Table of Contents
Gregor Mendel’s crossbreeding experiments laid the empirical foundation for what are now called the Mendelian laws of inheritance. In this chapter, the focus is on what Mendel actually did: his experimental organism, his breeding design, and the key numerical results he obtained. Interpretation of these results and formulation of the laws themselves belong to the following chapters.
Why Mendel Chose Pea Plants
Mendel worked with the garden pea, Pisum sativum. This species was particularly well suited for inheritance studies because:
- Many clearly distinguishable traits
Peas showed simple, easily noticed alternative forms of certain characteristics, for example: - Seed shape: round vs. wrinkled
- Seed color: yellow vs. green
- Flower color: purple vs. white
- Pod shape: inflated vs. constricted
- Pod color: green vs. yellow
- Flower position: axial (along the stem) vs. terminal (at the tip)
- Plant height: tall vs. dwarf
For each of these traits, Mendel chose forms that appeared clearly distinct and did not show intermediate stages in his breeding lines.
- Short generation time and many offspring
Each plant produces numerous seeds, allowing Mendel to work with large sample sizes. This made numerical regularities (like specific ratios) visible. - Possibility of self-fertilization and controlled cross-fertilization
- Pea flowers are usually self-pollinating, which helps maintain pure-breeding lines (plants that, when selfed, produce offspring identical for the trait in question).
- The flower structure also allows easy manual transfer of pollen from one plant to another, so Mendel could design specific crosses.
- Stable hereditary lines (true-breeding / pure lines)
Before starting the actual experiments, Mendel preselected lines that, when self-fertilized for several generations, always produced the same trait form. These pure lines were the starting point for his crosses.
How Mendel Performed Crosses
Establishing Parental (P) Generations
For each trait, Mendel first obtained two pure-breeding parental lines that differed in only that one characteristic. Examples:
- Pure-breeding round-seeded plants vs. pure-breeding wrinkled-seeded plants
- Pure-breeding yellow-seeded plants vs. pure-breeding green-seeded plants
- Pure-breeding tall plants vs. pure-breeding dwarf plants
These plants formed the P generation.
Controlled Cross-Pollination
To cross two different pure lines, Mendel:
- Removed the stamens (male organs) from flowers of one line before the pollen matured, preventing self-fertilization.
- Collected pollen from the second line.
- Transferred the pollen onto the emasculated flowers of the first line.
- Protected the pollinated flowers to prevent accidental pollen contamination.
The seeds obtained from these crosses were the first filial generation, or F₁.
Selfing and Producing Further Generations
Mendel then:
- Planted the F₁ seeds and allowed the resulting plants to self-fertilize.
- The seeds obtained from these self-pollinated F₁ plants formed the second filial generation, or F₂.
- In some experiments, he continued to self-fertilize selected F₂ plants to observe the third generation (F₃) and beyond, which helped him distinguish between different types of F₂ individuals (e.g., those that breed true and those that do not).
Monohybrid Crosses: One Trait at a Time
A monohybrid cross considers only a single characteristic that has two alternative forms.
Example Trait: Seed Shape
Parental generation (P):
- Pure-breeding round seeds
- Pure-breeding wrinkled seeds
Cross:
round × wrinkled
F₁ Generation
Observation:
- All F₁ seeds were round.
The wrinkled form disappeared in the F₁. Mendel noted that, for each trait he studied, one form masked the other in the F₁ generation.
F₂ Generation
When F₁ plants self-fertilized, the F₂ seeds showed both forms again:
- Approximately 3/4 round
- Approximately 1/4 wrinkled
In numbers, Mendel counted totals very close to a 3 : 1 ratio (dominant-like form : recessive-like form) for all the traits studied in monohybrid crosses.
Confirming Hidden Hereditary Factors
To analyze the F₂ further, Mendel cultivated F₂ plants separately and allowed them to self:
- Of the plants expressing the dominant-like trait (e.g., round):
- About 1/3 continued to produce only round offspring when selfed (these were true-breeding for round).
- About 2/3 produced both round and wrinkled offspring in about a 3 : 1 ratio when selfed (these were not true-breeding).
- All plants expressing the recessive-like trait (e.g., wrinkled):
- Produced only wrinkled offspring when selfed (always true-breeding for the recessive form).
This showed that F₁ individuals, although uniform in appearance, carried two different “factors” (hereditary units), one from each parent. The recessive factor was not lost; it was hidden in the F₁ and reappeared in the F₂.
Summary of Monohybrid Findings
Across different traits (seed shape, seed color, flower color, etc.), Mendel repeatedly observed:
- Uniform F₁ generation with only one of the two parental trait forms.
