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A-LEVEL REVISION & EXAM PREP IN A SNAP
MODULE 6Genetics, Evolution and Ecosystems
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TOPIC 1Types of Gene Mutations • Types of gene mutations
○ Substitution – Nucleotide replaced
○ Insertion – Causes frameshift
○ Deletion – Causes frameshift
• Consequences of gene mutations ○ Neutral ○ Harmful ○ Beneficial
• Point mutation ○ Only one base is affected ○ Silent mutation
– No change in amino acid sequence ○ Missense mutation
– 1 amino acid is changed in sequence ○ Nonsense mutation
– Triplet becomes stop codon • Gene expression regulation
○ Some genes constantly expressed ○ Others expressed only when needed
– E.g. lac operon – Presence of lactose and absence of glucose
triggers synthesis of enzymes that break down lactose
• Lac operon = section of E.coli DNA ○ Structural part codes for enzyme ○ Operon part switches structural genes on/off ○ Promoter region = region where RNA polymerase binds to begin transcription
• Lac operon in absence of lactose ○ Regulator gene expressed ○ Repressor protein synthesised and binds operator region
○ RNA polymerase cannot bind promoter region • Lac operon in presence of lactose
○ Lactose changes shape of repressor
○ Repressor breaks away from operator ○ RNA polymerase can bind to promoter
• Homeobox genes ○ Regulate body part development ○ Grouped in hox clusters ○ Precisely conserved due to importance ○ Expressed anterior posterior ○ Determine where limbs branch off
• Apoptosis = programmed cell death in multicellular organisms
○ Enzymes break down cytoskeleton and DNA ○ Remaining parts phagocytosed ○ Controlled by complex cell signals ○ Causes limbs to separate
• Transcription factors ○ Proteins/ noncoding RNA ○ Attach/detach from DNA
• Introns = unexpressed DNA (don’t code for genes) • Exons = expressed DNA
○ Excess amino acids deaminated in liver ○ Converted into ammonia then urea ○ Urea excreted in urine by kidney
TOPIC 2Patterns of Inheritence • Genotype = genetic makeup of an individual • Phenotype = observd/ expressed characteristics of an
individual ○ Result of genotype and environment
• Genetic factors contributing to phenotypic variation ○ Genetic random mutations
– Mutagenic agents – Chromosomal mutations – Aneuploidy/ polyploidy – Variation from sexual reproduction
• Environmental factors contributing to phenotypic variation
○ Environment alone (e.g. limb loss in accident) ○ Environment affecting gene expression ○ Epigenetics
• Genetic diagrams
• Monohybrid = genetic diagram with just 1 gene • Dihybrid = genetic diagram with 2 genes • Linkage = when 2 or more gene loci are on the same
chromosome ○ Autosomal linkage = when the two genes are on non-sex chromosomes
○ Sex linkage = when the two genes are on sex chromosomes
– More likely to be on X chromosome • Epistasis = interaction of non-linked genes where one
masks the other • Recessive epistasis
○ Homozygous recessive alleles mask expression of another allele at different locus
○ E.g. flower colour in Salvia • Codominance = when two different alleles at the same
locus are both expressed ○ E.g. human blood group type
• Chi-squared test ○ Determine whether expected vs observed allele frequency differences are due to chance
○ Use when... – Data variation is discrete – Large sample size
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