Inheritance

A **length of DNA that codes for one characteristic** (e.g. the gene for stem length).

One particular **version of a gene** (e.g. a 'tall' allele and a 'short' allele).

The **alleles an organism carries** for a gene — written as a pair of letters, e.g. **Tt**.

The **observable characteristic** an organism shows, produced by its genotype (e.g. 'tall').

An allele whose effect **shows with only one copy**; written as a **capital letter** (e.g. T).

An allele whose effect **shows only with two copies**; written as a **small letter** (e.g. t).

Having **two of the same allele** for a gene — e.g. **TT** (homozygous dominant) or **tt** (homozygous recessive).

Having **two different alleles** for a gene — e.g. **Tt**.

The **dominant allele is expressed and masks** the recessive one, so the recessive characteristic is not shown.

Only in a **homozygous-recessive** organism (e.g. tt) — one with **no dominant allele** to mask the recessive one.

A cross that follows the inheritance of **one gene** (with two alleles) from parents to offspring.

Because the two alleles **segregate** during meiosis — one goes into each gamete.

**Genotype** = the alleles you carry (e.g. Bb); **phenotype** = the observable characteristic those alleles produce.

Genotype **1 BB : 2 Bb : 1 bb**; phenotype **3 dominant : 1 recessive**.

**1/4 (25%)** each pregnancy, because only the homozygous-recessive (e.g. dd) cell is affected.

The disease allele is **recessive**: both parents are unaffected **carriers** (Dd), and if both pass on d the child is **dd** and affected.

Each of the four cells is equally likely, so count the matching cells out of 4 (e.g. **1 in 4 = 1/4 = 25%**).

Neither allele is fully dominant, so the **heterozygote shows a new, intermediate (blended)** phenotype (e.g. red × white → **pink**).

Both alleles are **fully expressed at the same time** in the heterozygote — you see **both** phenotypes together (not a blend).

A gene with **more than two** alleles in the population (e.g. ABO has **I^A, I^B and i**), though each individual still carries only **two**.

**1 red : 2 pink : 1 white** — the phenotype ratio equals the 1 : 2 : 1 genotype ratio because every genotype is visible.

**Incomplete dominance** = a single **blended** phenotype. **Codominance** = **both** phenotypes shown **together** (side by side).

Because **neither allele is recessive**, so neither should be written lower case — superscripts keep them equal.

**I^A and I^B are codominant** (both expressed → group AB); both are **dominant to i**, which is **recessive** (group O = i i).

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**.

Each genotype maps to one of only **four separate groups** (A, B, AB, O) with **no in-betweens** — distinct categories, not a continuous range.

**Female = XX**, **male = XY**.

The **22 pairs of chromosomes that are not the sex chromosomes** — the same in males and females.

The **father's sperm** — the egg always carries X, but a sperm carries **X or Y** (X→girl, Y→boy).

The gene is carried on a **sex chromosome (usually the X)**, so its inheritance is tied to the offspring's sex.

**On the X** — e.g. **Xᴮ** (dominant) and **Xᵇ** (recessive); the **Y has no matching allele**, written just **Y**.

Males have **only one X** and the **Y has no matching allele**, so a **single recessive allele shows**; a female needs it on **both** X chromosomes.

An **unaffected female (XᴮXᵇ)** who has one recessive allele, masked by the dominant allele on her other X; she can still pass it on.

**XᴮXᴮ (homozygous dominant)** or **XᴮXᵇ (carrier)** — you usually can't tell which from her phenotype.

Only **sons** — about **half** are colour-blind (XᵇY); **no daughter is affected**, though half are carriers.

**Square = male**, **circle = female**.

The person is **affected** — they show the condition. A clear symbol means unaffected.

An **unaffected** person who carries one copy of a recessive allele (e.g. genotype Dd) and can pass it on.

**Two unaffected parents have an affected child** — a dominant allele cannot hide in an unaffected parent.

The trait appears in **every generation** (no skipping) and affected children usually have an **affected parent**.

A father gives his **X to every daughter** and his **Y to every son** — so an **affected son** or an **unaffected daughter** of an affected father means the allele is **autosomal**.

His **Y** to a son; his **X** to a daughter.

She must be a **carrier (Dd)** — heterozygous.

**DD or Dd** — at least one dominant D allele.

Each **generation**, oldest at the **top**.

A chart of a cell's chromosomes **arranged in homologous pairs by size** (and banding pattern).

The **chromosome number**, the **sex** (XX/XY), and whether any chromosome is **extra or missing**.

Look at the **last (23rd) pair**: **XX = female**, **XY = male** (a large X next to a small Y).

Compare the **chromosome number**, the **relative sizes** of the chromosomes, and the **banding pattern**.

A pattern of **DNA bands** that is almost **unique to an individual**, used to identify people and their relatives.

**Every band in the child must match a band in one of its two parents** — a candidate missing a band is ruled out.

Remove the bands the child shares with the **mother**; the **true father must have all the remaining bands**.

**Crossing over** and **independent assortment** in meiosis, plus **random fertilisation**.

Meiosis makes genetically different **gametes**, and **random fertilisation** combines two of them into a new mix of alleles.