Introduction
Okay, let's talk inheritance. This is the chapter that answers the questions you've actually wondered about: why you've got your dad's nose, why two brown-eyed parents can have a blue-eyed kid, why colour blindness runs in the boys of a family. It's genetics, and once you crack the code it's honestly one of the most satisfying topics in biology.
We're going to break down how information is stored in DNA, how cells copy it (mitosis) and halve it (meiosis), and then the big one: how to work out what offspring you'll get from a cross, using a little grid that does most of the work for you. Let's get into it 🧬
The Blueprint: Chromosomes, Genes and DNAE17.1
Inside almost every cell, tucked in the nucleus, is your entire instruction manual, written in a molecule called DNA. Learn these three words biggest-to-smallest: a chromosome is a long thread of DNA; a gene is a short length of DNA that codes for one protein; an allele is just a different version of a gene (like a "free earlobe" version vs an "attached earlobe" version).
DNA itself is two strands twisted into a ladder (a double helix). The rungs are pairs of bases, and they always pair up the same way: A pairs with T, C pairs with G. That fixed pairing is why DNA can be copied perfectly, each strand is a template for the other.
> 🔑 Key idea: nucleus → chromosome (thread of DNA) → gene (codes for one protein) → allele (a version of a gene). In DNA, A pairs with T and C pairs with G.
Worked Example: Fill In the Missing StrandE17.1
Problem One strand of a DNA molecule reads `A C T T A G A T`. A pairs with T, and C pairs with G. What is the base sequence of the other (complementary) strand?
Step 1 Go letter by letter, in the same order, and write each base's partner underneath: A→T, C→G, T→A, G→C. $$\text{A C T T A G A T} \;\rightarrow\; \text{T G A A T C T A}$$
Step 2 So the complementary strand is `T G A A T C T A`. Just don't flip the order round, keep it left-to-right.
Step 3 Lock in the pairing rule: A always goes with T, C always goes with G. That's the whole trick to every "complete the DNA strand" question.
The Code: How DNA Runs the CellE17.1
Here's what genes are actually for: DNA controls the cell by controlling which proteins get made. And proteins are a huge deal, enzymes, haemoglobin, antibodies, keratin, they run basically everything. The order of bases in a gene decides the order of amino acids in the protein, and that decides what the protein does.
But the DNA never leaves the nucleus, while proteins are built at the ribosomes in the cytoplasm. So the cell makes a portable copy called mRNA that carries the message out. The flow to memorise: DNA (gene) → mRNA → ribosome → protein. Think of mRNA as a photocopy of one recipe, sent to the kitchen so the original cookbook never leaves the shelf.
> 🔑 Key formula: DNA (gene) → mRNA → ribosome → protein. mRNA carries the code from the nucleus to the ribosome, where the protein is built.
Worked Example: Spot the ProteinE17.1
Problem DNA controls the cell by controlling proteins. Which of these is a protein: starch, glycogen, haemoglobin, or glucose?
Step 1 Sort them by type: starch and glycogen are carbohydrates, glucose is a sugar. None of those are proteins.
Step 2 Haemoglobin is the red oxygen-carrier in your blood, and it is a protein.
Step 3 So the answer is haemoglobin. Remember the rule: genes code for proteins (enzymes, antibodies, haemoglobin, keratin), never for sugars, starch or fat.
Chromosome Count: Haploid vs DiploidE17.1
Chromosomes come in matching pairs, one from each parent. Two words: diploid means two full sets (chromosomes in pairs), and haploid means one single set. Your body cells are diploid; your gametes (sperm and eggs) are haploid.
Human numbers to lock in: a body cell has 46 chromosomes = 23 pairs; a gamete has 23. At fertilisation, sperm (23) + egg (23) → 46 again. That's exactly why gametes have to be haploid, if they were diploid, the number would double every generation. Oh, and one pair is the sex chromosomes: XX = female, XY = male.
> 🔑 Key idea: body cell = diploid (46 = 23 pairs); gamete = haploid (23). Female XX, male XY. Fertilisation brings the number back to 46.
Worked Example: Which Cell Is Haploid?E17.1
Problem A human body cell is diploid. Which of these is haploid: a liver cell, a skin cell, a sperm cell, or a nerve cell?
Step 1 Remember: haploid = a gamete (sex cell). Everything else in the body is diploid.
Step 2 Liver, skin and nerve cells are all ordinary body cells, so they're diploid (46).
Step 3 Only the sperm cell is a gamete, so the sperm cell is the haploid one (23 chromosomes).
Copy-Paste Division: Mitosis (and Stem Cells)E17.2
Mitosis is cell division that makes two identical copies of a cell, each with the same number of chromosomes as the parent. The trick: every chromosome is copied first, then one copy of each goes to each new cell, so both come out identical. Mitosis handles growth, repair, replacing worn-out cells, and asexual reproduction (like a strawberry plant cloning itself on runners).
