Genetics Definitions for Edexcel Biology GCSE

Allele

A version of a gene.

Amino acids

Small molecules from which proteins are assembled.

Anticodon

A triplet sequence of bases on a tRNA molecule that is complementary to a specific codon in mRNA.

Asexual reproduction

A form of reproduction involving a single parent that creates genetically identical offspring.

Base triplet

A sequence of three bases in a gene that code for a particular amino acid. Also known as a codon.

Blood group

Classification of blood type as A, B or O based on surface antigens on red blood cells and the presence of certain antibodies in the plasma.

Chromosome

A long, coiled molecule of DNA that carries genetic information in the form of genes.

Coding DNA

A sequence of DNA that codes for the production of a protein.

Codominance

When both alleles for a gene in a heterozygous organism equally contribute to the phenotype.

Codon

See 'Base triplet'.

Complementary base pairing

Describes how weak hydrogen bonds form between complementary base pairs. A pairs with T and C pairs with G.

Detergent

A chemical that is used to disrupt cell membranes in the extraction of DNA.

Diploid cell

A cell that contains two copies of each chromosome (i.e. a full set of chromosomes).

DNA

A double-stranded polymer, wound to form a double helix, that carries the genetic code.

Dominant

Describes an allele that is always expressed. Represented by a capital letter.

Environmental variation

Differences in phenotype acquired during the lifespan of an organism due to environmental factors such as diet, lifestyle, climate, exposure to light etc.

Family pedigree

A chart used to show the inheritance of genetic disorders in a family.

Fertilisation

The fusion of the nucleus of male and female gametes. Restores the full chromosome number.

Gametes

Sex cells (sperm and egg cells) with half the usual number of chromosomes.

Gene

A length of DNA on a chromosome that codes for the production of a specific protein.

Genetic variation

Differences in the genotypes of organisms of the same species due to the presence of different alleles, arising through mutations and sexual reproduction. It creates variation in phenotypes.

Genome

The complete genetic material of an organism.

Genotype

An organism's genetic composition. Describes all alleles.

Haploid cell

A cell that contains a single copy of each chromosome (i.e. half the number of chromosomes).

Heterozygous

When someone has two different alleles of a gene, e.g. Ff.

Homozygous

When someone has two identical alleles of a gene, e.g. ff.

Human genome project

An international research project involving thousands of scientists which successfully mapped the entire human genome.

Meiosis

A form of cell division that produces four genetically different daughter cells (gametes) with a haploid number of chromosomes. It involves two divisions.

Monohybrid inheritance

The inheritance of a single gene.

mRNA

A polymer of nucleotides that carries genetic information from the nucleus to the ribosomes during protein synthesis. It is a single helix and uses U instead of T.

Multiple alleles

The existence of more than two alleles of a gene, e.g. ABO blood groups are controlled by three alleles, I​A​, I​B​ and I​O​.

Mutation

A random change in the base sequence of DNA which may result in genetic variants. Mutations may be beneficial, damaging, or neutral.

Non-coding DNA

DNA which does not code for a protein but instead controls gene expression by influencing the binding of RNA polymerase to DNA.

Nucleotides

The monomers of DNA consisting of a common sugar, a phosphate group and one of four chemical bases (A, T, C, G) attached to the sugar.

Phenotype

An organism's observable characteristics due to interactions of the genotype and the environment.

Protein

A large molecule that is synthesised from amino acids.

Protein synthesis

The formation of a protein from a gene.

Punnett square

A grid used to predict the potential outcomes of a genetic cross.

Recessive

Describes an allele that is only expressed in the absence of a dominant allele. Represented by a small letter.

Reproductive cycle

The time required to produce independent offspring.

Ribosomes

Sub-cellular structures that are the site of protein synthesis.

RNA polymerase

An enzyme involved in transcription that binds to a region of non-coding DNA, unzips the DNA strands and joins free RNA nucleotides to complementary bases on the coding DNA strand.

Sex chromosomes

A pair of chromosomes responsible for the determination of gender. XY in males, XX in females.

Sex-linked characteristic

A characteristic that is coded for by an allele found on a sex chromosome.

Sex-linked genetic disorder

A disorder caused by a faulty allele located on a sex chromosome.

Sexual reproduction

A form of reproduction that creates genetic variation, involving the fusion of male and female gametes.

Transcription

The first stage of protein synthesis in which mRNA is formed from a DNA template.

Translation

The second stage of protein synthesis that takes place in the ribosomes. Amino acids are joined in a specific order dictated by mRNA to form a protein.

tRNA

An RNA molecule that carries specific amino acids to the ribosomes during protein synthesis.

Variation

The differences between individuals due to genes, the environment or a combination of both.

Zygote

A diploid cell formed by the fusion of the nucleus of a male gamete with the nucleus of a female gamete.