Level 2 Biology - Full Key Words & Definitions

Exam-Ready Edition

Allele

An allele is an alternative form of the same gene, caused by small differences in the DNA sequence. Different alleles produce variations in traits, such as hair colour or blood type.

Allele Frequency

The proportion of a specific allele compared to all alleles for that gene within a population. Allele frequencies can change over time due to natural selection, migration, mutation or genetic drift.

Asexual Reproduction

Reproduction involving only one parent, producing genetically identical offspring (clones). This means there is no mixing of alleles, so no new genetic variation is introduced.

Base Substitution / Point Mutation

A mutation where one base in the DNA sequence is replaced with another (e.g. A to G). This may change one amino acid in the protein, possibly affecting its structure and function.

Bottleneck Effect

When a population’s size is drastically reduced by an event (e.g. natural disaster), many alleles are lost, decreasing genetic variation. Future generations then inherit genes from a smaller genetic base.

Chiasma/Chiasmata

The physical point where non-sister chromatids cross over and exchange DNA during meiosis. This recombination produces new allele combinations in gametes, increasing variation.

Codon/Triplet

A codon is a group of three mRNA bases (or DNA triplet) that codes for one specific amino acid in a protein.

Co-dominance

When both alleles in a heterozygous genotype are fully expressed in the phenotype, so neither is dominant or recessive (e.g. AB blood group shows both A and B equally).

Coding Strand/Template Strand

During transcription, the template strand of DNA is used to build the mRNA sequence. The coding strand has the same base order as the mRNA except that thymine (T) in DNA is replaced by uracil (U) in RNA.

Crossing Over

A process during meiosis where homologous chromosomes exchange DNA segments. This creates new allele combinations in gametes and increases genetic diversity.

DNA (Deoxyribonucleic Acid)

A double-helix molecule made of nucleotide bases (A, T, C, G). It carries the genetic code that controls the structure and function of all living organisms.

Diploid (2n)

A cell that contains two sets of chromosomes - one set inherited from each parent. All body (somatic) cells are diploid.

Dominant Allele

An allele that is always expressed in the phenotype, even if only one copy is present in the genotype.

Downstream/Upstream

Terms used to describe positions on DNA or RNA: “upstream” means toward the promoter/start of transcription, “downstream” means toward the 3’ end or after the gene.

Enzyme

A protein that acts as a biological catalyst to speed up specific chemical reactions without being used up. Enzymes are vital for metabolism and gene expression.

Environmental Factor

Any external condition (such as temperature, pH, or chemicals) that can influence how genes are expressed without changing the DNA sequence itself.

Evolution

The change in inherited traits of a population over many generations, caused by natural selection action on genetic variation.

Expression/Gene Expression

The process where the information in a gene is used to make a functional product, usually a protein. This protein then determines an organism’s phenotype.

Founder Effect

When a small number of individuals start a new population in a new area. Because only some alleles are carried over, the new population has lower genetic diversity that the original one.

Gamete

A reproductive cells (sperm or egg) that carries half the normal number of chromosomes (haploid). When two gametes fuse during fertilisation, a zygote is formed.

Genetic Drift

Random changes in allele frequencies from one generation to the next, often affecting small populations and reducing genetic variation.

Gene

A specific section of DNA that codes for a protein or functional RNA molecule. Each gene determines a particular trait or function.

Gene Pool

The complete set of all alleles for all genes within a population. It represents the total genetic variation available.

Genetic Diversity

The range of different alleles and genes in a population. Greater diversity helps populations adapt and survive environmental change.

Genotype

The genetic makeup of an organism - the combination of alleles it has for on or more traits (e.g. Aa, AA or aa).

Haploid (n)

A cell with only one complete set of chromosomes (half the normal diploid number). Gametes are haploid.

Heterozygous

Having two different alleles for a gene (e.g. Aa).

Homozygous

Having two identical alleles for a gene (e.g. AA or aa).

Independent Assortment

During meiosis, homologous PAIRS of chromosomes line up and separate randomly. This creates many possible allele combinations in gametes, increasing genetic variation.

Incomplete Dominance

When neither allele completely masks the other. The heterozygous phenotype is a blend of both (e.g. red + white = pink).

Linked Genes

Genes located close together on the same chromosome. They tend to be inherited together because crossing over between them is rare.

Lethal Allele

An allele that causes death when present in certain genotypes, often when homozygous.

Metabolic Pathway

A series of enzyme controlled reactions in a cell where SUBSTRATE TURNS INTO A PRODUCT AND product becomes the next substrate, leading to a final useful END-product.

Migration/Gene Flow

The movement of individuals (and their alleles) from one population to another. This can introduce new alleles or change existing allele frequencies.

mRNA (Messenger RNA)

A single-stranded polynucleotide molecule that carries the copied genetic code from DNA in the nucleus to ribosomes in the cytoplasm, where proteins are made.

Multiple Alleles

A gene that has more than two possible alleles in the population (e.g. the ABO blood group has IA, IB and i).

Mutation

A RANDOM, permanent change in the DNA base sequence. Mutations create new alleles and are the original source of all genetic variation.

