Meiosis, Genetics & Inheritance

Topics Covered in This Guide

• D2.1 Meiosis — chromosome behaviour in meiosis I and II, crossing over at chiasmata, independent assortment, genetic variation
• D2.2 Inheritance — Mendel’s laws, monohybrid and dihybrid crosses, sex-linkage, codominance (ABO), incomplete dominance
• D2.3 Gene Pools & Speciation — allele frequency, Hardy-Weinberg principle (qualitative), allopatric and sympatric speciation
• MCQ Practice — styled like real IB Paper 1 questions
• Exam Alerts — the exact traps that cost marks in D2 questions

Aligned to IB Biology 2025 syllabus — D2.1 Meiosis — D2.2 Inheritance — D2.3 Speciation

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Jump to section: Meiosis · Inheritance Patterns · Gene Pools & Speciation · MCQ Practice · Exam Strategy

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Section 1: Meiosis (D2.1)

Overview

Meiosis is a specialised form of cell division that produces four genetically unique haploid cells from one diploid parent cell. It occurs in the gonads (testes and ovaries in animals) and is the basis of sexual reproduction.

$2n \xrightarrow{\text{meiosis}} 4 \times n$

Meiosis consists of two successive divisions — Meiosis I and Meiosis II — each with its own prophase, metaphase, anaphase, and telophase.

DNA Replication Before Meiosis

Before meiosis begins, the cell undergoes a normal S phase of interphase — DNA is replicated so that each chromosome now consists of two identical sister chromatids joined at the centromere. This replication is essential; it provides the material for crossing over.

Meiosis I — Reductional Division

The key distinction of meiosis I is that homologous chromosomes (not sister chromatids) are separated.

Prophase I

This is the longest and most complex phase of meiosis. Several critical events occur:

1. Chromosomes condense and become visible under a light microscope.
2. Homologous chromosomes pair up — each pair of homologues comes together in a process called synapsis, forming a bivalent (a tetrad of four chromatids: two from each homologue).
3. Crossing over occurs — non-sister chromatids of homologous chromosomes exchange corresponding segments of DNA at points called chiasmata (singular: chiasma).
4. The nuclear envelope breaks down.
5. Spindle fibres form and begin attaching to centromeres.

Metaphase I

• Bivalents (pairs of homologous chromosomes) align along the metaphase plate (cell equator).
• Spindle fibres from opposite poles attach to the centromeres of each homologue in a bivalent.
• The orientation of each bivalent is random — this is the physical basis of independent assortment.

Anaphase I

• Homologous chromosomes are pulled to opposite poles by spindle fibres.
• Sister chromatids remain joined at their centromeres — they do not separate yet.
• Each pole now has a haploid set of chromosomes, but each chromosome is still double-stranded (two chromatids).

Telophase I and Cytokinesis

• Nuclear envelopes may reform around each haploid set.
• Cytokinesis divides the cell into two haploid cells.
• The cells enter a brief interkinesis — there is no DNA replication.

Meiosis II — Equational Division

Meiosis II is essentially a mitotic division of the two haploid cells produced by meiosis I.

Phase	Key Events
Prophase II	Chromosomes recondense; spindle fibres reform; nuclear envelope breaks down
Metaphase II	Individual chromosomes (each with two chromatids) align at the equator
Anaphase II	Sister chromatids separate — pulled to opposite poles
Telophase II	Nuclear envelopes reform; cytokinesis produces four haploid cells

The final result: four haploid cells, each genetically unique.

Sources of Genetic Variation in Meiosis

Meiosis generates genetic variation through three mechanisms:

Meiosis Stages — Full Summary Diagram

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Section 2: Inheritance Patterns (D2.2)

Key Terminology

Before working through genetic crosses, master the vocabulary — IB mark schemes are precise about these terms.

Mendel’s Laws

Law of Segregation (Monohybrid Inheritance)

Each individual carries two alleles for each gene. During gamete formation, the two alleles segregate so that each gamete carries only one allele.

This is the basis of monohybrid crosses. Alleles are written as letters: dominant allele as upper case ($A$), recessive allele as lower case ($a$).

Law of Independent Assortment (Dihybrid Inheritance)

The alleles of two different genes assort into gametes independently of one another, provided the genes are on different (non-homologous) chromosomes.

This is the basis of dihybrid crosses. Note that genes on the same chromosome (linked genes) do not independently assort — this is a key HL concept.

Monohybrid Cross — Worked Example

Testcross

A testcross crosses an individual of unknown genotype with a homozygous recessive individual ($aa$). The ratio of offspring phenotypes reveals the unknown genotype.

Unknown Genotype	Cross	Offspring Ratio
Homozygous dominant ($AA$)	$AA \times aa$	All dominant phenotype (all $Aa$)
Heterozygous ($Aa$)	$Aa \times aa$	1 dominant : 1 recessive ($Aa : aa$)

Dihybrid Cross — Worked Example

Codominance — ABO Blood Groups

ABO blood groups are controlled by the $I$ gene with three alleles: $I^A$, $I^B$, and $i$.