Stem cells are the VIPs here: unspecialised cells that keep dividing by mitosis and can turn into specialised cells. Your red bone marrow has stem cells that pump out new blood cells; an early embryo is a ball of stem cells that becomes the whole body.
> 🔑 Key idea: mitosis = 2 identical cells, same chromosome number, for growth/repair/replacement/asexual reproduction. Stem cells = unspecialised cells that divide by mitosis and can specialise.
Worked Example: Mitosis Keeps the CountE17.2
Problem A parent cell with 12 chromosomes divides by mitosis. How many chromosomes does each new cell have, and how do they compare with the parent?
Step 1 In mitosis, each chromosome is copied, then shared out one-to-each. The number stays the same.
Step 2 So each new cell has 12 chromosomes, exactly like the parent, and is genetically identical to it.
Step 3 Watch the classic trap: the answer is not 6. Mitosis keeps the number the same. Only meiosis halves it.
Half-Set Division: MeiosisE17.3
Meiosis is the opposite vibe to mitosis. It halves the chromosome number (so it's called a reduction division) and makes four gametes that are all genetically different from each other and the parent. That's exactly what you need for sex cells.
Why not just use mitosis for gametes? Because gametes join at fertilisation. If they were diploid, every generation would double the chromosome number, chaos. Meiosis halves it first so fertilisation restores the normal number. Bonus: because meiosis shuffles the alleles, it creates the variation that makes every sibling different (well, except identical twins).
> 🔑 Key idea: meiosis = reduction division → 4 genetically different haploid gametes. Halves the number so fertilisation can restore it. Mitosis = identical; meiosis = different.
Worked Example: Meiosis Halves ItE17.3
Problem A leaf cell of a plant contains 20 chromosomes. A pollen grain nucleus (a gamete) is made from this plant by meiosis. How many chromosomes does the pollen grain nucleus have?
Step 1 The leaf cell is a body cell, so it's diploid (20 chromosomes). A pollen grain nucleus is a gamete, made by meiosis.
Step 2 Meiosis halves the number: $$20 \div 2 = 10$$
Step 3 So the pollen grain nucleus has 10 chromosomes. When it fertilises an egg cell (also 10), the new plant cell is back to 20.
Genetics Vocab: Say It RightE17.4
Before you can do crosses, nail this vocab, it's easy marks. Genotype = the alleles you have (the letters, like `Tt`). Phenotype = what you actually look like (like "tall"). Homozygous = two of the same allele (`TT` or `tt`). Heterozygous = two different ones (`Tt`). Dominant = shows up even with one copy (write it as a CAPITAL). Recessive = only shows up with two copies (write it lowercase).
Quick example with pea height (`T` tall dominant, `t` short recessive): `TT` and `Tt` are both tall (the `T` wins), and only `tt` is short. Notice two different genotypes can look the same, that's why you can't always read someone's genotype from the outside.
> 🔑 Key idea: genotype = the letters, phenotype = the look. Homozygous = same alleles, heterozygous = different. Dominant (capital) shows with one copy; recessive (lowercase) needs two.
Worked Example: Genotype vs PhenotypeE17.4
Problem In pea plants, `T` (tall) is dominant to `t` (short). A plant has the genotype `Tt`. State its genotype and its phenotype, and say whether it's homozygous or heterozygous.
Step 1 The genotype is literally the letters given: `Tt`. Don't overthink it.
Step 2 The phenotype is what it looks like. Since `T` (tall) is dominant and there's a `T` present, the plant is tall.
Step 3 Two different alleles (`T` and `t`) means it's heterozygous. So: genotype `Tt`, phenotype tall, heterozygous. Golden rule, if they ask for genotype, give letters; if they ask for phenotype, describe the look.
The Grid That Does the Work: Monohybrid CrossesE17.4
Time for the money skill: working out offspring from a cross. Always draw the Punnett square (grid). Put one parent's gametes across the top, the other's down the side, then fill in the four boxes by combining them. Each gamete carries only one allele.
Classic example, two heterozygous tall peas (`Tt × Tt`): the four boxes come out `TT`, `Tt`, `Tt`, `tt`. That's three tall to one short, the famous 3 : 1 ratio. And here's the probability hack: the chance of any phenotype = (boxes with it) ÷ 4. So a `Tt × Tt` cross gives a $\frac{1}{4}$ chance of a short (`tt`) offspring.
> 🔑 Key formula: two heterozygotes (Tt × Tt) → 3 : 1 phenotype ratio (1 : 2 : 1 genotypes). Chance of a phenotype = (boxes with it) ÷ 4.
