Genetics: 250 Core Concepts, Ratios and Assumptions

A modified ratio is not a broken rule. It is a second gene telling you something.

250 cardsLast updated 2026-08-31

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A nine to three to four ratio in an F2 generation is not a failed dihybrid cross. It is recessive epistasis, and reading it that way turns a confusing result into a diagnosis. The same is true of a pedigree with unaffected parents and an affected child: that single observation settles whether the allele is recessive. Genetics rewards students who can read a pattern backwards to its cause. This deck is 250 cards, one term, mechanism or law per card, with the back giving the definition and, where it applies, a line on the term it is confused with or the assumption that fails. The sections run from Mendelian inheritance through the patterns that depart from it, then chromosomes and linkage, molecular genetics, gene regulation and development, and population and applied genetics. Ratios appear where they follow from a definition, such as a monohybrid cross or a modified dihybrid ratio, and nowhere else. Once the deck is on a spaced-repetition schedule, the mechanisms you can already read stop coming back and the pairs you keep reversing return until they stop being guesswork.

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What's inside

Showing 100 representative cards from the full 250-card deck.

Front	Back
Gene — what is it?	A unit of heredity that occupies a specific position on a chromosome. A gene is not the same as an allele. A gene is the locus, an allele is one version of it.
Genotype — what is it?	The combination of alleles an organism carries. The genotype does not fully determine appearance, because environment and other genes contribute.
Homozygous — what is it?	Carrying two identical alleles at a locus. Homozygous recessive individuals always show the recessive phenotype, but homozygous does not mean recessive.
Dominant allele — what is it?	An allele whose effect appears in the heterozygote. Dominant does not mean common or advantageous. Many dominant alleles are rare.
Complete dominance — what is it?	The situation in which the heterozygote looks identical to the dominant homozygote. Any deviation from that ratio suggests something other than complete dominance.
Law of segregation — what is it?	Mendel's principle that the two alleles at a locus separate during gamete formation. Segregation happens in meiosis one, when homologues separate, not in meiosis two.
Law of dominance — what is it?	Mendel's observation that one allele can mask another in the heterozygote. It is the least general of Mendel's laws, because many genes show other patterns.
Monohybrid cross — what is it?	A cross tracking a single gene. The three to one phenotype ratio appears only under complete dominance.
Dihybrid cross — what is it?	A cross tracking two genes at once. That ratio breaks down if the two genes are linked or interact.
Back cross — what is it?	A cross of an offspring with one of its parents or an equivalent genotype. A back cross is a test cross only when the parent is homozygous recessive.
P, F1 and F2 generations — what is it?	The parental generation and the first two generations of offspring. The F1 generation is uniform, so ratios only become informative in the F2.
Product rule — what is it?	The rule that the probability of independent events both occurring is the product of their probabilities. It applies only when the events are genuinely independent.
Forked-line method — what is it?	A branching method for combining the outcomes of several genes. It assumes independent assortment, so it fails for linked genes.
Pedigree — what is it?	A diagram of family relationships used to trace inheritance. A pedigree shows phenotypes, so genotypes must be inferred rather than read off.
X-linked recessive pedigree — what is it?	A pattern in which affected individuals are mostly male. Father to son transmission never occurs, which rules the pattern out when it does.
Symbols in a pedigree — what is it?	The standard notation for individuals and relationships. Generations are numbered with Roman numerals and individuals within them with Arabic numerals.
Independent events across children — what is it?	The principle that each conception is independent of the ones before it. Having three affected children does not lower the risk for the fourth.
Incomplete dominance — what is it?	A pattern in which the heterozygote shows a phenotype between the two homozygotes. The heterozygote is a blend of appearance only. The alleles themselves are not blended.
Multiple alleles — what is it?	The existence of more than two alleles of a gene in a population. More alleles in the population do not mean more alleles per individual.
Epistasis — what is it?	An interaction in which one gene masks the effect of another. Epistasis involves two different genes, while dominance involves two alleles of one gene.
Complementary gene action — what is it?	A pattern in which two genes must both be functional for a phenotype to appear. It reflects two steps in one pathway, not two independent traits.
