × cabbage hybrid).

Significance: Polyploidy is a major mechanism of plant speciation and crop improvement.

Polyploidy in Plants:

Polyploidy is widespread in the plant kingdom and has played a major role in plant evolution and agriculture. It is estimated that 30–70% of angiosperms are polyploid.

Occurrence:

• Arises through somatic doubling, non-reduction of gametes, or hybridization + chromosome doubling.
• Can be induced artificially using colchicine, which inhibits spindle formation, doubling the chromosome number.

Significance in Plants:

• Gigas effect: Larger flowers, fruits, seeds, and vegetative parts.
• Seedless fruits: Triploid () plants like bananas and watermelons produce sterile, seedless fruits.
• Increased vigour and yield in many crops (e.g., wheat, cotton, sugarcane, potato).
• Restoration of fertility in interspecific hybrids (allopolyploids).
• Source of new species and genetic variability for evolution and breeding.

Why Common in Plants but Rare in Animals:

• Hermaphroditism and self-fertilization in plants allow polyploids to reproduce successfully.
• Asexual/vegetative reproduction lets plants bypass sterility problems.
• In animals, separate sexes and the disruption of the sex-determination balance (X:A ratio) by polyploidy cause sterility or lethality.
• Animals depend more on precise gene dosage balance, which polyploidy disrupts.
• Polyploidy interferes with animal development and delicate developmental regulation.

Thus, plant reproductive flexibility favours the establishment of polyploidy, unlike in animals.

Polyploidy in Animals:

Polyploidy is rare in animals compared to plants because it interferes with the delicate mechanisms of sex determination and developmental balance. However, it does occur in certain groups.

Reasons for Rarity:

• Most animals have separate sexes with a chromosomal sex-determination system (e.g., X:A balance). Polyploidy disrupts this ratio, leading to sterility or intersex conditions.
• Animals lack the reproductive flexibility (self-fertilization, vegetative propagation) that plants possess.
• Precise gene dosage requirements during development make animals sensitive to genome multiplication.

Conditions Under Which It Is Tolerated:

• Parthenogenesis (asexual reproduction): Species reproducing without fertilization can bypass sex-determination problems. Example: some lizards (Cnemidophorus), insects, and flatworms.
• Hermaphroditism: In earthworms and some flatworms.
• Absence of a rigid sex-chromosome mechanism: In organisms where sex is determined environmentally or where balance is less disturbed.

Examples in Animals:

• Certain fishes (salmonids, goldfish), amphibians (frogs like Xenopus, salamanders).
• Some reptiles and insects.
• Polyploid tissues occur naturally in specific organs (e.g., liver cells, salivary gland cells of Drosophila) as endopolyploidy, without whole-organism polyploidy.

Conclusion: True organismal polyploidy in animals is largely restricted to those with unisexual/parthenogenetic reproduction or lacking strict sex-chromosome-based determination.

Mutation is a sudden, heritable change in the genetic material (DNA) of an organism that is not due to segregation or genetic recombination. The term was coined by Hugo de Vries (1901).

Types of Mutations:

1. Based on the level (extent of change):

• Gene (Point) Mutations: Changes within a single gene at the nucleotide level (e.g., substitution, insertion, deletion).
• Chromosomal Mutations: Changes in chromosome structure (deletion, duplication, inversion, translocation) or number (aneuploidy, polyploidy).

2. Based on the type of cell affected:

• Somatic Mutations: Occur in body cells; not inherited by offspring.
• Germinal Mutations: Occur in gametes; heritable to the next generation.

3. Based on the direction:

• Forward Mutation: Wild-type → mutant.
• Reverse (Back) Mutation: Mutant → wild-type.

4. Based on the cause/origin:

• Spontaneous Mutations: Occur naturally without external agents (e.g., tautomeric shifts, replication errors).
• Induced Mutations: Caused by external agents (mutagens) like radiation and chemicals.

5. Based on effect on function:

• Loss-of-function, gain-of-function, lethal, conditional, and nutritional (auxotrophic) mutations.

Mutations are the ultimate source of all genetic variation and raw material for evolution.

