Principles of Inheritance and Variation

Lesson

Part 1 — how Gregor Mendel discovered the rules by which characters pass from parents to offspring.

In this lesson you will

Explain why Mendel chose the garden pea and how he designed his crosses
Work out monohybrid and dihybrid crosses with Punnett squares
Use a test cross to find an unknown genotype
State the laws of dominance, segregation and independent assortment

Concept

Why the garden pea?

Mendel crossed garden pea plants for seven years (1856–1863) and published his results in 1865. He worked with 14 true-breeding varieties, forming seven pairs of contrasting traits, and analysed large samples statistically.

Table of features of the garden pea and why each helped Mendel
Feature of the pea	Why it helped
True-breeding lines available	Parents give the same trait generation after generation
Clear contrasting traits	Easy to score, for example tall or dwarf
Self-pollinates naturally but can be crossed by hand	Crosses are controlled
Many seeds per generation	Large samples for counting

Concept

The seven pairs of contrasting traits

In each pair, one form appeared in all the F1 plants: the dominant trait.

Table of the seven pea characters with their dominant and recessive forms
Character	Dominant	Recessive
Stem height	Tall	Dwarf
Flower colour	Violet	White
Flower position	Axial	Terminal
Pod shape	Inflated	Constricted
Pod colour	Green	Yellow
Seed shape	Round	Wrinkled
Seed colour	Yellow	Green

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