2.5 Genetic Variation and Change

Population bottleneck

A sudden, drastic reduction in population size caused by a random event, resulting in reduced genetic diversity and altered allele frequencies.

Genetic drift

Random changes in allele frequency due to chance events, with a greater effect in small populations.

Founder effect

A type of genetic drift where a small group breaks away from a larger population to form a new population with a small, non-representative sample of alleles.

Gene pool

The total collection of alleles present in a population.

Allele frequency

The proportion of a particular allele in a population’s gene pool.

Genetic change

A change in allele frequencies in a population over time.

Gene flow (migration)

The movement of alleles into or out of a population due to individuals migrating.

Locus

The specific position of a gene on a chromosome.

Chiasma

The point where non-sister chromatids overlap and exchange DNA during crossing over.

Crossing over

The exchange of DNA between non-sister chromatids of homologous chromosomes during prophase I of meiosis.

Segregation (meiosis)

The separation of homologous chromosomes in anaphase I and the separation of sister chromatids in anaphase II so alleles separate into different gametes.

Independent assortment

The random orientation and separation of homologous chromosome pairs during metaphase I of meiosis.

Random fertilisation

The random fusion of any sperm with any egg, increasing genetic variation.

How processes increase genetic variation

Crossing over, independent assortment, segregation and random fertilisation create new combinations of alleles in gametes and offspring.

Mutation

A permanent change in the base sequence that can create new alleles (can be made more likely by mutagens)

Point (gene) mutation

A mutation involving a change in a single nucleotide base.

Deletion

A mutation where a section of DNA is removed from a chromosome.

Duplication

A mutation where a section of DNA is copied and repeated.

Inversion

A mutation where a section of DNA is reversed within the chromosome.

Translocation

A mutation where a section of DNA moves to a different chromosome.

Complete dominance

One allele completely masks the expression of another allele in a heterozygote.

Incomplete dominance

Neither allele is fully dominant, resulting in an intermediate phenotype.

Co-dominance

Both alleles are fully and equally expressed in the phenotype.

Multiple alleles

A gene that has more than two alleles present in a population.

Sex-linked genes

Genes located on the sex chromosomes, usually the X chromosome.

Linked genes

Genes located close together on the same chromosome that are usually inherited together unless separated by crossing over.

Lethal alleles

Alleles that produce a phenotypic effect that causes the death of the organism (usually arise due to a mutation in an essential gene; can cause a disability in heterozygous form)

Phenotype ratio of a heterozygous dihybrid cross

9 : 3 : 3 : 1

Natural selection

The process where individuals with advantageous traits are more likely to survive and reproduce, increasing the frequency of those alleles.

Artificial selection (selective breeding)

Humans deliberately breed organisms to increase the frequency of desirable traits.

Directional selection

Selection that favours one extreme phenotype.

Stabilising selection

Selection that favours the mean phenotype.

Disruptive selection

Selection that favours both extreme phenotypes.

Sexual selection

Selection based on traits that increase mating success rather than survival.

Section of DNA (gene) codes for…

A protein, which determines a trait.

Locus vs chiasma

A locus is the position of a gene on a chromosome, while a chiasma is the point where crossing over occurs.

Founder effect example

A small group migrates to an island and forms a population with reduced genetic variation.

Bottleneck effect example

A natural disaster drastically reduces a population, leaving a non-representative sample of alleles.

Founder effect vs bottleneck

A founder effect involves migration of a small group, while a bottleneck involves sudden population reduction.

Neutral mutation

A mutation that has no effect on the organism’s phenotype or fitness.

Silent mutation

A mutation that does not change the amino acid sequence of the protein and therefore has no effect on phenotype.

Harmful mutation

A mutation that reduces an organism’s fitness or survival by producing a less effective or non-functional protein.

Beneficial mutation

A mutation that increases an organism’s fitness by producing a protein that gives a survival or reproductive advantage.