Inheritance

Inheritance is the transmission of genetic information from generation to generation.

This topic sits on a small number of definitions. Get those exactly right and the crosses, ratios and pedigrees become mechanical. Get them fuzzy and everything downstream is guesswork.

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1. The hierarchy: cell → chromosome → DNA → gene

The single most useful thing to fix first is how the terms nest inside each other.

cell → nucleus → chromosomes → DNA → gene

• A cell contains a nucleus
• The nucleus contains chromosomes
• Each chromosome is made of a long molecule of DNA
• A gene is a length of that DNA

The relationship to hold on to: a gene is a part of a DNA molecule, in the way that your hand is part of your body. They are not two different substances.

The definitions that earn marks

Term	Definition
Chromosome	A thread-like structure of DNA, carrying genetic information in the form of genes
Gene	A length of DNA that codes for a protein
Allele	A different version of the same gene

“Codes for a protein” is the part students drop. “A gene is a piece of DNA” is only half the definition and often only half the marks.

Alleles, concretely

A gene occupies a particular position and does a particular job — say, determining eye colour. But the same gene can exist in different versions. One version might give brown eyes, another blue. Those versions are alleles.

You inherit two alleles of every gene — one from each parent. What you end up looking like depends on which two you have.

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2. DNA structure

For Cambridge 0610 you need a specific and fairly short list.

• DNA is a double helix — two strands twisted around each other
• Each strand is made of units called nucleotides
• Each nucleotide contains a phosphate group, a sugar (deoxyribose) and a base
• There are four bases: adenine (A), thymine (T), cytosine (C), guanine (G)

The base-pairing rule

A always pairs with T. C always pairs with G.

These are called complementary base pairs. A base will only ever pair with its partner — A never pairs with C or G. Diagrams usually show this by giving each base a jigsaw-like shape that only fits one other.

So if one strand reads A T G C, the other strand must read T A C G.

How DNA codes for a protein

The sequence of bases in a gene determines the sequence of amino acids in a protein. Different sequences of amino acids give proteins different shapes, and therefore different functions.

Proteins built this way include enzymes, antibodies, haemoglobin, some hormones, and structural proteins such as keratin in skin and hair.

Scope warning — this is where a lot of study time gets wasted. Cambridge IGCSE 0610 stops at “the sequence of bases codes for the sequence of amino acids.” It does not require mRNA, tRNA, rRNA, ribosomes, codons, transcription, translation, template vs non-template strands, or DNA polymerase. That material belongs to A Level and to some other specifications. If you are sitting 0610, you can safely skip it.

Same genes, different cells

Every body cell in an organism contains the same genes. A skin cell and a liver cell carry identical genetic information. They differ because only some genes are expressed in each cell type — the rest are switched off. That is what makes cells specialised.

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3. Chromosome numbers

Humans have 46 chromosomes in each body cell — 23 pairs. One of each pair came from each parent.

• 22 pairs are ordinary body chromosomes
• 1 pair is the sex chromosomes

Term	Meaning	In humans
Diploid	Nucleus containing two sets of chromosomes	46 — all body cells
Haploid	Nucleus containing one set of chromosomes	23 — gametes only

Why gametes are haploid: if sperm and egg each carried 46, the zygote would have 92, and the number would double every generation. Halving the number in the gametes keeps it constant.

Sex determination

• Female: XX
• Male: XY

Every egg cell carries an X. A sperm carries either an X or a Y. So which sperm fertilises the egg determines the sex of the offspring:

Egg	Sperm	Zygote	Sex
X	X	XX	Female
X	Y	XY	Male

A Punnett square of X X × X Y gives XX : XY = 1 : 1, which is why the ratio is roughly 50:50.

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4. Mitosis

Mitosis is nuclear division giving rise to genetically identical cells, in which the chromosome number is maintained.

What happens, in order:

1. Chromosomes are copied (replicated) before division — 46 becomes 92
2. The copies line up along the centre of the cell
3. They are pulled apart to opposite ends
4. The cytoplasm divides, giving two genetically identical daughter cells, each with 46 chromosomes

Where it matters:

• Growth
• Repair of damaged tissue
• Replacement of worn-out cells (skin cells are shed and replaced constantly)
• Asexual reproduction

Mitosis is also how the zygote develops into an embryo, which is why every cell in your body carries the same genes.

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5. Meiosis

Meiosis is a reduction division in which the chromosome number is halved, from diploid to haploid, resulting in genetically different cells.

Meiosis produces gametes — sperm in the testes, egg cells in the ovaries.

