Causes of variation and phenotype development
Inheritance, variation and evolution • Variation and evolution
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Extent of genetic variation within populations
Genetic variation describes differences in DNA sequences between individuals of the same species. Populations usually show extensive genetic variation because sexual reproduction and inheritance combine different alleles in offspring, making identical genomes rare outside identical twins. The genome contains thousands of genes and two copies of each gene in diploid organisms; small differences in DNA sequences across those genes create variation in traits . Limiting factor: measurement of genetic variation requires sampling across many individuals; observed variation depends on population size and sampling method, so conclusions about ‘extensive’ variation use sufficiently large samples to be reliable .
Causes of variation: genetic, environmental, and combined
Genetic causes: inherited alleles determine traits such as blood group and many simple inherited disorders. Different allele combinations (genotypes) produce different physical characteristics (phenotypes) when genes act largely alone, for example some single-gene traits such as cystic fibrosis or polydactyly . Environmental causes: non-genetic factors alter phenotype without changing DNA sequence. Examples include suntans, scars, tattoos, and hair dye; these produce visible differences that are not inherited. Environmental effects can be direct (damage, staining) or developmental (nutritional deficits affecting growth) . Combined effects: many traits arise from genome–environment interaction. Genotype sets a tendency or range (for example, genetic tendency for tall stature), while environmental inputs such as diet, disease, or mineral availability determine the final phenotype (for example, adequate calcium needed for full genetic height potential) .
Mutations as the source of variants and their effect on phenotype
Mutation is a permanent change to DNA sequence. All new heritable variants arise from mutations that alter bases or change the number of bases in a gene. Single-base substitutions may change one amino acid in a protein; insertions or deletions can shift the reading frame and alter every downstream amino acid, causing major protein changes or loss of function . Effect on phenotype: mutations alter the protein produced by a gene, which can change enzyme activity, structural protein strength, or regulatory functions. Consequences vary: some mutations have no effect, some are harmful (disease-causing), and some are advantageous. The phenotype changes when protein structure or levels change enough to affect cell or organism function .
Genome structure, gene expression and environmental influence on phenotype
The genome is the complete DNA content of an organism and contains coding and non-coding regions. Genes code for proteins via transcription and translation; proteins determine many physical traits. Genotype is the genetic makeup; phenotype is the set of observable characteristics produced by the genotype in a given environment . Environmental modulation: the environment influences which genes are expressed and how proteins are used. Examples include nutritional limits on growth and environmental triggers that change pigmentation. Gene expression can be switched on or off at different life stages, so the same genome can produce different phenotypes under different conditions. Therefore phenotype results from both the genome’s coding potential and environmental modulation of gene expression and development .
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