**Core Concept**
The Hardy-Weinberg law is a fundamental principle in population genetics that describes the expected frequencies of alleles and genotypes in a population under certain conditions. It assumes a large, randomly mating population with no mutation, gene flow, or natural selection.

**Why the Correct Answer is Right**
The Hardy-Weinberg law states that the frequencies of alleles and genotypes will remain constant from generation to generation if the following conditions are met: the population is large, there is no mutation, no gene flow, no genetic drift, and no natural selection. This equilibrium is achieved when the frequency of a dominant allele is p and the frequency of the corresponding recessive allele is q, such that p + q = 1.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not related to the Hardy-Weinberg law.
**Option B:** This option is incorrect because the Hardy-Weinberg law is not related to the concept of genetic code.
**Option C:** This option is incorrect because the Hardy-Weinberg law is not related to the concept of gene expression.

**Clinical Pearl / High-Yield Fact**
The Hardy-Weinberg law is a crucial concept in population genetics that helps in understanding the genetic structure of populations and the effects of evolutionary forces on allele frequencies. It is essential for understanding the genetic basis of diseases and developing effective genetic counseling.

**Correct Answer: D. Hardy-Weinberg law is related to population genetics.**