What happens:

1. Chromosomes are copied
2. The pairs line up alongside each other, and sections are exchanged between them — producing new combinations
3. Division one separates the pairs
4. Division two separates the copies
5. The result is four genetically different haploid cells

Why meiosis produces variation

Two things make every gamete different: the exchange of sections between chromosome pairs, and the fact that which chromosome of each pair ends up in which gamete is random.

With 23 pairs shuffled independently, the number of possible combinations is enormous. This is why siblings with the same two parents are not identical — each was formed from a different sperm and a different egg.

Mitosis vs meiosis

Examiners ask for this comparison directly, and every row is a potential mark.

	Mitosis	Meiosis
Number of divisions	One	Two
Daughter cells produced	2	4
Chromosome number	Maintained (diploid → diploid)	Halved (diploid → haploid)
Daughter cells are	Genetically identical	Genetically different
Where	All body cells	Testes and ovaries only
Purpose	Growth, repair, replacement, asexual reproduction	Production of gametes

Note how cheap some of these marks are. “Mitosis produces 2 daughter cells, meiosis produces 4” is a full comparison point that takes seconds to write.

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6. The vocabulary of genetics

Term	Definition
Genotype	The genetic make-up of an organism — the alleles it has (e.g. Bb)
Phenotype	The observable features of an organism (e.g. brown eyes)
Homozygous	Having two identical alleles of a gene (BB or bb)
Heterozygous	Having two different alleles of a gene (Bb)
Dominant	An allele that is expressed if it is present
Recessive	An allele that is only expressed when no dominant allele is present

Notation: dominant alleles get a capital letter, recessive alleles the same letter in lower case. Using two different letters is a common and costly slip — B and b, never B and w.

The consequence to internalise:

Genotype	Phenotype shown
BB homozygous dominant	Dominant
Bb heterozygous	Dominant
bb homozygous recessive	Recessive

A recessive phenotype appears only in a homozygous recessive individual. That single fact solves most pedigree questions.

A heterozygous organism shows the dominant characteristic. This is the key that unlocks “which allele is dominant?” questions: if an individual has two different alleles and shows characteristic X, then X is dominant.

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7. Monohybrid crosses and Punnett squares

A monohybrid cross follows the inheritance of a single gene.

How to set one out

Always show your working — marks are given for the steps, not just the answer.

1. State the parental genotypes
2. State the gametes (each gamete gets one allele)
3. Draw the Punnett square
4. State the offspring genotypes
5. State the offspring phenotypes and the ratio

The two crosses you must know cold

Heterozygous × heterozygous → 3 : 1

	B	b
B	BB	Bb
b	Bb	bb

Genotypes 1 BB : 2 Bb : 1 bb. Phenotypes 3 dominant : 1 recessive.

Heterozygous × homozygous recessive → 1 : 1

	b	b
B	Bb	Bb
b	bb	bb

Genotypes 2 Bb : 2 bb. Phenotypes 1 dominant : 1 recessive. (This is the test cross — crossing an unknown individual with a homozygous recessive one reveals whether it was homozygous or heterozygous.)

Genotype ratio is not phenotype ratio

Questions almost always ask for one specifically. In the 3:1 cross above, the genotype ratio is 1:2:1 but the phenotype ratio is 3:1. Answering with the wrong one loses the mark even though the working is right.

Also: results can be given as a ratio (3:1), a percentage (75%) or a fraction (¾). Any is acceptable unless the question specifies.

A trap worth expecting: the dominant allele is not always the “normal” one. Some questions make the affected condition dominant — polydactyly (extra fingers or toes) is the standard example. Read which is which every single time rather than assuming.

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8. Codominance

Codominant alleles are alleles that are both expressed in the phenotype of a heterozygous individual — neither is recessive to the other.

The syllabus example in humans is the ABO blood group system.

The gene has three alleles: I^A, I^B and I^O.

• I^A and I^B are codominant with each other
• I^O is recessive to both

Genotype	Blood group
IAIA or IAIO	A
IBIB or IBIO	B
IAIB	AB ← the codominant one
IOIO	O

Group AB is the proof of codominance: a person with one I^A and one I^B allele shows both, rather than one masking the other.

In other organisms codominance may show as a blend or a mixture — red and white flowers giving pink, or black and white feathers giving a speckled bird.

Codominance is not the same as “both alleles are dominant.” It means both are expressed together in the heterozygote.

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9. Sex-linked inheritance

A sex-linked characteristic is one in which the gene responsible is located on a sex chromosome.

The syllabus example is red-green colour blindness, carried on the X chromosome.

