Every one of your genes came in two copies — one from each parent. That single fact is the whole of inheritance. Once you can see where those two copies come from, Punnett grids stop being a memory test and start being simple counting.
📚 What you need to know
Gametes (sperm and egg) are made by meiosis, so they are haploid — one copy of each chromosome, and therefore one allele of each gene.
Fertilisation is the fusion of two gamete nuclei. The zygote it makes is diploid, with two alleles of every gene.
Two identical alleles = homozygous. Two different alleles = heterozygous.
A monohybrid cross follows one gene. A Punnett grid lists every gamete from each parent and pairs them up.
Homozygous parents that differ (e.g. BB × bb) give a 100% heterozygous F1; crossing two of that F1 gives the famous 3 : 1 phenotype ratio in the F2.
Ratios are probabilities, not promises — they only show up clearly in large numbers of offspring.
Where the two alleles come from
Body cells are diploid. In humans that means 46 chromosomes, arranged as 23 matching pairs. One chromosome of each pair came from your mother, the other from your father.
Meiosis halves that number when gametes are made. A sperm cell or an egg cell carries just 23 chromosomes — one from each pair. So a gamete carries one allele of every gene, never two.
At fertilisation the two haploid nuclei fuse. The full number is restored, and the new zygote has two alleles of every gene again: one from mum, one from dad.
The egg is much bigger because it carries food stores for the early embryo — but it passes on exactly the same number of chromosomes as the tiny sperm.
Students often say “the sperm gives half the DNA and the egg gives the rest of it”. That is right for the nucleus, and it is what exam questions mean. Mitochondrial DNA is the odd one out — it comes from the egg only.
The words you must use precisely
Marks are lost here more than anywhere else in genetics, so nail the vocabulary early.
Word
What it means
Example
Gene
A length of DNA that codes for one polypeptide, sitting at a fixed place on a chromosome
The gene for coat colour in mice
Allele
One version of that gene, differing by a few bases
B (black) or b (brown)
Genotype
The two alleles an individual carries
BB, Bb or bb
Phenotype
The feature you can actually see or measure
Black fur or brown fur
Homozygous
The two alleles are the same
BB or bb
Heterozygous
The two alleles are different
Bb
Dominant and recessive
A dominant allele shows up in the phenotype whenever it is present, even if there is only one copy. A recessive allele only shows in the phenotype when there is no dominant allele to hide it — so you need two copies of it.
That is why Bb and BB look identical from the outside. Both make black mice. Only bb makes a brown one.
Why one copy is enough. The dominant allele usually codes for a working protein. One working copy is often plenty to do the job, so the phenotype looks normal. The recessive allele often codes for a protein that does not work — and you only notice that when there is no working copy left.
Building a Punnett grid
🧩 The five steps
Write the parental phenotypes and, under them, the parental genotypes.
Write the gametes each parent can make. Circle them — it reminds you they are haploid.
Put one parent’s gametes along the top of a grid and the other parent’s down the side.
Fill each box by combining the gamete at the top with the gamete at the side.
Count the genotypes, then convert them to phenotypes and give both ratios.
Show the gametes even if you can do the cross in your head. The gamete line is usually worth its own mark, and a missing line is the most common way to drop marks on an otherwise perfect answer.
The brown phenotype disappears in the F1 and comes back in the F2. It was never lost — the recessive allele was just hidden inside the heterozygotes.
Worked examples
WORKED EXAMPLE 1
A pure-breeding black mouse (BB) is crossed with a pure-breeding brown mouse (bb). Give the genotype and phenotype of the F1.
Step 1: parental genotypes
BB (black) × bb (brown)
Step 2: gametesBB can only make B. bb can only make b.A homozygous parent makes just one kind of gamete — that is what “pure-breeding” is telling you.Step 3: combineB + b → Bb, every single time100% Bb, all blackGenotype ratio and phenotype ratio are both 1 : 0 here, because there is only one possible outcome.
WORKED EXAMPLE 2
Two F1 mice from that cross are bred together. They have 24 pups. How many would you expect to be brown?
Step 1: parents and gametes
Bb × Bb, so each parent makes B or bStep 2: grid
B×B = BB, B×b = Bb, b×B = Bb, b×b = bb
Step 3: read off the ratio1 BB : 2 Bb : 1 bb → 3 black : 1 brownStep 4: apply it to 24 pups24 ÷ 4 = 6 brown, 18 blackAbout 6 brown pupsSay “about” or “expected”. Each pup is an independent 1-in-4 chance, so a real litter might give 4 or 8.
Gametes
B
b
B
BB — black
Bb — black
b
Bb — black
bb — brown
Crosses in flowering plants
Mendel worked with pea plants for a good reason: he could control exactly which pollen reached which flower. Pollen carries the male gamete, the ovary holds the female gametes, and moving pollen by hand meant he always knew both parents.
Many plants can self-pollinate, because one flower carries both male and female parts. Growers use this to keep a useful feature. To combine two useful features from different plants, they cross-pollinate instead. Genetic crosses let them predict the outcome before they plant a whole field.
💡 Exam tip
Always write the full working: parental phenotypes, parental genotypes, gametes, grid, then offspring ratios. Each line can be worth a mark.
Give the phenotype ratio as well as the genotype ratio unless the question clearly only wants one.
Use letters that look different in upper and lower case. B and b are fine; S and s are risky. If you are forced to use a risky pair, write them large and clearly.
Use the same letter for both alleles of a gene. B and b, never B and g — that would look like two different genes.
Watch the word “expected”. Ratios are probabilities, so real results wobble around them.
Read whether the question wants a ratio, a fraction or a percentage and answer in that form.
⚠ Common mix-up
Gene and allele used as if they mean the same thing. The gene is the instruction; the alleles are the different versions of it.
Putting two alleles in one gamete. Gametes are haploid — one allele each, always.
Thinking the recessive allele disappears in the F1. It is still there in every heterozygote, just hidden.
Assuming dominant means “common”. Plenty of dominant alleles are rare in a population; dominance is about masking, not about frequency.
Reporting 3 : 1 as a genotype ratio. 3 : 1 is the phenotype ratio; the genotype ratio is 1 : 2 : 1.
Forgetting that a heterozygous parent makes two kinds of gamete in equal numbers. That equality is what makes the grid work.
Up next: Inheritance: Key Terms — we slow right down on genotype, phenotype, locus and the two ways alleles can share a phenotype.
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