---
title: "Genetics & Evolution"
description: "61 topics in Science, in the order they build on each other."
canonical: https://lightmysky.com/learn/science/areas/genetics-and-evolution
source: https://lightmysky.com/learn/science/areas/genetics-and-evolution.md
retrieved: 2026-09-02
---

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# Genetics & Evolution

61 topics in Science, in the order they build on each other.

Page: https://lightmysky.com/learn/science/areas/genetics-and-evolution

- [Meiosis and Why Gametes Are Haploid](https://lightmysky.com/learn/science/meiosis-and-why-gametes-are-haploid-mt_hvu0bczcfX): Meiosis halves the chromosome number and makes four cells that are genetically different, so fertilisation puts the full set back. The contrast with mitosis is what explains why offspring vary and clones do not.
- [DNA Structure and Base Pairing](https://lightmysky.com/learn/science/dna-structure-and-base-pairing-mt_PK4uNJeg_k): DNA is two strands of nucleotides wound into a double helix, with A always pairing to T and C to G. That pairing rule is what makes an exact copy possible before a cell divides.
- [From Gene to Protein](https://lightmysky.com/learn/science/from-gene-to-protein-mt_PdNyzTl8Ye): A gene is a length of DNA whose base order fixes the amino acid order of one protein, read three bases at a time. Covers the messenger copy, the ribosome, and reading a codon table.
- [Mutations and Their Effect on Proteins](https://lightmysky.com/learn/science/mutations-and-their-effect-on-proteins-mt_dTOQH3vGSz): A change to one base may change one amino acid, and a changed amino acid may change the shape the protein folds into. Explains why most mutations do nothing while a few change everything.
- [Punnett Squares and Monohybrid Ratios](https://lightmysky.com/learn/science/punnett-squares-and-monohybrid-ratios-mt_T4ZC45-77w): Draw a Punnett square for a cross between two known genotypes and read off the expected ratio of offspring. Covers dominant and recessive alleles, homozygous and heterozygous, and genotype against phenotype.
- [Sex Determination and Inherited Disorders](https://lightmysky.com/learn/science/sex-determination-and-inherited-disorders-mt_PJQnvtnO0D): The X and Y chromosomes decide sex, which is why crossing XX with XY gives an even chance either way. The same squares, plus a family tree, give the probability that a child inherits a recessive disorder.
- [Selective Breeding and Genetic Engineering](https://lightmysky.com/learn/science/selective-breeding-and-genetic-engineering-mt_a3zTtiYrR2): Selective breeding decides which individuals reproduce, generation after generation, while genetic engineering moves a gene from one organism straight into another. Compares what each can do, how long it takes, and the risks people raise.
- [Cloning: Cuttings, Tissue Culture and Adult Cell Cloning](https://lightmysky.com/learn/science/cloning-cuttings-tissue-culture-and-adult-cell-cloning-mt_3NaPCGrZTg): A clone is an organism with the same genome as another, and plants have been making them without help for as long as there have been plants. Doing it from an adult animal cell is harder because the cell has to be persuaded to forget what it had become.
- [Speciation and the Rate of Evolution](https://lightmysky.com/learn/science/speciation-and-the-rate-of-evolution-mt_OdcdfeRyo1): Two populations kept apart pile up different changes until they can no longer breed together and produce fertile offspring. Covers isolation, the evidence in DNA and fossils, and why the rate is not steady.
- [Semi-conservative DNA Replication](https://lightmysky.com/learn/science/semi-conservative-dna-replication-mt_wIcBfUw295): Each new DNA molecule keeps one original strand and one newly built one. Helicase, DNA polymerase and the split into leading and lagging strands explain how base pairing gets copied.
- [Taxonomy, the Three Domains and What a Species Name Encodes](https://lightmysky.com/learn/science/taxonomy-the-three-domains-and-what-a-species-name-encodes-mt_12ucjuHtHE): A two-word species name places an organism inside a nested hierarchy, and every rank in that hierarchy is a claim about shared ancestry. The three-domain split was forced by molecular evidence that contradicted the older picture, which is why classification is treated as a hypothesis rather than a filing system.
- [Transcription and RNA Splicing in Eukaryotes](https://lightmysky.com/learn/science/transcription-and-rna-splicing-in-eukaryotes-mt_j2vc70vH5t): RNA polymerase copies one strand into pre-mRNA, and introns are cut out before the message leaves the nucleus. Splicing is why one gene can give more than one protein.
