(p+q)²= p²+2pq+q²=1 represents Hardy Weinberg equation. It is used in:

Population genetics

The correct answer is Option (1) -Population genetics

The Hardy-Weinberg principle, also known as the Hardy-Weinberg equilibrium or law, is a fundamental concept in population genetics that describes the conditions under which the frequency of alleles in a population remains constant from generation to generation. Five factors are known to affect Hardy-Weinberg equilibrium. These are gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.

This principle underscores the equilibrium in the relative proportions of alleles within a sizable population over time, assuming the following conditions:

(1) random mating, (2) absence of natural selection, (3) no migration, and (4) no mutation.

In such a stable population, if we consider a gene with two alleles, A (dominant) and a (recessive), and denote their frequencies as p and q respectively, then the frequencies of the three possible genotypes (AA, Aa, and aa) can be expressed using the Hardy-Weinberg equation: p² + 2pq + q² = 1.

In essence, this principle falls under the domain of 'Population Genetics.'