Mendelian Genetics Made Simple

Mendelian Genetics Made Simple: How to Actually Predict Inherited Traits

Did you know you can predict the chance of inheriting traits like eye color using a simple diagram?
Mendelian genetics introduces tools like the Punnett square to map out how traits are passed from parents to children based on principles discovered by Gregor Mendel.
This guide will show you how genes work, how traits are passed on, and why genetics isn’t guesswork—it’s science.

Key Takeaways

Genetics is the study of how traits like eye color, flower shape, or risk of diseases are passed from one generation to the next.
Early theories of inheritance, including blending and pangenesis, were later disproven by scientific research.
Gregor Mendel used pea plants to show that traits follow predictable patterns of inheritance, not random blending.
Mendel introduced the idea that traits are determined by “factors” (now called genes), and that these factors come in different forms (now called alleles).
Mendel developed three foundational principles of inheritance:
Principle of Dominance: In a cross between two purebred parents with contrasting traits (e.g. purple vs. white flowers), the dominant trait will always appear in the first (F1) generation.
Principle of Segregation: Every organism carries two alleles for each trait, and these alleles separate (segregate) during reproduction. The offspring inherits one allele from each parent.
Principle of Independent Assortment: Alleles for different traits (e.g. seed shape and color) are inherited independently of each other.
A dominant trait is one that appears even when paired with a different version, while a recessive trait can be hidden in one generation and reappear in the next.
Genotype refers to an organism’s genetic makeup (e.g., PP, Pp, or pp), while phenotype is the observable expression of that genotype (e.g., purple or white flowers).
Punnett squares are used to predict possible genetic outcomes of a cross and to visualize how alleles combine from each parent.
A monohybrid cross examines the inheritance of one trait (e.g., flower color). Crossing two heterozygous parents (e.g., Pp × Pp) produces a 3:1 phenotypic ratio (dominant:recessive) in the F2 generation.
A dihybrid cross looks at two traits at the same time (e.g., seed shape and color). Crossing two dihybrid parents (e.g., RrYy × RrYy) results in a 9:3:3:1 phenotypic ratio, showing all possible trait combinations.
Mendel’s findings form the foundation of modern genetics and help us understand how inherited traits appear in humans, plants, and animals.

A Brief History of the Understanding of Inheritance

Genetics is the study of the traits of a particular organism and how those traits are passed down, or inherited, between generations. In humans, traits might include eye color, body structure, or risk of certain diseases like sickle cell anemia, cystic fibrosis or diabetes. It’s possible that humans have understood something of the concept of inheritance for thousands of years, using it in the development of cultivated crops and domesticated animals. However, our understanding of the mechanisms behind genetics and inheritance are relatively new, beginning in the mid-19th century.

Prior to the 1800s, understanding of the mechanics of heredity was largely speculative. For instance, Aristotle (384–322 BCE) believed that blood played a big part in the passing of physical traits between generations, and though that turned out to be incorrect, we still talk about “blood lines” to talk about heritable qualities.

Hippocrates (c. 460–c. 375 BCE) believed in a theory of inheritance of acquired characteristics (IAC), which persisted for two millennia, and posited that attributes acquired

From Hippocrates to Darwin, the understanding of inheritance was initially largely speculative. It was Gregor Mendel who laid the foundations for genetics as it is known today, earning him the title “Father of Genetics.”

Another of Hippocrates’ long-lasting theories was that of pangenesis — that parts of the male body gave off invisible “seeds” that were transmitted to and assembled inside the female womb after sexual intercourse. Over two thousand years later, Charles Darwin (1809–1882) brought back Hippocrates’ idea, although with some major modifications. Both he and his scientific counterpart Alfred Russel Wallace, both of whom contributed to developing the theory of evolution by natural selection, realized that natural selection would require variations within a population, and that individuals of the same species would have to require subtle differences in traits. As a result, in 1868 Darwin proposed his own concept of pangenesis, featuring tiny units of inheritance contained in the cells of the body, which he called gemmules. These could be modified based on the organism’s environment, but when it was time to reproduce, the gemmules concentrated in the reproductive organs of both parents to be passed along to their offspring.

Gregor Mendel

While Darwin was drumming up support amongst mid-19th century experts for his version of pangenesis, an Austrian monk named Gregor Mendel was quietly experimenting on a foundational new theory of inheritance using the common pea plant, Pisum sativum, resulting in a plausible hypothesis for inheritance that formed the foundations for the discipline we now call Genetics.

