A. Incomplete dominance → II (Antirrhinum): Snapdragon flower colour — RR (red) × WW (white) → RW (pink) heterozygote shows intermediate phenotype.
B. Co-dominance → IV (ABO blood groups): IᴬIᴮ heterozygote expresses BOTH A and B antigens simultaneously — classic co-dominance.
C. Pleiotropy → III (PKU — Phenylketonuria): Single gene mutation → multiple effects (intellectual disability, seizures, skin depigmentation, etc.).
D. Polygenic inheritance → I (Human skin colour): Multiple genes (6+) contribute additively → continuous variation from very dark to very light.
Gregor Mendel (1822-1884), the "Father of Genetics," formulated the laws of inheritance from his pea plant experiments. Law of Segregation (Law of Purity of Gametes): alleles of a gene separate during gamete formation and each gamete receives only one allele. Law of Independent Assortment: genes for different traits assort independently during gamete formation (only if on different chromosomes or far apart on same chromosome). Post-Mendel discoveries revealed exceptions to simple dominance: incomplete dominance, co-dominance, multiple alleles (e.g., ABO blood groups), pleiotropy (one gene affects multiple traits), polygenic inheritance (multiple genes affect one trait), epistasis (one gene masks another).
Incomplete dominance occurs when the heterozygote shows a phenotype intermediate between the two homozygous phenotypes — neither allele is completely dominant over the other. Classic example: Antirrhinum majus (snapdragon) flower colour. RR (red homozygous) × WW (white homozygous) → F₁: RW (pink) — intermediate. F₁ × F₁ → F₂: 1 RR (red) : 2 RW (pink) : 1 WW (white). The phenotypic ratio is 1:2:1 (same as genotypic ratio) — different from Mendel's 3:1. Explanation: one R allele doesn't produce enough red pigment for full red colour → pink (intermediate). Other examples: Mirabilis jalapa (4 O'clock plant) — same mechanism. The key feature: 3 distinct phenotypes from 2 alleles in heterozygote.
Co-dominance occurs when BOTH alleles are expressed simultaneously in the heterozygote — no blending, both products are present. Classic example: ABO blood group system. Three alleles: Iᴬ, Iᴮ, i. Iᴬ and Iᴮ are co-dominant (both expressed when present together). i is recessive (produces no antigen). Genotypes: Iᴬ Iᴬ or Iᴬ i → blood group A (A antigens on RBC). Iᴮ Iᴮ or Iᴮ i → blood group B. Iᴬ Iᴮ → blood group AB (BOTH A and B antigens — classic co-dominance!). ii → blood group O (no antigens). The AB individual has both IA and IB expressed → both A and B antigens on RBC → both A-enzyme and B-enzyme produced. This is co-dominance — both alleles expressed, not intermediate. Discovered by Karl Landsteiner (1901, Nobel 1930).
Pleiotropy (from Greek: pleion = more, tropos = affecting) is the phenomenon where a single gene affects multiple, seemingly unrelated traits. Classic example: Phenylketonuria (PKU) in humans. The phenylalanine hydroxylase gene mutation causes: intellectual disability, seizures, hyperactivity, lighter skin and hair (reduced melanin — tyrosine needed for melanin is also reduced), musty body odour. All caused by ONE gene mutation. Other examples: Marfan syndrome (fibrillin gene mutation) → affects height (very tall), eyes (lens dislocation), cardiovascular (aortic aneurysm), limbs (long fingers), connective tissue everywhere. Sickle cell anaemia: one β-globin mutation → anaemia, sickling, vaso-occlusion, organ damage — multiple organ effects from one gene. The phenotypic effects of a pleiotropic gene may all trace back to one biochemical pathway disruption that has downstream effects throughout the organism.
Polygenic inheritance (polygenism) occurs when multiple genes (polygenes) contribute additively to a single trait, usually producing continuous variation. Classic example: Human skin colour. Controlled by multiple genes (at least 6 known: OCA2, SLC45A2, TYRP1, TYR, HERC2, others — all on different chromosomes). Each gene contributes a certain amount of melanin pigment. More dominant alleles = more melanin = darker skin. Continuous variation: from very dark to very light skin — the many possible combinations of alleles at multiple loci produce a near-continuous spectrum. Statistical properties: normal (bell-shaped) distribution in a population. Mean affected by environment (sun exposure). Height, weight, IQ, blood pressure in humans are also polygenic. F₂ phenotypic distribution from AaBb × AaBb is 1:4:6:4:1 (like the binomial expansion) for two genes with additive effect.
ABO blood group antibodies are naturally occurring (no prior sensitisation needed): Group A: has A antigens on RBC, produces anti-B antibodies in plasma. Group B: has B antigens on RBC, produces anti-A antibodies in plasma. Group AB: has A and B antigens, NO antibodies in plasma (universal recipient for blood). Group O: no antigens on RBC, produces BOTH anti-A AND anti-B antibodies (universal donor for blood — though universal donor concept is now more nuanced). Transfusion compatibility: if incompatible blood is transfused, antibodies in recipient react with antigens on donor RBCs → agglutination (clumping) → haemolysis → transfusion reaction → potentially fatal. Cross-matching before transfusion is essential. Rh factor: another important antigen — Rh+ (has D antigen, ~85% people) or Rh− (no D antigen). Rh incompatibility in pregnancy: erythroblastosis foetalis — Rh− mother carries Rh+ foetus.
Epistasis: one gene (epistatic gene) masks the expression of another gene (hypostatic gene) at a different locus. Different from dominance (which is between alleles of the same gene). Types of epistasis: Recessive epistasis (9:3:4 ratio): homozygous recessive at one locus masks expression at other. Example: Labrador retriever coat colour — ee (yellow) masks B/b (black/brown). Dominant epistasis (12:3:1): one dominant allele masks another gene. Example: Summer squash colour. Duplicate dominant epistasis (15:1): either gene can mask — dominant at either locus produces same phenotype. Duplicate recessive epistasis (9:7): both dominant alleles needed for expression. Example: flower colour in sweet pea (two genes needed for colour; either gene homozygous recessive → white). Supplementary epistasis (9:3:4): similar to recessive epistasis but different ratios.
Genes on the same chromosome violate Mendel's law of independent assortment — they tend to be inherited together (linked genes). Complete linkage: genes always inherited together (never separated by crossing over). Incomplete linkage: genes occasionally separated by crossing over between them → recombinant offspring. Recombination frequency (RF) = (recombinant offspring / total offspring) × 100%. RF = genetic distance in centiMorgans (cM). 1% RF = 1 cM = 1 map unit. Maximum observable RF = 50% (genes appear unlinked if very far apart due to multiple crossovers). Morgan mapped first chromosomes using Drosophila (fruit flies). Chromosome mapping: uses RF values to place genes in linear order on chromosome. Sex-linked inheritance: genes on X chromosome — males are hemizygous (only one copy). X-linked recessive: haemophilia, colour blindness — appear more in males (need only one copy since no second X to mask it).