Gregor Mendel's pioneering work on the principles of inheritance fundamentally transformed our understanding of how traits are transmitted from generation to generation. His experiments with pea plants laid the groundwork for the discovery of genes, discrete units within organisms that control heredity.

Each gene exists in pairs, and the combination of these genes from both parents forms an individual's genotype. This genotype is a blueprint of potential traits. Examples of genotype traits include blood type, eye color, and the presence of hereditary diseases. For instance, the genes inherited from one's parents might dictate whether they have brown, blue, or green eyes. Phenotype refers to the observable characteristics of an organism, such as physical appearance and behavior, which result from both genetic makeup and environmental influences. Factors like nutrition and climate can influence these characteristics.

In Mendel's model, the concept of dominance plays a crucial role in gene expression. Dominant genes mask the effects of others in pairings where the two genes differ. For instance, in humans, the gene for brown eyes is dominant over the gene for blue eyes. As a result, a child inheriting a brown-eye gene from one parent and a blue-eye gene from the other will typically have brown eyes. However, if both parents, despite having brown eyes themselves, carry the recessive gene for blue eyes, they can produce a blue-eyed child.

This principle extends to other physical traits. For example, curly hair is a dominant trait over straight hair. This means that if a child receives at least one gene for curly hair from either parent, the child will likely exhibit curly hair, regardless of the other gene.

However, not all traits adhere to such simple inheritance patterns. Psychological characteristics, such as intelligence, temperament, and predispositions for certain behaviors, are influenced by multiple genes, each contributing to a single trait. Each gene involved has a small additive effect, contributing to the vast diversity observed in human personalities and behaviors. This complexity makes it challenging to predict psychological traits based solely on parental genes.

The detailed explanation of gene expression does not conclude with the genetic code alone; it extends into the domain of epigenetics, a complex layer that modulates gene activity without changing the DNA sequence itself. This additional regulatory mechanism is crucial for understanding how genes are expressed differently across various cells and stages of development.