- Reappearance of the other form in the F₂ in a roughly 3 : 1 ratio (visible form : hidden form).
- Among F₂ individuals showing the visible form, about 1/3 bred true, and 2/3 did not.
These regularities formed the empirical basis of what is later called the first Mendelian law (law of uniformity) and part of the second Mendelian law (segregation), which are discussed conceptually elsewhere.
Dihybrid Crosses: Two Traits at Once
A dihybrid cross follows the inheritance of two different characteristics simultaneously, each with two alternative forms.
Example: Seed Color and Seed Shape
Parental generation (P):
- Line 1: yellow, round seeds (pure-breeding for both traits)
- Line 2: green, wrinkled seeds (pure-breeding for both traits)
Cross:
yellow-round × green-wrinkled
Both parent lines were chosen so that:
- Yellow vs. green in previous monohybrid tests had shown a 3 : 1 pattern.
- Round vs. wrinkled had also shown a 3 : 1 pattern.
- Each line bred true for both traits.
F₁ Generation
Observation:
- All F₁ seeds were yellow and round.
Again, the forms yellow and round behaved like dominant traits in appearance, while green and wrinkled were hidden in F₁.
F₂ Generation and Phenotypic Classes
When F₁ plants self-fertilized, the F₂ generation showed four combinations of traits:
- Yellow, round
- Yellow, wrinkled
- Green, round
- Green, wrinkled
Mendel counted large numbers of seeds and obtained an approximate 9 : 3 : 3 : 1 ratio in the F₂:
- 9/16 yellow, round
- 3/16 yellow, wrinkled
- 3/16 green, round
- 1/16 green, wrinkled
This distribution can be viewed as a combination of two independent 3 : 1 segregations (one for color, one for shape). The key finding from the experiment itself is the appearance of novel trait combinations:
- New combinations (yellow, wrinkled and green, round), which were not present in the pure-breeding P generation, appeared in the F₂.
Thus, Mendel’s dihybrid crosses demonstrated that traits can recombine in different ways from one generation to the next.
Further Testing: Breeding Behavior of F₂ Types
Mendel examined the breeding behavior of different F₂ phenotypes by self-fertilizing them and observing their offspring:
- Some F₂ individuals for a given trait combination bred true for both traits.
- Others bred true for one trait but segregated for the other.
- A third group segregated for both traits again.
By analyzing these patterns, Mendel inferred that the hereditary “factors” for color and shape behaved as separate units that could reassort in offspring. The conceptual consequence (independent assortment) is treated in the next chapter, but the essential experimental observation is that the distribution of one trait did not seem to influence the distribution of the other in the F₂ numbers.
Backcrosses and Test Cross–Like Experiments
To clarify the nature of the F₁ plants and the origin of different F₂ types, Mendel also performed crosses where F₁ individuals were crossed back to recessive parental types. For example:
- F₁ (yellow, round in appearance) × pure-breeding green, wrinkled
He then examined the combinations of traits that appeared. These backcrosses helped confirm that:
- F₁ individuals carried latent factors for both possible trait forms for each characteristic.
- The recessive parental type contributed only one kind of hereditary factor for each trait.
The numerical patterns of these crosses again supported the idea that each trait was represented by discrete “factors” that could be tracked across generations.
Scope and Limits of Mendel’s Experiments
Mendel’s regular 3 : 1 and 9 : 3 : 3 : 1 ratios were obtained under specific experimental conditions:
- He used carefully selected traits with two contrasting, clearly distinguishable forms.
- He worked with pure-breeding lines at the outset.
- The traits he chose in peas behaved, under his conditions, as if determined by single hereditary factors with simple dominance.
Mendel himself was aware that not all traits in nature would behave in such a simple manner, but within his studied system, the consistency of the ratios was striking and reproducible.
Summary of Mendel’s Empirical Findings
From his crossbreeding experiments with peas, Mendel established several core empirical observations:
- Uniform F₁ when crossing pure lines that differ in one or two traits.
- Reappearance of the hidden trait form in the F₂ generation.
- Characteristic numerical ratios in the F₂:
- About 3 : 1 for monohybrid crosses.
- About 9 : 3 : 3 : 1 for dihybrid crosses.
- New combinations of traits that did not exist in the original parental lines emerged in the F₂ of dihybrid crosses.
- F₂ individuals varied in their breeding true or segregating behavior upon selfing, revealing underlying differences in their hereditary constitution.
These experimental results formed the empirical basis from which the Mendelian laws of inheritance were later derived and formulated.