Mendel's Laws

Genetics

Genetics can be defined as a branch of Biology that studies how hereditary traits are passed from generation to generation.

It is undoubtedly a fascinating discipline, made possible through the hard work of many scientists and researchers who concluded that the gene is the unit of information used by living organisms to transfer traits from one generation to the next. These traits are expressed during the ontogeny of individuals and help define species.

But even before the chromosomal theory of inheritance was established and the concept of the gene was defined, scientists already had a fair understanding of how heritable traits worked — thanks largely to the work of Gregor Mendel in the early 19th century.

Gregor Mendel

Johann Mendel was an Augustinian monk born in the former Austrian Empire (now the Czech Republic). After a childhood marked by poverty and hardship, he joined an Augustinian monastery near Brünn, where he became a priest and adopted the name Gregor. He was later sent to Vienna to pursue further education, earning a doctorate in mathematics and science. In 1868, Mendel was appointed abbot of his monastery and ceased scientific research to focus on his administrative duties.

However, it was within the monastery where he conducted his most famous experiments, leading to what we now consider the three laws of inheritance. These laws were later clarified and popularized by scientists such as Thomas Hunt Morgan, who is regarded as the father of modern experimental genetics.

While at the University of Vienna, Mendel became interested in the inheritance of traits in plant hybrids, having previously worked with mice and insects like bees.

The Pea Plant

Mendel’s primary work involved the pea plant (Pisum sativum), a legume native to the Mediterranean basin but now widespread worldwide. The plant has compound leaves ending in tendrils, racemose inflorescences, and some varieties can behave like vines if provided with support.

Choosing this species was a brilliant decision for several reasons:

• The plants were cheap and easy to obtain.
• They took up little space and had a short generation time.
• They produced many offspring.
• They displayed clear genetic variability in traits like flower color, plant height, and seed shape.

Perhaps most importantly:

• The plant is self-pollinating (autogamous), meaning the pollen from a flower’s anthers can fertilize the ovary of the same flower.

Why is that important?

Because inbreeding increases the likelihood of homozygosity — where individuals have identical alleles (either AA or aa). Mathematically, we can calculate the variance in gene frequency over generations in a population at equilibrium:

This tells us that, over time, inbreeding leads to populations where genotypes become predominantly homozygous and the frequency of heterozygotes (Aa) approaches zero. This can be summarized in the table below:

Mendel used this self-fertilization trait to establish pure lines for specific traits — meaning that after several generations, all the plants were either AA or aa, with very few or no heterozygotes (Aa).

Mendel’s Experiments

Before diving into Mendel’s experiments, let’s define some key terms necessary to understand them. One important concept is the allele.

Though there are cases of multiple alleles in nature, usually, a gene has two versions. For example, the gene for flower color has one version for purple and another for white. Each version is called an allele, and one is inherited from the male parent’s gamete, the other from the female.

What happens if an individual has both alleles (Aa)?

This individual is heterozygous, and if there is complete dominance, the dominant allele (A) will be expressed — in this case, purple flowers.

Key Concepts:

• Genotype: the genetic makeup of an individual for a specific trait.
  • AA: homozygous dominant
  • aa: homozygous recessive
• Phenotype: the observable expression of the genotype, influenced by genes and environmental factors.

With these concepts in mind, let’s go over Mendel’s experiments.

Mendel’s Method

Mendel used a consistent method:

1. Crossed pure lines that differed in one trait.
2. Self-fertilized the resulting generation.
3. Repeated the process to a third generation.
4. Reversed the direction of the cross to ensure results were consistent regardless of pollen donor.

First Law: Law of Uniformity

In the first filial generation (F1), all offspring were identical for the trait studied. This led to the Law of Uniformity:

When two pure lines are crossed, all offspring will be genetically and phenotypically identical for the specific trait.

For example, crossing AA (purple) x aa (white) = all Aa (purple), as A dominates a.

Second Law: Law of Segregation

Crossing the heterozygous F1 plants (Aa x Aa) results in:

• Genotypes: AA, Aa, and aa
• Phenotypes: 3 purple : 1 white

So Mendel’s Second Law is:

When two heterozygous individuals are crossed, the recessive phenotype reappears in one-quarter of the offspring.

Third Law: Law of Independent Assortment

Mendel then examined two traits at once, e.g., flower color and seed shape. These crosses involved dihybrids — heterozygous for both traits (AaBb).

Each individual can produce four gametes (AB, Ab, aB, ab), yielding 16 possible genotypes. The Punnett square below shows the combinations:

The resulting phenotypes followed a 9:3:3:1 ratio:

• Purple flowers & smooth seeds: 9/16
• Purple flowers & wrinkled seeds: 3/16
• White flowers & smooth seeds: 3/16
• White flowers & wrinkled seeds: 1/16

So Mendel’s Third Law is:

When crossing individuals differing in more than one trait, the inheritance of one trait is independent of the other.

Legacy and Modern Impact

Although Mendel’s arguments were sound, his work was forgotten until rediscovered by scientists like Hugo de Vries who independently reached similar conclusions. Today, Mendel’s laws are seen as a scientific milestone — foundational to the chromosomal theory of inheritance, which states that genes are arranged linearly on chromosomes and occupy specific positions called loci.

Later research confirmed that Mendel’s laws are broadly applicable across species — although certain conditions (like gene linkage) must be met for them to hold true.

Bibliography & Web Sources

• Alberts et al. Molecular Biology of the Cell
• Lacadena J.R. General Genetics: Fundamental Concepts
• genotipia.com
• khanacademy.com

If you want to learn more about Mendel’s Laws, don’t miss this video on my channel: