Mendelian & Non-Mendelian Inheritance
⭐ Classic 9:3:3:1 Derivations
7 Parameters
Mendelian Dihybrid Ratio Modifications (Epistasis Summary Chart)
Comprehensive reference table of classic dihybrid cross modifications, epistatic interactions, biochemical mechanisms, and genetic examples.
| Interaction Type | Modified F2 Ratio | Biochemical Basis | Classic Example |
|---|---|---|---|
| Standard Mendelian Dihybrid | 9 : 3 : 3 : 1 | Two independent genes acting on unrelated traits | Seed Shape and Color in Pea (Pisum sativum) |
| Recessive Epistasis (Supplementary Gene) | 9 : 3 : 4 | Homozygous recessive alleles of one gene mask the other (aa masks B_ and bb) | Coat color in mice (Agouti 9 : Black 3 : Albino 4) |
| Dominant Epistasis (Masking Gene) | 12 : 3 : 1 | Dominant allele of one gene (A_) completely masks both alleles of gene B | Fruit color in summer squash (White 12 : Yellow 3 : Green 1) |
| Duplicate Recessive Epistasis (Complementary Gene) | 9 : 7 | Dominant alleles of BOTH genes (A_B_) required for phenotype expression | Flower color in Sweet Pea (Lathyrus odoratus; Bateson & Punnett) |
| Duplicate Dominant Epistasis (Duplicate Gene) | 15 : 1 | Dominant allele at EITHER locus (A_ or B_) produces the phenotype | Capsella bursa-pastoris (Triangular 15 : Ovoid 1 seed capsules) |
| Duplicate Genes with Cumulative Effect (Polymeric Gene) | 9 : 6 : 1 | Two genes produce same effect separately, but compound when together | Fruit shape in summer squash (Disc 9 : Spherical 6 : Long 1) |
| Dominant and Recessive Interaction (Inhibitory Gene) | 13 : 3 | Dominant allele at one locus (I) inhibits dominant allele at another locus (C) | Feather color in domestic fowls (White 13 : Colored 3) |
F2 Epistatic Fraction Map
Recessive Epistasis
Ratio: 9 : 3 : 4
Grouping: 9 : 3 : (3+1)
Dominant Epistasis
Ratio: 12 : 3 : 1
Grouping: (9+3) : 3 : 1
Complementary
Ratio: 9 : 7
Grouping: 9 : (3+3+1)
Duplicate
Ratio: 15 : 1
Grouping: (9+3+3) : 1
Molecular & Pedigree Genetics
⭐ Medical Genetics Pedigree Rules
6 Parameters
Human Pedigree Inheritance Patterns (Diagnostic Matrix)
Rules of thumb, transmission traits, generation skips, and sex bias across all four major Mendelian pedigree modes.
| Diagnostic Feature | Autosomal Dominant (AD) | Autosomal Recessive (AR) | X-Linked Recessive (XLR) | X-Linked Dominant (XLD) |
|---|---|---|---|---|
| Generation Skipping | Never skips generations (vertical transmission) | Frequently skips generations (horizontal transmission) | May skip generations through carrier females | Does NOT skip generations |
| Sex Bias in Affected Offspring | Males and females affected equally (1:1) | Males and females affected equally (1:1) | Marked male preponderance (males >> females) | Females affected ~2x more than males |
| Father-to-Son Transmission | Possible and common | Possible and common | IMPOSSIBLE (fathers pass Y to sons) | IMPOSSIBLE (fathers pass Y to sons) |
| Affected Father's Daughters | 50% risk if mother is unaffected | Only carriers if mother is normal | 100% obligate carriers | 100% affected daughters (Hallmark rule) |
| Consanguinity Association | Rarely relevant | Frequently elevated in rare conditions | Unrelated to parental consanguinity | Unrelated to consanguinity |
| Classic Disease Examples | Huntington's, Marfan, Achondroplasia, Familial Hypercholesterolemia | Cystic Fibrosis, Sickle Cell Anemia, Phenylketonuria, Tay-Sachs | Duchenne Muscular Dystrophy, Hemophilia A/B, Red-Green Colorblindness | Hypophosphatemic Rickets, Rett Syndrome, Fragile X syndrome |
Pedigree Diagnostic Shortcuts
Key Point
Pattern: Autosomal Dominant
Generation Skipping: No
Male To Male: Yes
Sex Ratio: Equal (1:1)
Key Point
Pattern: Autosomal Recessive
Generation Skipping: Yes
Male To Male: Yes
Sex Ratio: Equal (1:1)
Key Point
Pattern: X-Linked Recessive
Generation Skipping: Yes
Male To Male: NEVER
Sex Ratio: Males >> Females
Key Point
Pattern: X-Linked Dominant
Generation Skipping: No
Male To Male: NEVER
Sex Ratio: Females > Males
Chromosomal Aberrations
⭐ Cytogenetics & Meiotic Loops
6 Parameters
Chromosomal Inversions & Translocations: Paracentric vs Pericentric vs Robertsonian
Structural comparison of chromosomal rearrangements, loop formations, and meiotic crossover consequences.
| Feature | Paracentric Inversion | Pericentric Inversion | Robertsonian Translocation |
|---|---|---|---|
| Centromere Involvement | DOES NOT include centromere (confined to one arm) | INCLUDES centromere (breaks occur on both arms) | Centric fusion of two acrocentric chromosomes |
| Arm Ratio Alteration | Arm ratio (p/q) remains unchanged | Can significantly change arm ratio and centromere position | Creates one large metacentric and one minute lost fragment |
| Meiotic Crossover Product | Produces Dicentric bridge + Acentric fragment | Produces Duplication/Deletion chromatids with 1 centromere each | Trivalent formation during pachytene |
| Acentric Fragment Fate | Lost during anaphase (no kinetochore for spindle attachment) | No acentric fragment formed; both have centromeres | Small p-arm heterochromatic fragment lost without penalty |
| Chromosomes Involved | Single chromosome (intrachromosomal) | Single chromosome (intrachromosomal) | Acrocentric chromosomes: 13, 14, 15, 21, 22 |
| Clinical Impact | Inviable recombinant gametes → semi-sterility | Duplications/deletions cause severe congenital malformations | Familial Down Syndrome (rob(14;21) carrier has 45 chromosomes) |
Centromeric Anatomy & Anaphase Products
Paracentric
Centromere Included: No
Anaphase Structure: Dicentric bridge + Acentric
Gamete Viability: 50% non-recombinant survive
Pericentric
Centromere Included: Yes
Anaphase Structure: Monocentric Duplication/Deletion
Gamete Viability: Recombinants inviable
Robertsonian
Centromere Included: Centromeric Fusion
Anaphase Structure: Metacentric translocated
Gamete Viability: Down syndrome risk