- [Translation at the Ribosome](https://lightmysky.com/learn/science/translation-at-the-ribosome-mt_1EdZzeqts1): The ribosome reads codons and tRNA brings the matching amino acid, turning a base sequence into an amino acid sequence. The code is degenerate, non-overlapping and read from a fixed start.
- [Transcription Factors and Epigenetic Control](https://lightmysky.com/learn/science/transcription-factors-and-epigenetic-control-mt_nF4JJ_EDa4): Transcription factors bind DNA and decide which genes are read, while methylation and histone changes decide which genes can be reached at all. Both are reversible and neither alters the base sequence.
- [Gene Mutations and Their Consequences](https://lightmysky.com/learn/science/gene-mutations-and-their-consequences-mt_CDcaL6-iVp): Substitutions, insertions and deletions change a gene in different ways, and only some of them change the protein. A frame shift does the most damage because every codon after it is misread.
- [Dihybrid Crosses and the 9:3:3:1 Ratio](https://lightmysky.com/learn/science/dihybrid-crosses-and-the-9-3-3-1-ratio-mt_9IlVo04Dbt): Two gene pairs on different chromosomes assort independently, so a cross between two double heterozygotes gives four phenotypes in a 9:3:3:1 ratio. The grid is four by four and the ratio is a prediction, not a law.
- [Testing Genetic Ratios with Chi-squared](https://lightmysky.com/learn/science/testing-genetic-ratios-with-chi-squared-mt_YCO6jijrNQ): Observed offspring counts never match a predicted ratio exactly, so chi-squared measures whether the gap is bigger than chance would give. The test hands back a decision, not a proof.
- [The Hardy-Weinberg Principle and Allele Frequencies](https://lightmysky.com/learn/science/the-hardy-weinberg-principle-and-allele-frequencies-mt_eJjwZ2qVAE): In a population that meets a short list of conditions, allele and genotype frequencies stay put from one generation to the next. Departures from that prediction are how selection gets detected.
- [Recombinant DNA and Gene Transfer](https://lightmysky.com/learn/science/recombinant-dna-and-gene-transfer-mt_L9yncU_x2P): Restriction enzymes cut DNA at named sequences, ligase joins the pieces, and a vector carries the gene into a host cell. Marker genes are how the few successful cells get found.
- [PCR, Electrophoresis and DNA Profiling](https://lightmysky.com/learn/science/pcr-electrophoresis-and-dna-profiling-mt_Sv1jV0XnsX): PCR copies a chosen stretch of DNA through repeated heating and cooling, and electrophoresis then sorts the fragments by length. A profile compares repeat lengths rather than reading whole genes.
- [Linkage, Recombination Frequency and Genetic Maps](https://lightmysky.com/learn/science/linkage-recombination-frequency-and-genetic-maps-mt_xBjZXt3TVn): Genes on the same chromosome are inherited together unless a crossover separates them, so the proportion of recombinant offspring measures how far apart they sit. Frequencies from several crosses can be assembled into a map order.
- [Epistasis and Reading Pathway Order from Double Mutants](https://lightmysky.com/learn/science/epistasis-and-reading-pathway-order-from-double-mutants-mt_eE2WRPyYPk): When one gene masks the effect of another, the offspring ratios change in ways that say which gene acts first. Combining two mutations in one organism is how the order of steps in a pathway is worked out.
- [Quantitative Traits, Variance Components and Heritability](https://lightmysky.com/learn/science/quantitative-traits-variance-components-and-heritability-mt_orJhIlYnKM): Traits governed by many genes vary continuously, and the observed variance can be split into genetic and environmental parts. Heritability is the share of variance that is genetic in one population and one environment, which is not the same as how much of the trait is inherited.
- [Chromatin: Nucleosomes and Higher-Order Packing](https://lightmysky.com/learn/science/chromatin-nucleosomes-and-higher-order-packing-mt_5q3Hsdqm9x): Two metres of DNA fit in a nucleus by wrapping around histone cores and coiling those into thicker fibres. The packing is not just storage: how tightly a region is packed decides whether it can be read.
- [Selection Coefficients and Predicting Allele Frequency Change](https://lightmysky.com/learn/science/selection-coefficients-and-predicting-allele-frequency-change-mt__JS2XJxOXV): Giving each genotype a relative fitness turns selection into arithmetic: the frequency after one generation follows from the frequencies before and the fitnesses. Working the change out generation by generation shows how slowly a rare recessive allele is removed.
- [Genetic Drift and Effective Population Size](https://lightmysky.com/learn/science/genetic-drift-and-effective-population-size-mt_lbB1GbKv_t): Allele frequencies wander from generation to generation because gametes are a sample, and the smaller the population the wider that wandering. The effective size, which counts breeding individuals unevenly, is usually well below the number of individuals present.
