Genetics & Inheritance

DNA (deoxyribonucleic acid) is the molecule of inheritance. It is a double helix made of nucleotides (base + sugar + phosphate). The four bases are: A (adenine), T (thymine), G (guanine), C (cytosine). A pairs with T; G pairs with C.

A gene is a section of DNA that codes for a protein. Humans have approximately 20,000 genes on 23 pairs of chromosomes (46 total). Alleles are different versions of a gene.

Dominant allele (capital letter) is expressed when present. Recessive allele (lowercase) is only expressed when two copies are present (homozygous recessive).

Genetic crosses using Punnett squares:

• Homozygous dominant (BB) × Homozygous recessive (bb) → all Bb (carriers, all show dominant trait)
• Heterozygous (Bb) × Heterozygous (Bb) → BB:Bb:Bb:bb = 3 dominant : 1 recessive

Sex determination: Females = XX, Males = XY. Father's sperm (X or Y) determines sex.

Genetic disorders:

• Sickle cell anaemia: recessive (ss). Common in populations from malaria-endemic areas including Pakistan.
• Down's syndrome: extra chromosome 21 (trisomy 21) — caused by non-disjunction during meiosis.

Mastery Extension: Genetics & Inheritance

This extension turns the lesson into an active study session for Biology (AKU-BIO) on Aga Khan Board. It does not replace the current specification or a teacher's instructions. Use it after reading the main notes, and check the official syllabus linked from the subject page whenever a live rule, paper structure, or assessment detail matters.

Learning Targets

By the end of a focused revision session, you should be able to define the central idea in your own words, connect it to the lesson's supporting concepts, apply it to a new question, and explain why your method or evidence is appropriate. Use the topic description as your boundary: DNA, genes, alleles, Mendelian inheritance and genetic crosses. If an answer wanders outside that boundary, return to the command word and remove material that does not earn a mark.

Use these lesson-specific checkpoints:

• Explain this accurately without copying: DNA: double helix, A-T and G-C base pairs.
• Explain this accurately without copying: Dominant expressed over recessive; recessive needs two copies.
• Explain this accurately without copying: Punnett square: Bb × Bb gives 3:1 ratio.
• Explain this accurately without copying: Father determines sex (X or Y sperm).

A Reliable Answer Method

Write the relationship or rule first, keep units and symbols consistent, show each transformation, and check whether the final magnitude is sensible. For a practical, graph, or data question, separate the observation from the explanation and use the values or pattern supplied rather than guessing a cause.

Before writing, spend a few seconds planning. Circle or note the command word, identify the exact knowledge being tested, and decide what evidence, working, example, or quotation is needed. During the answer, make every line do a job. At the end, reread the question and verify that your conclusion follows from the steps you actually showed.

Apply the Pakistan Context Carefully

The lesson's local case is Thalassaemia — Pakistan's Genetic Crisis. Pakistan has one of the highest rates of thalassaemia (a blood disorder) in the world — approximately 9 million carriers. Thalassaemia major occurs when a child inherits the recessive allele (tt) from both carrier parents (Tt × Tt). This gives a 25% chance for each pregnancy. AKU-EB Biology Punnett square questions regularly use thalassaemia as the Pakistani context Use this example to make an abstract idea memorable, but do not force it into an exam response when the question supplies a different context. A good application names the relevant feature, explains the connection, and also states where the comparison stops.

Try this three-part application drill:

1. Summarise the local example in one accurate sentence.
2. Identify which key point it demonstrates and quote or calculate the evidence available in the lesson.
3. Change one condition in the scenario and explain how the result, interpretation, or decision might change.

Guided Retrieval and Practice

Change one value, condition, or input in a worked example and solve it again. Then explain which step changed and which principle stayed constant. Work without notes first; retrieval is the test. Mark the attempt only after finishing, using the lesson and the explanation rather than memory or confidence.

1. Question lens: A-T and G-C describes:

   After answering, justify the choice using this check: In DNA, Adenine always pairs with Thymine, and Guanine always pairs with Cytosine — complementary base pairing

1. Question lens: Two carriers (Tt × Tt) — probability of affected child (tt)

   After answering, justify the choice using this check: Punnett square: TT, Tt, Tt, tt — one in four chance of tt (affected). 25%

1. Question lens: Males have sex chromosomes:

   After answering, justify the choice using this check: Males are XY — they have one X and one Y chromosome. The Y chromosome carries genes that trigger male development

For every missed question, keep an error log with four fields: what I chose, why it was tempting, the rule or evidence I missed, and the cue I will notice next time. This turns a wrong answer into a reusable revision asset instead of a score you immediately forget.

Seven-Day Revision Loop

• Day 1 — Understand: read the lesson and reduce each section to one sentence.
• Day 2 — Recall: reproduce the key points with the page closed.
• Day 3 — Apply: complete the local-context drill and one unfamiliar example.
• Day 4 — Practise: answer the inline quiz, showing or saying your reasoning before selecting an option.
• Day 5 — Repair: revisit only the weak steps recorded in your error log.
• Day 6 — Mix: combine this topic with an earlier topic from the same subject so that you must choose the correct method.
• Day 7 — Check: complete a timed response, self-mark conservatively, and plan the next review date.

You are ready to move on when you can explain the topic without the notes, answer the lesson-specific checks with reasons, apply the idea to a fresh context, and identify one common mistake before making it. If one of those checks fails, repeat the relevant stage rather than rereading everything.