AQA Biology A-level Topic 7: Genetics and Evolution

Inheritance

• Genotype: all the alleles an organism carries on its chromosomes
• Phenotype: observable characteristics of an organism, result of genotype and environmental factors
• Alleles: alternative forms of a gene
• Locus: specific position of a gene on a chromosome
• Types of alleles:
  • Dominant: expressed in phenotype when one or two copies are present
  • Recessive: expressed in phenotype when two copies are present
  • Codominant: both alleles expressed equally in phenotype

Monohybrid Inheritance

• Studying a single gene trait
• Example: cystic fibrosis, where doubly recessive individuals are affected
• Cross between a pure breeding green podded pea plant (dominant) and a yellow podded pea plant (recessive) results in a 3:1 ratio of dominant to recessive traits in the offspring

Dihybrid Inheritance

• Studying two characteristics at once
• Example: crossing round, yellow seeds (both dominant) with a plant pure breeding for wrinkled, green seeds (both recessive)
• Results in a more complex ratio of traits in the offspring

Codominance and Multiple Alleles

• Codominance: two alleles are expressed equally in the phenotype
• Example: human ABO blood groups, with three alleles (IA, IB, and Io) that lead to different antigens on the surface of red blood cells
• Multiple alleles: more than two alleles for a gene

Sex Linkage

• The expression of an allele dependent on the gender of the individual
• Sex chromosomes: X and Y chromosomes that determine sex
• Females: XX, carry two alleles of sex-linked genes
• Males: XY, carry only one allele of sex-linked genes
• Example: haemophilia, a sex-linked disease that affects males

Autosomal Linkage

• The presence of two or more genes on the same autosomal chromosome
• Example: a cross between two genes, A and B, can result in different combinations of alleles depending on whether they are on the same or different chromosomes

Epistasis

• The interaction of different genes, where one gene affects the expression of another
• Example: two genes in mice that control fur color, with one gene (B) controlling the expression of another gene (A)

Chi-Squared Test

• A statistical test used to establish whether the difference between observed and expected results is due to chance
• Criteria for use: sample size > 20, discrete categories, raw counts only
• Formula: χ² = (observed - expected)² / expected
• Comparison to critical value (p = 0.05) to determine significance

Populations

• A group of organisms of the same species occupying a particular space at a particular time
• Gene pool: total number of alleles present in a population
• Allelic frequency: proportion of a certain allele in a gene pool
• Hardy-Weinberg Equation: used to estimate allele frequencies and detect changes in a population over time

Variation in Phenotype

• Causes of variation: random fertilization, meiosis, mutation, and environmental influences
• Example: two plants with the same alleles for flower color, but one grows in a soil lacking a certain mineral, resulting in a different flower color

Evolution and Natural Selection

• The niche of a species: its role within the environment
• Natural selection: the process by which better-adapted species survive and reproduce
• Increase in variation in genotypes and phenotypes within a population increases the chance of survival in a changing habitat

Types of Selection

• Directional selection: the selection of individuals with advantageous traits in a changing environment
• Stabilizing selection: the preservation of successful characteristics and the reduction of diversity
• Disruptive selection: the favoring of both extremes of a normal distribution

Speciation

• The process by which new species arise
• Causes of speciation: allopatric speciation (physical barrier), sympatric speciation (chromosomal error or reproductive isolation)

Populations in Ecosystems

• An ecosystem: all living organisms and non-living elements in a particular environment
• Distribution and abundance of organisms controlled by biotic and abiotic factors
• Each species has a niche, consisting of its biotic and abiotic interactions with the environment

Variation in Population Size

• Factors affecting population size: abiotic (temperature, light, pH, water, humidity) and biotic factors (competition, predation, disease)
• Carrying capacity: the maximum population size supported by an ecosystem

Competition

• Intraspecific competition: competition between members of the same species
• Interspecific competition: competition between members of different species
• Examples: male robins competing for territory, red and grey squirrels competing for resources

Predation

• The relationship between predators and prey
• Predators affect prey populations, which in turn affect predator populations
• Example: a predator-prey relationship between a predator and its prey, with oscillations in population sizes

Measuring Population Size

• Methods: randomly placed quadrats, quadrats along a belt transect, mark-release-recapture
• Estimating population size using the mark-release-recapture method

Succession

• The change of one community of organisms into another
• Ecosystems are dynamic, with succession occurring over time### Primary Succession
• Occurs in areas previously devoid of life, such as after volcanic eruptions that form a rock surface
• Pioneer species like lichens, adapted to harsh conditions, are the first to colonize the area
• As organisms die, microorganisms decompose them, adding humus and forming soil
• Soil formation makes the environment more suitable for complex organisms
• Over time, the soil becomes richer in minerals, enabling larger plants like shrubs to survive
• Eventually, a climax community is established, a self-sustaining and stable community of organisms

Secondary Succession

• Occurs in previously colonized areas where an existing community has been cleared
• Examples include areas affected by forest fires
• As a soil layer is already present, succession begins at a later stage

Conservation

• Human management of the Earth's resources
• Involves managing succession
• Examples include controlled burning of land to prevent the formation of a climax community