Blood Groups & Mendel's Peas

Blood Groups & Mendel's Peas 🩸

Genetics began not with a microscope but with a garden of **pea plants**. And some of its neatest rules show up in your own **blood**. This module meets the founder of genetics and then tackles the ABO blood groups, where the usual dominant-or-recessive rule bends.

Gregor Mendel 🌱

In the 1860s — long before anyone knew about DNA — a monk called **Gregor Mendel** cross-bred thousands of **pea plants** and counted the offspring. He worked out that characteristics are passed on as separate **units** (now called **genes**), and that some are **dominant** over others. His work was ignored for decades, then rediscovered — founding the whole science of genetics.

Mendel's discovery

What did Gregor Mendel work out from his pea-plant experiments?

• Characteristics are inherited as separate units (genes), some dominant over others
• That DNA is shaped like a double helix
• That cells divide by mitosis
• That human blood comes in four groups

When alleles share 🅰️

Usually one allele is dominant and one recessive. But sometimes **both alleles are expressed together** — this is **codominance**. The **ABO blood group** gene shows this, and it has **three alleles** (multiple alleles): **I^A**, **I^B** and **i** (we\'ll write them A, B and o). I^A and I^B are **codominant**; i is **recessive**: • A A or A o → group **A** • B B or B o → group **B** • A B → group **AB** (both shown) • o o → group **O**

Match each genotype to its blood group

What is codominance?

What does it mean that the I^A and I^B alleles are codominant?

• Both alleles are fully expressed together, giving blood group AB
• They blend to make a group halfway between A and B
• I^A is always dominant over I^B
• Neither allele has any effect

A blood-group cross 🔲

You can predict blood groups with a **Punnett square**, just like any other cross. Remember there are three alleles in the population, but each person carries only **two**. Let's cross a **group-A parent (I^A i)** with a **group-B parent (I^B i)** and see what their children could be.

Cross: I^A i × I^B i

An interactive activity.

True about the ABO system

Select ALL THREE statements about blood groups and inheritance that are TRUE.

• The ABO gene has three alleles (multiple alleles)
• I^A and I^B are codominant, giving group AB together
• Mendel discovered the rules of inheritance from pea plants
• Codominant alleles blend together into a mixture
• Each person carries all three ABO alleles
• The i allele is dominant

Group O

Which genotype gives blood group O?

• o o (ii) — two recessive alleles
• A o (I^A i)
• A B (I^A I^B)
• B B (I^B I^B)

Match each term to its meaning

Inheritance summary

Gregor Mendel worked out the rules of inheritance from _____ plants. The ABO blood-group gene has three alleles — an example of _____ alleles. I^A and I^B are _____, so together they give blood group AB. The genotype ii gives blood group _____.