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Genetics

Agricultural genetics

Agricultural genetics is the same allele logic as Life Sciences, used to predict a calf, a lamb or a seed.

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Grade 12 · Agricultural Sciences · Genetics

Agricultural Sciences splits animals from plants, soil and management.

  • The papers

    Paper 1 is animal nutrition, animal production, reproduction and protection. Paper 2 is soil science, plant studies, and agricultural management. Each paper is 150 marks.

  • How to write it

    Use the farming term, then the reason: a deficient mineral, a symptom, a management practice, and why it changes the yield or the health of the animal. A list with no cause does not explain.

  • How a marker reads it

    A common mix-up is to cull only the animal that shows a recessive trait and keep the parents who carry it.

Data-response questions want you to read the table first. A general paragraph about farming that ignores the numbers scores as if you did not see them.

Diagram

Worked theory

Worked theory. A recessive coat defect in a stud. The ram looks normal and carries the allele. Half his sperm carry it. A ewe who also carries it, mated to him, has a one-in-four chance of an affected lamb. Two affected lambs in a small flock do not “disprove” the one in four. They are what probability looks like in a handful of births. Culling only the affected lambs leaves the carriers, and the defect returns. The square tells you to look at the parents, not only at the sick lamb.

More worked examples

  • A carrier ram

    A ram looks normal and carries a recessive defect. He is mated to 20 ewes who do not carry it. None of the lambs show the defect. About half of them carry it, unseen. Next year one of those daughters is mated back to him. Now both parents carry it, and about one lamb in four is affected. Culling only that lamb leaves the ram in the flock. The defect returns the following year.

  • Hybrid maize seed

    A seed company crosses two inbred lines. The F1 seed is uniform and yields well. The farmer plants it. If she saves grain from that crop and plants it next year, the next generation segregates: tall and short, strong and weak, and the yield drops. The good year was the cross, not a new pure line. That is why hybrid maize is bought again. The recessive-ram problem is a different pattern. This one is the loss of the cross.

In ordinary words

A trait can be dominant or recessive. A parent can carry a recessive allele and not show it. A Punnett square gives the chance of each kind of offspring. On a farm the interesting traits, milk or growth, are usually many genes plus the environment, so a square is the teaching model, not the whole prediction. A pedigree still matters: it shows which parent handed down a known defect. Variation is the raw material. Without it, selection has nothing to choose.

In South Africa

A stud pedigree with one known recessive is the classroom square in a real flock.

Part 1 / 4

What it is

Agricultural genetics is the same allele logic as Life Sciences, used to predict a calf, a lamb or a seed.

More worked examples

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

Example 1 / 20

In cattle, polled (P) is dominant to horned (p). Cross Pp × pp.

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 cattle, polled (P) is dominant to horned (p). Cross Pp × pp.

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: Cross Pp × pp. Circle that word — that is the missing variable.

Find

We are required to find: Cross Pp × pp. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

What we need to calculate: Punnett square. Then substitute the given, one line at a time, to get Cross Pp × pp.

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

If you have never seen this before: 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?

Pp × pp.

3. Which method — and why this one?

Punnett square.

4. Step-by-step working

  1. 01 / 08

    Step A — Read the question twice

    Slow down. Circle the word that names the unknown. Underline every number and its unit. Do not grab a formula yet — first decide what the paper is actually asking.

    Why this step: If you can say the method in a sentence, you will pick the right formula. First-timers skip this and grab the first equation they remember.

  2. 02 / 08

    Step A2 — Hunt: Given, Missing, Find, Calculate

    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: Cross Pp × pp. Circle that word — that is the missing variable. Find: We are required to find: Cross Pp × pp. Write that as a symbol on the left, with a question mark, before you touch a formula. Calculate: What we need to calculate: Punnett square. Then substitute the given, one line at a time, to get Cross Pp × pp.

    Why this step: Do not name the letters yet. First hunt: what already has a number (Given), what has no number (Missing), the word the paper wants (Find), and which formula still contains the missing letter (Calculate).

  3. 03 / 08

    Step B — Write the given in a column

    Pp × pp.

    Why this step: A column with units stops you mixing cm with m, or minutes with seconds. Markers can also see you started correctly.

  4. 04 / 08

    Step C — Name the method in words

    Punnett square.

    Why this step: If you can say the method in a sentence, you will pick the right formula. First-timers skip this and grab the first equation they remember.

  5. 05 / 08

    Calculate

    Pp, Pp, pp, pp → 50% polled, 50% horned.

    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.

  6. 06 / 08

    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.

  7. 07 / 08

    Step F — Box the answer with its unit

    1 polled : 1 horned.

    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.

  8. 08 / 08

    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

1 polled : 1 horned.

Copy onto your answer sheet — in this order

Answer: 1 polled : 1 horned.

6. Check it like a marker

This is a test cross.

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…”

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