Point mutations are changes involving a single or a few nucleotides in the DNA sequence.

1. Base Substitution Mutations:

• Transition: Purine → Purine () or Pyrimidine → Pyrimidine ().
• Transversion: Purine ↔ Pyrimidine (e.g., ).

Effects of Substitution on Codons:

• Silent (Synonymous) Mutation: Codon changes but codes for the same amino acid (due to degeneracy of the genetic code); no effect on protein.
• Missense Mutation: Codon changes to code for a different amino acid (e.g., sickle-cell anemia: GAG → GTG, Glu → Val).
• Nonsense Mutation: Codon changes into a stop codon (e.g., UAA, UAG, UGA) → premature termination → truncated, non-functional protein.

2. Frameshift Mutations:

• Caused by insertion or deletion of nucleotides (not in multiples of three).
• Shifts the reading frame during translation.
• All codons downstream of the mutation are altered → completely different amino acid sequence and often a premature stop codon.
• Generally produce non-functional proteins with severe effects.

Summary of Consequences:

Thus, the severity depends on the type and position of the mutation.

Spontaneous mutations arise naturally due to inherent errors in DNA metabolism and chemical instability of bases, without external mutagens.

1. Tautomeric Shifts:

• DNA bases exist in alternative chemical forms called tautomers (keto ↔ enol, amino ↔ imino).
• The rare tautomeric forms have altered base-pairing properties.
• Example: Enol form of thymine pairs with guanine instead of adenine, causing a mispairing that becomes a mutation after replication.
• Leads to transition mutations.

2. Depurination:

• The spontaneous loss of a purine base (A or G) due to breakage of the glycosidic bond between the base and sugar.
• Creates an apurinic (AP) site.
• During replication, a random base may be inserted opposite the gap, causing mutation.

3. Deamination:

• Loss of an amino group from a base.
• Cytosine → Uracil: Uracil pairs with adenine, causing a transition after replication.
• 5-Methylcytosine → Thymine: Creates mutational hotspots in the genome.
• Adenine → Hypoxanthine and Guanine → Xanthine can also occur.

4. Replication Errors:

• DNA polymerase occasionally inserts wrong nucleotides.
• Slippage during replication of repetitive sequences causes insertions/deletions.

5. Oxidative Damage:

• Reactive oxygen species produce 8-oxoguanine, which mispairs with adenine.

Most of these errors are corrected by DNA repair mechanisms (proofreading, mismatch repair); those that escape become permanent mutations.

Radiation-induced mutations are caused by exposure to different types of radiation, which damage DNA either directly or indirectly.

1. Ionizing Radiation:

• Includes X-rays, gamma rays, cosmic rays, and particles like alpha and beta particles, neutrons.
• High energy that ejects electrons from atoms, producing ions and free radicals.

Mechanism:

• Direct effect: Directly breaks the sugar-phosphate backbone → single-strand and double-strand breaks.
• Indirect effect: Ionizes water to form reactive free radicals (e.g., ) that damage bases and DNA strands.
• Causes chromosomal aberrations (deletions, translocations, inversions) and gene mutations.

2. Non-Ionizing Radiation:

• Mainly Ultraviolet (UV) light (most effective at 260 nm, absorbed by DNA).
• Lower energy; does not produce ions but excites molecules.

Mechanism:

• Induces formation of pyrimidine (thymine) dimers — covalent bonds between adjacent pyrimidines on the same strand.
• Dimers distort the DNA helix, blocking replication and transcription.
• Repaired by photoreactivation (photolyase) and excision repair; errors during repair cause mutations.

Comparison:

Significance: H.J. Muller demonstrated X-ray-induced mutations in Drosophila, earning a Nobel Prize.

Chemical mutagens are chemical substances that induce mutations by altering the structure or pairing behaviour of DNA bases.

1. Base Analogues:

• Structurally similar to normal bases; incorporated into DNA during replication.
• 5-Bromouracil (5-BU): An analogue of thymine that pairs with guanine in its enol form → causes transitions.
• 2-Aminopurine (2-AP): Analogue of adenine; can pair with cytosine.

