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SingaporeBiology

Inheritance and Evolution

7 dot points across 7 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

How are two genes inherited together, and what is the basis of the 9:3:3:1 dihybrid ratio?

Explain dihybrid inheritance and the law of independent assortment, including the use of the chi-squared test
A focused answer to the H2 Biology Inheritance and Evolution outcome on dihybrid inheritance. The law of independent assortment, constructing a dihybrid cross to give the 9:3:3:1 ratio, and testing observed ratios against expected ones using the chi-squared test.
10 min answer →

How do linkage and gene interactions such as epistasis cause departures from the expected Mendelian ratios?

Explain autosomal linkage, recombination by crossing over, and epistasis as causes of departure from expected ratios
A focused answer to the H2 Biology Inheritance and Evolution outcome on linkage and gene interaction. Autosomal linkage and recombination by crossing over, why linked genes give non-Mendelian ratios, and epistasis where one gene masks another.
9 min answer →

How are single-gene traits inherited, and how do we predict the outcomes of a monohybrid cross?

Explain monohybrid inheritance using genetic diagrams, including dominant, recessive, codominant and sex-linked alleles
A focused answer to the H2 Biology Inheritance and Evolution outcome on monohybrid inheritance. Alleles, genotype and phenotype, dominant and recessive inheritance, codominance, sex linkage, and how to construct genetic diagrams and predict ratios.
9 min answer →

How does natural selection lead to adaptation and changes in allele frequencies over time?

Explain natural selection and how it brings about evolution and adaptation, including directional, stabilising and disruptive selection
A focused answer to the H2 Biology Inheritance and Evolution outcome on natural selection. The logic of selection from variation and differential survival, the three types (directional, stabilising and disruptive), and how selection changes allele frequencies and produces adaptation.
9 min answer →

Where does the genetic variation that natural selection acts on come from?

Explain the sources of genetic variation: mutation, meiosis (crossing over and independent assortment) and random fertilisation
A focused answer to the H2 Biology Inheritance and Evolution outcome on variation. Mutation as the source of new alleles, the role of meiosis (crossing over and independent assortment) and random fertilisation in shuffling alleles, and the contrast with environmental variation.
8 min answer →

How do new species arise, and what is the difference between allopatric and sympatric speciation?

Explain the concept of a species and the mechanisms of allopatric and sympatric speciation
A focused answer to the H2 Biology Inheritance and Evolution outcome on speciation. The biological species concept, reproductive isolation, allopatric speciation by geographical separation and sympatric speciation without it, and how isolation plus selection produce new species.
9 min answer →

How can we calculate allele and genotype frequencies in a population, and what conditions keep them constant?

Use the Hardy-Weinberg principle to calculate allele and genotype frequencies and state the conditions for equilibrium
A focused answer to the H2 Biology Inheritance and Evolution outcome on population genetics. The Hardy-Weinberg equations, calculating allele and genotype frequencies including carriers, the conditions required for equilibrium, and how departures indicate evolution.
9 min answer →