GED Science: Understanding Inheritance: Dominant and Recessive Traits

Before Mendel’s experiments, most people believed that traits in offspring resulted from a blending of the traits of each parent.  However, when Mendel crossed pea plants with purple flowers to those with white flowers, he did not get plants with lavender colored flowers in the F1 generation. In general, if the offspring of crosses between purebred plants looked like only one of the parents with regard to a specific trait, Mendel called the expressed parental trait the dominant trait.

Mendel noted that while the plants of the F1 generation looked like one parent of the P generation, they were actually hybrids of the two parent plants. To understand whether non-dominant traits were hidden in the F1 generation, Mendel crossed F1 plants with one another (F1 x F1). The resulting F2 generation had plants that showed the dominant trait, just like the F1 generation, but it also had some plants that showed the non-dominant, or recessive, trait. Furthermore, he noticed that, regardless of which trait he looked at, the F2 generation always had a 3:1 ratio of dominant to recessive traits. For the flower color trait, for example, he had three purple flowers for every white flower. Mendel concluded that the white flower trait had been present in the purple flowered F1 plants, but had not been directly observable because it was masked by the dominant purple flower trait.

Mendel carried on his pea plant breeding experiments for over eight years, collecting and analyzing significant amounts of data. From this work, Mendel proposed three foundational principles, or laws, of inheritance.

These are:

  1. Fundamental Theory of Heredity: Inheritance involves the passing of discrete units of inheritance, or genes, from parents to offspring.
  2. Law of Segregation: During reproduction, the two inherited factors for a trait that an organism received (one from each parent) must separate into reproductive cells, so that only one factor is passed onto the organism’s offspring.
  3. Law of Independent Assortment: Inherited factors separate independently from one another. For example, in pea plant, how the factors for seed color separate is completely independent from how the factors for pod color separate.

Today, whether you are talking about pea plants or human beings, genetic traits that follow the rules of inheritance that Mendel proposed are called Mendelian traits.


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