2. Base-Modifying Agents (Chemical Modifiers):

• Nitrous acid (): Causes oxidative deamination — converts cytosine → uracil, adenine → hypoxanthine, leading to transitions.
• Hydroxylamine: Reacts specifically with cytosine → causes transitions.
• Alkylating agents (e.g., EMS – Ethyl methane sulfonate, mustard gas): Add alkyl groups to bases; alkylated guanine mispairs with thymine → transitions; may also cause depurination.

3. Intercalating Agents:

• Flat molecules that insert between adjacent base pairs of DNA, distorting the helix.
• Acridine dyes, ethidium bromide, proflavin: Cause insertions or deletions of single nucleotides during replication → frameshift mutations.

Summary Table:

Chemical mutagens are widely used in experimental mutagenesis and crop improvement.

Phenotypic Effects of Mutations:

Mutations may affect the phenotype in various ways, ranging from no effect to lethality.

1. Morphological Mutations:

• Alter the external appearance (form, colour, size).
• Example: White eye colour in Drosophila, dwarfism in plants.

2. Lethal Mutations:

• Cause death of the organism.
• May be dominant lethal or recessive lethal.
• Example: Yellow lethal gene in mice (homozygous lethal).

3. Conditional Mutations:

• Expressed only under certain conditions (e.g., temperature).
• Temperature-sensitive mutants function normally at permissive temperature but not at restrictive temperature.

4. Biochemical (Nutritional) Mutations:

• Affect metabolic pathways; the organism loses the ability to synthesize an essential compound.
• Example: Auxotrophic mutants of Neurospora (require supplements to grow).

5. Loss-of-function and Gain-of-function Mutations:

• Loss-of-function: Reduced or absent gene product (often recessive).
• Gain-of-function: New or enhanced activity (often dominant).

6. Beneficial, Neutral, and Harmful Mutations:

• Beneficial: Confer advantage (e.g., antibiotic resistance in bacteria).
• Neutral: No effect on fitness (e.g., silent mutations).
• Harmful: Reduce fitness (most mutations).

Human Examples:

• Sickle-cell anemia (missense mutation, HbS).
• Phenylketonuria (PKU) – enzyme defect.

Role in Evolution:

• Mutations are the ultimate source of new alleles and genetic variation.
• Natural selection acts on this variation → adaptation and evolution.
• Without mutation, there would be no raw material for evolutionary change.

Thus, though most mutations are harmful, they are essential for the origin of biological diversity.

Chromosomal variations are broadly classified into structural aberrations (changes in chromosome structure) and numerical aberrations (changes in chromosome number).

A. Structural Variations

Arise due to breakage and abnormal rejoining of chromosome segments.

1. Deletion: Loss of a chromosome segment.

• Consequence: Loss of genes, pseudodominance; e.g., Cri-du-chat syndrome (5p−).

2. Duplication: Presence of an extra copy of a segment.

• Consequence: Gene dosage imbalance, raw material for evolution; e.g., Bar eye in Drosophila.

3. Inversion: A segment reverses its orientation by 180°.

• Paracentric (excludes centromere) and Pericentric (includes centromere).
• Consequence: Acts as crossover suppressor; maintains gene combinations.

4. Translocation: Exchange of segments between non-homologous chromosomes.

• Consequence: Semi-sterility, position effect; e.g., Philadelphia chromosome t(9;22) in CML.

B. Numerical Variations

Arise mainly due to non-disjunction or errors in cell division.

1. Aneuploidy ( few chromosomes):

• Monosomy (): Turner syndrome (45,X).
• Trisomy (): Down (21), Edward (18), Patau (13), Klinefelter (XXY).
• Nullisomy (): Usually lethal.

2. Euploidy (Change in complete sets):

• Haploidy (), Triploidy (), Tetraploidy ().
• Autopolyploidy (same genome) and Allopolyploidy (different genomes, e.g., wheat).

Comparison Table

Genetic Significance

• Provide variation for evolution and speciation.
• Structural changes affect gene order and linkage.
• Numerical changes cause genetic disorders in animals but are important for crop improvement in plants.

Thus, chromosomal variations, though often deleterious, are vital drivers of genetic diversity and evolution.