Chapter 2 — Match and the Punnett Square That Predicts the Next Generation

Match is a careful vole. He wears a tiny lab coat. It has small pockets for his cards. He always carries his Punnett cards. He also has a dominance tracker. It’s a little dial.

Match is cream-colored. His fur is soft grey-brown. He is super careful. He loves to compare things. He especially loves comparing gene pairs. He squints at them closely.

“Two parents, two alleles each,” Match squeaks. He holds up two cards. One is blue. One is red. “Punnett square predicts offspring.” He taps his cards together.

Match is all about allele pairing. He teaches you the special skill of PREDICTING-OFFSPRING.

He sets down his cards. “Imagine a mom and a dad,” he says. He places a blue card for the mom. A red card for the dad. “Each parent gives one part of a gene for every trait.” He points to the cards. “Like fur color. Or eye color.”

Match pulls out a bigger card. It has a square with four boxes. “This is a Punnett square,” he explains. He moves the blue and red cards around the boxes. “It shows the four ways those gene parts can mix up.”

“Let’s try one,” Match says. He picks up two new cards. “Imagine a mom vole with brown fur. She has one strong gene for brown, ‘B’. And one weak gene for white, ‘b’. So she’s ‘Bb’.” He places a ‘B’ card and a ‘b’ card.

“Now, the dad vole,” Match continues. “He also has brown fur. And he’s also ‘Bb’.” Match places another ‘B’ card and ‘b’ card.

“Okay, watch!” Match carefully fills in the Punnett square.

First box: Mom’s ‘B’ and Dad’s ‘B’. That makes ‘BB’. “Brown fur!” Match chirps. Second box: Mom’s ‘B’ and Dad’s ‘b’. That makes ‘Bb’. “Still brown fur, because ‘B’ is strong!” Third box: Mom’s ‘b’ and Dad’s ‘B’. That also makes ‘Bb’. “More brown fur!” Fourth box: Mom’s ‘b’ and Dad’s ‘b’. That makes ‘bb’. “Aha! White fur!”

Match points to the ‘bb’ box. “See? One out of four chances for a white fur baby. Even though both parents had brown fur!” He looks very pleased. “That’s the magic of the Punnett square.”

“Some genes are strong,” Match says. He flips a card. It shows a bold ‘A’. “We call them dominant.” He puts a smaller ‘a’ card next to it. “Strong genes can hide weak ones. Weak genes are called recessive.”

He shows an ‘Aa’ pair. “If you get one strong and one weak, the strong one wins.” He holds up two ‘aa’ cards. “The weak gene only shows up if both gene parts are weak. Like this. Both weak. So the weak trait shows.”

Match sighs a little. “But real genes are often much trickier,” he admits.