Lecture 2 The genetic Model for Quantitative Traits

1
Animal Breeding and Genetics
Instructor Dr. Jihad Abdallah Simply inherited
and polygenic Traits
2

• Simply inherited traits are traits affected by
  one or few genes (coat color, presence of horns,
  genetic defects like spider syndrome in sheep).
• Phenotypes of simply inherited traits are placed
  into categories (qualitative or categorical
  traits)
• Very little affected by the environment.

3

• Polygenic traits are traits affected by many
  genes (no single gene having an overriding
  effect) like growth rate, milk production, birth
  weight, etc.
• Generally described in numbers.
• Typically quantitative or continuous in their
  expression (an exception is Dystocia which is
  affected by many genes but the phenotype is
  described in categories)
• Polygenic traits are affected by the environment.

4
Different breeding approaches

• Different breeding approaches are used to improve
  simply inherited traits and polygenic traits.
• This is due to the different number of genes
  involved in each type.

5

• For polygenic traits
• It is more difficult to observe the effects of
  individual genes and search specific information
  on each one (we do not try to identify the
  genotype at many loci).
• In contrast, we try to identify the net effect of
  the many genes the individual carries (we try to
  quantify the individuals performance and genetic
  merit).

6

• For simply inherited traits
• We search to identify specific genotypes
• Test mating (or test cross) a test designed to
  reveal the genotype of an individual for one gene
  or few genes.