IGCSE›Edexcel IGCSE Biology›Mind mapsInheritanceEdexcel IGCSE Biology: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsDNA and genesGene = section of DNA coding for one proteinGenome = entire DNA of an organismGenes sit on chromosomes in the nucleusDNA is a double helix of two strandsRNA is single-stranded with uracil, not thymineProtein synthesisTranscription: gene copied to mRNA in nucleusTranslation: mRNA read at a ribosomeThree-base codon codes for one amino acidtRNA anticodon pairs with the codonAmino acids joined in order to build proteinAllelesAllele = different form of a geneDominant shows with one copyRecessive needs two copiesHomozygous = two identical allelesHeterozygous = two different allelesCodominance: both alleles expressedInheritancegenes, DNA and variationA-TC-GXXXY46CrossesMonohybrid cross tracks one geneBb×Bb\text{Bb} \times \text{Bb}Bb×Bb gives 1 BB : 2 Bb : 1 bbThat is a 3:1 phenotype ratioPedigree shows a trait across generationsFemales XX, males XY, 50:50 ratioVariationMitosis: two identical cells, for growth and repairMeiosis: four different haploid gametesHumans: diploid 46, haploid 23Mutation = rare, random DNA changeNatural selection favours best-suited individualsExam tipsGive base pairing A-T, C-G, not just complementaryBacteria do not develop resistanceAntibiotic selects for existing resistant mutants