Genetics

Genetics and inheritance

Genetics is how alleles, versions of a gene, pass from parents to offspring and show up as traits.

CAPS

Grade 12 · Life Sciences · Genetics

Life Sciences is two papers. Use the strand to know which one.

Where it is examined
Paper 1 is reproduction, responding to the environment, and homeostasis. Paper 2 is DNA, meiosis, genetics and evolution. Each paper is 150 marks.
How to write it
Name the structure, then say what it does. A labelled diagram only scores if the labels are the biological names. “It helps the body” is not a function.
How a marker reads it
A common mix-up is to treat a 3:1 ratio as a guarantee, or to say a dominant allele is the common one. Dominant does not mean frequent.

A 2-mark “explain” wants two facts, or one fact and its reason. Extra sentences that contradict the first one can cancel a mark.

Diagram

Worked theory

Worked theory. Tongue-rolling, treated as a simple dominant for the classroom. Two parents who roll their tongues but carry the recessive allele can have a child who cannot roll. The square gives three rollers to one non-roller, as a probability, not a promise that four children will split three and one. If a pedigree shows two non-rolling parents with a rolling child, the “rolling is recessive” idea fails for that chart, because two recessive parents would have no dominant allele to give. Blood groups are the same logic with more alleles. You do not invent a new letter in the middle of a cross.

More worked examples

Blood groups in one family
A mother is blood group O, genotype ii. A father is AB, genotype IA IB. Every child gets i from the mother and either IA or IB from the father. The children are group A or group B, each about half the time. None can be O, because the father has no i to give. None can be AB, because the mother has no A or B to give. Two children who are both A do not break the cross. They are what a small family looks like.
Albinism in one family
Both parents have typical pigmentation and both are carriers, Aa. Each child has a 1 in 4 chance of aa, which is albinism, a 1 in 2 chance of being a carrier, and a 1 in 4 chance of AA. A family of three children with no albinism has not broken the cross. Small numbers do not have to match the fraction. The fourth pregnancy still has the same 1 in 4 chance. The earlier children did not use it up.

In ordinary words

Genotype is the alleles you carry. Phenotype is what shows. A dominant allele shows even if only one copy is present. A recessive allele shows only when both copies are recessive. In a monohybrid cross, one gene with two alleles, a Punnett square lays out the possible offspring. A pedigree chart follows a trait through a family. If the trait skips a generation and appears in both sexes, it is often recessive.

In South Africa

A family tree drawn for albinism or blood type is the pedigree you are learning to read.

Part 1 / 4

What it is

Genetics is how alleles, versions of a gene, pass from parents to offspring and show up as traits.

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

In peas, tall (T) is dominant to short (t). Cross Tt × Tt and give the phenotypic ratio.

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: In peas, tall (T) is dominant to short (t). Cross Tt × Tt and give the phenotypic ratio.

Before any working — hunt first

Given

Numbers and units already in the sentence: no numbers yet — only a name, a diagram, or a worded unknown. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: phenotypic ratio. Circle that word — that is the missing variable.

Find

We are required to find: phenotypic ratio. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

What we need to calculate: Use a Punnett square. Then substitute the given, one line at a time, to get phenotypic ratio.

In other words — as if you have never seen this before

If you have never seen this before: Use a Punnett square. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

Tt × Tt.

3. Which method — and why this one?

Use a Punnett square.

4. Step-by-step working

01 / 05
Identify the given
Gametes T, t × T, t.
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Calculate
Offspring: TT, Tt, Tt, tt → genotypes 1 : 2 : 1; phenotypes 3 tall : 1 short.
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Step G — Sense-check the size
Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.
Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.
04 / 05
Step F — Box the answer with its unit
3 tall : 1 short.
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

3 tall : 1 short.

Copy onto your answer sheet — in this order

Answer: 3 tall : 1 short.

6. Check it like a marker

75% of 400 offspring ≈ 300 tall.

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

Share on WhatsApp

Next lesson →

Responding to the environment: humans

Life Sciences

Take topic quiz