- [Mutation, Gene Flow and the Balance That Holds Variation](https://lightmysky.com/learn/science/mutation-gene-flow-and-the-balance-that-holds-variation-mt_A2UgDE0ORv): Mutation makes new alleles slowly and migration moves existing ones between populations quickly, while selection and drift remove them. What is observed is the balance of those inputs and outputs rather than any of them alone.
- [The Neutral Theory and Reading a Molecular Clock](https://lightmysky.com/learn/science/the-neutral-theory-and-reading-a-molecular-clock-mt_yIidVpa703): Most substitutions between species change nothing about fitness, so they accumulate at a rate set by the mutation rate rather than by selection. That regularity is what lets sequence differences be read as elapsed time, once the clock is calibrated.
- [Selection on Quantitative Traits and Predicting the Response](https://lightmysky.com/learn/science/selection-on-quantitative-traits-and-predicting-the-response-mt_M-czTLDmfl): When selection acts on a continuous trait, the response over one generation is the strength of selection multiplied by the heritable share of variation. That single relation covers artificial breeding and natural selection alike.
- [Speciation: Isolation, Hybrid Zones and Where the Boundary Blurs](https://lightmysky.com/learn/science/speciation-isolation-hybrid-zones-and-where-the-boundary-blurs-mt_ngZxpFlwo1): Populations become separate species as barriers accumulate before and after mating, and those barriers can be geographic, behavioural or genetic. Hybrid zones show that the boundary is often partial, which is why one species definition does not fit every case.
- [Building a Phylogeny: Homology, Shared Derived Characters and Parsimony](https://lightmysky.com/learn/science/building-a-phylogeny-homology-shared-derived-characters-and-parsimony-mt_GFmiVZ8g-F): A tree is built from characters that are shared because of common ancestry, not because of similar conditions, and only the derived states group taxa together. Parsimony picks the tree that needs the fewest changes to explain the data.
- [Homologous Recombination and Double-Strand Break Repair](https://lightmysky.com/learn/science/homologous-recombination-and-double-strand-break-repair-mt_yWlwqAsXEf): A break across both strands is repaired by invading a matching sequence and copying from it, which is also how crossovers form in meiosis. The same mechanism is what makes targeted gene replacement possible.
- [Cancer as Clonal Evolution: Drivers, Passengers and Selection in a Tissue](https://lightmysky.com/learn/science/cancer-as-clonal-evolution-drivers-passengers-and-selection-in-a-tissue-mt_3qIJZwD0m5): A tumour is a cell population under selection inside a body, so population genetics applies to it directly. This stop separates driver from passenger mutations and reads a tumour's history out of its mutation data.
- [Oncogenes and Tumour Suppressors: Why One Allele Is Sometimes Enough](https://lightmysky.com/learn/science/oncogenes-and-tumour-suppressors-why-one-allele-is-sometimes-enough-mt_7WTgv1dLNx): Gain-of-function and loss-of-function lesions behave differently in a pedigree and in a tumour: a single altered allele can drive, while a suppressor usually needs both copies lost. This stop reads that difference out of inheritance patterns and tumour genotypes.
- [Enhancers, Accessibility and Combinatorial Control](https://lightmysky.com/learn/science/enhancers-accessibility-and-combinatorial-control-mt_BTMCj6V6jD): Sequences far from a gene raise its transcription by looping into contact with the promoter, and they only work when the chromatin there is open. Which genes a cell expresses is decided by which combination of factors is present, not by one master switch per gene.
- [Small RNAs and Silencing After Transcription](https://lightmysky.com/learn/science/small-rnas-and-silencing-after-transcription-mt_1AozDyS1Mn): Short RNAs pair with a matching transcript and mark it for cutting or block its translation. A gene can therefore be fully transcribed and still produce almost no protein.
- [Methylation, Histone Marks and Heritable Expression States](https://lightmysky.com/learn/science/methylation-histone-marks-and-heritable-expression-states-mt_QIEjt77tjV): Chemical marks on DNA and on histone tails are copied when a cell divides, so a pattern of gene activity can be passed to daughter cells without any change in sequence. The marks are readable, writable and erasable, which is what makes them a control system.
- [Sequencing Reads and How a Genome Is Assembled](https://lightmysky.com/learn/science/sequencing-reads-and-how-a-genome-is-assembled-mt_4mUJIUAZGd): Sequencing instruments produce many short reads rather than one long sequence, and a genome is rebuilt by finding where reads overlap. Coverage and read length decide which regions can be resolved and which stay ambiguous.
