Genetics | Extension of Mendelian

Extensions of Mendelian Genetics

• The inheritance patterns extend beyond simple Mendelian genetics.

• Offspring produced at F2 do not show a typical Mendelian ratio of 3:1 or 9:3:3:1.

• Key concepts in extensions include:

  • Incomplete dominance

  • Co-dominance

  • Multiple alleles

  • Epistasis

  • Polygenic inheritance

Complete Dominance

• Definition: The phenotype of a heterozygote (F1) is identical to that of the homozygous dominant parent.

• Example:

  • Purple-flowered pea plant (homozygous dominant) crossed with white-flowered (homozygous recessive).

  • Gametes: Capital P (purple) and small p (white).

  • F1 generation: All heterozygous plants produce purple flowers.

Incomplete Dominance

• Definition: When neither allele is completely dominant, resulting in an intermediate phenotype in heterozygotes.

• Example: Snapdragon plants

  • Alleles: CR (red) and CW (white).

  • Cross: CRCR (red) × CWCW (white) yields F1 with genotype CRCW (pink).

  • Self-fertilization yields:

    • F2 Genotype Ratio: 1 CRCR : 2 CRCW : 1 CWCW.

    • F2 Phenotype Ratio: 1 Red : 2 Pink : 1 White.

Co-Dominance

• Definition: Both alleles are equally dominant and expressed simultaneously.

• Example: Cattle coat color

  • Alleles: R (red) and W (white).

  • Cross: RR (red) × WW (white) yields RW (roan) heterozygotes.

  • Self-fertilization yields:

    • F2 Genotype Ratio: 1 RR : 2 RW : 1 WW.

    • F2 Phenotype Ratio: 1 Red : 2 Roan : 1 White.

• Examples in humans:

  • Blood types: IA (A antigen), IB (B antigen), I (no antigen)–with IA and IB being co-dominant.

  • Sickle cell trait: HBA (normal) and HBS (sickle cell)—co-dominance manifests where both types of hemoglobin are present in heterozygotes.

Multiple Alleles

• Definition: A gene exists in three or more alleles occupying the same locus.

• Example: ABO blood types in humans with alleles IA, IB, and I

  • Leads to six genotypes corresponding to four phenotypes (A, B, AB, and O).

Epistasis

• Definition: Gene interaction affecting phenotype; one gene can mask the expression of another.

• Example: Labrador Retrievers

  • Two gene interactions:

    • Gene for pigment (black: B, brown: b) and gene for deposition (E for deposit, e for no deposit).

  • Yellow color occurs when homozygous recessive (ee) regardless of pigmentation gene.

Polygenic Inheritance

• Definition: Multiple genes affect a single trait, leading to continuous variation.

• Example: Human skin pigmentation controlled by multiple genes (e.g., A, B, C).

  • Dark skin alleles contribute to darkness; cumulative effect observed.

  • Environmental factors (e.g., sun exposure) also influence the phenotype.

• Phenotypic variation resembles a bell-shaped curve, indicating a normal distribution, like human heights.