• $I^A$ and $I^B$ are codominant to each other — both alleles are fully expressed
• $i$ is recessive to both $I^A$ and $I^B$

Incomplete Dominance

In incomplete dominance, the heterozygote shows a phenotype intermediate between the two homozygous phenotypes. Neither allele is dominant or recessive.

Example: Snapdragon flower colour

• $R^1 R^1$ = red flowers
• $R^2 R^2$ = white flowers
• $R^1 R^2$ = pink flowers (intermediate)

Sex-Linked Inheritance (X-linked Traits)

In humans, the sex chromosomes are $X$ (larger, gene-rich) and $Y$ (smaller, fewer genes). Genes located on the X chromosome but not on the Y chromosome are called X-linked (or sex-linked).

Because males have only one X chromosome (hemizygous), they express X-linked recessive traits whenever the allele is present on their single X chromosome — there is no second X to “mask” a recessive allele.

Pedigree Analysis

IB often presents a pedigree chart (family tree diagram) and asks you to determine the mode of inheritance.

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Section 3: Gene Pools & Speciation (D2.3) HL

Gene Pools and Allele Frequency

A gene pool is the total set of all alleles of all genes present in a population at a given time.

Allele frequency is the proportion of a particular allele among all alleles of that gene in the population.

$\text{Allele frequency of } A = \frac{\text{number of } A \text{ alleles}}{\text{total number of alleles for that gene in population}}$

Example: In a population of 100 diploid individuals, if 60 carry the $A$ allele out of 200 total alleles: allele frequency of $A = \frac{60}{200} = 0.30$

Hardy-Weinberg Principle (Qualitative)

The Hardy-Weinberg principle states that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary influences.

Speciation

Speciation is the evolutionary process by which new species arise. A species is a group of organisms that share common characteristics and can interbreed to produce fertile offspring.

Reproductive isolation is the key mechanism — once populations can no longer exchange genes, they diverge.

Allopatric Speciation

Allopatric speciation occurs when a population is divided by a geographical barrier (mountain range, ocean, river), creating two isolated sub-populations.

Process:

1. Original population is split by a geographic barrier.
2. Gene flow between populations ceases.
3. Each sub-population is subject to different selection pressures, mutations, and genetic drift.
4. Over time, allele frequencies diverge.
5. Populations accumulate enough genetic differences that, if brought back together, they can no longer interbreed → reproductive isolation is complete → two species.

Example: Darwin’s finches on the Galápagos Islands — ancestral finches colonised different islands; geographical isolation led to divergence in beak morphology and feeding behaviour.

Sympatric Speciation

Sympatric speciation occurs within the same geographical area — no physical barrier separates the populations. It is less common in animals but well-documented in plants and insects.

Mechanisms include:

• Polyploidy — a sudden multiplication of chromosome sets (common in plants). An allopolyploid organism (with chromosome sets from two different species) cannot reproduce with either parent species → instant speciation.
• Ecological specialisation — subpopulations exploit different resources in the same area, reducing gene flow between them (e.g., insects specialising on different host plants).

Example: Many crop plants (wheat, cotton) arose through allopolyploidy.

Feature	Allopatric Speciation	Sympatric Speciation
Geographic barrier	Required	Not required
Gene flow interrupted by	Physical separation	Reproductive/ecological isolation
Common in	Animals and plants	Primarily plants (polyploidy); some insects
Speed	Gradual (thousands of generations)	Can be rapid (polyploidy can be near-instantaneous)

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Section 4: MCQ Practice (IB Paper 1 Style)

Question 1. Which of the following events occurs during prophase I of meiosis but NOT during prophase of mitosis?

A. Chromosomes condense and become visible.

B. The nuclear envelope breaks down.

C. Homologous chromosomes pair and crossing over occurs.

D. Spindle fibres begin to form.

Reveal answer

C — Synapsis (pairing of homologous chromosomes) and crossing over at chiasmata are exclusive to prophase I of meiosis. Options A, B, and D occur in both prophase I of meiosis and prophase of mitosis.

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Question 2. In a population of rabbits, the allele for brown coat ($B$) is dominant over the allele for white coat ($b$). Two brown rabbits are crossed and produce 3 brown offspring and 1 white offspring. What are the genotypes of the parent rabbits?

A. $BB \times BB$

B. $BB \times Bb$

C. $Bb \times Bb$

D. $Bb \times bb$

Reveal answer

C — A 3:1 phenotype ratio (3 brown : 1 white) is the hallmark of a cross between two heterozygotes ($Bb \times Bb$). The offspring genotypes are 1 $BB$ : 2 $Bb$ : 1 $bb$, giving the 3 brown : 1 white phenotype ratio. Option D ($Bb \times bb$) would give a 1:1 ratio; option B ($BB \times Bb$) would give all brown (no white offspring possible).

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Question 3. A woman with type AB blood and a man with type O blood have children. Which of the following blood types is POSSIBLE for their children?

