Quantitative Genetics and Breeding Notes

Key Concepts in Quantitative Genetics and Breeding

  • Genetic Diversity: Understanding genetic variation within species is crucial for breeding programs and conservation.

Important Formulas

  • Number of Possible Genotypes:

    • Calculated using the formula: n(n+1)/2

    • Where n represents the number of alleles (variant DNA sequences). This indicates a potentially vast genetic pool.

Phenotypic Diversity

  • Multigenic Influences:

    • Multiple genes with multiple alleles contribute to the variety of genotypes, leading to significant phenotypic diversity.

    • Genetic variation is common in certain genes which can complicate genetic analysis.

Case Study: Coat Colour in Mammals

Mice Coat Colour

  • Key Genes: Five principal genes influence coat colour in mice:

    • A (Agouti): Controls the distribution of pigment in the fur.

    • B (Black): Determines the colour of the pigment.

    • C (Colour): Regulates colour expression.

    • D (Dilution): Affects the intensity of the pigment.

    • S (Spotting): Influences pigment distribution across the body.

Dogs Coat Colour

  • Gene Analysis:

    1. Agouti Gene (A): 4 alleles - Fawn (ay), Wolf Grey (aw), Tan/Black/Brown (at), Recessive Black (a).

    2. Brown Gene (B): 2 alleles - Black Eumelanin (B), Brown Eumelanin (b).

    3. Extension Gene (E): 3 alleles - Melanistic Mask (EM), Eumelanin (E), only Phaeomelanin (e).

    4. Dilution Gene (D): 2 alleles - No dilution (D), Diluted pigmentation (d).

    5. Dominant Black Gene (K): 3 alleles - Dominant Solid Black/Brown/Blue (K), Brindle (Kbr), Recessive Yellow (k).

  • Total Genotypes:

    • Assuming unlinked genes, the calculated total possible genotypes: 10imes15imes6imes3imes6=16,20010 imes 15 imes 6 imes 3 imes 6 = 16,200

Genetic Effects in Horses

  • Example Genes: Several genes control pigmentation in horses which demonstrate unique inheritance patterns.

    1. W (White): WW - Lethal; Ww - Lacks pigmentation; ww - Fully pigmented.

    2. G (Grey): GG - Progressively greys with age; Gg - Similar to GG; gg - No grey.

    3. E (Extension): Controls black pigment distribution.

    4. A (Agouti): Influences black points on the horse - AA or Aa results in distinct patterns based on base pigments.

    5. C (Cream): Dilutes colours in various combinations.

    6. D (Dilution): Alters the body coat colour intensity.

Human Genetics: Eye Colour

  • Key Genes:

    • OCA2: Reduces P protein resulting in lighter eye colours.

    • HERC2: Regulates the expression of OCA2.

  • Allelic Variants: Different combinations of these and other genes lead to diverse human eye pigmentation.

Notable Conditions

  • Congenital Heterochromia: Can be inherited through autosomal dominant patterns, often resulting from genetic mosaicism, leading to individuals with differently coloured eyes (e.g., Mila Kunis).