Mendelian inheritance describes the principles of how genetic traits are passed from parents to their children. These patterns were first identified by Gregor Mendel, an Austrian monk who conducted extensive experiments in the 19th century. His work established that heredity follows specific, predictable patterns rather than being a simple blending of parental traits. The principles derived from his research explain how characteristics are transmitted across generations.
The Experiments of Gregor Mendel
Gregor Mendel’s work in genetics was conducted using the common garden pea plant, Pisum sativum. This plant was an ideal subject because it grows quickly, displays a variety of distinct characteristics, and its pollination can be easily controlled. Pea plant flowers contain both male and female reproductive parts, allowing them to self-pollinate. Mendel used this to create purebred lines, which consistently produce offspring identical to the parent.
His experimental process was quantitative. Mendel performed hybridizations by manually cross-pollinating purebred plants with contrasting traits, such as tall versus short stems or purple versus white flowers. This first cross produced the F1 generation. After recording the traits of the F1 offspring, he allowed these plants to self-pollinate to produce a second (F2) generation.
Mendel counted the exact number of offspring exhibiting each trait. For example, when he crossed purebred purple-flowered plants with purebred white-flowered plants, all F1 offspring had purple flowers. When the F1 generation self-pollinated, the white-flower trait reappeared in the F2 generation. This occurred at a predictable ratio of approximately three purple-flowered plants to one white-flowered plant.
The Fundamental Laws of Inheritance
From his experiments, Mendel derived three principles that govern the inheritance of traits. The first is the Law of Dominance, which states that when an organism has two different forms of a gene, one can mask the presence of the other. The expressed allele is called dominant, while the masked allele is termed recessive. In a cross between purebred tall and short pea plants, all offspring in the first generation were tall because the allele for tallness is dominant.
The second principle is the Law of Segregation. This law describes how the two alleles for a single trait separate from each other during the formation of gametes (sperm and egg cells). As a result, each gamete receives only one allele for each trait. When the gametes unite during fertilization, the offspring inherits one allele from each parent. This law explained why a recessive trait could reappear in the F2 generation.
The Law of Independent Assortment explains how different traits are inherited relative to one another. It states that the alleles for different traits are sorted into gametes independently. For example, the allele a pea plant inherits for flower color does not influence the allele it inherits for seed shape. This independent sorting allows for new combinations of traits to appear in the offspring. Mendel observed this in dihybrid crosses, which involve tracking two different traits simultaneously.
Key Genetic Concepts and Tools
To apply Mendel’s laws, it is important to understand some basic genetic vocabulary. A gene is a segment of DNA that codes for a specific trait, while alleles are the different versions of that gene. An organism’s genetic makeup is its genotype, and the physical expression of that genotype is its phenotype.
An individual’s genotype for a particular gene can be either homozygous or heterozygous. An organism is homozygous if it has two identical alleles for a trait (e.g., TT or tt). If it has two different alleles (e.g., Tt), it is heterozygous, and the dominant allele determines the phenotype.
A Punnett square is a tool for predicting the outcomes of a genetic cross. This diagram visualizes all the possible combinations of alleles that offspring can inherit from their parents. For a monohybrid cross, which tracks a single trait, a 2×2 grid is used. The alleles from one parent are written across the top, and the alleles from the other parent are written down the side.
By filling in the boxes of the square, one can determine the probable genotypes of the offspring. For example, crossing two heterozygous tall pea plants (Tt) reveals that the offspring have a 25% chance of being homozygous dominant (TT), a 50% chance of being heterozygous (Tt), and a 25% chance of being homozygous recessive (tt). This results in a phenotypic ratio of three tall plants to one short plant, matching Mendel’s results.
Beyond Basic Mendelian Genetics
While Mendel’s principles provide the foundation for understanding heredity, the inheritance of many traits is more complex. These patterns, known as non-Mendelian inheritance, do not follow the simple dominant/recessive model. They reveal a more nuanced picture of how genetic information translates into physical traits.
One such pattern is incomplete dominance, where the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For example, in snapdragon plants, crossing a red-flowered plant with a white-flowered plant results in pink-flowered offspring. Neither the red nor the white allele is completely dominant.
Another variation is codominance, where both alleles are fully and separately expressed in the heterozygous individual. A classic example in humans is the AB blood type, where both the A and B alleles are expressed simultaneously on the surface of red blood cells. The individual has both A-type and B-type antigens.
Many characteristics are polygenic traits, meaning they are controlled by the interaction of multiple genes rather than just one. This often results in a continuous spectrum of phenotypes. Human traits like height, skin color, and eye color are determined by the combined effect of many genes, which is why there is such a wide range of variation.