# Inheritance Patterns

> Mendel's work, alleles, genotype and phenotype, Punnett squares, codominance, incomplete dominance, sex-linked inheritance and pedigrees.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/inheritance-patterns/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The monk and his peas
In a monastery garden in Brno, in the 1850s and 1860s, a monk named Gregor Mendel grew and crossed about 28 000 pea plants. He crossed tall plants with short ones and counted the results in each generation.

The first generation was all tall. But when those tall plants were crossed with each other, short plants came back, about one in every four. Mendel realised that each plant must carry two "factors" for height, one from each parent, and that one factor can hide another. Nobody paid much attention for 35 years. Today we call his factors **genes**, and he is called the father of genetics.

**In short:** Inheritance patterns show how traits pass from parents to offspring through alleles, the different versions of a gene. A dominant allele shows with one copy and a recessive allele needs two, so two Tt parents give a 3 : 1 ratio of dominant to recessive phenotypes.

## Starting point: what you already know

- Body cells have pairs of chromosomes, one from each parent (see DNA, Chromosomes and Cell Division).
- Gametes are haploid and carry one of each pair.
- Genes code for features.

## Key words

| Term | Meaning |
|---|---|
| **Allele** | A different version of a gene (for example, tall or short) |
| **Dominant** | An allele that shows its effect even if only one copy is present. Capital letter: **T** |
| **Recessive** | An allele that shows only if two copies are present. Lower case: **t** |
| **Genotype** | The alleles an organism has: **TT**, **Tt** or **tt** |
| **Phenotype** | The feature you can see: tall or short |
| **Homozygous** | Two identical alleles: **TT** or **tt** |
| **Heterozygous** | Two different alleles: **Tt** (a "carrier" of the recessive allele) |

Phenotype depends on genotype **and** the environment. A pea plant with tall alleles will still be short if it has poor soil.

## Monohybrid crosses with Punnett squares

### Worked example: Crossing two heterozygous tall plants
Parents: **Tt × Tt**. Each parent makes gametes **T** and **t**.

| | **T** | **t** |
|---|---|---|
| **T** | TT | Tt |
| **t** | Tt | tt |

Genotype ratio: 1 TT : 2 Tt : 1 tt.
Phenotype ratio: **3 tall : 1 short**. The chance of a short plant is $\frac{1}{4}$ or 25%.

**Test cross.** To find out if a tall plant is TT or Tt, cross it with a short (tt) plant. If any offspring are short, the tall parent must be Tt.

## When neither allele is dominant

**Codominance**: both alleles show fully in the heterozygote. A roan cow, genotype $C^R C^W$, has both red hairs and white hairs mixed.

**Incomplete dominance**: the heterozygote is a **blend**. Red snapdragon × white snapdragon gives pink flowers. Pink × pink gives 1 red : 2 pink : 1 white.

**Multiple alleles**: some genes have more than two alleles. Human ABO blood groups have three: $I^A$ and $I^B$ are codominant, and $i$ is recessive.

| Blood group | Possible genotypes |
|---|---|
| A | $I^A I^A$ or $I^A i$ |
| B | $I^B I^B$ or $I^B i$ |
| AB | $I^A I^B$ |
| O | $i\,i$ |

## Sex-linked inheritance

Genes on the **X chromosome** are **sex-linked**. Males (XY) have only one X, so a single recessive allele on it will always show. This is why red-green colour blindness and haemophilia are much more common in males.

### Worked example: A carrier mother
Mother: $X^B X^b$ (normal vision, carrier). Father: $X^B Y$ (normal).

| | $X^B$ | $Y$ |
|---|---|---|
| $X^B$ | $X^B X^B$ | $X^B Y$ |
| $X^b$ | $X^B X^b$ | $X^b Y$ |

Daughters: all normal vision (half are carriers). Sons: half normal, **half colour blind**.

## Pedigrees

A **pedigree** is a family tree showing a trait. Squares are males, circles are females, shaded shapes show the trait.

### Exam tip: Reading a pedigree
- Two unaffected parents with an affected child: the trait is **recessive**.
- Two affected parents with an unaffected child: the trait is **dominant**.
- Mostly males affected, passed from carrier mothers to sons: likely **X-linked recessive**.

