biology definitions

DNA

The molecule that carries genetic instructions for the development, functioning, and reproduction of living organisms.

Chromosome

A threadlike structure made of coiled DNA and proteins that holds many genes.

Gene

A specific section of DNA on a chromosome that codes for a particular trait (e.g., eye colour).

Allele

An alternative or different version of a gene.

Gamete

A specialised sex cell (e.g., sperm or egg) containing half the usual number of chromosomes.

Fertilisation

The fusion of two haploid gametes to form a diploid zygote.

Zygote

A single diploid cell resulting from the fertilisation of an egg by a sperm.

Embryo

An early developmental stage consisting of a ball of cells formed from repeated mitotic divisions of a zygote.

Foetus

An unborn offspring that has developed beyond the embryo stage with recognizable structural features.

Homologous chromosomes

A pair of matching chromosomes with the same gene loci, one inherited from each parent.

Meiosis

A type of cell division that produces four genetically unique haploid gametes.

Mitosis

A type of cell division producing two genetically identical diploid somatic (body) cells.

Sexual reproduction

A reproductive process involving the combination of genetic material from two parents to produce genetically varied offspring.

Asexual reproduction

A reproductive process involving only one parent that produces genetically identical offspring.

Genetic variation

The differences in DNA sequences and observable traits among individuals within a population or species.

Continuous variation

Variation that exhibits a full spectrum of values without distinct categories (e.g., height, body mass).

Discontinuous variation

Variation where traits fall into distinct, discrete categories with no intermediates (e.g., tongue rolling, ABO blood group).

Monohybrid inheritance

The inheritance of a single characteristic controlled by one gene.

Genotype

The specific combination of alleles an individual carries for a particular gene.

Phenotype

The observable physical or physiological traits expressed by an organism, determined by its genotype and environment.

Dominant allele

An allele that is always expressed in the phenotype when at least one copy is present.

Recessive allele

An allele that is only expressed in the phenotype when an individual is homozygous for that allele.

Homozygous

Having two identical alleles for a specific gene.

Heterozygous

Having two different alleles for a specific gene (one dominant and one recessive).

Punnett square

A grid diagram used to predict the possible genotypic and phenotypic outcomes of a genetic cross.

Pure breeding

An organism that is homozygous for a particular trait and always passes that same trait to its offspring when self-crossed.

Test cross

A cross between an individual showing a dominant phenotype and a homozygous recessive individual, used to determine the dominant individual's genotype.

Natural selection

The process by which organisms possessing traits best suited to their environment survive and reproduce at higher rates.

Mutation

A permanent change in the nucleotide base sequence of DNA.

Point mutation

A genetic mutation affecting only a single nucleotide base or very few bases within a single gene.

Chromosomal mutation

A large-scale structural change or numerical alteration affecting whole chromosomes or large segments of DNA, impacting multiple genes.

Selective breeding

The human-directed practice of choosing specific individuals with desirable traits to reproduce.

Inbreeding

The mating of closely related individuals, often increasing the frequency of homozygous genotypes.

DNA sequencing

The laboratory method of determining the precise order of nucleotide bases (adenine, thymine, cytos