Normal distribution about the mean Mode (most frequent) = median (mid) = mean (average value) Bell-shaped/ even distributions of values above and below mean Standard error (SE) True mean of SE is ±1.96 In a number of samples each sample will have its own mean Standard error measures how much the value of a sample mean is likely to vary The greater the standard error, the greater the variation of the mean Standard deviation (σ) Measure of the spread of results about the mean of a normal distribution curve Thinner bell-shape / smaller standard deviation / less variation Same pattern with bigger bell-shape Causes of variation Independent assortment of bivalents at the equator during anaphase I Chromosomes of bivalents pulled to opposite poles at random 2n different combinations of chromosomes in four haploid cells produced where n is the haploid number of chromosomes Crossing over between non-sister chromatids during prophase I At synapsis, non-sister chromatids of homologous pairs cross over at chiasmata Homologous chromatids (corresponding pieces of genetic material) break and exchange equivalent segments between maternal and paternal chromatids Results in new combination of genes from the two parents Fertilization / random fusion of gametes Genetic difference amongst the zygote New combinations of alleles Gene mutation / increased by environmental factors (eg radiation) Addition / at least one base is added during DNA replication Deletion / at least one base is not copied (frameshift) Substitution / at least one base is copied wrongly Interferences with normal base pairing (A-T;C-G) Degenerate code / different triplets can code for same amino acids Discontinuous variation IMG 5-14-2 Limited number of distinct phenotypes / categories (e.g. blood group) Strong genetic factor controlled by alleles on one gene Frequency histogram has separate bars Unaffected by the environment Continuous variation Continuous range of values / class intervals (e.g. human height) Alleles on many genes located on different chromosomes / polygenic inheritance Frequency histogram is a smooth (normal distribution) curve Phenotype is affected by environmental factors Lower skin temperature activates a gene for pigment production Diet affects individual's size and health. Malnourishment results in shorter height Therefore, genes + environment → phenotype (continuous variation) Advantageous of variation to species Allows different adaptations / some better adapted Some survive / reproduce / pass on gene/allele Allows for changing environment / different environment