This mini-lesson covers cell division and inheritance from CCEA GCSE Biology (Unit 2): mitosis and meiosis, monohybrid inheritance, drawing Punnett squares, working out genotype and phenotype, how sex is determined, and how genetic conditions are inherited.
Make a guess: why doesn't the chromosome number keep doubling?
Mission 1Why doesn't a baby get double the chromosomes?
Mission 2How can a child show a feature neither parent has?
Work through each screen, answer the questions as you go (some are wordy, some use Punnett squares) and collect ⭐ stars. Press Start when you're ready.
Cell division · mitosis
Mitosis
Mitosis is the exact duplication of chromosomes to produce daughter cells that are genetically identical to the parent cell (they are clones).
Mitosis is part of the cell cycle and allows organisms to grow, to replace worn-out cells, and to repair damaged tissue.
The two new cells have the same number of chromosomes as the parent cell (in humans, 46).
Mitosis → two genetically identical daughter cells for growth, replacement and repair.
CCEA note: you need the outline of mitosis — you do not need the names of the individual phases or the detail of DNA replication.
Cell division · meiosis
Meiosis
Meiosis is a reduction division: one cell produces four genetically different, haploid daughter cells (the gametes — sperm and egg).
Reduction division means the chromosome number is halved — human gametes have 23 single chromosomes, not 23 pairs.
Meiosis reassorts the chromosomes (by independent assortment), so the four gametes are all different — this creates variation.
At fertilisation two haploid gametes fuse to restore the full number (23 + 23 = 46).
Meiosis → 4 different, haploid gametes; halves the chromosome number and adds variation.
Mitosis vs meiosis: mitosis = 2 identical diploid cells (growth/repair); meiosis = 4 different haploid cells (gametes).
Sort it
Mitosis or meiosis?
Tap a feature, then tap the type of division it belongs to.
🔁 Mitosis
🧬 Meiosis
✅ Both
Quick check
Which makes gametes?
?Which type of cell division produces gametes (sperm and egg cells) that are haploid and genetically different from one another?
Mission 1 check
Why doesn't a baby get double the chromosomes?
?A dog's body cells each have 78 chromosomes. Puppies also have 78. How many chromosomes are in each dog sperm cell, and why?
Mission 2 of 2
How can a child show a feature neither parent has?
Make a guess: how can a child show a feature neither parent shows?
Genetics · key terms
The words you must know
Gene — a length of DNA controlling a characteristic. Allele — a version of a gene.
Dominant allele — shows its effect even if only one copy is present (written as a capital letter, e.g. B).
Recessive allele — only shows when two copies are present (a lower-case letter, e.g. b).
Genotype — the alleles an organism has (e.g. Bb). Phenotype — the characteristic you see (e.g. black fur).
Homozygous — two same alleles (BB or bb). Heterozygous — two different alleles (Bb).
Rule: a dominant allele always uses a capital letter; the recessive form is the same letter in lower case. So in rabbits, B (black fur) is dominant and b (brown fur) is recessive.
Match it
Match each term to its meaning
Tap a term on the left, then its matching meaning on the right.
Term
Meaning
Genetics · monohybrid cross
Monohybrid crosses & Punnett squares
A monohybrid cross follows one characteristic controlled by one gene with two alleles. A Punnett square shows every way the parents' alleles can combine.
Worked example: cross two heterozygous black-furred rabbits (Bb × Bb). B = black fur (dominant), b = brown fur (recessive).
B
b
B
BB
Bb
b
Bb
bb
Genotypes: 1 BB : 2 Bb : 1 bb
Phenotypes: 3 black (BB, Bb, Bb) : 1 brown (bb) — a 3 : 1 ratio.
So there is a 75% (3 in 4) chance of black fur and a 25% (1 in 4) chance of brown.
Remember: a brown-furred rabbit must be bb — they need two recessive alleles, one from each parent.
Calculate
Your turn — offspring ratio
1In the cross Bb × Bb above, the rabbits have 4 offspring. Based on the 3 : 1 ratio, how many are expected to have brown fur (genotype bb)?
rabbit
Hint: the ratio 3 black : 1 brown means 1 in every 4 is brown. 4 offspring × ¼ = ?
Calculate
Your turn — probability as a percentage
2A different cross gives a 3 : 1 ratio of tall to short plants. What is the probability of a tall plant, as a percentage?
%
Hint: 3 tall out of every 4 plants = 3 ÷ 4 = 0.75 = 75%.
Mission 2 check
How can a child show a feature neither parent has?
?Two black guinea pigs have a baby with brown fur. Black fur (B) is dominant; brown fur (b) is recessive. What must the parents' genotypes be?
Genetics · sex determination
How sex is determined
One of the 23 pairs of chromosomes is the sex chromosomes. Females are XX; males are XY.
Every egg carries an X. A sperm carries either X or Y.
Sperm with X → XX (girl). Sperm with Y → XY (boy).
So there is a 50 : 50 (1 : 1) chance of a boy or a girl.
X
Y
X
XX
XY
X
XX
XY
It's the father: because eggs are always X, it is the sperm (X or Y) that decides the sex of the baby.
Quick check
Boy or girl?
?From the Punnett square above, what is the probability that a baby will be a boy (XY)?
Genetics · genetic conditions
Inherited genetic conditions
Some conditions are caused by faulty alleles. CCEA names four:
Cystic fibrosis — caused by a recessive allele; affects the lungs and digestion. A child needs two faulty alleles (one from each carrier parent).
Huntington's disease — caused by a dominant allele; symptoms usually appear in middle age. Only one copy is needed.
Haemophilia — caused by a recessive allele on the X chromosome (sex-linked); the blood does not clot properly, so cuts bleed for a long time. Males (XY) need only one copy, so it is far more common in boys.
Down's Syndrome — caused by an extra chromosome (not a single faulty allele).
Dominant vs recessive conditions: a recessive condition (cystic fibrosis) can be carried silently by healthy carriers; a dominant condition (Huntington's) shows whenever the allele is present.
Quick check
Two carrier parents
?Cystic fibrosis is recessive (allele f). Two healthy carrier parents are both Ff. What is the chance that a child has cystic fibrosis (genotype ff)?
Quick check
Homozygous or heterozygous?
?A pea plant has the genotype Tt for height (T = tall, dominant). Which statement is correct?