● Life Science · Quarter 1

Three in Four Is a Chance, Not a Count

The square takes a minute to fill. Reading it correctly takes the rest of the week.

Coins on the desk, one per pair. Say nothing about them yet. 1 min

A real Philippine example

Before the 1960s, Philippine rice grew tall.
Tall rice falls over in wind and rain, and the grain is lost.
At IRRI in Los Baños, a semi-dwarf variety was bred: shorter, stiffer, and it stayed standing.
That variety was IR8, and it changed how much rice this country could grow.

This is real, local, and it is genetics. Use it all week. 5 min

Two versions of one gene

Call tall T and semi-dwarf t.
Tall is dominant. One T is enough to make the plant tall.
Semi-dwarf is recessive. It only shows when there is no T at all — tt.

How many different pairs can a plant have?

Three: TT, Tt, tt. Write all three on the board. 4 min

Genotype and phenotype

Genotype

The letters. TT, Tt or tt. What the plant carries.

Phenotype

What you can see. Tall or semi-dwarf. Nothing else.

Which two genotypes look the same?

TT and Tt are both tall. You cannot tell them apart by looking, and that is the whole of item 3 later. 4 min

Filling the square

One parent's two letters across the top.
The other parent's two down the side.
Each box takes one letter from each.
Four boxes. Four combinations. That is all it is.

Do Tt × Tt on the board slowly, once. 5 min

Tt × Tt

Genotype ratio — 1 TT : 2 Tt : 1 tt
Phenotype ratio — 3 tall : 1 semi-dwarf

TT · Tt · Tt · tt

Why are the two ratios different?

Because Tt and TT look the same. Ask before answering. 5 min

Now the hard question

A farmer plants four seeds from that cross. How many will be semi-dwarf?

Take the answers. Most classes say one. Write the count on the board. 3 min

The honest answer

Nobody knows.
Each seed has a 1 in 4 chance, on its own.
All four could be semi-dwarf. None of them could be.
The square gives a chance for each, not a share of the four.

Then why is the square useful at all?

Because with four hundred seeds it is close, and with four it is not. 5 min

Flip to find out

Take a coin. Heads is T. Tails is t.
Flip twice — once for each parent.
TT, Tt or tt. Record it.
Do it ten times. Then bring your ten to the front.

Ten each, forty pairs in a class of forty. This is the slide the lesson turns on. 7 min

What the class got

Ten flips

Almost nobody gets exactly 3 and 1. Twos, fours and zeros are ordinary.

Four hundred flips

The whole class together lands close to 3 : 1 — usually within a few per cent.

What does that tell you about the square?

Fill in the real class numbers here. They are better than any printed example. 4 min

Four things the square does not say

It does not say three of your four children.
It does not say dominant is more common in the population.
It does not say Tt is halfway between TT and tt.
It does not say the next one is more likely because the last three were tall.

These four are on the wall chart. They are the week's misconceptions. 5 min

The recessive allele is unharmed

In Tt, the t is not damaged, weakened or overwritten.
It is simply not the one you can see.
Pass it on and it can show up whole, generations later.

How do two tall parents have a semi-dwarf offspring?

Both were Tt. This is the question that makes heredity click. 4 min

Traits in this room

Tongue rolling · attached or free earlobes · widow's peak
Count the class for one of them.
Does the dominant trait have more people?

Should it?

Not necessarily. Dominance is about which shows, not about how many. Slide 14. 4 min

Dominant does not mean common

Having extra fingers is dominant and very rare.
Blood type O is recessive and the most common type in the Philippines.
Dominance describes what happens in one individual, not how many people there are.

This is the misconception most likely to survive the week. Say it twice. 4 min

One sentence to leave with

Each offspring is a fresh throw of the same dice.

Say it together. It is on the worksheet and on the wall chart. 2 min