Question

What is meant by allele frequency?

Answer 1

An allele is one of two or more variants of a given gene. For instance, the ABO blood grouping is controlled by the ABO gene having six common alleles.

Complete answer:
Allele frequency or gene frequency is the relative frequency of an allele (i.e., variant of a gene) at a particular locus in a population It is expressed as a fraction or percentage. Specifically, it is the fraction of all chromosomes in the population that carry that allele. Consider the following information:
A particular locus on a chromosome and a given allele at that same locus.
A population of N individuals with ploidy n (i.e., an individual carries n copies of each chromosome in their somatic cells).
The allele exists in i chromosomes in the given population.
Here, the allele frequency is the fraction of all the allele occurrences i and the total number of chromosome copies across the population, i/(nN). The actual frequency calculations depend on the ploidy of the species for autosomal genes.
Monoploids
The frequency (p) of an allele A is the fraction of the number of copies (i) of the A allele and the population or sample size (N), so
p=i/N
Diploids
If f(AA), f(AB) and f(BB) and are the frequencies of the three genotypes at a particular locus with two alleles, then the frequency p of the A-allele and the frequency q of the B-allele in the population are obtained by counting alleles.
p = f(AA) + ½ f(AB) = frequency of A
q = f(BB) + ½ f(AB) = frequency of B
Because p and q are the frequencies of the only two alleles that is present at that locus, they must sum to 1, i.e.,
p + q = f(AA) + f(BB) + f(AB) = 1
q = 1-p
p = 1-q
If there are more than two different alleles, then the frequency for each allele is simply the frequency of its homozygote plus half the sum of the frequencies for all the heterozygotes in which it appears.
Allele frequency can always be calculated from genotype frequency, whereas the reverse requires that the Hardy–Weinberg conditions of random mating apply.

Note:
Microevolution is the change in allele frequencies that occurs over time within a population. In population genetics, allele frequencies are used to describe the amount of variation at a particular locus or across multiple loci. When considering the ensemble of allele frequencies for many distinct loci, their distribution is called the allele frequency spectrum. The allele frequency is distinct from the genotype frequency, although they are related, and allele frequencies can be calculated from genotype frequencies.