What is the allele frequency

The number of copies of a specific version of a gene divided by the number of copies of all versions of that gene. This number can be used to show the amount of biological diversity in an individual, a population, or a species.

Explanation: Allele frequency is a measure of the relative frequency of an allele on a genetic locus in a population. Usually it is expressed as a proportion or a percentage. In population genetics, allele frequencies show the genetic diversity of a species population or equivalently the richness of its gene pool.

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"a.) well–developed vascular system."

it's diluted or concentrated

in a diluted solution, you can add a lot more solute but in concentrated only a little more can be added