Life Sciences · Lesson 02

DNA & Heredity

DNA stores the instructions for building and maintaining living things. In this lesson, trace the path from the double helix to genes, alleles, replication, and Punnett squares so you can explain how traits are encoded, copied, and passed from parent to offspring.

Double helix Genes Alleles Replication Punnett squares

Investigate Use base-pairing rules and Punnett squares to model how biological information moves.

Predict Make a claim about offspring outcomes before running simulated trials.

Explain Use evidence from the model to explain why probability is not a guarantee.

3D DNA Model by artisan3dventures on Sketchfab

Starter

Four ideas to keep straight

DNA

The molecule that stores hereditary information using the bases A, T, C, and G.

Gene

A segment of DNA that carries instructions related to a trait or cell function.

Allele

A version of a gene. Different alleles can produce different trait outcomes.

Trait

An observable characteristic, such as eye color, seed shape, or blood type.

Big Idea

Heredity is information transfer

Parents pass DNA to offspring. Offspring receive combinations of alleles, and those combinations help shape their traits. The central question in genetics is simple: what information was passed on, and how does it show up in the organism?

Parents -> gametes -> offspring Genotype vs phenotype

Structure

How the double helix works

The DNA double helix looks like a twisted ladder. The sides are sugar-phosphate backbones. The rungs are base pairs. Adenine pairs with thymine, and cytosine pairs with guanine. That matching rule matters because it lets DNA copy itself accurately.

Part	Job	Why it matters
Backbone	Supports the molecule	Keeps the long DNA strand stable
Base pairs	Store coded information	The order of bases is the message
Complementary pairing	Matches A-T and C-G	Makes DNA replication possible

Hands-on model

Replication base-pairing lab

DNA can copy itself because each base has a specific partner. Complete the new strand by matching each exposed base, then check your evidence.

Template strand

New strand

Select a blank slot, then choose A, T, C, or G.

Copying

Replication

Before a cell divides, DNA unzips and each strand acts like a template. New matching bases are added, producing two nearly identical DNA molecules.

Instructions

Genes and proteins

Genes are not traits themselves. They are instructions cells use to build proteins, and proteins help create trait outcomes.

Variation

Alleles

Different versions of the same gene can lead to different outcomes, which is why offspring are similar to parents but not identical.

Compare

Genotype vs. phenotype

Genotype

The allele combination an organism carries, such as TT, Tt, or tt.

Phenotype

The visible or measurable trait outcome, such as tall pea plant or short pea plant.

Two different genotypes can sometimes lead to the same phenotype. For example, both TT and Tt can produce a dominant trait.

Tutorial

How to build a Punnett square

A Punnett square is a 2 × 2 grid that maps every possible allele combination two parents can produce. Follow these five steps with any monohybrid cross.

Identify the parents' genotypes

Write the two alleles for each parent. For example, a heterozygous tall parent is Tt and a short parent is tt. Capital letters represent dominant alleles; lowercase letters represent recessive alleles.

Place Parent 1's alleles across the top

Split the first parent's genotype into its two separate alleles and write one above each column of the grid. For Tt that's T over column 1 and t over column 2.

Place Parent 2's alleles down the left side

Split the second parent's genotype and write one allele to the left of each row. For tt that's t on row 1 and t on row 2.

Fill each cell by combining the row allele + column allele

Take the column allele and the row allele for each cell and write them together. Always write the dominant allele first. Row 1 × Col 1 = Tt, Row 1 × Col 2 = tt, and so on.

Read the results — count genotypes and phenotypes

Count how many times each genotype appears. For Tt × tt: 2 cells show Tt (tall) and 2 show tt (short). That gives a 1 : 1 phenotype ratio — half tall, half short. Every cell is an equally likely outcome.

Mathematics

Punnett squares are probability models

Each of the four cells represents one equally likely outcome. That means you can convert any Punnett square directly into fractions, decimals, percentages, and ratios — the same tools used in any probability problem.

Cross: Tt × Tt The 1 : 2 : 1 genotype ratio

When both parents are heterozygous, the four cells produce three genotypes. TT appears once, Tt appears twice, tt appears once.

25%

50%

25%

Cross: Tt × Tt The 3 : 1 phenotype ratio

Both TT and Tt produce the dominant (tall) phenotype. Only tt shows the recessive (short) phenotype. So 3 of 4 offspring look tall.

Tall

75%

Short

25%

Ratio 3 : 1 means for every 1 short plant, expect 3 tall plants.

All crosses Fraction → Decimal → Percent

Fraction	Decimal	Percent
1 / 4	0.25	25%
2 / 4	0.50	50%
3 / 4	0.75	75%
4 / 4	1.00	100%

These are the only four fractions a 2 × 2 Punnett square can produce.

Cross	Genotype ratio	Phenotype ratio	Dominant probability
`TT × TT`	4 TT : 0 : 0	4 dominant : 0 recessive	4/4 = 100%
`TT × tt`	4 Tt : 0 : 0	4 dominant : 0 recessive	4/4 = 100%
`Tt × Tt`	1 TT : 2 Tt : 1 tt	3 dominant : 1 recessive	3/4 = 75%
`Tt × tt`	2 Tt : 2 tt	2 dominant : 2 recessive	2/4 = 50%
`tt × tt`	4 tt : 0 : 0	0 dominant : 4 recessive	0/4 = 0%

Practice

Fill in the Punnett square

Choose a cross below. Click an empty cell in the grid to select it, then pick the correct genotype from the right panel. Click Check answers when all four cells are filled.

Cross

Choose genotype for selected cell

Breeding Lab

Genetic Breeding Lab

Pick a trait, choose the parent genotypes, and watch the square re-calculate. Then click any offspring box to turn the genotype into a visible pea plant phenotype.

Trait

Parent 1

Parent 2

Empirical test

Run offspring trials

Use the current cross to simulate many offspring. Small samples may wander; larger samples usually move closer to the Punnett square prediction.

Phenotype	Expected	Observed
Run trials to collect data.

Prediction comes from the square. Evidence comes from repeated trials.

Apply

Three quick case studies

Pea plants

Mendel tracked visible traits like seed color and plant height to infer rules about dominant and recessive alleles.

Family resemblance

Children inherit allele combinations from both parents, which helps explain why siblings share traits but still differ.

DNA evidence

Because DNA sequences differ across individuals, genetics can also be used in medicine, ancestry, and forensic science.

Limits of simple models

Many traits are influenced by multiple genes and the environment, so real heredity is often more complex than one Punnett square.

Discuss

Reflection questions

Why does complementary base pairing make DNA replication possible?
What is the difference between a gene, an allele, and a trait?
Why is a Punnett square a probability model instead of a guarantee?
What are some examples where environment changes how a trait appears?