2025 Biology Exam Revision: Genetics, Evolution, Adaptations, and Ecosystems

Genetics Revision

Definitions

• Gamete: A haploid reproductive cell (sperm or egg) that fuses with another gamete during fertilization.
• Somatic Cell: Any biological cell forming the body of a multicellular organism other than gametes, germ cells, gametocytes or undifferentiated stem cells.
• Genotype: The genetic makeup of an organism, describing all the alleles it contains.
• Phenotype: The observable characteristics of an organism, resulting from the interaction of its genotype with the environment.
• Haploid: Having a single set of chromosomes (n).
• Diploid: Having two sets of chromosomes (2n).
• Zygote: A diploid cell resulting from the fusion of two haploid gametes; a fertilized ovum.
• Homozygous: Having two identical alleles for a particular gene.
• Heterozygous: Having two different alleles for a particular gene.
• Sex-linked: A gene located on a sex chromosome (X or Y).
• X-linked: A gene located on the X chromosome.

DNA

• DNA stands for deoxyribonucleic acid.
• The four nitrogenous bases in DNA are Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).
• Complementary base pairs: A pairs with T, and G pairs with C.
• Basic structure of DNA: A double helix, with a sugar-phosphate backbone and nitrogenous bases forming the rungs.
• Role of DNA: Contains the genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.

Relationships

• Nucleus: The organelle within a cell that contains chromosomes.
• DNA: The molecule that makes up chromosomes; carries genetic information in the form of genes.
• Chromosome: A structure made of DNA that contains many genes.
• Gene: A segment of DNA that codes for a specific protein.
• Protein: A molecule made of amino acids, responsible for various functions in the body.
  • These words link together as follows: DNA is organized into genes, many genes are located on a chromosome and chromosomes are found in the nucleus, genes encode instructions to make proteins.

Mendel's Pea Plants

• When Mendel crossed two heterozygous individuals (Pp x Pp), the ratio of phenotypes in the offspring was 3:1 (3 dominant phenotype, 1 recessive phenotype).

Butterfly Wings

• Green wings are dominant (G) and white wings are recessive (g).
  • Possible genotypes and phenotypes:
    • GG: Green wings
    • Gg: Green wings
    • gg: White wings

Human Chromosomes

• A human somatic cell has 46 chromosomes (23 pairs).
• A human gamete cell has 23 chromosomes.

Mitosis and Meiosis

• Mitosis and meiosis both involve cell division. Both processes begins with a single cell. Both involve stages where chromosomes are duplicated.

Functions of Mitosis and Meiosis

• Main function of mitosis: cell division for growth, repair, and asexual reproduction; produces two identical diploid cells.
• Main function of meiosis: sexual reproduction; produces four genetically different haploid gametes.

Sex determination

• Genotypes of male mammals: XY.
• Genotypes of female mammals: XX.
• Sex determination: Determined by the sperm cell; if it carries an X chromosome, the offspring will be female (XX); if it carries a Y chromosome, the offspring will be male (XY).

Karyotype

• A karyotype is an organized profile/ visual display of an individual's chromosomes, arranged in homologous pairs and ordered by size and centromere position. A karyotype will show if there are any chromosomal abnormalities (e.g., Down syndrome, Turner syndrome).

Pedigree Analysis

• Pedigree symbols:
  • Square: Male
  • Circle: Female
  • Filled symbol: Affected individual
  • Unfilled symbol: Unaffected individual
  • Horizontal line connecting a male and female: Mating
  • Vertical line down from mating line: Offspring

Albinism Pedigree

• Albinism is a recessive trait.
• Genotypes:
  • Bill: aa (affected)
  • Don: aa (affected)
  • Karen: Aa (unaffected carrier)
  • Brian: Aa (unaffected carrier)
  • Sam: aa (affected)
• Justification for Karen and Brian's genotypes: Since they have affected children (aa) but are unaffected themselves, they must both be heterozygous carriers (Aa).

Polydactyly Pedigree

• Polydactyly is a dominant trait.
• Genotypes:
  • Ted: pp (unaffected)
  • Joan: Pp (affected)
  • Peter: Pp (affected)
  • Dennis: pp (unaffected)
  • Mavis: Pp (affected)
  • Gordon: Pp (affected)
  • Ann: pp (unaffected)

Allele

• An allele is a variant form of a gene.

