Genetics and Evolution — IB Biology HL Study Guide (HL Extension)

For: IB Biology HL candidates sitting IB Biology HL.

Covers: IB Topic 10 (HL only) — full meiosis with crossing-over, dihybrid crosses, autosomal and sex linkage, gene mapping by recombination frequency, polygenic inheritance, mechanisms of evolution and speciation.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Biology HL style for educational use. They are not reproductions of past IBO papers.

1. Why Genetics & Evolution Matters in HL

Topic 10 (HL only) extends SL Topic 3 (Genetics) and Topic 5 (Evolution & Biodiversity) with mechanisms: how meiosis generates variation, how Punnett squares work for two genes, why some genes don't follow independent assortment (linkage), and how populations evolve.

About 7-10% of HL Paper 1+2.

2. Meiosis details

Meiosis halves chromosome number: diploid (2n) → haploid (n) gametes. Two divisions:

Meiosis I — homologous chromosomes separate (reductional division):

1. Prophase I: chromosomes condense; homologous pairs synapse (form bivalents); crossing-over exchanges DNA at chiasmata.
2. Metaphase I: bivalents line up at equator; independent assortment — each pair orientates independently of others.
3. Anaphase I: homologues pulled to opposite poles. Sister chromatids stay together.
4. Telophase I: two haploid cells, each with chromosomes still as sister-chromatid pairs.

Meiosis II — sister chromatids separate (similar to mitosis): two more divisions, end result 4 haploid cells, each genetically unique.

Sources of variation: independent assortment, crossing over, random fertilisation.

3. Dihybrid crosses

Cross involving two genes. If genes assort independently:

• Heterozygous AaBb × AaBb → 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb.
• 4 phenotypes in 9:3:3:1 ratio.

Use a 4×4 Punnett square with gametes AB, Ab, aB, ab (each with frequency 1/4 in heterozygote).

4. Linkage

Genes on the same chromosome are linked — tend to be inherited together. Their inheritance violates Mendel's law of independent assortment.

Recombinant offspring arise from crossing-over between linked loci. The further apart two genes are on a chromosome, the more likely a crossover occurs between them.

Recombination frequency (RF): $$\text{RF}=\frac{\text{recombinant offspring}}{\text{total offspring}}\times 100\%$$

• RF = 0%: completely linked (no crossover ever observed).
• RF = 50%: behave as if unlinked (independent assortment, e.g. on different chromosomes).
• 0% < RF < 50%: linked, with frequency proportional to map distance.

1 cM (centimorgan) = 1% RF. Used to make genetic maps.

5. Sex linkage

In humans, females are XX, males XY. Genes on the X (or Y) chromosome are sex-linked.

X-linked recessive (e.g. haemophilia, red-green colour blindness):

• Females need both X's affected to express; can be carriers (X^aX^A) without symptoms.
• Males need only their single X affected (X^aY) → much higher male prevalence.

Punnett with sex linkage uses notation like X^A and X^a; cross example: carrier mother (X^AX^a) × normal father (X^AY) gives 1/4 carrier daughters, 1/4 normal daughters, 1/4 affected sons, 1/4 normal sons.

6. Polygenic inheritance

Some traits (skin colour, height, weight) are determined by multiple genes (often each with additive effect). Phenotype shows continuous variation with a normal-distribution shape.

If 3 genes each contribute additively (each with 2 alleles), F2 of an AABBCC × aabbcc cross gives 7 distinct phenotypes from very dark (6 dominant alleles) to very light (0 dominant), in a binomial distribution.

7. Hardy-Weinberg equilibrium

For a single locus with two alleles (frequency $p$ for A, $q$ for a, $p+q=1$):

Genotype frequencies in equilibrium: $p^2$ (AA) + $2pq$ (Aa) + $q^2$ (aa) = 1.

Five conditions for equilibrium: large population, no migration, no mutation, no selection, random mating. Violation of any → evolution occurs (allele frequencies change).

Example: 9% of population is aa (recessive disease). Then $q^2=0.09$, $q=0.3$, $p=0.7$. Frequency of carriers (Aa) = $2pq=0.42$.

8. Speciation

A species is a group whose members can interbreed and produce fertile offspring (biological species concept).

Reproductive isolation prevents two populations from interbreeding:

• Pre-zygotic (no zygote forms): habitat, behavioural, temporal, mechanical, gametic.
• Post-zygotic (zygote forms but unsuccessful): hybrid inviability, hybrid sterility (e.g. mules), hybrid breakdown.

Allopatric speciation: geographic separation (mountain rises, river forms) prevents gene flow → independent evolution → eventual reproductive isolation.

Sympatric speciation: speciation in same geographic area, often via polyploidy in plants (a polyploid offspring cannot breed with diploid parent).

9. Worked Example

In peas, gene T (tall, dominant) and t (dwarf) are on chromosome 2; gene W (white flowers, dominant) and w (purple) are on chromosome 4. Cross TtWw × ttww and predict offspring phenotype ratio.

Solution.

Genes are on different chromosomes → independent assortment.

For T (tall): TtWw × ttww gives 1/2 Tt (tall) : 1/2 tt (dwarf). For W (flowers): 1/2 Ww (white) : 1/2 ww (purple).

Combined: 1/4 tall white : 1/4 tall purple : 1/4 dwarf white : 1/4 dwarf purple. Ratio 1:1:1:1.

If T and W were linked, the ratio would be skewed toward parental combinations (TW and tw if those were the parental linkages) and against recombinants.

10. Common Pitfalls

• Independent assortment vs linkage: only genes on different chromosomes (or far apart on same chromosome) assort independently. Close linkage gives ratios deviating from 9:3:3:1.
• Sex-linked carrier confusion: a carrier female has the recessive allele but doesn't express the trait. Sex-linked recessive traits are more common in males.
• Hardy-Weinberg conditions: the equation describes a non-evolving population. If allele frequencies change, the population is evolving.
• Recombination > 50%: RF cannot exceed 50% because once half the gametes are recombinant, the ratio caps at 1:1 like independent assortment.

11. Practice Questions

1. A cross AaBb × aabb produces 100 offspring: 40 AaBb, 38 aabb, 11 Aabb, 11 aaBb. Are these genes linked? Calculate RF.
2. In a population, 1 in 10,000 newborns has cystic fibrosis (autosomal recessive). Calculate the expected carrier frequency.
3. Explain why polyploidy is a more common route to sympatric speciation in plants than in animals.

12. Quick Reference Cheatsheet

• Meiosis: 2 divisions, halves chromosome number, generates variation by crossing over, independent assortment, random fertilisation.
• Dihybrid AaBb × AaBb → 9:3:3:1 (independent).
• Linked genes: RF < 50%; closer = lower RF.
• Sex-linked recessive: males more affected.
• Polygenic: continuous variation, normal distribution.
• HW: $p^2+2pq+q^2=1$. 5 conditions for no evolution.
• Speciation: pre/post-zygotic isolation; allopatric vs sympatric.

13. What's Next

Genetics & Evolution feeds Topic 11 (Animal Phys) for hormonal genetics and Ecology for population genetics over time. Use Ollie for any specific Punnett or Hardy-Weinberg problem.