Selective breeding increases the probability that offspring will have the desired trait because it limits narrows the gene pool to only include desired variants of a gene. Not all thoroughbreds may express the desired trait because they may inherit an unwanted variant of the gene & mutations may occur.
The traits and characteristics of an animal are determined by the genes they possess. When two animals breed, the offspring inherits one variant of each gene from both parents, known as alleles. This means that all animals have 2 variants for any specific trait.
When a gene in an allele is expressed, it means that the genetic code in that allele is the one that can be seen on the offspring. Gene variants can be classified based on how likely they are to be expressed as:
When one allele is expressed and the other is not, it is because that allele is dominant and the other is recessive. Recessive variants for a gene will only be expressed if both alleles inherited by the offspring possess are recessive. dominant genes are symbolized by a capital letter and recessive genes with a lower-case letter.
Selective breeding is used to continuously breed animals that have similar variants of the desired trait. If breeding were to be natural, then many of what we consider desirable traits may be lost behind the expression of common, more dominant genes. Selective breeding is done by narrowing the gene pool, meaning that we only allow breeding between animals known to possess the desired trait.
Not all thoroughbreds may express the desired traits because sometimes even though a parent has the desired variant of a trait, they may also contain a different recessive or dominant variant of a trait. Since selective breeding does not control what variant exactly the offspring inherits, it does not always work with each offspring. Genetic mutations can also affect the outcome.
for more information on genes see:
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