Genetic Variation

Allele

Different versions of a gene.

Asexual reproduction

Reproduction involving only one parent - producing identical offspring.

Bases

Adenine

Carrier

Has inherited a genetic trait or mutation

Characteristic

An attribute of an individual

Chromosome

Strand of DNA which contains many genes; a normal human cell has 46 (23 pairs).

Complementary base pairing

Used to describe how the bases bond

Continuous variation

Variation that shows a range/gradual change

Diploid

A cell with the full complement of chromosomes (46 in humans).

Discontinuous variation

Variation that is either/or e.g. blood groups

DNA

The chemical which carries genetic information in the nuclei of cells. Made up of phosphate units

Dominant

The allele which will be expressed in the phenotype if it is present. Shown as an uppercase letter.

Double Helix

Name given to the twisted ladder shape of DNA.

Fertilisation

When a male sex cell (gamete) combines with a female sex cell (gamete) to form a zygote.

Gamete

A sex cell e.g. egg or sperm in animals

Genetic variation

The differences among parents and their offspring or among individuals in a population.

Genotype

Piece of DNA which codes for a particular protein and therefore a characteristic

Haploid

Cell with half the complement of chromosomes (23 in humans).

Heterozygous

Having a dominant and a recessive allele

Homozygous

A pair of alleles that are the same

Hydrogen bonds

Attractive forces that hold the two strands of DNA together.

Inheritable

The information can be passed on to the offspring.

Karyotype

A photograph or diagram of the chromosomes of the cell arranged in an orderly fashion.

Meiosis

Cell division that reduces the number of chromosomes to half the normal number

Mitosis

Cell division that maintains the normal number of chromosomes

Monohybrid cross

A straightforward cross involving only one pair of genes or alleles.

Natural selection

Organisms with characteristics best suited to their environment become more successful

Mutation

A sudden

Non-inheritable

Information that cannot be passed on to the offspring as it is due to a change in the somatic cells or due to the environment.

Nucleotide

A unit built up of a base

Pedigree chart

A family tree drawn with standard genetic symbols

Phenotype ratio

Ratio of phenotypes e.g. 3 brown hair : 1 blonde hair (a prediction of the phenotypes and their occurrence as a result of a cross).

Phenotype

The expression of the genotype. The physical appearance of a trait.

Phosphate

Group that alternates with the ribose sugar in the DNA backbone.

Punnett square

Name given to the grid of squares that may be drawn to show the possible combinations of alleles in a genetic cross.

Pure breeding

A group of identical individuals that always produce offspring of the same phenotype when interbred. They are homozygous for a trait

Recessive

The allele which is only expressed in a phenotype if there are 2 of them present

Ribose

Sugar group that alternates with the phosphate group on the DNA backbone.

Semi conservative

Each double-stranded DNA molecule is composed of one parental strand

Sexual reproduction

Reproduction involving 2 parents

Somatic

Body cells.

Trait

Another word for characteristic

Triplet code

Variation

Differences in DNA/alleles/genes. Differences within a species.

Zygote

Cell formed when a sperm cell fuses with an egg cell.

The Bottleneck Effect

A reduction in the genetic diversity of a population caused by a reduction in its size due to a disaster.

Founder effect

When a small number of individuals colonize a new area; the new gene pool is not reflective of original population.

Gene flow

Movement of alleles into or out of a population due to the migration of individuals to or from the population

Immigration

Where individuals more into a population

Population

A group of individuals of the same species that live in the same area.

Crossing over

The exchange of genes between homologous chromosomes

Species

A group of similar organisms that can breed and produce fertile offspring.

Chiasma

The site of crossing over.

Recombination

The genetic process by which one chromosome breaks off and attaches to another chromosome during reproductive cell division.

Directional selection

Occurs when natural selection favors one of the extreme variations of a trait.

Stabilising selection

Selection for the middle range against the extremes.

Centromere

Region of a chromosome where the two sister chromatids attach.

Segregation

Separation of alleles during gamete formation.

Adaptation

Characteristic that increases an organism's chance of survival.

Evolution

The gradual change in a species over time.

Gene pool

Combined genetic information of all the members of a particular population.

Genetic drift

A change in the allele frequency of a population as a result of chance events rather than natural selection.

Genetic equilibrium

situation in which allele frequencies in a population remain the same.

Mate selection

The process by which individuals choose each other for mating.

Speciation

The formation of new and distinct species in the course of evolution.

Selective pressure

When the environment pushes an individual or population to adapt or evolve.

Locus

Location of a gene on a chromosome.

Disruptive selection

Favors individuals at both extremes of the phenotypic range.

Allele frequency

How often a form of a gene appears in a gene pool.

Independent assortment

The random distribution of the pairs of genes from different chromosomes to the gametes.