Notation matters here. Because the allele sits on the X chromosome, you write it as a superscript on the X:

• X^B = normal colour vision (dominant)
• X^b = colour blind (recessive)
• Y carries no allele for this gene — write nothing on it

Genotype	Phenotype
XBXB	Female, normal vision
XBXb	Female, normal vision (carrier)
XbXb	Female, colour blind
XBY	Male, normal vision
XbY	Male, colour blind

Why it affects males more often

A male has only one X chromosome. If that single X carries the recessive allele, there is no second X to carry a dominant allele and mask it — so he is colour blind. A female needs the recessive allele on both X chromosomes.

This also gives the standard exam answer: a colour-blind mother (X^bX^b) will have colour-blind sons, because each son inherits his only X from her, and his Y from his father carries no allele.

Write the X and Y in your genetic diagram. For ordinary monohybrid crosses you write just the alleles; for sex linkage you must show the chromosomes too. Omitting them is a frequent lost mark.

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10. Pedigree diagrams

A pedigree shows a characteristic through several generations of a family. You are usually given the phenotypes and asked to deduce the genotypes.

The method:

1. Find the recessive individuals first. Anyone showing the recessive phenotype must be homozygous recessive — that is the only genotype that produces it. This is your anchor.
2. Work back to the parents. If two unaffected parents have an affected child, both parents must be heterozygous — each must have carried a recessive allele to pass on.
3. Establish which allele is dominant. If two individuals showing a characteristic produce a child without it, the characteristic is dominant and the child is homozygous recessive.
4. Mark genotypes you cannot fully determine. An individual showing the dominant phenotype may be BB or Bb; write it as B_ unless the diagram lets you narrow it down.

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11. Mistakes that cost marks

Defining a gene without “codes for a protein.”

Using two different letters for the two alleles. Use B and b.

Giving the genotype ratio when the question asked for the phenotype ratio, or the reverse.

Assuming the dominant allele is the “normal” one. Check the question.

Forgetting the X and Y in sex-linked genetic diagrams.

Writing an allele on the Y chromosome in a colour-blindness question. The Y carries no allele for that gene.

Saying codominance means “both alleles are dominant.” It means both are expressed.

Confusing “chromosome number is halved” with “the cell divides in half.” Meiosis halves the chromosome number — that’s a different claim.

Saying mitosis produces gametes. Meiosis produces gametes; mitosis produces everything else.

Not showing gametes and working in a cross. Marks are awarded for the steps.

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Frequently asked questions

What is the difference between a gene and an allele? A gene is a length of DNA coding for a protein — for example, the gene for eye colour. An allele is a particular version of that gene — for example, the brown version or the blue version.

What is the difference between genotype and phenotype? Genotype is the alleles you have (Bb). Phenotype is what you can observe (brown eyes).

What’s the difference between mitosis and meiosis in one sentence? Mitosis makes two genetically identical diploid cells for growth and repair; meiosis makes four genetically different haploid cells as gametes.

Why are gametes haploid? So that when two gametes fuse, the zygote has the correct diploid number. Otherwise the chromosome number would double each generation.

Who determines the sex of a baby? All eggs carry an X. Sperm carry either X or Y, so it is the sperm that determines it.

Why is colour blindness more common in males? Males have only one X chromosome, so a single recessive allele is enough to produce the condition. Females need it on both X chromosomes.

Do I need to know transcription and translation for 0610? No. Cambridge IGCSE 0610 requires only that the sequence of bases codes for the sequence of amino acids. mRNA, tRNA, ribosomes and codons are beyond the syllabus.

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Quick revision checklist

• I can define inheritance, chromosome, gene and allele — including “codes for a protein”
• I can put cell, nucleus, chromosome, DNA and gene in the right order of size
• I know DNA is a double helix of nucleotides with four bases, and that A–T and C–G
• I can write the complementary strand for a given base sequence
• I know that 0610 stops short of transcription and translation
• I know the human chromosome number, and what haploid and diploid mean
• I can explain why gametes must be haploid
• I can explain sex determination and why the ratio is 1:1
• I can describe mitosis and state where it happens and why
• I can describe meiosis and explain how it produces variation
• I can compare mitosis and meiosis across all six rows
• I can define genotype, phenotype, homozygous, heterozygous, dominant, recessive
• I can set out a monohybrid cross with parents, gametes, Punnett square, genotypes and phenotypes
• I know the 3:1 and 1:1 crosses without having to derive them
• I can explain what a test cross is and why it works
• I can explain codominance using the ABO blood groups
• I can write sex-linked genotypes correctly with X and Y
• I can deduce genotypes from a pedigree, starting with the recessive individuals

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These notes cover topic 17 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.