Quantitative Genetics and Breeding Notes

Quantitative Genetics Overview

  • Previous Take Home Messages:

    • Successful breeding practices existed for millennia before understanding of genetics emerged.

    • Understanding DNA's role in genetics and breeding is a recent development.

    • In quantitative genetics, it is difficult to pinpoint specific genes related to a trait.

    • Accurate measurement is essential for achieving genetic improvement.

Case Study: Milk Production

  • Historical Breeding:

    • Cows have been bred for milk production for over 10,000 years.

    • Modern cows can produce approximately 12,000 liters of milk/ year.

  • Mutation and Breeding:

    • New mutations that significantly increase milk production are unlikely.

    • Instead, favorable alleles related to milk production traits have been selected over generations.

Case Study: Wool Fiber Diameter

  • Breeds Examined:

    • Merino:

      • Produces high-quality fine wool (12-24 microns).

      • Hardy and well-adapted to varying terrains.

    • Romney:

      • Mid micron wool (26-31 microns).

      • Good fertility and thriving on various terrains.

    • Corriedale:

      • Medium wool and quality meat producer.

  • Genetic Variation in Sheep:

    • DNA mutations are rare between sheep breeds, while allelic variants are more easily identified.

    • Focus is on understanding how alleles affect phenotype through variation in genes such as KAP6.

Genetics of the KRTAP6.1 Gene in Sheep

  • Gene Overview:

    • Synonyms: KAP6-1

    • Located on chromosome OAR1 (reverse strand).

    • Comprises a single transcript with one exon and many allelic variants (437 variants recorded).

  • Allelic Variants:

    • Most variations occur in neighboring genes, with significant mutations in untranslated regions (UTRs).

    • Only a few mutations directly affect the translation of the gene (missense variants).

Understanding Genetic Diversity

  • Sources of Genetic Diversity:

    • Primarily comes from allelic combinations rather than mutations.

    • The process of meiosis generates significant genetic diversity necessary for species adaptation.

  • Analysis of Genetic Diversity in Breeds:

    • Example from cattle and dog breeds shows diverse phenotypes originate from common ancestors.

    • Despite documented breeding histories being sparse, genetic analysis can illustrate lineage and diversity.

Genetic Diversity and Phenotype Variability

  • **Diversity Influences:

    • Phenotypic diversity arises from gene expression variability under different environmental conditions.

    • Multiple genes often contribute to quantitative traits, allowing for substantial variability even with limited gene diversity.

Genotype Calculation for Multiple Alleles

  • Formula for NUMBER OF POSSIBLE GENOTYPES FOR MULTIPLE ALLELES

    • The formula for the number of possible genotypes for multiple alleles:
      Genotypes=n(n+1)2Genotypes = \frac{n(n + 1)}{2}

      = number of genotypes for n alleles

    • Examples:

    • n = 1 1 genotype.

    • n = 2 3 genotypes.

    • n = 3 6 genotypes.

    • n = 32 496 genotypes.

Conclusion on Genetics and Breeding Techniques

  • Understanding genetics requires depth in biochemistry, physiology, and trait genetics.

  • Effective breeding is complex, necessitating accurate tracking and understanding of allelic variants rather than relying solely on mutation-based improvements.