- [Sequence Alignment and Reading a Search Result](https://lightmysky.com/learn/science/sequence-alignment-and-reading-a-search-result-mt_jNbEMFqgZ5): Two sequences are compared by scoring matches, mismatches and gaps, and the best alignment is the one with the highest total score. A database search returns hits ranked by that score with a statistic saying how often such a match would appear by chance.
- [Distance and Likelihood Trees, and What Branch Support Means](https://lightmysky.com/learn/science/distance-and-likelihood-trees-and-what-branch-support-means-mt_mEO9PUEdh6): Sequence-based trees are built either from pairwise distances or by asking which tree makes the observed data most probable under a model of change. Support values report how often a branch survives resampling, which is a statement about the data rather than about the truth of the branch.
- [Using Trees: Ancestral States, Dates and Comparative Tests](https://lightmysky.com/learn/science/using-trees-ancestral-states-dates-and-comparative-tests-mt_lEYWID9Ffs): Once a tree exists it becomes a tool: it reconstructs what an ancestor probably looked like, dates splits against fossils and corrects comparisons between species for shared ancestry. Species are not independent data points, which is the mistake the comparative method exists to avoid.
- [Annotation, Orthologues and Comparative Genomics](https://lightmysky.com/learn/science/annotation-orthologues-and-comparative-genomics-mt_eBRge2bLeC): An assembled genome is only useful once genes are located and their likely functions assigned, usually by similarity to genes already studied elsewhere. Comparing species shows which sequences have been conserved, which is a strong hint that they matter.
- [Association Studies and What They Can and Cannot Show](https://lightmysky.com/learn/science/association-studies-and-what-they-can-and-cannot-show-mt_IFFUFX3xrv): Scanning many variants across many people finds positions where one allele is more common in affected individuals. The hit is usually a marker near the causal variant rather than the cause, and the effect of any one variant is normally small.
- [Measuring Genetic Variation: Heterozygosity and Population Structure](https://lightmysky.com/learn/science/measuring-genetic-variation-heterozygosity-and-population-structure-mt_Prn7vgkyLl): Variation within a population is summarised by how often individuals carry two different alleles, and variation between populations by how much of the total sits between rather than within groups. Those two numbers turn samples of sequence into a statement about how populations are connected.
- [Alignment Scoring: Substitution Matrices, Gap Penalties and Significance](https://lightmysky.com/learn/science/alignment-scoring-substitution-matrices-gap-penalties-and-significance-mt_k--A8iF5SC): An alignment is only as meaningful as the score that produced it. This stop derives substitution scores from observed substitution frequencies, sets gap penalties deliberately, and asks whether a score that good would arise by chance.
- [Hidden Markov Models for Sequence: Profiles, States and Decoding](https://lightmysky.com/learn/science/hidden-markov-models-for-sequence-profiles-states-and-decoding-mt_nk14IaZTcw): A profile hidden Markov model represents a whole family by position-specific emission and transition probabilities rather than by one representative sequence. This stop covers the decoding step that assigns states to a new sequence and what it buys.
- [Models of Sequence Evolution and Maximum Likelihood Trees](https://lightmysky.com/learn/science/models-of-sequence-evolution-and-maximum-likelihood-trees-mt_Ujtg0xul9N): Estimating a tree by likelihood needs an explicit model of how sites change: a rate matrix, an account of rate variation across sites, and a way to compare candidate models. This stop connects the model chosen to the tree that comes out.
- [Bayesian Phylogenetics: Priors, Sampling and Judging Convergence](https://lightmysky.com/learn/science/bayesian-phylogenetics-priors-sampling-and-judging-convergence-mt_7jWhYh1n5X): A Bayesian analysis returns a distribution over trees rather than a single tree, sampled by a Markov chain. This stop covers what the priors do to the answer and how to tell a converged chain from one that only looks settled.
- [Phylogenomics: Gene Trees, Species Trees and Why They Disagree](https://lightmysky.com/learn/science/phylogenomics-gene-trees-species-trees-and-why-they-disagree-mt_rg7feIg1D4): Different genes give different trees for the same set of species, through incomplete lineage sorting, introgression, duplication and plain estimation error. This stop separates those causes and uses methods that expect the conflict instead of hiding it.
- [Differential Expression from Counts: Dispersion, Fit and Shrinkage](https://lightmysky.com/learn/science/differential-expression-from-counts-dispersion-fit-and-shrinkage-mt_nEV9547kSN): Sequencing gives counts, not measurements, and counts carry a mean-variance relationship that a normal model gets wrong. This stop fits a count model, borrows information across genes to estimate dispersion, and reads the result honestly.