Mendel’s discoveries resulted in the understanding that inheritance of specific traits is determined by what Mendel called “factors,” and we now call genes. He was also the first to postulate that there are multiple versions of each gene, which we now call alleles, and that the expression of different alleles results in the expression of different traits in the organism.

Mendel began his research with a curiosity about how different forms of a characteristic, or traits, like flower color, seed color, seed shape, etc. were passed down through generations of pea plants. His choice of peas as a model subject was a lucky one, as multiple generations of pea plants could be observed in a short amount of time. Also, flowers could be self-pollinated or pollinated with a flower from another plant, and the fertilization of the plants could be easily controlled with the use of a small paintbrush for pollen transfer.

The seven traits Mendel observed. Mendel observed seven distinct traits in his pea plant experiment, with each trait having a dominant and recessive version.

Mendel began by observing a specific plant for two years running, ensuring that its outward characteristics were consistent before introducing the pollen from another plant. Then he crossed pure-breeding parent plants — those that always make offspring like themselves when self-fertilized over many generations — with those with different characteristics, like purple and white flowers or smooth and wrinkled peas.

Through his experimentation, Mendel developed a plausible explanation for the transmission of genetic traits, based on three principles he developed, using statistics to analyze the results of his cross-pollination of pea plants with different physical traits.

The three foundational principles of inheritance. Mendel developed three principles from his pea plant experiment: Principle of Dominance, Principle of Segregation, and Principle of Independent Assortment

Prior to Mendel’s experiments, scientists believed offspring inherited a blended version of each parent’s traits. Mendel, however, observed something very different: when cross-pollinating his purebred (homozygous) pea plants, he found that crosses always yielded offspring with traits of either one or the other parent. For instance, a white-flowered pea plant crossed with a purple-flowered pea plant always resulted in offspring with either white or purple flowers — never pale purple flowers.

Furthermore, Mendel noticed that some crosses between purebred parents always yielded a specific trait across the board. For instance, homozygous, white-flowered pea plants crossed with homozygous, purple-flowered plants always yielded purple flowers; the white flower trait completely disappeared. He called the visible trait — in this case, the purple flower — the dominant trait, and the invisible or lost trait — the white flowers — the recessive trait.

Dominant and recessive alleles. Whether an organism will exhibit the dominant or recessive trait is determined by the pair of alleles it has for the gene.

The crossing of two organisms to influence a single trait is today called a monohybrid cross.

Monohybrid cross. A cross designed to observe a single trait is known as a monohybrid cross.

Dominant vs Recessive Traits

From the simple observation that some heritable traits overpower others, Mendel proposed his first principle, the Principle of Dominance, which posits that all the offspring of a cross between two homozygous parents that differ by only one trait will exhibit the dominant trait. Although some exceptions to the Principle of Dominance have been discovered after Mendel’s time, it still laid the foundation for 20th century genetics.

Principle of Dominance from P to F1. Mendel spent two years confirming his parent plants were true-breeding (homozygous), showing consistent traits before performing monohybrid crosses. All F1 offspring uniformly expressed just one parental trait, never the alternative or a blend. This expressed trait is now called the dominant trait.

Having discovered that some traits are dominant, Mendel continued his experimentation to see if the white flowers would show back up in successive generations. He let the first generation (F1) offspring of the purebred parent generation (P) self-fertilize. The result was that their offspring — the F2 generation — had a mix of purple and white flowers. In fact, the distribution of flower color was predictable: two-thirds of them had the purple flowers representing the dominant allele, and one third expressed the recessive white flower allele. He found that this 3:1 ratio worked similarly for the other characteristics he studied, like the shape and color of the pea: one trait disappeared completely when two P generations were crossed, but the hidden recessive trait reappeared in the F2 generation at a predictable ratio of 3:1.

Principle of Dominance from F1 to F2. When Mendel self-pollinated the F1 generation, he observed a 3:1 ratio of purple to white flowers in the offspring (75% purple, 25% white).

Genotype vs Phenotype

This information allowed Mendel to create a framework by which he could understand the concept of genes, which he called factors and alleles, which he called particles. Furthermore, he began to understand that the full complement of alleles carried by an organism — what we now know as its genotype — can’t be observed, but within the genotype can be found a phenotype, the collection of features that can be observed.

Genotype vs phenotype. The dominant trait will be expressed as long as one of the alleles is the dominant allele. In Mendel’s experiment, one of the parents is homozygous dominant and the other is homozygous recessive.

