Mendel’s Monohybrid Cross

Lesson notes

Overview

Crossing two heterozygous parents gives a 1:2:1 genotype ratio and 3:1 phenotypes. This topic is rated beginner and is number 8 of 110 in Biology. Core ideas and first intuitions. No prior knowledge needed.

What you are seeing

The panel above shows a living system animated over time. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

Gregor Mendel crossed pea plants differing in one trait. Crossing two heterozygous parents, Aa × Aa, gives offspring genotypes in the ratio 1 AA : 2 Aa : 1 aa, and, since A is dominant over a, phenotypes in the ratio 3 dominant : 1 recessive. He deduced that each parent carries two copies of each factor, now called alleles, and passes on one at random, the law of segregation. The cross is the basis of classical genetics.

Reading the relationship

The expression "Aa × Aa → 1AA : 2Aa : 1aa" is a compact summary of the idea. It uses the symbols A, a; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.

Try this

Start by only moving one slider at a time. Say aloud what you expect, then check.

Common pitfalls

Biological processes are rarely all-or-nothing. Look for the feedback, the timing and the exceptions, rather than learning a single fixed sequence by heart.

How it connects

It builds on “Mutations”, the topic before it in Biology. Once you are comfortable, continue with “Punnett Squares”.

Why it matters

Biology shows how living things are built and how they work, grow, inherit traits and adapt.