Milestones in Genetics: History, Concepts, and Disorders

Posted on Nov 21, 2025 in Biology

Key Historical Discoveries in Genetics (8000 B.C. – 1962)

Year	Discovery / Event	Scientist(s) Involved
8000–1000 B.C.	Humans knew sexual reproduction caused variation (used in selective breeding).	Ancient humans
1856–1863	Hybridization experiments on garden peas (basis of inheritance laws).	Gregor Johann Mendel
1865	Publication of Mendel’s work, “Experiments on Plant Hybridization”.	Mendel
1866	First description of Down Syndrome.	Langdon Down
1891	Discovery of the X-body (X chromosome) during spermatogenesis in insects.	Henking
1900	Rediscovery of Mendel’s work.	Hugo de Vries, Carl Correns, Erich von Tschermak
1902	Chromosome movement during meiosis explained (basis of chromosomal theory).	Walter Sutton, Theodore Boveri
1910–1915	Experimental verification of chromosomal theory (using fruit flies, Drosophila).	Thomas Hunt Morgan
1913	Gene mapping using recombination frequency.	Alfred Sturtevant
1928	Birth of James D. Watson.	Watson
1937	Francis Crick obtained B.Sc. in Physics.	Crick
1950	Watson completed Ph.D. on bacteriophage multiplication.	Watson
1953	Discovery of the DNA double-helix structure.	James Watson & Francis Crick
1954	Crick completed Ph.D. on “X-ray Diffraction: Polypeptides and Proteins”.	Crick
1959–1962	Multiple awards (including Nobel Prize 1962) for the DNA model.	Watson & Crick

Scientist	Contribution
Gregor Mendel	Laws of Inheritance – Law of Dominance, Law of Segregation, Law of Independent Assortment.
Reginald C. Punnett	Developed the Punnett Square for predicting genetic outcomes.
Walter Sutton & Theodore Boveri	Proposed the Chromosomal Theory of Inheritance (1902).
Thomas Hunt Morgan	Verified chromosomal theory using Drosophila; discovered linkage and recombination.
Alfred Sturtevant	Created genetic linkage maps using recombination frequency.
Henking	Discovered the X chromosome (1891).
Langdon Down	Described Down Syndrome (1866).
Watson & Crick	Proposed the double-helical structure of DNA (1953).

Concept	Ratio / Number	Description
Monohybrid Cross (F₂)	3:1 (Phenotypic), 1:2:1 (Genotypic)	Example: Tall (T) × Dwarf (t) pea plants.
Dihybrid Cross (F₂)	9:3:3:1	Phenotypes: Round–Yellow, Wrinkled–Yellow, Round–Green, Wrinkled–Green.
ABO Blood Group Genotypes	6 Genotypes, 4 Phenotypes	I^AI^A or I^Ai → A; I^BI^B or I^Bi → B; I^AI^B → AB; ii → O.
*Linkage (Morgan’s Drosophila)*	1.3% (white–yellow), 37.2% (white–miniature wing)	Shows strength of linkage (recombination frequency).
Polygenic Trait Example	3 genes (A, B, C) for skin colour	Each dominant allele contributes to darkness.
Sex Determination in Humans	23 pairs of chromosomes (XX or XY)	22 autosomes + 1 pair of sex chromosomes.
Honey Bee Chromosomes	♀ = 32 (diploid), ♂ = 16 (haploid)	Haplodiploid system of sex determination.
Mutation Example (Sickle Cell)	1 base change: GAG → GUG	Glutamic acid → Valine substitution (a point mutation).
Down Syndrome	Trisomy 21, 47 chromosomes	Extra copy of chromosome 21.
Turner Syndrome	45, XO	Female missing one X chromosome.
Klinefelter Syndrome	47, XXY	Male with an extra X chromosome.

Concept	Example
True-breeding lines	14 pea varieties with 7 contrasting traits (tall/dwarf, round/wrinkled, etc.).
Incomplete Dominance	Flower colour in Snapdragon (Antirrhinum sp.) → Red (RR), Pink (Rr), White (rr).
Co-dominance	ABO blood groups in humans.
Multiple Alleles	Gene I → I^A, I^B, i (blood groups).
Polygenic Inheritance	Human skin colour, height.
Pleiotropy	Phenylketonuria (PKU) – a single gene affects mental ability and pigmentation.
Mutation Example	Sickle-cell anaemia, caused by a point mutation.
Linked Genes	White and yellow body colour in Drosophila.
Sex Determination Types	XO (Grasshopper), XY (Human/Drosophila), ZW (Birds), Haplodiploid (Honey Bee).
Mendelian Disorders	Haemophilia, Colour Blindness, Sickle-cell Anaemia, Phenylketonuria, Thalassemia.
Chromosomal Disorders	Down, Turner, Klinefelter syndromes.

Equation	Meaning
(½T + ½t)² = ¼TT + ½Tt + ¼tt	Binomial expression for monohybrid cross F₂ generation genotypes.
(3 Round : 1 Wrinkled) × (3 Yellow : 1 Green) = 9:3:3:1	Dihybrid phenotypic ratio derivation (Law of Independent Assortment).

Laws of Inheritance: Dominance, Segregation, Independent Assortment.
Chromosomal Theory: Sutton & Boveri (1902).
Experimental Proof of Chromosomal Theory: Morgan (1910, using Drosophila).
Gene Mapping: Sturtevant (1913).
DNA Structure Discovery: Watson & Crick (1953).
Important Genetic Diseases: Haemophilia (X-linked), Sickle-cell Anaemia (autosomal recessive), Down Syndrome (trisomy 21).
Key Ratios/Numbers: F₂ phenotypic ratios = 3:1 or 9:3:3:1; Human chromosomes = 46 (23 pairs).

Disorder	Type	Genetic Basis	Key Features
Haemophilia	X-linked recessive	Defective clotting factor gene	Excessive bleeding; failure of blood to clot properly.
Colour Blindness	X-linked recessive	Cone pigment gene mutation	Inability to distinguish certain colours (often red–green).
Sickle-cell Anaemia	Autosomal recessive	Point mutation (GAG→GUG) leading to abnormal haemoglobin	Sickle-shaped Red Blood Cells (RBCs); severe anaemia.
Phenylketonuria (PKU)	Autosomal recessive	Deficiency of the enzyme phenylalanine hydroxylase	Accumulation of phenylalanine, leading to mental retardation if untreated.
Thalassemia	Autosomal recessive	Globin chain gene defect (reduced synthesis)	Severe anaemia due to abnormal haemoglobin production.
Down Syndrome	Chromosomal (Trisomy 21)	Extra copy of chromosome 21	Retarded physical growth and intellectual disability.
Turner Syndrome	Chromosomal (Monosomy X: 45, XO)	Missing X chromosome in females	Sterile female; short stature; webbed neck.
Klinefelter Syndrome	Chromosomal (Aneuploidy: 47, XXY)	Extra X chromosome in males	Sterile male; tall stature; feminized characteristics (gynecomastia).