GENETIC VARIATION AND GENETIC CHANGE VOCABULARY

crossing over

when parts of two chromatids cross over each other before meiosis I at a point of crossing over called the chiasma

recombination

the exchange of genetic material between homologous chromosomes producing recombinant chromosomes

independent assortment

each chromosome pair is sorted independently of the other pairs during meiosis

segregation

the two members of any pair of alleles at a given locus separate and pass unchanged into different gametes

incomplete dominance

the action of one allele does not completely mask the action of the other and heterozygous offspring is intermediate in phenotype between the parental phenotypes

codominance

both alleles in a heterozygous organism are dominant so both alleles are fully and equally expressed

multiple alleles

three or more forms of ONE gene leading to 3 or more possible phenotypes for a particular trait

lethal alleles

alleles that cause an organism to die but only when present in homozygous condition

linked genes

genes that are located close together on the same chromosome and tend to be inherited together

genetic diversity

the range of different genes and different alleles present in the gene pool of a population AND their relative frequency.

allele frequency

% or number of each allele in a gene pool

gene pool

all the alleles of all the genes that exist in a population at any one time

genetic change

the change in the frequency of alleles in a population's gene pool

natural selection

the differential survival and differential reproductive success of individuals whose characteristics are best suited to the environment at a given time

stabilising natural selection

selection for those individuals with average phenotypes and selection against those with extreme phenotypes

directional natural selection

selection for one end of a phenotypic range at the expense of the other

disruptive natural selection

selection against the average and for the extremes

genetic drift

the change in allele frequency in a gene pool due to chance

founder effect

loss of genetic variation when only a few individuals colonise a new area AND allele frequencies in the new gene pool are not representative of the original population

bottleneck effect

loss of genetic variation [change in allele frequencies in a population's gene pool] when a large population decreases in numbers to become a small population and then increases again

migration

when individuals move from one population to another

gene

a length of DNA, bounded by start and stop codes, that codes for a specific polypeptide chain, that then folds to form a functional protein

somatic mutations

not inheritable and occur in any cell in the body except the gametes

gametic mutations

occur in the gonads when gametes are formed so can be passed onto offspring

gene mutation

sudden and permanent change to the base sequence of a gene

evolution

the change in allele frequency in a population's gene pool from generation to generation

meiosis

cell division to produce gametes

homologous chromosomes

chromosomes that have the same genes and the same structure

recombination

one chromosome breaks off and attaches to another shuffling allele combinations

phenotype

physical characteristics of an organism

genotype

genetic makeup of an organism

selection pressure

environmental factors that favour certain phenotypes

allele

alternative form of a gene.

haploid cell

cell containing only one set of chromosomes (n).

diploid cell

cell with chromosomes that come in homologous pairs, one set inherited from each parent

unlinked genes

genes that are found on different chromosomes