AQA A-Level Biology I Unit 7 Genetics Populations Evolution I Primrose Kitten

GENE

A length of DNA or a sequence of nucleotide bases, that codes for a specific polypeptide and therfore a specific characteristic.

ALLELE

One of the alternative or different forms of the same gene e.g. in pea plants there is a gene for the colour of the seed pod. This gene has two different alleles, an allele which codes for green pods and another allele which codes for yellow pods.

PHENOTYPE

The observable characteristics of an organism, which may be visual or biochemical. The environment can alter an organism's phenotype.

GENOTYPE

The alleles present in the cells of an individual organism which code for a particular characteristic. It is also used to refer to the entire complement of alleles that an organism has.

HOMOLOGOUS CHROMOSOMES

Pairs of chromosomes in a diploid cell. Each chromosome within the pair, carry the same sequence of genes occupying the same loci along their length, but not necessarily the same alleles.

LOCUS

The position of a gene on a particular DNA molecule.

HOMOZYGOUS

The state of having two identical alleles for a specific gene in a diploid cell.

HETEROZYGOUS

The state of having two different alleles for a specific gene in a diploid cell.

DOMINANT

An allele that is always expressed in the phenotype, even when a diferent allele is also present.

RECESSIVE

An allele that is only expressed in the phenotype if it is present in the homozygous state.

DIPLOID

Containing two sets of chromosomes e.g. human somatic cells produced by mitosis.

HAPLOID

Containing one set of chromosomes e.g. gametes produced during meiosis.

CODOMINANCE

When two alleles of the same gene influence the phenotype of a heterozygous organism because both are dominant over any other alleles of the gene, but neither allele is dominant over the other e.g inheritance of human ABO blood groups.

INCOMPLETE DOMINACE

When neither of two alleles of a gene dominates so there is blending of the two alleles to form an intermediate e.g. if two alleles for petal colour in flowers were red and white, all heterozygotes would be pink.

MULTIPLE ALLELES

A gene has more than two types of alleles. However, as there are only two chromosomes in a homologous pair, only two of the three or more different alleles can be inherited e.g. human ABO blood groups where there are three different alleles IA; IB; IO

MONOHYBRID

The inheritance of a single gene which controls a specific characteristic e.g.wing length in the fruit fly, Drosophila.

DIHYBRID

The inheritance of two characteristics, determined by two different genes located on different chromosomes.

PURE BREEDING

A group of organisms that always produce offspring with the same phenotype and genotype e.g. TT x TT; tt x tt

WILD TYPE

The phenotype of the standard form of a species as it exists in nature. The wild type is considered to be the 'normal' allele at a locus, in contrast to that produced by a 'non-standard' mutant allele.

FIRST FILIAL GENERATION

Often referred as the F1 generation. The first set of offspring produced by the parent generation.

RANDOM ASSSORTMENT

The random organisation of homologous chromosomes on the spindle equator during metaphase 1 of meiosis. This also occurs with the chromatids during metaphase II of meiosis.

RANDOM SEGREGATION

The random separation of homologous chromosomes during anaphase 1 and chromatids during anaphase II of meiosis.

CHIASMATA

The point where non sister chromatids in a homologous pair of chromosomes [bivalent] cross over [and join] during prophase 1 of meiosis.

CHROMATID

One copy of a duplicated chromosome generally joined to the other copy by a single centromere. Chromatids of a single chromosome occur after DNA replication during the S phase of interphase.

TEST CROSS

A genetic cross carried out with a double recessive phenotype/genotype to distinguish homozygous dominant and heterozygous dominant genotypes for a particular characteristic.

SEX CHROMOSOME

A chromosome involved in determining the sex of an organism. In humans, there are X and Y chromosomes; females have two X chromosomes; males have one X and one Y chromosome.

SEX LINKAGE

Refers to genes carried on the X chromosome e.g. haemophilia.

AUTOSOMAL LINKAGE

When two or more genes are located on the same chromosome, other than a sex chromosome.

AUTOSOME

Any chromosome which is not a sex chromosome.

EPISTASIS

When genes interact in the expression of a characteristic. One gene supresses the expression of another gene

CHI-SQUARED TEST

A statistical test used to test categoric data to see if there are differences between expected and observed data.

