Heredity

Lesson

Part 1 — how variations build up from generation to generation, and the simple rules Mendel discovered with pea plants.

In this lesson you will

Explain why sexual reproduction produces more variation than asexual reproduction
Distinguish dominant and recessive traits, and genotype and phenotype
Use a Punnett square to predict the results of monohybrid crosses
Use a Punnett square to predict the 9 : 3 : 3 : 1 ratio of a dihybrid cross

Concept

Variation builds up over generations

Every generation adds a few new variations from DNA copying. In asexual reproduction the offspring differ only by these small copying changes. Sexual reproduction also mixes DNA from two parents, so it produces far more variation.

Table comparing the sources and amount of variation in asexual and sexual reproduction
	Asexual reproduction	Sexual reproduction
Source of variation	Small DNA copying errors only	Copying errors plus mixing of two parents’ DNA
Amount of variation	Very little	Much more
Example	Bacteria dividing	Humans, pea plants

Diagram

How variations accumulate

Sexual reproduction mixes the variations of two parents and creates even more diversity.

Each offspring inherits its parent’s variations and adds new ones of its own, so the diversity of a population grows with every generation.

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