Variation during reproduction:

Organisms show similarities to parents but also small differences called variations.

Asexual reproduction: Very little variation (only due to minor DNA copying errors).
Sexual reproduction: Large variation because offspring get genes from both parents.
Variations help some individuals survive better and become the basis of evolution.

Exam tips for this topic:

Be ready to write difference between asexual and sexual variation in 2–3 lines.
For 3-mark answers, always mention variation → survival → evolution.
Keep an example ready, like heat-resistant bacteria survive a heat wave.

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Variations

What are inherited traits?

Traits are characteristics that are passed from parents to offspring.

Example:

Height, eye colour, earlobe type.

All offspring show a basic body design but also differences.

Exam tips for this topic:

Write a short definition of trait and variation—these often come in 1-mark questions.
Keep examples ready: free vs. attached earlobe, tall vs. short plants.
Don’t call acquired traits “inherited”—they are NOT passed to next generation.

Mendel’s experiments & Laws

a. Monohybrid cross – Law of dominance

Mendel crossed tall (\(T\)) and short (\(t\)) pea plants.

F1: All tall → tall is dominant.
F2: \(3\) tall : \(1\) short ratio → both traits were present but tall was expressed.

Monohybrid cross

Exam Tips

Always write F1 = \(100%\) dominant trait.
Always write F2 phenotype ratio = \(3:1\) and genotype ratio = \(1:2:1\).
Use a Punnett square if the question is for 3 or 5 marks—gets full marks.

b. Dihybrid cross – Law of Independent Assortment

Mendel crossed plants with two traits:

Seed shape: round (\(R\)) / wrinkled (\(r\))
Seed colour: yellow (\(Y\)) / green (\(y\))

F2 ratio: \(9:3:3:1\), showing traits assort independently.

Dihybrid cross

Exam Tips

Memorize the \(9:3:3:1\) ratio — it is asked very often.
Use the phrase “new combinations appear in \(F2\) (round-green, wrinkled-yellow)”.
Diagram of a dihybrid Punnett square can fetch 1–2 extra marks.

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Dominant & Recessive traits

Dominant trait = expressed even if one allele is present (T).
Recessive trait = expressed only when both alleles are recessive (tt).

Exam Tips

Always give an example: T = tall, t = short.
Use key words: allele, expression, genotype, phenotype.
For 1-mark MCQs, remember: dominant hides recessive.

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How traits get expressed

Genes are portions of DNA that code for proteins. Proteins control physical traits.

Example:

A gene may produce an enzyme that controls plant height.

Exam Tips

Write “genes → proteins → traits” as a flow.
Define gene as a segment of DNA.
In 3-mark questions, mention how efficient/inefficient enzyme leads to tall/short.

Chromosomes and Inheritance

Each human cell has \(23\) pairs of chromosomes. Gametes (sperm/egg) contain only one set (23). During fertilization, the child receives one set from each parent → equal genetic contribution.

Exam Tips

In 2–3 marks, mention “gametes are haploid”.
Use the word “equal genetic contribution”.
Draw a simple labelled diagram of chromosome pairs for extra marks.

Sex determination in humans

Females: \(XX\)
Males: \(XY\)
Mother always contributes X.
Father contributes X or Y → decides sex of child.

XX-XY type sex determination system in humans

Exam Tips

Must conclude: “Father determines the sex of the child.”
Draw the\(\ XX–XY\) chart—this alone can earn full marks in a 3-mark question.
If asked for reasoning, include:
“Because father produces two types of gametes (X and Y), mother produces only one (X).”

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