- [Testing Thousands of Genes at Once: False Discovery Rate and What q Means](https://lightmysky.com/learn/science/testing-thousands-of-genes-at-once-false-discovery-rate-and-what-q-means-mt_mIQ27rphrC): With twenty thousand tests, a five percent error rate on each one is meaningless. This stop separates family-wise error control from false discovery rate control and reads a q-value for exactly what it claims.
- [From a Gene List to Biology: Enrichment Tests and Their Backgrounds](https://lightmysky.com/learn/science/from-a-gene-list-to-biology-enrichment-tests-and-their-backgrounds-mt_M9eHEXq6iA): Enrichment analysis asks whether a gene list is unusually full of some annotated category, and the answer depends entirely on the background set chosen. This stop runs the test properly and names the biases that make it lie.
- [Gene Regulatory Networks: Motifs and What They Compute](https://lightmysky.com/learn/science/gene-regulatory-networks-motifs-and-what-they-compute-mt_QWMKReB10f): A regulatory network is more than a list of activators and repressors: small recurring wiring patterns such as autoregulation, feed-forward loops and mutual repression each produce a characteristic response over time. This stop reads a network diagram as a computation rather than as a picture.
- [Stochastic Gene Expression: Bursting, Noise and Cell-to-Cell Variation](https://lightmysky.com/learn/science/stochastic-gene-expression-bursting-noise-and-cell-to-cell-variation-mt_YD6DOmrvng): Transcription happens in bursts, so genetically identical cells in one flask hold different amounts of the same protein. This stop separates intrinsic from extrinsic noise and works out what each does to the distribution across a population.
- [Single-Cell RNA Sequencing: From Droplets to a Count Matrix](https://lightmysky.com/learn/science/single-cell-rna-sequencing-from-droplets-to-a-count-matrix-mt_QCoqY7w_De): A single-cell protocol captures one cell per droplet, tags its transcripts with a cell barcode and a molecular identifier, and returns a sparse matrix of counts. This stop follows the sample through the bench steps that decide what that matrix can support.
- [Mapping Regulatory Elements: Accessibility, Occupancy and Contact](https://lightmysky.com/learn/science/mapping-regulatory-elements-accessibility-occupancy-and-contact-mt_W2YKGBADJ-): Three assay families answer three different questions about a piece of DNA: which regions are open, which protein sits there, and which distant regions touch it. This stop matches the question to the assay and to the controls its claim needs.
- [Single-Cell Analysis: Normalisation, Embedding and Naming Cell States](https://lightmysky.com/learn/science/single-cell-analysis-normalisation-embedding-and-naming-cell-states-mt_diYHLaLmij): A count matrix becomes a cell atlas through normalisation, feature selection, a linear reduction, a neighbour graph and a clustering step, each with choices that move the answer. This stop makes the choices explicit and tests how much the conclusion rests on them.
- [Modelling a Biological System: Rate Equations, Parameters and Fit](https://lightmysky.com/learn/science/modelling-a-biological-system-rate-equations-parameters-and-fit-mt_Al6sNjUOW5): A mechanistic model turns a pathway diagram into rate equations whose parameters have to be estimated from data and whose behaviour has to be checked against data held back. This stop asks what the data can identify before any conclusion is drawn from a fit.
- [Genome Engineering: Cut Site, Repair Choice and Editing Outcome](https://lightmysky.com/learn/science/genome-engineering-cut-site-repair-choice-and-editing-outcome-mt_3DqvkO4wdE): The nuclease only makes a break; the cell decides the edit. This stop works out how guide design, the repair pathway available in that cell type and the donor template together determine which allele you end up with.
- [Base and Prime Editing: Changing a Letter Without Breaking Both Strands](https://lightmysky.com/learn/science/base-and-prime-editing-changing-a-letter-without-breaking-both-strands-mt_roM4eWoWVn): Base editors tether a deaminase to a disabled nuclease and rewrite one letter inside a narrow window, while prime editors write from a template carried on the guide itself. This stop compares what each class can install and how each one goes wrong.
- [Pooled Genetic Screens: Library Design, Selection and Reading a Hit List](https://lightmysky.com/learn/science/pooled-genetic-screens-library-design-selection-and-reading-a-hit-list-mt_0n-a7GfLOL): A pooled screen turns a biological question into a counting problem: which guides are enriched or depleted after a selection. This stop covers library coverage, the choice of selection pressure, and how a real hit is told apart from a noisy guide.