**Mind map**

```text
Inheritance
  Words
    Allele, dominant, recessive
    Genotype and phenotype
    Homozygous and heterozygous
  Monohybrid cross
    Punnett square
    Tt × Tt gives 3 : 1
    Test cross with tt
  Other patterns
    Codominance: both show
    Incomplete: blend
    Multiple alleles: ABO
  Sex-linked
    On the X chromosome
    More common in males
  Pedigrees
    Recessive: skips generations
    Dominant: in every generation
```

### Year 11: Dihybrid crosses
A **dihybrid cross** follows two genes at once. If the genes are on different chromosomes, they are inherited independently.

Pea seeds: Round (R) is dominant to wrinkled (r). Yellow (Y) is dominant to green (y). Cross **RrYy × RrYy**. Each parent makes four gametes: RY, Ry, rY, ry. A 4 × 4 Punnett square gives 16 boxes and the famous ratio:

$$9 \text{ round yellow} : 3 \text{ round green} : 3 \text{ wrinkled yellow} : 1 \text{ wrinkled green}$$

If results are far from 9 : 3 : 3 : 1, the genes may be **linked** (close together on the same chromosome) and are inherited together unless crossing over separates them.

**Recombination.** If two genes on the same chromosome are separated by crossing over in 8% of gametes, they are **linked** and fairly close together. The further apart two genes are, the more often crossing over separates them.

**Multiple alleles in a dominance series.** Rabbit coat: agouti > chinchilla > Himalayan > albino. Two agouti rabbits can have an albino kit if both carry the albino allele hidden underneath.

### Year 12: Probability shortcuts for the HSC
For independent genes, multiply probabilities. What is the chance that a child of two parents who are both **Aa Bb** has genotype **aa bb**?

$$P(aa) \times P(bb) = \frac{1}{4} \times \frac{1}{4} = \frac{1}{16}$$

Each child is an independent event. If two carrier parents already have one child with cystic fibrosis, the chance that the next child has it is still $\frac{1}{4}$.

**Carrier chance when unaffected.** Two carriers (Aa × Aa) have an unaffected child. Of the unaffected outcomes (AA, Aa, Aa), two are carriers, so the chance is $\frac{2}{3}$.

**Exactly one of two children.** Affected then unaffected, or unaffected then affected: $2 \times \frac{1}{4} \times \frac{3}{4} = \frac{3}{8}$.

**Dominant disease.** A heterozygous parent with Huntington's disease and an unaffected partner: Hh × hh gives $\frac{1}{2}$ for each child.

**X-linked patterns.**
- X-linked **dominant**: an affected father passes it to **all** his daughters and none of his sons.
- A carrier mother and an affected father ($X^H X^h \times X^h Y$): half the daughters and half the sons are affected.
- Tortoiseshell cats: $X^B X^O \times X^O Y$ gives tortoiseshell or orange daughters, and black or orange sons.
- In birds, females are **ZW** and males ZZ, so females have only one Z and show sex-linked recessive traits more often.

**Small samples mislead.** One family where only males are affected is too few results to prove X-linkage; it could happen by chance.

## Quick summary

- Alleles are versions of a gene. Dominant alleles show with one copy, recessive alleles need two.
- Genotype is the alleles. Phenotype is what you see, shaped by genes and environment.
- Tt × Tt gives 3 dominant : 1 recessive phenotypes.
- Codominance shows both alleles. Incomplete dominance blends them. ABO blood groups have multiple alleles.
- X-linked recessive traits are more common in males.
- Pedigrees show how a trait passes through a family.

## Quiz

Try these questions from the chapter quiz.

1. Different versions of the same gene are called
   - A. alleles
   - B. chromosomes
   - C. phenotypes
   - D. gametes
   - Answer: A. alleles. Alleles are gene variants.

2. A plant has genotype Tt. It is
   - A. homozygous dominant
   - B. heterozygous
   - C. homozygous recessive
   - D. haploid
   - Answer: B. heterozygous. Two different alleles.

3. Which describes a phenotype?
   - A. Tt
   - B. TT
   - C. Tall
   - D. An allele
   - Answer: C. Tall. The phenotype is the visible feature.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/inheritance-patterns/#quiz