In selective breeding, when crossing individuals that express the desired phenotype but are heter0zygous for this character, their offspring might express the undesired trait.
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The main goal of selective breeding is to get individuals -plants or animals- with the desired traits. These traits make them better, according to the breeder's interest. The interest is to produce individuals with the same reproductive capability, adaptability, capacity, and general phenotype as the original pure line.
To get a generation of individuals expressing the desired traits, the breeder must select parents that have the characteristics of interest. Many or some animals of the progeny might inherit the desirable phenotype.
In general, pure and almost identical animals with the desired traits are produced after many generations of inbreeding. These are the pure-bred individuals and are the product of many selective crosses.
Pure-bred (true-bred) animals are h0m0zyg0us dominant or recessive for the desirable traits. The progeny of the same line express similar phenotype and performance to the ones of their progenitors.
Many problems are associated with selective breeding and the production of pure lines.
In the exposed example, let us say that the desired trait is running speed and that there is only one autosomal gene that codes for run speed.
- the dominant allele F expresses the trait fast runner
- the recessive allele f does not express the trait fast runner
So, FF and Ff individuals will be able to run fast, while ff individuals will not.
Probably the breeder does not know the genotype of the animals, so this person crosses the horses according to how fast they run -phenotype-. If the breeder wants to get fast runner animals, he/she will only choose horses with this trait, which might be either FF or Ff.
If the breeder chooses two heter0zyg0us individuals, there are 75% of getting horses with the desired trait and 25% of chances to get horses with the undesired trait -h0m0zyg0us recessive genotype ff-.
The breeder needs many generations of inbreeding to get only horses that are fast runners.
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