Gregor Mendel conducted thousands of experiments using pea plants because they were easy to cultivate and possessed easily distinguishable characteristics such as flower color, seed shape, seed color, pod shape, pod color, flower position, and plant height. By carefully crossing plants with contrasting traits and analyzing the offspring over several generations, Mendel discovered predictable patterns of inheritance.

Genes are units of heredity that control specific characteristics. Each gene exists in different forms called alleles. Every individual inherits one allele from each parent. Some alleles are dominant, meaning they are expressed even when only one copy is present, while others are recessive, meaning they are expressed only when two copies are inherited.

Mendel proposed three fundamental laws that explain the inheritance of genetic traits.

1. Law of Dominance (Law of Uniformity)

The first law states that when two pure-breeding individuals with contrasting traits are crossed, all offspring of the first filial generation (F₁) are uniform and express only the dominant trait.

For example, if a pure tall pea plant (TT) is crossed with a pure short pea plant (tt), all offspring in the F₁ generation will be tall (Tt). Although each offspring carries one allele for tallness and one for shortness, the dominant allele masks the expression of the recessive allele.

This law explains why only one trait appears in the first generation even though both parental traits are inherited.

2. Law of Segregation

The second law states that each individual possesses two alleles for every trait, but these alleles separate during the formation of gametes (sperm and egg cells). As a result, each gamete carries only one allele for each characteristic.

During fertilization, the alleles combine again, restoring the pair in the offspring.

When two heterozygous individuals (Tt × Tt) are crossed, the offspring show predictable ratios:

Genotypic ratio:

• 1 TT
• 2 Tt
• 1 tt

Phenotypic ratio:

• 3 Tall plants
• 1 Short plant

This law explains why recessive traits can reappear in later generations after disappearing in the F₁ generation.

3. Law of Independent Assortment

The third law states that alleles of different genes assort independently during gamete formation, provided that the genes are located on different chromosomes or are far apart on the same chromosome.

For example, when studying two characteristics simultaneously, such as seed color and seed shape, the inheritance of one characteristic does not influence the inheritance of the other.

A typical dihybrid cross between two heterozygous individuals produces the famous phenotypic ratio:

9 : 3 : 3 : 1

This means:

• 9 offspring show both dominant traits.
• 3 show the first dominant trait and the second recessive trait.
• 3 show the first recessive trait and the second dominant trait.
• 1 shows both recessive traits.

Importance of Mendel's Laws

Mendel's laws are fundamental in many areas of biology and medicine. They help scientists understand hereditary diseases, predict genetic traits, improve crop production, develop better livestock breeds, perform genetic counseling, and conduct research in molecular biology and biotechnology.

Modern genetics has shown that some inheritance patterns do not strictly follow Mendel's laws. Examples include incomplete dominance, codominance, multiple alleles, polygenic inheritance, linked genes, and mitochondrial inheritance. Nevertheless, Mendel's principles remain the foundation for understanding classical genetics.

Today, Mendel's discoveries continue to be applied in agriculture, medicine, biotechnology, forensic science, and evolutionary biology. They provide the basic framework for studying how genetic information is transmitted from one generation to the next.

Conclusion

Mendel's Laws explain how hereditary traits are inherited through dominant and recessive alleles. The three laws—the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment—form the basis of classical genetics. Although modern genetics has expanded our understanding of inheritance, Mendel's discoveries remain among the most important achievements in biological science.