Genetics, populations, evolution and ecosystems

Alleles

Alternative forms of a gene with different base sequences

Heterozygous

A genotype in which the 2 alleles for a characteristic are different

Homozygous

A genotype in which the 2 alleles for a characteristic are the same

Dominant allele

Always expressed in the phenotype

Recessive allele

Expressed only in homozygous organisms

Locus

The fixed location of an allele on a chromosome

Monohybrid inheritance

Inheritance of a single pair of alleles

Codominant alleles

Both alleles are expressed in the phenotype

Multiple alleles

When a gene halls more than 2 alleles

Dihybrid inheritance

2 characteristics are inherited by 2 different genes with different alleles

Autosomes

All chromosomes except sec chromosomes

Sex chromosomes

Chromosomes that determine the sex of an individual (X or Y)

Linkage group

A group of genes located on the same chromosome but at different loci, often inherited together

When are linkage groups not inherited together

Crossing over occurs and a chiasma is formed between the genes

Recombinant chromosomes

When crossing over occurs and separates a linkage group

Epistasis

When the alleles of one gene affects the phenotype expression of another gene at a different locus

Hardy-Weinburg Equations (2)

p + q = 1

p² + 2pq + q² = 1

Natural selection

The process by which organisms better adapted to their environment survive and reproduce and pass on advantageous alleles to their offspring

How natural selection drives evolution

1. Variation in a population

2. Mutations cause individuals with advantageous alleles to survive and reproduce

3. Favourable alleles passed onto offspring, so Increased frequency

4. A new species may be formed if no interbreeding occurs

Directional selection

Environmental changes lead to individuals with extreme phenotypes to be favoured

Stabilising selection

Stable environment so individuals with average characteristics are favoured

Disruptive selection

Fluctuating environment so multiple phenotypes are favoured

Speciation

The evolution of a new species from an existing one

Allopatric Speciation

Speciation that has occurred due to geographical isolation

Sympatric Speciation

Speciation that has occured due to reproductive isolation

Genetic Drift

The change in allele frequency in a population between generations

Ecosystem

Communities of organisms interacting with the habitat and each other

Habitat

The abiotic aspects of an ecosystem

Community

Populations of different species living in the same environment/habitat

Population

All the members of a particular species that live in a habitat

Niche

The role of a species within its ecosystem/community

Population size

The total number of organisms of one species

Carrying capacity

The maximum stable population size that an ecosystem can support

Succession

The change in an environment over time due to changes in the living organisms present over time

Primary Succession

Occurs when there were previously no living organisms present there

Secondary Succession

Occurs when there were previously living organisms present there

Pioneer species

The first species to arrive for primary succession, often adapted to harsh conditions so that they can grow and colonise the land

Climax community

The largest, most complex and stable community