Worked Example: What Are the Odds?E17.4
Problem Cystic fibrosis is caused by a recessive allele `f` (the dominant `F` gives no cystic fibrosis). Two parents are both heterozygous (`Ff`). What is the probability that a child has cystic fibrosis?
Step 1 Draw the grid for `Ff × Ff`, filling the four boxes: `FF`, `Ff`, `Ff`, `ff`.
Step 2 Only `ff` has cystic fibrosis. That's 1 box out of 4. $$P(\text{cystic fibrosis}) = \frac{1}{4}$$
Step 3 So there's a $\frac{1}{4}$ (25%) chance for each child. This is why two healthy carriers can still have an affected child, the recessive allele was hiding in both of them.
Both Alleles Show: Codominance & Blood GroupsE17.4
Sometimes neither allele bosses the other, and both show up together in a heterozygote, that's codominance. Classic example: in some cattle a red allele and a white allele are codominant, so a heterozygote is roan (red hairs and white hairs together, not pink). Cross two roan cattle and you get 1 red : 2 roan : 1 white, a 1 : 2 : 1 ratio, not 3 : 1, because the heterozygote has its own look.
The human version is ABO blood groups, which has three alleles: $I^A$ and $I^B$ are codominant, and $I^O$ is recessive to both. So $I^A I^B$ gives blood group AB (both antigens show, that's the codominance), and only $I^O I^O$ gives group O.
> 🔑 Key idea: codominance = both alleles show together (roan cattle; blood group AB). ABO has three alleles: $I^A$, $I^B$ (codominant) and $I^O$ (recessive). Codominant cross → 1 : 2 : 1.
Worked Example: Roan Times RoanE17.4
Problem In cattle, a red allele and a white allele are codominant, and heterozygous animals are roan. Two roan cattle ($C^R C^W$) are crossed. What ratio of coat colours is expected in the offspring?
Step 1 Draw the grid for $C^R C^W \times C^R C^W$. The four boxes are $C^R C^R$, $C^R C^W$, $C^R C^W$, $C^W C^W$.
Step 2 Read the phenotypes: $C^R C^R$ = red, the two $C^R C^W$ = roan, $C^W C^W$ = white.
Step 3 So the ratio is 1 red : 2 roan : 1 white. Codominance always gives this 1 : 2 : 1 look because the heterozygote (roan) is its own separate colour.
Boys, Girls and Colour Blindness: Sex LinkageE17.4
Sex is decided by the sex chromosomes: XX = female, XY = male. All eggs carry X; sperm are half X, half Y, so the sperm decides the sex (giving a 1 : 1 boy-to-girl chance).
A sex-linked feature is controlled by a gene on the X chromosome. Since boys have only one X, a recessive allele on it shows up in a boy with just one copy, there's no second X to hide it. That's why red-green colour blindness (a recessive X-linked condition) is way more common in males. A girl needs two copies ($X^b X^b$) to be colour blind; a boy needs only one ($X^b Y$). A girl with one copy ($X^B X^b$) is a carrier, she can pass it on.
> 🔑 Key idea: sex-linked = gene on the X. Boys have one X, so recessive X-linked conditions (colour blindness) show up more in males. Notation: $X^B$ normal, $X^b$ colour blind, Y carries neither.
Worked Example: Colour Blindness CrossE17.4
Problem A colour-blind woman ($X^b X^b$) has children with a man who has normal vision ($X^B Y$). What will their sons and daughters be like?
Step 1 The mother can only pass on $X^b$. The father passes $X^B$ (to daughters) or Y (to sons). Draw the grid.
Step 2 Daughters get $X^b$ from mum and $X^B$ from dad → $X^B X^b$: normal vision, but all carriers.
Step 3 Sons get $X^b$ from mum and Y from dad → $X^b Y$: all colour blind. So every son is colour blind here, because his only X came from his colour-blind mother.
Key Points to Remember
- Nucleus → chromosome (thread of DNA) → gene (codes for one protein) → allele (a version of a gene). DNA: two strands, A pairs with T, C pairs with G.
- DNA controls the cell by controlling proteins: DNA → mRNA → ribosome → protein. All cells have the same genes but switch on different ones.
- Diploid = two sets (body cell, 46 = 23 pairs); haploid = one set (gamete, 23). Fertilisation restores 46. Female XX, male XY.
- Mitosis = 2 identical cells, same chromosome number (growth, repair, replacement, asexual reproduction). Stem cells divide by mitosis and can specialise.
- Meiosis = reduction division → 4 genetically different haploid gametes.
- Genotype = alleles/letters; phenotype = the look. Dominant (capital) shows with one copy; recessive (lowercase) needs two.
- Monohybrid cross: draw the Punnett square. Two heterozygotes → 3 : 1 phenotype ratio. Codominance (roan cattle, blood group AB) → 1 : 2 : 1. Sex-linked conditions (colour blindness) are commoner in males.