Punnett Squares

• Green (G) is dominant over yellow (g).

• Cross between two heterozygous plants (Gg x Gg):

  	G	g
  G	GG	Gg
  g	Gg	gg

• Cross between a heterozygous plant and a homozygous recessive plant (Gg x gg):

  	G	g
  g	Gg	gg
  g	Gg	gg

• Cross between a heterozygous plant and a homozygous dominant plant (Gg x GG):

  	G	G
  G	GG	GG
  g	Gg	Gg

Coat Color in Cows

• White cow: WW
• Red cow: RR
• Roan cow: RW

Roan Cow Production

• A roan cow is produced by codominance. The offspring (RW) expresses both the red and white phenotypes simultaneously.

Codominance vs. Incomplete Dominance

• Codominance: Both alleles are expressed equally in the phenotype (e.g., roan cow).
• Incomplete Dominance: The phenotype of the heterozygote is a blend or intermediate of the two homozygous phenotypes (e.g., a red flower and a white flower producing a pink flower).

Mutations

• Mutations are changes in the DNA sequence.
• Causes of mutations: replication errors, exposure to mutagens (radiation, chemicals), spontaneous mutations.

Chromosomal Disorders

• Disorder caused by abnormal chromosome number: Down syndrome (Trisomy 21 - having an extra copy of chromosome 21).

Haemophilia

• Haemophilia is an X-linked recessive disorder.

• Carrier mother (XHXh) and a father with haemophilia (XhY):

  	XH	Xh
  Xh	XHXh	XhXh
  Y	XHY	XhY

• Genotypic ratio: 1 XHXh : 1 XhXh : 1 XHY : 1 XhY.

• Phenotypic ratio: 1 carrier female : 1 affected female : 1 unaffected male : 1 affected male.

• More males have haemophilia because males only have one X chromosome (XhY), so if they inherit the affected X, they will have the disorder. Females need to inherit two copies of the affected X chromosome (XhXh) to have haemophilia.

Blood Types

• Mother with blood type B (IBi) and a father with blood type A (IAi):

  	IA	i
  IB	IAIB	IBi
  i	IAi	ii

• Possible genotypes: IAIB , IBi, IAi, ii.

• Genotypic Ratio: 25% IAIB : 25% IBi : 25% IAi : 25% ii.

• Possible phenotypes: Blood type AB, Blood type B, Blood type A, Blood type O.

• Phenotypic Ratio: 25% AB : 25% B : 25% A : 25% O.

Evolution Revision

Charles Darwin

• Charles Darwin was a British naturalist, geologist and biologist, best known for his contributions to evolutionary biology.

Darwin's Theory of Evolution

• Darwin's theory of evolution: Species change over time through natural selection.

Gondwana, Laurasia, and Pangaea

• Pangaea: A supercontinent that existed millions of years ago.
• Gondwana: The southern part of Pangaea.
• Laurasia: The northern part of Pangaea.
• Pangaea split into Gondwana and Laurasia. Gondwana then further split into South America, Africa, Australia, Antarctica, and India. Laurasia split into North America, Europe, and Asia.

Marsupials

• Marsupials: Mammals that carry their young in a pouch.
• Geographic Distribution: Mainly found in Australia and the Americas.
• Explanation: Marsupials evolved in Gondwana before the continents separated. Australia became isolated, allowing marsupials to thrive without competition from placental mammals.

Plate Tectonics

• Plate tectonics: The theory that the Earth's crust is divided into several plates that move over the mantle.

Plate Tectonics and Evolution

• Plate tectonics supports evolution by:
  • Explaining the distribution of species across continents.
  • Creating new environments, leading to speciation.
  • Isolating populations, causing them to evolve independently.

Fossils

• Fossils: Preserved remains or traces of ancient organisms.

Living Fossils

• Living fossils: Extant organisms that closely resemble fossils from ancient times (e.g., the coelacanth).

