Allele frequency in a population can remain constant only if individuals of the population randomly interbreed. Generally it never happens in nature and species populations exist in small groups of randomly breeding subpopulations. Thus allele frequency between two subpopulations may differ but allele frequency within the subpopulation will remain constant.

•Such subpopulations are termed Mendelian populations or deme. Exchange of genes between demes takes place occasionally. Mutation, gene flow (due to migration), etc. may tend to change the allele frequency of the subpopulation.

Explanation:

The gene ratio, also known as the gene frequency or allele frequency, refers to the proportion of a particular gene or allele within a population. It represents how common or rare a specific gene variant is within a population.

The condition in which the gene ratio remains constant for any species is random mating. Random mating refers to the process where individuals within a population have an equal chance of mating with any other individual in the population.

Importance of Random Mating:
Random mating is essential for maintaining genetic equilibrium within a population. It prevents any specific gene or allele from becoming more or less common due to mating preferences or selective mating. Here's why random mating helps in maintaining a constant gene ratio:

1. Prevents Sexual Selection: Sexual selection occurs when individuals choose their mates based on specific traits or characteristics. This can lead to an increase or decrease in the frequency of certain genes or alleles within the population. However, in random mating, there is no selective preference for specific traits, leading to a balanced gene ratio.

2. Minimizes Assortative Mating: Assortative mating occurs when individuals preferentially mate with others who have similar traits or characteristics. This can lead to an increase in homozygosity and a decrease in heterozygosity within the population. Random mating minimizes assortative mating, ensuring a more diverse gene pool.

3. Reduces Inbreeding: Inbreeding occurs when individuals with similar genetic backgrounds mate, leading to an increase in homozygosity and a decrease in genetic variation. Random mating reduces the likelihood of inbreeding, maintaining a more balanced gene ratio.

4. Preserves Genetic Variation: Random mating allows for the preservation of genetic variation within a population. It ensures that all genetic variants have an equal chance of being passed on to the next generation, preventing the loss of alleles and maintaining a constant gene ratio.

5. Eliminates the Effect of Mutation: While mutation can introduce new genetic variants into a population, random mating ensures that the gene ratio remains constant despite the occurrence of mutations. This is because random mating allows for equal chances of mating between individuals with and without mutations.

Therefore, random mating is the condition that ensures a constant gene ratio for any species. It promotes genetic equilibrium, preserves genetic variation, and prevents the selective increase or decrease of specific genes or alleles within a population.