Biotechnology: Class 11 — Chapter 8: Genetic Disorder

8.1 Chromosomal Abnormalities and Syndromes

Introduction to Chromosomal Aberrations

Chromosomes can suffer structural damage or numerical changes due to environmental radiation, food intake, or internal genetic conditions. These changes often lead to significant phenotypic alterations, diseases, or syndromes.

Structural Chromosomal Abnormality: A change in the physical structure of the chromosome (aberration).
Numerical Chromosomal Abnormality: A change in the total number of chromosomes.
Aneuploidy: A condition where one or a few chromosomes are absent or present in extra copies. This includes Monosomy ($2n-1$, e.g., monosomy of chromosome 1) and Trisomy ($2n+1$, e.g., trisomy of chromosome X).
Polyploidy: A condition where the entire set of chromosomes is multiplied (e.g., triploid $3n$, tetraploid $4n$, hexaploid $6n$, octoploid $8n$). Highly common in plants and artificial breeding (e.g., hexaploid bread wheat, triploid banana/apple, octoploid strawberry/sugar cane).

Structural Chromosomal Abnormalities

Deletion: A segment of a chromosome breaks away, leading to shortening. Example: Retinoblastoma (deletion of a portion of chromosome 13). Deletion of both ends can lead to the formation of a ring chromosome.
Duplication: A segment of the chromosome is repeated, resulting in a longer chromosome. Example: Charcot-Marie-Tooth disease (duplication of genes on chromosome 17).
Inversion: A segment breaks away, completely reverses by 180 degrees, and reattaches. The overall length remains the same, but the gene orientation is reversed. Example: RCAD syndrome (inversion of a segment of chromosome 17).
Translocation: A segment breaks away and reattaches to another chromosome. Includes reciprocal translocation (mutual exchange of segments, e.g., Burkitt's lymphoma between chromosomes 8 and 14) and Robertsonian translocation (one-way attachment without mutual exchange, decreasing the cell's chromosome count).

Numerical Chromosomal Syndromes Matrix

Syndrome	Incidence	Chromosomal Basis & Genotype	Clinical Symptoms	Diagnosis & Treatment
Down's Syndrome	~1 per 800 live births	Trisomy 21 ($47, XX, +21$ in females; $47, XY, +21$ in males). Caused by non-disjunction (failure of chromosomes to separate during cell division). Risk increases with maternal age (85% born to mothers over 35).	Flat face, slanting eyes, small mouth, protruding tongue, flattened nose, short neck, short limbs, single deep palmar crease, low IQ, stunted growth, muscular hypotonia, underdeveloped gonads, breathing/heart/hearing issues.	Diagnosed via karyotype showing extra chromosome 21. No standard cure; managed with speech therapy, physiotherapy, and nutritional supplements. Life expectancy has increased to 60+ years.
Klinefelter's Syndrome	~1 in 1000 newborn males	Genotype $47, XXY$. Affects males. Arises from non-disjunction of X chromosome during meiosis. Not genetically transmitted from father.	Unusually tall, reduced facial/body hair, smaller testes, gynecomastia (enlarged breasts), coarse voice, osteoporosis, feminine abdomen, small penis, depression/aggression.	Diagnosed via Barr body test of buccal smear (shows one Barr body, normal males have zero). Treated with testosterone therapy and psychological counselling.
Turner's Syndrome	1 in 2,500 newborn girls	Monosomy X ($45, X$). Affects females. Arises from meiotic error producing an ovum with no X chromosome, which fuses with an X-bearing sperm. Not inherited.	Short stature, webbed neck (loose neck skin), small breasts, low-set ears, swollen hands/feet, underdeveloped ovaries, absent menstrual periods (amenorrhea).	Prenatal diagnosis via amniocentesis or chorionic villus sampling. At puberty, diagnosed by absence of Barr body in buccal smear. Managed with controlled administration of androgen and estrogen. No permanent cure.

Famous People with Syndromes (Box 1)

Isabelle Springmühl: Famous fashion designer with Down syndrome; showcased talent in London, Rome, and Mexico.
George Washington: Commonly believed to have had Klinefelter Syndrome; 6'2" tall, had no biological children.
Lauren Foster: South African model born with XXY Klinefelter condition; transitioned to female phenotype, appeared in Vogue.
Linda Hunt: Hollywood actress diagnosed with Turner syndrome; won 13 awards including an Oscar.
Dr. Catherine Ward Melver: Medical genetics doctor with Turner syndrome; adopted a daughter with the same condition.

8.2 Monogenic Disorders and Pedigree Mapping

Introduction to Monogenic Diseases

Monogenic diseases are caused by an error or mutation in a single gene, inherited according to Mendel's Laws. Over 10,000 human diseases are estimated to be monogenic.

Pedigree Analysis Symbols

Pedigree analysis is the interpretation of family tree information to diagnose inherited genetic diseases.