Modifier gene — what is it?	A gene that alters the expression of another gene without producing the trait itself. A modifier is not the cause of the trait, and looking for it as such is misleading.
Polygenic trait — what is it?	A trait influenced by many genes each with a small effect. Polygenic means many genes affecting one trait, the reverse of pleiotropy.
Quantitative trait locus — what is it?	A region of the genome associated with variation in a continuous trait. A mapped region usually contains many genes, so identifying the causal one takes further work.
Penetrance — what is it?	The proportion of individuals with a genotype who show the associated phenotype. An unaffected individual in a pedigree may still carry the allele.
Norm of reaction — what is it?	The range of phenotypes a genotype produces across environments. Two genotypes can rank differently in different environments.
Sex-limited trait — what is it?	A trait expressed in only one sex although both carry the genes. The gene is not on a sex chromosome, so both sexes transmit it equally.
Maternal effect — what is it?	A phenotype determined by the mother's genotype rather than the offspring's own. The offspring phenotype can contradict its own genotype in the first generation.
Mitochondrial inheritance — what is it?	Transmission of mitochondrial DNA from mother to all her children. Affected fathers do not transmit it, which distinguishes it from autosomal dominance.
Genetic background — what is it?	The rest of an organism's genome, which influences how one gene is expressed. Results from one strain may not transfer to another with a different background.
Genomic imprinting overview — what is it?	Expression of a gene depending on the parent it came from. The DNA sequence is unchanged, so it is an epigenetic rather than genetic difference.
Deviations from Mendelian ratios — what is it?	Departures from the expected proportions in a cross. The total number of classes usually still sums to sixteen, which helps identify the pattern.
Interpreting a nine to seven ratio — what is it?	A modified dihybrid ratio in the F2. Seven parts share one phenotype because any block in the pathway gives the same result.
Interpreting a fifteen to one ratio — what is it?	A modified dihybrid ratio in the F2. Only the double homozygous recessive shows the alternative phenotype.
Chromosome — what is it?	A single long DNA molecule packaged with proteins. A chromosome has one DNA molecule before replication and two identical ones after it.
Sister chromatids — what is it?	The two identical copies of a chromosome after replication. Sister chromatids are identical copies, while homologues carry different alleles.
Centromere — what is it?	The constricted region where spindle fibres attach. It is a structural region, not a gene-rich one.
Karyotype — what is it?	An ordered display of an individual's chromosomes. It cannot detect single-gene mutations, which are far too small to see.
Mitosis — what is it?	Nuclear division producing two genetically identical cells. Mitosis produces two diploid cells, not four haploid ones.
Meiosis one — what is it?	The first meiotic division, in which homologues separate. Sister chromatids stay together through this division.
Crossing over — what is it?	The exchange of segments between non-sister chromatids of homologues. It occurs between homologues, not between sister chromatids, which are identical.
Independent assortment in meiosis — what is it?	The random orientation of homologue pairs at the metaphase plate. It shuffles whole chromosomes, while crossing over shuffles within them.
Genetic linkage — what is it?	The tendency of genes close together on a chromosome to be inherited together. Linkage is never complete over a whole chromosome, because crossing over separates distant loci.
Map unit — what is it?	A unit of genetic distance equal to one percent recombination. Map distance is not physical distance, because recombination rates vary along a chromosome.
Interference — what is it?	The reduction in double crossovers caused by one crossover suppressing another nearby. Interference means map distances are not simply additive over long intervals.
Sex linkage — what is it?	Inheritance of genes located on a sex chromosome. Sex linkage is not the same as a sex-influenced trait, which is autosomal.
Hemizygous — what is it?	Having only one copy of a gene rather than a pair. Hemizygous individuals cannot be heterozygous or homozygous for those genes.
Dosage compensation — what is it?	Mechanisms equalising the expression of sex-linked genes between the sexes. Different species solve the problem in different ways, so mammalian rules do not generalise.
Barr body — what is it?	The condensed, inactive X chromosome visible in a female cell nucleus. The number of Barr bodies is one fewer than the number of X chromosomes.
Nondisjunction — what is it?	The failure of chromosomes to separate properly during cell division. It can occur in either meiotic division, and the resulting gamete patterns differ.
Polyploidy — what is it?	Having more than two complete sets of chromosomes. It is usually lethal in animals, unlike in plants.
