How did Mendel discover dominant and recessive traits through controlled cross-breeding?

Mendel discovered dominant and recessive traits through controlled cross-breeding experiments with pea plants. He carefully selected which plants to breed and observed how traits like flower colour were passed from one generation to the next. This process helped him understand how some traits are stronger (dominant) while others are weaker (recessive).

There are two main steps that led Mendel to discover dominant and recessive traits:

Step 1: Choosing the Parent Plants and Cross-Pollinating Them

The first step in Mendel's experiment was selecting two purebred pea plants with easily distinguishable traits:

• One plant had purple flowers, which was the dominant trait.
• The other plant had white flowers, which was the recessive trait.

He carefully crossed these plants to produce the first generation of offspring, called the F1 generation. Since Mendel controlled the cross-pollination (by transferring pollen from one plant to another), he knew exactly which plants were involved in the process.

Mendel's cross-pollination method was very specific. He took pollen from the purple-flowered plant and transferred it to the white-flowered plant to ensure that the plants' genes would mix. This way, he could track which traits were inherited.

After crossing the purple-flowered plant with the white-flowered plant, all the plants in the F1 generation had purple flowers. This surprised Mendel because he thought the offspring would have both purple and white flowers. However, all of them had purple flowers, which told him that purple flowers were the dominant trait.

This result showed that even though one parent had white flowers (a recessive trait), the purple flower trait appeared in all the offspring. This led him to conclude that the purple flower trait was stronger and could "hide" the white flower trait in the offspring.

Step 2: Self-Pollinating the F1 Generation and Observing the F2 Generation

Mendel then took these F1 plants (all with purple flowers) and let them pollinate themselves. This was the next controlled step because Mendel was now able to see how the traits were passed on when the plants reproduced.

The plants in the F2 generation (the offspring of the F1 plants) showed a 3:1 ratio. This means:

• About three-fourths of the plants had purple flowers.
• About one-fourth had white flowers.

Mendel concluded that the purple flower trait was dominant and the white flower trait was recessive. For white flowers to show up, both parents must carry the recessive gene. When a purple-flowered plant has one dominant and one recessive gene, the dominant gene (purple) "covers" the effect of the recessive gene (white). This meant that the recessive trait (white flowers) was only seen when both alleles for the trait were recessive (i.e., both parents contributed a recessive allele).

What Did Mendel Learn from This?

Through this controlled cross-breeding process, Mendel learned several important things:

• Dominant traits are the ones that appear in the F1 generation and "cover up" the expression of the recessive traits. For example, purple flowers were dominant over white flowers.

• Recessive traits are only expressed when both alleles are recessive. That's why white flowers only appeared when both alleles were recessive (pp).

Mendel discovered that each trait is controlled by two alleles (versions of a gene), one from each parent. The dominant allele will always express its trait, while the recessive allele will only express its trait if the plant has two copies of the recessive allele.