Natural Selection

The process where individuals with BENEFICIAL ALLELES, traits, ADAPT (better suited) to their environment, survive and reproduce more successfully, passing their advantageous alleles to offspring. THOSE WITHOUT BENEFICIAL ALLELES, WON’T BE ABLE TO ADAPT TO SELECTION PRESSURES, WON’T SURVIVE OR REPRODUCE SUCCESSFULLY AND THEIR ALLELES REDUCE IN FREQUENCY IN THE GENE POOL OR DISAPPEAR.

Pedigree Chart

A diagram that shows how a trait or disorder is inherited across generations within a family. It can be used to identify carriers or predict inheritance patterns.

Phenotype

The observable traits or physical characteristics of an organism, determined by both its genotype and environment.

Population Bottleneck

A sharp drop in population size that reduces genetic variation, leaving the surviving population with fewer alleles.

Promoter Region

A DNA sequence located before (upstream of) a gene, where RNA polymerase bind to begin transcription 

Protein

A large molecule made of amino acids, folded into a specific shape. Proteins carry out most of the functions in cells and are the product of gene expression.

Regulatory Gene

A gene that controls the activity of other genes - it can turn genes on or off, controlling when and how much protein is made.

Redundant Code

The genetic code is redundant because several different codons can code for the same amino acid. This reduces the effect of some mutations.

Segregation

During meiosis, the HOMOLOGOUS CHROMOSOMES separates so each gamete receives only one allele FOR EACH GENE IN THE GAMETE CELL.

Sex Linkage/Sex-linked Traits

Traits controlled by genes located on sex chromosomes (usually the X). Males are more likely to show recessive X-linked traits because they only have one X-chromosome.

Silent Mutation

A mutation that changes a base but does not change the amino acid due to the redundancy of the genetic code. It has no effect on the phenotype.

Stop Codon

A codon that signals the end of translation - it tells the ribosome to stop adding amino acids to the growing protein chain.

Template Strand

The DNA strand used as a guide to make a complementary mRNA strand during transcription.

Transcription

The process of copying a gene from DNA into mRNA inside the nucleus.

Translation

The process where ribosomes read mRNA codons and build a chain of amino acids, forming a protein.

tRNA (Transfer RNA)

A small RNA molecule that carries amino acids to the ribosome and matches its anticodon to the codons on mRNA during translation.

Variation (Genetic/Phenotypic)

Genetic variation refers to differences in DNA or alleles between individuals. Phenotypic variation is the physical differences these genes produce, influenced by both genes and envrionment.

Zygote

The first cell formed when a sperm and egg fuse during fertilisation. It contains two sets of chromosomes (diploid) - one from each parent. 

Spontaneous Mutation

Mutation caused by mistake happening in DNA REPLICATION.

Induced Mutation

Mutations caused by MUTAGES (e.g. viruses, radiation, chemicals)

Insertion mutation

Insert base(s) in DNA

Deletion Mutation

Remove base(s) from DNA

Same-sense Mutation

Change of base doesn’t change amino acid being coded for.

Missense Mutation

Change in bases changes amino acid, but not over protein.

Nonsense Mutation

Change in base completely changes DNA (stop codon).

Frame Shift

Deletion or insertion bases changes reading of triplets (usually lethal).

Denatured

Change the shape of an enzyme so that it can no longer speed up a reaction

Substrate

The reactant on which an enzyme works

RNA Polymerase

Enzyme causes DNA helix to unwind ahead of transcription.

Pyrimidine

Bases such as thymine and cytosine (T+C). Has a single ring structure.

Nucleotide

Monomer of nucleic acids made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base.

Purine

Classification of bases such as adenine and guanine (A+G). Has a double ring structure.

Polypeptide

Long chain of amino acids joined together by peptide bonds.

Locus (loci)

Place on a chromosome where particular gene is found for a TRAIT (characteristic).

Gametic Mutation

A mutation in the sex cells. Can be inherited.

Duplication Mutation

A type of chromosomal mutation in which a segment of DNA is copied multiple times in the genome.

Inverse Mutation

A genetic mutation in which the order of a segment of genetic material is reversed. This type of mutation can involve a small number of nucleotides as well as larger sections of a chromosome containing more than one gene.

Translocation Mutation

Mutation in which one part of one chromosome breaks off and attaches to another.

Autosome

All non-sex chromosomes in a cell.

Pure Breeding

Homozygous (dominant or recessive) for the phenotypes being questioned about.

Test Cross

The crossing of an individual of unknow genotype (dominant phenotype) with a homozygous recessive individual to determine the unknown genotype.

Stabilising Selection

A type of natural selection which favours the mean of the distribution because the extremes are at a selective disadvantage. Occurs when environment is stable.

Directional Selection

Is where individuals of one extreme type are more likely to survive and reproduce. 

This happens when the environment changes, and causes corresponding genetic change to the population.

Disruptive Selection

Is where two different extreme types are selected for perhaps because they live in two different habitats.

This leads to two distinct types developing, and eventually these may become different species.

Selection Pressure

The abiotic and/or biotic factors in an environment.

Similar to saying change in the environment.

Interbreeding (interbreed)

Reproduction (breeding) between different populations that are closely related to one another (subspecies - have same genus name, but different species name).

This causes gene flow and introduces new alleles into a population increasing allele frequency and genetic diversity.

Inbreed (inbreeding)

Reproduction (breeding) within same population (same species).

This can reduce allele frequency (fixed alleles).

This can reduce further variation unless a mutation happens to create new alleles.