A. Type O

B. Type AB

C. Type A

D. Neither A nor B can be produced

Reveal answer

C — The mother with type AB has genotype $I^A I^B$. The father with type O has genotype $ii$. Possible gametes: mother produces $I^A$ and $I^B$; father produces only $i$. Offspring genotypes: $I^A i$ (type A) and $I^B i$ (type B). Type A and Type B are both possible. Type O (requiring $ii$) and Type AB (requiring $I^A I^B$) cannot be produced. The answer is C (Type A is possible).

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Question 4. Haemophilia is an X-linked recessive condition. A non-haemophiliac woman whose father had haemophilia marries a non-haemophiliac man. What is the probability that their first son will have haemophilia?

A. 0 (impossible)

B. 1/4

C. 1/2

D. 1 (certain)

Reveal answer

C — The woman’s father had haemophilia ($X^h Y$), so she must have received $X^h$ from him. Since she is unaffected, she is a carrier ($X^H X^h$). Her husband is normal ($X^H Y$). The cross $X^H X^h \times X^H Y$ produces $X^H X^H$, $X^H X^h$, $X^H Y$, $X^h Y$ in equal proportions. Among sons, half will be $X^H Y$ (normal) and half will be $X^h Y$ (haemophiliac). The probability that their first son has haemophilia is 1/2.

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Question 5. Which of the following is NOT a condition required for a population to be in Hardy-Weinberg equilibrium?

A. No migration into or out of the population

B. Random mating within the population

C. Large population size

D. High rate of mutation at the gene locus in question

Reveal answer

D — A high mutation rate would alter allele frequencies, violating Hardy-Weinberg equilibrium. Hardy-Weinberg requires the absence of mutation (or at least mutation rates so low they do not measurably alter allele frequencies). Options A, B, and C (no migration, random mating, large population size) are all required conditions for equilibrium.

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Question 6. A new species of plant arose when a hybrid formed between two different species underwent chromosome doubling (polyploidy). This is an example of which type of speciation?

A. Allopatric speciation due to geographic isolation

B. Allopatric speciation due to reproductive isolation

C. Sympatric speciation

D. Adaptive radiation

Reveal answer

C — Polyploidy-driven speciation occurs without geographic separation — the new species arises within the same area as both parent species. This is the definition of sympatric speciation. Allopatric speciation (A, B) requires a geographic barrier. Adaptive radiation (D) refers to rapid diversification of one ancestral species into many niches, which is a pattern — not a mechanism of speciation.

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Quick Recall — All Sections

Try to answer without scrolling up:

1. What separates during anaphase I of meiosis versus anaphase II?
2. What is the phenotype ratio expected from a monohybrid cross of two heterozygotes?
3. Why are X-linked recessive conditions more common in males than females?
4. Name the two types of speciation and their key distinguishing feature.
5. State two conditions required for Hardy-Weinberg equilibrium.

Reveal answers

1. Anaphase I: homologous chromosomes separate (sister chromatids remain joined). Anaphase II: sister chromatids separate (same as mitotic anaphase).
2. 3:1 (3 dominant phenotype : 1 recessive phenotype).
3. Males are hemizygous for X-linked genes — they have only one X chromosome and will express any allele (dominant or recessive) present on it. There is no second X allele to mask a recessive allele.
4. Allopatric — populations separated by a geographic barrier; Sympatric — speciation within the same geographic area (no physical barrier), typically via polyploidy or ecological specialisation.
5. Any two from: random mating, no mutation, large population size (no genetic drift), no migration (no gene flow), no natural selection.

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Section 5: Exam Strategy

Top Mistakes on D2 Questions

1. Confusing what separates in meiosis I vs meiosis II. In meiosis I: homologous chromosomes separate. In meiosis II: sister chromatids separate. A diagram question showing double-stranded chromosomes moving apart = meiosis I. Single-stranded = meiosis II or mitosis.

2. Forgetting to show all gamete types in a dihybrid cross. For $AaBb$ parents, there are four gamete types ($AB$, $Ab$, $aB$, $ab$). Students often write only two gametes. In a 4×4 Punnett square, you must list all four.

3. Writing “dominant” instead of “codominant” for ABO blood groups. $I^A$ is not dominant over $I^B$ — they are codominant. Saying ”$I^A$ is dominant and $I^B$ is recessive” is factually incorrect.

4. Confusing incomplete dominance with codominance. Use the test: can you detect both original phenotypes in the heterozygote? If yes (e.g., roan cattle have both red and white hairs) → codominance. If the heterozygote is a blend (e.g., pink snapdragons) → incomplete dominance.

5. Failing to use notation correctly for X-linked alleles. Always write X-linked alleles as superscripts on X: $X^H$ and $X^h$, not simply $H$ or $h$. The IB mark scheme requires the X to appear in the genotype.

6. Confusing allopatric and sympatric speciation. “Allopatric” = geo (geographic) separation. “Sympatric” = same place, reproductive/ecological isolation.

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