Genetic Variation Key Words

Allele

One of the alternative forms of a gene.

Amino acids

Any of a group of water-soluble organic compounds that possess both a carboxyl(-COOH) and an amine (-NH2) group attached to the same carbon atom.

Anticodon

A sequence of three nucleotides on a tRNA molecule that matches with a specific codon on a strand of mRNA during translation.

Centromere

The part of a chromosome that attaches to the spindle during cell division.

Chromatid

A threadlike strand formed from a chromosome during the stages of cell division.

Chromosome

A threadlike structure made of DNA and (in eukaryotes) associated histone proteins on which the genetic information of an individual is stored.

Cloning

artificial production of genetically identical individuals

Co-dominance

The condition that arises when both alleles in a heterozygous organism are dominant and fully expressed - eg. the human blood group AB.

Codon

A triplet of nucleotides within a molecule of messenger RNA that specifies a particular amino acid during the synthesis of proteins in a cell.

Diploid

Describing a cell or organism with twice the haploid number of chromosomes.

DNA

Deoxyribonucleic acid. The genetic material of most living organisms. It plays a central role in the determination of hereditary characteristics by controlling protein synthesis in cells.

Dominant allele

The allele that is expressed when two different alleles of a gene are present in the cells of an organism. It is denoted by a capital letter.

Double Helix

ladder-like molecule twisted into a helix

Eukaryotic cells

Cells that have a distinct nucleus where the cell’s genetic information is stored.

F1 generation

The first generation of offspring resulting from an arranged cross between selected parents in breeding experiments.

F2 generation

The second generation of offspring in breeding experiments, obtained by crosses between individuals of the F1 generation.

Fertilization

fusing of an egg and sperm

Gametes

A reproductive cell that fuses with another to form a zygote - eg. eggs (ova) and sperm (spermatozoa).

Gene             

A section of DNA that codes for a particular protein and cannot be broken by recombination.

Genetic code

The genetic information in DNA which controls the manufacture of specific proteins by the cell. It takes the form of a series of triplets of bases in DNA.

Genome

All the genes contained in a single (haploid) set of chromosomes.

Genotype

The genetic composition of an organism,  i.e. the combination of alleles it possesses.

Haploid

Describing a cell or organism with a single set of unpaired chromosomes.

Heterozygous

Describes an organism that has two different alleles controlling a particular feature.

Histone

Water-soluble proteins found in association with the DNA of eukaryotic chromosomes. They serve as a scaffold around which the DNA coils.

Homologous Chromosomes

Matching pairs of chromosomes, i.e. Chromosomes that have the same structural features. In diploid cells, one chromosome comes from the female parent, the other from the male parent.

Homozygous

Describes an organism that has two of the same alleles for a trait.

Incomplete dominance

The condition where neither allele controlling a characteristic is dominant and the resulting phenotype is partially influenced by both alleles.

Independent assortment

The separation of the alleles of one gene into gametes independently of the way in which the alleles of other genes have segregated.

Inheritance

The transfer of genetic traits from one generation to the next.

Lethal genes

A mutant form of a gene that eventually results in the death of an organism if expressed in the phenotype. Most of these genes are recessive, eg. Sickle-cell anaemia.

Meiosis

A type of cell division that gives rise to four gametes each with half the chromosome number of the parent cell.

Mitosis

A type of cell division that results in two daughter cells, each having a nucleus containing the same number and kind of chromosomes as the mother cell.

Monohybrid

The offspring of a cross between parents that differ in the alleles they possess for one particular gene, one parent having two dominant alleles, the other two recessives.

mRNA

RNA that carry’s the genetic code transcribed from DNA to the ribosomes.

Multiple alleles

Three or more alleles that produce different versions of the same protein. Eg. The human ABO blood group system has three alleles IA, IB and i. IA and IB are co-dominant alleles.

Nucleotide

An organic compound consisting of a nitrogen-containing purine or pyrimidine base linked to a sugar and a phosphate group.

Pedigree Chart

a chart used to identify genotypes in families

Phenotype

The observable characteristics of an organism.

Prokaryotic cells

Cells where the genetic material is not enclosed in a cell nucleus.

Protein synthesis

The process by which living cells assemble amino acids into proteins based on the genetic information carried in the DNA of the chromosomes.

Punnett Square

A grid used to calculate the expected offspring phenotype ratios resulting from a cross between two parents of known genotypes.

Pure-breeding

A lineage where characteristics are passed on unchanged from generation to generation.

Purines

The chemical family to which the organic nitrogenous bases adenine and guanine belong.

Pyrimidines

The chemical family to which the organic nitrogenous bases uracil, thymine and cytosine belong.

Recessive allele

An allele whose effects are masked by those of a dominant allele. It is denoted by a lower case letter.

rRNA

A type of RNA that is present in the ribosomes.

Self-Replication

ability of DNA molecules to make identical copies of themselves

Somatic cells

All cells in the body other than the reproductive cells.

Test-cross

An organism with a dominant phenotype may possess two dominant alleles, or one dominant and one recessive. This can be determined by crossing it with an organism showing the recessive characteristic (ie. homozygous recessive).

Trait

feature whose appearance is determined by genes (inherited)

Transcription

The process in living cells in which the genetic information of DNA is transferred to a molecule of mRNA as the first step in protein synthesis.

Translation

The process in living cells in which a sequence of mRNA triplets (codons) is used to build a polypeptide chain during protein synthesis.

Triplet

A sequence of three nucleotides coding for a specific amino acid during protein synthesis.

tRNA

RNA that is responsible for bringing specific amino acids to the site of translation on the ribosome.

Variation

Differences between organisms created by their different genetic make-up. It can either be discrete, where a trait is controlled by a single gene; or continuous, where a trait is influenced by several genes.

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