Genetics — Recap & Recall
Whiteboards out — 1, 2, 3 or 4?
Don’t recap yet. Use the spread of answers to decide what to dwell on. Click Reveal for the answer and what each wrong choice tells you.
The shape of DNA
Recall first
(1) Sketch the shape of DNA. (2) What are the two sides made of? (3) Write the four bases and which one pairs with which.
- DNA is found in the nucleus, packaged into chromosomes.
- Its shape is a double helix — a twisted ladder.
- The two sides (the backbone) are made of sugar and phosphate.
- The rungs are pairs of bases. There are four: A T C G.
- The bases pair up: A–T and C–G (complementary base pairing).
- One repeating unit — sugar + phosphate + base — is a nucleotide.
- The order (sequence) of the bases is the genetic code.
Quick task · pair the bases
Tap a base, then tap the slot to show its partner. Then press Check.
Independent practice A
The language of inheritance
Build the schema · chromosome → gene → alleles
Try it · how dominance decides the phenotype
Pick the two alleles a person inherits and watch the eye colour. Brown (B) is dominant.
Recall first
Define each in your own words: gene · allele · dominant · recessive · genotype · phenotype · homozygous · heterozygous.
- Gene — a section of DNA that codes for a characteristic.
- Allele — a different version of a gene.
- Dominant (F) — shows in the phenotype with just one copy.
- Recessive (f) — only shows when two copies are present (ff).
- Genotype — the alleles an organism has (e.g. Ff).
- Phenotype — the characteristic you actually see.
- Homozygous — two of the same allele (FF or ff). Heterozygous — two different (Ff).
Quick task · match the term to its meaning
Tap a card, then tap where it goes. Tap a filled space to send it back. Then Check.
Independent practice B
Predicting the offspring
Recall first
Write the steps for completing a Punnett square. Then predict: for Ff × Ff, what is the ratio of offspring?
- Write both parents’ genotypes.
- Find each parent’s gametes (one allele each).
- Draw a 2×2 grid — gametes on top and side.
- Combine to fill every cell, then count the ratio.
- State the probability for each pregnancy.
Quick task · build the Punnett square (Ff × Ff)
The gametes are given. Tap a genotype card, then tap a cell. Then Check.
Independent practice C
Boy or girl?
Recall first
Which sex chromosomes make a female and a male? And who determines the baby’s sex — the mother or the father?
- Sex chromosomes: XX = female, XY = male.
- Mother is XX → every egg carries an X.
- Father is XY → sperm carries an X or a Y.
- So the father’s sperm determines the sex.
- XX × XY → 50% XX : 50% XY → roughly 50:50.
Independent practice D
When alleles cause disorders
Recall first
What is a carrier? What is the difference between a dominant and a recessive disorder? What does embryo screening involve?
- Genetic disorders are caused by faulty alleles passed from parents.
- Recessive — cystic fibrosis: need two faulty alleles (ff) to be affected.
- Carrier (Ff): one faulty allele, healthy, can pass it on. Two carriers → 1 in 4 affected.
- Dominant — polydactyly (extra fingers/toes): one faulty allele is enough.
- Embryo screening (IVF + PGD): IVF embryos are tested for the faulty allele before one is implanted. Raises ethical questions on both sides.
Independent practice E
“1 in 4” — what it really means
“A 1 in 4 chance means one in every four of their children will have cystic fibrosis.”
1 in 4 is the probability for each pregnancy, worked out fresh every time — like flipping a coin. Two carriers could have four affected children, or none at all. The dice don’t remember the last roll.
Say it, then write it
Explain to your partner: what is the difference between a carrier and an affected person?
Use the words allele · recessive · genotype.