Mendel began to understand that pairs of factors (dominant and recessive alleles) influenced the phenotype, which led to his Principle of Segregation, which states that an organism carries two alleles for a trait, and during reproduction, their alleles separate, or segregate, and each parent contributes only one allele to their offspring. The phenotype of the offspring will be determined by whether one parent has contributed a dominant allele.

Principle of Segregation. Organisms possess two alleles per gene that separate during reproduction, with each parent contributing one allele to their offspring.

Later, scientists discovered this prediction could be made using a Punnett square, a diagram used to determine the probability of an offspring having the phenotype for a specific trait. These diagrams are based on whether an organism has two copies of the same allele, or is homozygous — for instance GG would represent double dominant alleles for the same gene, and gg would represent two recessive alleles. A heterozygous allele pair would have two different copies of the gene, and could be represented by an uppercase and lowercase pair (Gg) in a Punnett square.

Punnett squares for various monohybrid crosses. A Punnett square predicts both the genotype and phenotype probabilities for an offspring’s trait by showing all possible allele combinations from parental alleles.

Punnett square for the P generation cross and F1 generation cross. Punnett squares demonstrate why the recessive trait is absent in the F1 generation, how the recessive trait reappears in the F2 generation, and why the F2 generation exhibits a 3:1 ratio of dominant to recessive traits.

Dihybrid Crosses

With his Principle of Segregation Mendel showed that it was possible to predict how a single trait associated with a single gene is inherited. However, he went on to discover a mechanism by which it was possible to predict the inheritance of two separate features of the pea plant, associated with two different genes.

His concept of segregation found that individual traits are attached to discrete bits of genetic information, but he wanted to know whether the inheritance of one trait affected the inheritance of another. In other words, were genes somehow connected to each other as they passed between generations, or did they ignore each other completely?

To test this, Mendel created pea plants that were purebred for two traits — like seed color (green or yellow) and seed shape (round or wrinkled). He crossed the plants with wrinkled and yellow seeds (rrYY) with plants with round, green seeds (RRyy). Mendel knew from previous experiments which traits were dominant — round and yellow. Because the parents are both homozygous, the law of segregation tells us that the phenotype and genotype are the same in the P generation: the wrinkled, green plants are all ry, and the round, yellow plants are all RY. This means that if the two are cross-pollinated, the F1 offspring would all be RrYy, and the seeds would all be round and yellow, but genetically they are dihybrids, containing the alleles for both traits (RrYy).

Dihybrid cross of P generation. Dihybrid crosses examine the inheritance of two traits simultaneously. When an organism that is homozygous dominant for two traits is crossed with an organism that is homozygous recessive for the same traits, a dihybrid is produced. A dihybrid is an organism that is heterozygous for two traits.

Mendel created these dihybrids in the F1 generation, so he then crossed individual F1 plants with each other, creating a dihybrid cross. He found that when he crossed two dihybrid crosses, the offspring exhibited four distinct categories of pea seeds: yellow and round, yellow and wrinkled, green and round, and green and wrinkled. The phenotypes broke down into a ratio of approximately 9:3:3:1

Principle of Independent Assortment. The dihybrid cross of F1 generation plants produced offspring with a 9:3:3:1 phenotypic ratio when considering both traits together, but a 3:1 ratio is still observed when each trait is examined separately. This demonstrates that alleles for different traits segregate independently during reproduction.

When broken down further, this still yields a ratio of 3:1 for both seed shape and color, individually. In other words, there is no difference between the outcome of crossing two dihybrid crosses and two monohybrid crosses — the individual characteristics acted independently of one another. Which led Mendel to develop the Principle of Independent Assortment, which states that the inheritance of one trait does not influence the inheritance of another trait.

Conclusion: Peas to Predictions

Mendel’s pea plant experiments may seem simple at first glance, but they unlocked a powerful truth: the inheritance of traits follows clear, predictable rules. Concepts like dominant and recessive traits, genotype vs. phenotype, and Punnett squares allow us to predict inherited characteristics such as eye color, hair type, or even genetic disorders.

What began with questions about flower color led to the foundational principles of heredity: uniformity, segregation, and independent assortment. These ideas paved the way for modern genetics, impacting everything from medicine to agriculture.

So the next time you wonder why you have your dad’s eye color or your grandmother’s curly hair, remember: it’s not luck—it’s science. And yes, a simple diagram can help explain it.