Square: Male
Circle: Female
Filled Square/Circle: Affected individual
Horizontal line connecting male and female: Mating
Suspended branch: Offspring in birth order (generations labeled I, II; offspring numbered II-1, II-2)
Diverging lines from a single point: Identical twins
Diverging lines with a crossbar or separate origins: Non-identical twins

Classification of Monogenic Disorders

Inheritance Category	Genetic Mechanism	Key Examples & Pathophysiology
Autosomal Recessive	Requires two copies of the mutated gene (homozygosity) to manifest. Heterozygotes are unaffected carriers.	Sickle Cell Anemia: Mutation in hemoglobin-$eta$ gene on chromosome 11. Defective Hb molecules cluster into rod-like structures under low oxygen, causing red blood cells to assume a rigid, sickle shape. Common in sub-Saharan Africa, India (Deccan plateau, Kerala, Tamil Nadu), etc. Genotype $s/s$ is affected; $S/s$ is carrier; $S/S$ is normal. Cystic Fibrosis: Produces abnormally thick, sticky mucus that damages organs, especially lungs, leading to chronic infections. Tay-Sachs Disease: Absence of enzyme hexosaminidase A, leading to fatty substance accumulation in nerve cells, affecting the brain. Fatal in childhood. High incidence in European Ashkenazi Jewish populations. Phenylketonuria: Mutation in phenylalanine hydroxylase gene, causing toxic accumulation of phenylalanine in the blood.
Autosomal Dominant	Only one copy of the abnormal allele is needed to manifest the disease. Normal individuals are homozygous recessive ($d/d$).	Achondroplasia: A form of dwarfism. Genotype $D/d$ represents mild disease (surviving heterozygotes); $D/D$ is severely affected and often lethal; $d/d$ is normal. Huntington's Disease: A rare autosomal dominant disorder affecting the nervous system.
X-linked Recessive	Mutated gene is on the X chromosome. Females ($XX$) are typically carriers if they have one mutated copy; males ($XY$) are affected because they have only one X chromosome. Affected fathers pass the gene to all daughters (carriers) but no sons.	Haemophilia: Bleeding disorder caused by mutation in coagulation factor VIII gene (Type A) or factor IX gene (Type B), preventing effective blood clotting and causing spontaneous bleeding. Duchenne Muscular Dystrophy (DMD): Mutation in the dystrophin gene, leading to reduction/absence of dystrophin protein, causing muscle degeneration and fragility.
X-linked Dominant	Affected males pass the mutated dominant gene to all daughters but no sons. Affected females pass it to half of their sons and daughters.	Hypophosphatemia: A type of vitamin D resistant rickets. Alport Syndrome: Associated with progressive hearing loss and kidney disease.

8.3 Polygenic Disorders

Characteristics of Polygenic Disorders

Polygenic disorders are caused by the combined action or defects of multiple genes. They depend on the simultaneous association of several genes and environmental factors, and cannot be explained by simple Mendelian monogenic inheritance.

Major Polygenic Disorders

Diabetes Mellitus: A heterogeneous group of disorders characterized by persistent high blood sugar (hyperglycemia).
- Type 1 Diabetes (T1D): Insulin-dependent diabetes (IDDM). Caused by immunological destruction of pancreatic beta cells. Accounts for ~10% of cases. Requires lifelong insulin injections.
- Type 2 Diabetes (T2D): Non-insulin-dependent diabetes (NIDDM). Commonest form (~90% of cases). Caused by impaired insulin secretion and peripheral insulin resistance.
Hypertension: Persistent increase in blood pressure. Major risk factor for renal failure, heart disease, and stroke.
- Normal BP: $<120$ (systolic) / $<80$ (diastolic) mm Hg
- Elevated BP: 120-129 / $<80$ mm Hg
- Hypertension Stage 1: 130-139 or 80-89 mm Hg
- Hypertension Stage 2: $\ge 140$ or $\ge 90$ mm Hg
Coronary Heart Disease (CHD): Narrowing of the coronary artery due to gradual build-up of fatty material (plaque) on its wall, a condition called atherosclerosis. This reduces oxygen-rich blood supply to heart muscles, causing ischemia (formerly called ischaemic heart disease).

Mitochondrial Inheritance and Diseases

Mitochondrial Genetics

Mitochondria are cytoplasmic organelles responsible for ATP production. They contain circular DNA, making them the only animal organelle besides the nucleus to contain DNA and genes.

Maternal Inheritance: Sperm contains very few mitochondria, so offspring inherit mitochondrial genes exclusively from the mother. An affected father cannot transmit mitochondrial diseases to his offspring.
Heteroplasmy & Severity: Disease severity and symptoms depend on the ratio of normal to abnormal (mutated) mitochondria within the cells.
Affected Organs: High-energy demanding organs like the brain, heart, kidneys, skeletal muscle, and liver are most severely affected by mitochondrial gene defects.