STUDENT T-TEST

A statistical test used to find out whether the means of two sets of data are significantly different. To use the t-test, both sets of data must show a normal distribution.

DEGREES OF FREEDOM

The number of classes or categories minus one e.g. if humans can have blood group A, B, AB or O, there are four categories but three degrees of freedom.

GENE POOL

All the allelesof all the genes of all the individuals in a population at a given time.

ALLELE FREQUENCY

The number of times an allele occurs within the gene pool.

HARDY WEINBERG PRINCIPLE

A mathematical equation that can be used to calculate the allele frequencies of a particular gene in a population. The principle is based on the assumption that the proportion of dominant and recessive alleles of any gene in a population remains constant from one generation to the next. p+q = 1, where p = the frequency of the dominant allele; q = frequency of the recessive allele. In a single population, the genotype frequency can be represented as p2 + 2pq + q2 = 1, where p2 = the frequency of the homozygous dominant genotype; 2pq = the frequency of the heterozygous dominant and q2 = the frequency of the homozygous recessive genotype.

SELECTION PRESSURE

A factor that gives a greater chance of survival to some members of the population than others.

INTRASPECIFIC COMPETITION

Individuals of the same species complete with one another for resources such as light, food, water, breeding sites, mates.

INTERSPECIFIC COMPETITION

Individuals of different species complete with one another for resources such as light, food, water, breeding sites, mates.

NATURAL SELECTION

The mechanism for evolution.It is the survival to reproductive age of those organisms with characteristics best suited to their environment. This increases the chances of their alleles being passed on to subsequent generations.

STABLISING SELECTION

The type of selection that operates against the extremes of a range of phenotypes, so the population remains the same over time e.g. body mass of human children at birth.

DIRECTIONAL SELECTION

Type of selection that favours one extreme of the range of phenotypes so the population changes over time. This type of selection pressure favours a combination of alleles that results in the mean moving to either the left or the right of its original position. e.g. antibiotic resistance in bacteria.

DISRUPTIVE SELECTION

The least common form of selection but the most important in bringing about evolutionary change. This type of selection favours the extremes of the range of phenotypes at the expense of the intermediate phenotypes e.g. coho salmon. Large and small males each have a different competitive advantage over the intermediate sized males.

SPECIATION

The evolution of new species from existing ones. A species is a group of individuals that have a common ancestry and so share the same genes but different alleles and are capable of breeding with one another to produce fertile offspring. Members of a species are reproductively separated from other species.

GENETIC DRIFT

A change in allele frequency in the absence of natural selection.

GENETIC BOTTLENECK

A sharpt reduction in population size due to environmental events such as earthquake, flood, fire, drought, disease or human activity. Any subsequent population increase is based on the limited gene pool available from the surviving individuals.

FOUNDER EFFECT

When a small population colonises a new area, any population descending from this small 'ancestor' group will have a much reduced genetic diversity and a different allele frquency from other similar populations.

ADAPTIVE RADIATION

The adaptation [via genetic mutation] of an organism, which enables it to successfully spread into other environments. Adaptive radiation leads to speciation.

ALLOPATRIC SPECIATION

Speciation by any geographical mechanism that separates a population into physically distinct and separate sub groups which then become isolated from each other.

SYMPATRIC SPECIATION

Speciation by reproductive differences that cause members of a population to become isolated while living in close proximity.

BIOTIC FACTOR

Any factor that influences populations in a community that is the result of the activities of organisms; examples include competition; predation; mutualism; disease; allelopathy.

ABIOTIC FACTOR

Any physical or chemical factor that influences populations in a community. Examples include temperature; wind speed; light intensity; water availability and humidity.

EDAPHIC FACTOR

Any abiotic factor of the soil. Examples include soil structure; salinity; temperature; oxygen content. These can all influence the composition of soil microbes and the distribution/abundance of plants and animals.

ECOSYSTEM

An ecological term for the interaction of the all the living [biotic] and non-living [abiotic] factors in a specific environment.

POPULATION

All the members of the same species living in a particular area at the same time. In species which reproduce sexually, there is interbreeding between males and females to produce fertile offspring.

COMMUNITY

An ecological term for a group of populations of two or more species which occupy the same geographical area at the same time.

