Mendel's Experiments: Inheritance Patterns in Pea Plants

Mendel's Pea Plant Inheritance Studies

What is a Pure Race for a Character?

Pea flowers can be white or purple. When crossing two plants with purple flowers, some offspring plants had white flowers, and others had purple flowers. After repeating this process for several generations (remembering there is only one crop of peas per year), offspring were obtained solely from plants with purple flowers. This outcome persisted in subsequent generations. These purple-flowered plants represent a pure race for that specific character.

Character Study: Crossing Pure Breeds

Mendel studied several characters by crossing pure breeds (varieties):

• Seed Color

  • Dominant Alternative: Yellow
  • Recessive Alternative: Green

• Seed Aspect

  • Dominant Alternative: Smooth
  • Recessive Alternative: Wrinkled

• Flower Color

  • Dominant Alternative: Purple
  • Recessive Alternative: White

• Stem Length

  • Dominant Alternative: High (Tall)
  • Recessive Alternative: Dwarf

Selfing of Hybrids (F1 Generation)

Mendel allowed the F1 hybrids (resulting from crossing pure breeds) to self-pollinate. After waiting a year, he studied the offspring produced (the F2 generation). He found that the F2 generation was not uniform; the dominant character appeared in approximately 3 out of 4 seeds obtained, while the recessive character appeared in 1 out of 4. He repeated the experiment with other characters and obtained similar results, consistently observing ratios close to 3:1 (dominant:recessive). To explain these results, Mendel proposed the existence of internal factors (now known as genes) responsible for the characters expressed by the plants.

Artificial Hybridization

An animal or plant resulting from the fertilization between two different breeds or varieties of a species (or between two closely related species) is called a hybrid. The process to obtain it is called hybridization. The flowers of the pea plant (Pisum sativum) are hermaphroditic and usually self-pollinate, but cross-fertilization (pollination between flowers of different plants) can also be induced.

Technique

1. Pollen is taken from the stamen (male part) of one fertile flower.
2. This pollen is transferred to the pistil (female part) of another flower from which the stamens have been removed (to prevent self-pollination).
3. Seeds develop from this cross-fertilization, and pure hybrid plants are grown from these seeds.

Mendel's Research Process

Why Pea Plants?

Mendel chose pea plants (Pisum sativum) for his investigations for several reasons:

• Certain characters were easily visible and distinguishable.
• These characters manifested in only two distinct alternative forms (phenotypes).
• The plant grew well in the monastery garden in Brno (Czech Republic).
• Peas were a common food source (reportedly liked by the monks).

Experimental Steps

Mendel's scientific background allowed him to make a suitable choice of characters in Pisum sativum for his inheritance studies. His process involved several key steps:

1. He first needed to obtain what he called pure races (pure-breeding lines) for each character.
2. He then crossed pure breeds that differed in one form of a character (this was the parental generation, P) and observed the presence of traits in their offspring (the first filial generation, F1).
3. Next, he crossed the F1 hybrid plants with each other (or allowed them to self-pollinate) and studied how the characters appeared in the subsequent generation (the second filial generation, F2).