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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