HABITAT

A place where a species lives, expressed in one or few words e.g. pond, lake, river, coral reef, grassland; sand dune; deciduous woodland; heathland.

NICHE

The role of a species in an ecosystem. A niche includes a description of all a species' interactions with the different factors in its environment [i.e. its' address and its profession'.]

CARRYING CAPACITY

The maximum population size of a species that a particular habitat can support over time.

PREDATOR

An organism that feeds on another organism.

PREY

An organism that is eaten by another organism

QUADRAT

A square wooden or metal frame [of varying dimensions] used in ecology, to isolate a standard unit area to study the distribution and abundance of very slow moving and sessile organisms.

SYSTEMATIC SAMPLING

This is used in ecology when the study area includes an environmental gradient or transition in the plant and animal communities e.g rocky shore. A line or belt transect is used to sample along the environmental gradient.

FREQUENCY

This is the likelyhood of a particular species occuring in a quadrat. If a species occurs in 10 quadrats out of a total of 40, the frequency of its occurrence is 25%.

PERCENTAGE COVER

An estimate of the area within a quadrat that a particular plant species occupies. The technique is useful where a species is particularly abundant or difficult to count.

MARK RELEASE RECAPTURE

A method of estimating abundance of motile animal species, which is based on various assumptions. Marked individuals have the same probability of being recaptured as unmarked individuals; the mark must not rub off or wash off; the mark must not harm the individual or make it more susceptible to predation; there must be no immigation into or emigration out from the population; there are no deaths or births within the population between the mark and recapture times.

SUCCESSION

The slow change in communities that occurs when new land becomes available to colonise [primary succession] or when vegetation is cleared [secondary succession].

PRIMARY SUCCESSION

The colonisation of land that has never been colonised before, followed by the progression of changes in the abiotic environment, until a climax community is established. Primary succession leads to a greater number of habitats and niches and increased biodiversity.

SERAL STAGE

One of the communities during a succession e.g. pioneer community; grasses, scrubland.

PIONEER SPECIES

The first organisms in the process of succession to colonise an inhospitable environment. Pioneer species often reproduce asexually, produce large quantities of wind dispersed seeds or spores; germinate rapidly; can photosynthesise, fix nitrogen from the atmosphere and are tolerant of extreme conditions.

CLIMAX COMMUNITY

The community that is the final stage of a natural succession, which remains more or less stable over a long period of time e.g. deciduous oak woodland.

DEFLECTED SUCCESSION

A succession that does not progress to the natural climax community. Factors that can cause this are grazing, mowing and fire.

CONSERVATION

The maintenance of biodiversity and sustainability of biological resources using different methods of in situ and ex situ management.

VARIATION

A range of differences in characteristics between organisms.

INTERSPECIFIC VARIATION

The range of differences in characteristics of organisms across different species.

INTRASPECIFIC VARIATION

The range of differences in characteristics of organisms within a species.

DISCONTINUOUS VARIATION

Features or characteristics which cannot be measured or observed across a complete range so they form distinct classes or categories using discrete or categoric data. Characteristics are often controlled by one pair of alleles. The influence of the environment is small on this type of variation e.g. ABO blood groups; ability to tongue roll.

CONTINUOUS VARIATION

Features or characteristics which can be measured or observed across a complete range from one extreme to the other. There are many intermediate phenotypes between the two extremes. The data collected is quantitative. Characteristics are often controlled by many pair of alleles situated at different loci on different chromosomes. The influence of the environment is large on this type of variation e.g. height in humans; milk yield in cattle.

POOTER

Equipment used to suck up small invertebrate animals into a glass or plastic tube, when investigating local biodiversity.

SWEEP NET

Large net which is often used to catch flying insects or insects which live in long grass.

PITFALL TRAPS

Cans or jars buried in the ground, filled with paper or cardboard to provide shelter and covered with a lid or stone to keep out the rain. These are used to collect ground dwilling insects, which are often nocturnal.

TULLGREN FUNNEL

A piece of equipment which is used to extract small invertebrates especially arthropods from soil or leaf litter.

BEATING TRAY

A large white sheet is placed on the ground or supported by struts and held below a tree. The tree is shaken to dislodge the animals. Very small animals can be collected from the beating tray with a pooter.