Definitions

• Species: A group of organisms that can interbreed and produce fertile offspring.
• Speciation: The process by which new species arise.
• Hybrid: The offspring of two different species.

Stages of Speciation

• The stages of speciation includes:
  • Variation: Variation arises within a population.
  • Isolation: A population separates into two separate groups in different areas.
  • Selection: Natural selection favours some variations in each group.
  • Speciation: eventually results, which means that the two groups can no longer interbreed to produce fertile offspring.

Theory of Natural Selection

• Natural selection: The process by which organisms better adapted to their environment tend to survive and reproduce more successfully.

Peppered Moths

• Peppered moths: During the Industrial Revolution, the frequency of dark-coloured moths increased due to pollution darkening tree bark. This provided better camouflage against predators. When pollution decreased, the frequency of light-coloured moths increased again.

Divergent Evolution

• Divergent evolution: The accumulation of differences between groups which can lead to the formation of new species, usually as a result of different populations adapting to different environments.

Convergent Evolution

• Convergent evolution: The independent evolution of similar features in different lineages.

Homologous Structures

• Homologous structures: Structures in different species that have a similar underlying anatomy but different functions. Evidence of common ancestry. Example: the pentadactyl limb in vertebrates (human arm, bat wing, whale flipper).

Analogous Structures

• Analogous structures: Structures in different species that have similar functions but different underlying anatomy; evidence of adaptation to similar environments. Example: wings of insects and birds.

Types of Evolution

• Analogous structures are linked to convergent evolution.
• Homologous structures are linked to divergent evolution.

Adaptations Revision

Adaptation

• Adaptation: A trait/ feature that helps an organism survive and reproduce in its environment.

Ecosystems Revision

Definitions

• Ecosystem: A community of interacting organisms and their physical environment.
• Habitat: The natural home or environment of an organism.
• Range: The geographical area where a species can be found.

Biotic vs. Abiotic Factors

• Biotic factors: Living components of an ecosystem (e.g., plants, animals, bacteria).
• Abiotic factors: Non-living components of an ecosystem (e.g., sunlight, water, temperature, soil).

Population vs. Community

• Population: A group of individuals of the same species living in the same area.
• Community: All the different populations of organisms living in the same area.

Autotroph vs. Heterotroph

• Autotroph: An organism that produces its own food (e.g., plants, algae).* Heterotroph: An organism that obtains food from other organisms (e.g., animals, fungi).

Producer vs. Consumer

• Producer: An autotroph that forms the base of the food chain (e.g., plants).
• Consumer: A heterotroph that feeds on other organisms (e.g., herbivores, carnivores, omnivores).

Trophic Level

• Trophic level: The position an organism occupies in a food chain or food web.

Trophic Levels of Organisms

• Producers: First trophic level.
• Herbivores: Second trophic level.
• Carnivores: Higher trophic levels (third, fourth, etc.).
• Decomposers: Can be found at any trophic level. Decomposers play a role in every level.

Multiple Trophic Levels

• An organism can occupy more than one trophic level if it has a varied diet (e.g., an omnivore).

Self-Sufficient Food Web

• Requirements:
  • A constant source of energy.
  • A population of decomposers.

Source of Energy

• The source of energy for most ecosystems is the sun.

Energy Loss

• Energy is lost from an ecosystem through heat, respiration, waste, and incomplete consumption.

Food Chain and Food Web

• Food chain: A linear sequence of organisms through which nutrients and energy pass as one organism eats another.
• Food web: A network of interconnected food chains.

Arrows in Food Chain/Web

• Arrows represent the flow of energy and nutrients from one organism to another.

Removing a Producer

• Consequences of removing a producer: The entire food web will be affected, as consumers will lose their primary food source, potentially leading to population declines and ecosystem collapse.

Food Chain Example

• Example Food Chain: Grass → Grasshopper → Frog → Snake → Hawk
  • Producer: Grass
  • 1st order consumer/Primary consumer: Grasshopper
  • 2nd order consumer/Secondary consumer: Frog
  • Third order consumer/Tertiary consumer: Snake
  • Quaternary consumer: Hawk
  • Apex predator: Hawk

Competitor

• Competitor: An organism that competes with another for resources (e.g., food, water, shelter).