Genetics Decoded

Genetics Decoded 🧬

Why do you have your grandmother's eyes? Inheritance follows rules you can actually **predict** — with a little genetic vocabulary and a simple grid. Learn the language of alleles, run a cross yourself, and work out the odds of inheriting a trait.

Alleles 🔤

A **gene** can come in different versions called **alleles**. For each gene you inherit **two** alleles — one from each parent. • A **dominant** allele shows its effect with just **one** copy (written as a **capital**, e.g. **B**). • A **recessive** allele only shows its effect when you have **two** copies (written **lowercase**, e.g. **b**).

Match the terms

Genotype and phenotype 👥

Two more pairs of words: • Your **genotype** is the alleles you have (**BB**, **Bb** or **bb**); your **phenotype** is the characteristic they produce. • **Homozygous** means two of the **same** allele (BB or bb); **heterozygous** means two **different** alleles (Bb).

Name the genotype

Genotype or phenotype?

A pea plant has the alleles **Bb**. Is "Bb" its genotype or its phenotype?

• Genotype — it is the combination of alleles
• Phenotype — it is the visible characteristic
• Both mean the same thing
• Neither — it is an allele

The Punnett square ▦

To predict a cross, draw a **Punnett square**. Put one parent's two gametes across the **top** and the other parent's down the **side**. Each cell combines a top allele with a side allele to give an offspring **genotype**. Write each genotype with the **dominant (capital) allele first** — so **Bb**, never **bB**.

Cross them: Bb × Bb

An interactive activity.

Work out the odds

An interactive activity.

The phenotype ratio

In that Bb × Bb cross (B dominant), what is the ratio of dominant to recessive **phenotype** in the offspring?

• 3 : 1
• 1 : 1
• 1 : 3
• 1 : 2 : 1

Inherited disorders 🩺

Some disorders are caused by a single faulty allele: • **Polydactyly** (extra fingers or toes) — caused by a **dominant** allele, so one copy is enough to have it. • **Cystic fibrosis** (a disorder of cell membranes) — caused by a **recessive** allele, so you need **two** copies. A person with just one copy is an unaffected **carrier**.

Dominant or recessive?

Two carriers

Both parents are unaffected carriers of cystic fibrosis (each Ff, where f is the recessive CF allele). What is the chance their child HAS cystic fibrosis?

• 1 in 4 (25%)
• 1 in 2 (50%)
• None — carriers are healthy
• All of their children

How sex is decided ⚧️

Of your 23 pairs of chromosomes, the 23rd pair are the **sex chromosomes**: • **XX** = female • **XY** = male The mother is XX, so every egg carries an **X**. The father is XY, so his sperm carries **either X or Y** — and that is what decides the sex.

Cross for sex: XX × XY

An interactive activity.

In the exam 🎓

Inheritance decoded. Grade-9 habits: • Use the vocabulary precisely: **allele, dominant/recessive, homozygous/heterozygous, genotype/phenotype**. • Lay Punnett squares out properly — **gametes shown**, genotypes with the capital first — and always **state the ratio or probability**. • **Polydactyly** = dominant, **cystic fibrosis** = recessive; and the XX × XY cross gives a **50 : 50** ratio of females to males.