Mendelian Genetics Explained: The Laws of Inheritance

Gregor Mendel and His Experiments

Gregor Mendel was an Augustinian friar who conducted breeding experiments with garden peas (Pisum sativum) at his monastery in Brno, now in the Czech Republic. Between 1856 and 1863, Mendel crossed thousands of pea plants, carefully tracking seven distinct traits across multiple generations. He chose pea plants because they had clearly distinct trait variants (such as purple vs. white flowers, tall vs. short stems), could be easily cross-pollinated or self-pollinated, and produced large numbers of offspring for statistical analysis.

Mendel brought a mathematical approach to biology that was unusual for his time. Rather than simply observing that offspring resembled their parents, he counted the exact ratios of different trait variants in each generation. This quantitative approach revealed consistent numerical patterns that pointed to discrete hereditary units following predictable rules. His paper, published in 1866, was largely ignored until 1900, when three scientists independently rediscovered his work.

The Law of Segregation

Mendel observed that when he crossed pure-breeding purple-flowered plants with pure-breeding white-flowered plants, all offspring in the first generation (F1) had purple flowers. When these F1 plants were crossed with each other, the second generation (F2) contained approximately three purple-flowered plants for every one white-flowered plant, a consistent 3:1 ratio.

This pattern led Mendel to propose his Law of Segregation: each organism carries two copies of each hereditary factor (alleles), and these two copies separate during the formation of gametes so that each gamete receives only one copy. The purple color allele is dominant (expressed when at least one copy is present), while the white color allele is recessive (expressed only when two copies are present). The F1 plants carry one of each allele but appear purple because the purple allele is dominant.

In modern terminology, the Law of Segregation reflects the behavior of homologous chromosomes during meiosis. When cells undergo meiosis to produce gametes, the paired chromosomes (each carrying one allele at a given locus) are separated into different daughter cells. This physical separation of chromosomes is the mechanism behind Mendel observed segregation of alleles.

The Law of Independent Assortment

When Mendel tracked two traits simultaneously (dihybrid crosses), he found that the inheritance of one trait did not influence the inheritance of another. For example, seed color (yellow vs. green) was inherited independently of seed shape (round vs. wrinkled). This led to his Law of Independent Assortment: alleles for different traits segregate independently during gamete formation.

Independent assortment produces a 9:3:3:1 ratio in the F2 generation of a dihybrid cross. Of every 16 offspring, approximately 9 show both dominant traits, 3 show one dominant and one recessive trait, 3 show the other combination, and 1 shows both recessive traits. This ratio results from the independent probability of inheriting each trait multiplied together.

We now know that independent assortment applies to genes on different chromosomes. Genes located on the same chromosome tend to be inherited together (genetic linkage), violating independent assortment. However, recombination (crossing over) during meiosis can separate linked genes, with the frequency of recombination proportional to the physical distance between genes on the chromosome.

The Law of Dominance

Mendel observed that in heterozygous organisms (those carrying two different alleles for a trait), one allele completely masks the expression of the other. The expressed allele is dominant, and the masked allele is recessive. An organism must carry two copies of the recessive allele (be homozygous recessive) for the recessive trait to appear in its phenotype.

Dominance is not a fixed property of alleles but depends on molecular mechanisms. In many cases, one functional copy of a gene produces enough protein for normal function, so having one functional allele (dominant) and one non-functional allele (recessive) produces the same phenotype as having two functional alleles. This explains why many genetic diseases are recessive: one working copy of the gene is sufficient for health.

Punnett Squares: Predicting Offspring

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a cross. The possible gametes from each parent are arranged along the top and side of a grid, and the squares are filled in with all possible combinations. For a monohybrid cross between two heterozygotes (Aa x Aa), the Punnett square predicts a genotype ratio of 1 AA : 2 Aa : 1 aa, producing the observed 3:1 phenotype ratio.

For dihybrid crosses, 4x4 Punnett squares with 16 cells are used, representing all combinations of gametes carrying two independently assorting genes. More complex crosses can be analyzed using probability rules (the product rule for independent events and the sum rule for mutually exclusive events) rather than unwieldy Punnett squares.

Beyond Simple Mendelian Genetics

While Mendel described the simplest patterns of inheritance, many traits do not follow these clean ratios. Incomplete dominance produces intermediate phenotypes (red crossed with white producing pink). Codominance shows both alleles simultaneously (AB blood type). Multiple alleles provide more than two possible versions of a gene in a population (ABO blood groups have three alleles: IA, IB, and i). Polygenic traits are influenced by many genes, producing continuous variation rather than discrete categories.

Pleiotropy occurs when a single gene affects multiple seemingly unrelated traits. Epistasis occurs when one gene influences the expression of another gene. Environmental effects can modify the expression of genotypes, producing different phenotypes from the same genotype under different conditions. Despite these complications, Mendelian principles remain fundamental: alleles segregate during meiosis and assort independently when on different chromosomes.