DNA structure — what is it?	A double helix of two antiparallel strands of nucleotides. The two strands run in opposite directions, which constrains how replication and transcription work.
Base pairing rules — what is it?	The specific pairing of bases across the double helix. In RNA, uracil replaces thymine as the partner of adenine.
Semiconservative replication — what is it?	Replication in which each new molecule keeps one original strand. Neither daughter molecule is entirely new or entirely old.
Helicase — what is it?	An enzyme that unwinds the double helix. Unwinding creates strain ahead of the fork, which other enzymes must relieve.
Primer — what is it?	A short RNA sequence that provides a starting point for DNA polymerase. The primer is RNA and is later removed and replaced with DNA.
Okazaki fragments — what is it?	The short pieces of DNA made on the lagging strand. They exist only on the lagging strand, and both strands are complete when replication finishes.
Transcription — what is it?	The synthesis of RNA from a DNA template. Only one strand serves as the template, and which one differs between genes.
Promoter — what is it?	A DNA sequence where transcription begins. The promoter is not transcribed into the RNA product.
Transfer RNA — what is it?	The RNA that brings amino acids to the ribosome. Each transfer RNA carries one specific amino acid, matched by a dedicated enzyme.
Codon — what is it?	A group of three bases specifying one amino acid or a stop. Codons are read in the messenger RNA, not directly from DNA.
Start codon — what is it?	The codon that begins translation. It also codes for methionine at internal positions, so not every occurrence is a start.
Degeneracy of the genetic code — what is it?	The property that most amino acids are specified by several codons. Degenerate does not mean ambiguous. Each codon still specifies only one amino acid.
Universality of the genetic code — what is it?	The near-identical use of the same codon assignments across life. A few exceptions exist, notably in mitochondria.
RNA processing — what is it?	The modification of a eukaryotic transcript before it leaves the nucleus. Bacteria do not process transcripts this way, and translation can begin before transcription ends.
Exon — what is it?	A sequence retained in the mature transcript. Not every exon is coding, since untranslated regions are also exonic.
Point mutation — what is it?	A change affecting a single base pair. Many point mutations have no effect at all because of code degeneracy.
Missense mutation — what is it?	A base change that substitutes one amino acid for another. A missense change can be harmless, so it should not be assumed damaging.
Frameshift mutation — what is it?	An insertion or deletion that shifts the reading frame. Inserting or deleting a multiple of three bases does not shift the frame.
Mutagen — what is it?	An agent that increases the rate of mutation. Mutagens raise the rate of random changes rather than directing specific ones.
Mismatch repair — what is it?	A system that corrects base pairing errors after replication. Loss of this system causes a strong increase in mutation rate.
Telomere — what is it?	A repetitive sequence capping the end of a linear chromosome. Telomeres carry no genes, so their loss does not remove coding sequence directly.
Gene regulation — what is it?	The control of when and how much a gene is expressed. Regulation acts at several stages, not only at transcription.
Inducible gene — what is it?	A gene switched on in response to a signal. Induction usually means removing a block rather than adding an activator.
Operon — what is it?	A cluster of bacterial genes transcribed from a single promoter. Operons are characteristic of bacteria and are rare in eukaryotes.
Repressor protein — what is it?	A protein that binds the operator and prevents transcription. A repressor blocks transcription. It does not degrade the product.
Catabolite repression — what is it?	The suppression of alternative sugar operons when glucose is available. It reduces expression indirectly, by removing an activator rather than adding a repressor.
Corepressor and inducer — what is it?	Small molecules that control repressor binding. The two act in opposite directions on the same kind of protein.
Eukaryotic promoter — what is it?	The region where transcription of a eukaryotic gene begins. A eukaryotic promoter alone gives only a low basal level of transcription.
Transcription factor — what is it?	A protein that binds DNA and influences transcription. Transcription factors regulate transcription. They do not synthesise RNA themselves.
Chromatin remodelling — what is it?	The repositioning of nucleosomes to change access to DNA. Remodelling changes packaging, not sequence.
Euchromatin and heterochromatin — what is it?	Loosely and tightly packed chromatin. The distinction is dynamic, and regions can switch between states.
Genomic imprinting — what is it?	Expression of a gene depending on which parent contributed it. The silenced copy differs between the sexes, so imprints are reset each generation.
RNA interference — what is it?	Regulation of gene expression by small RNA molecules. It acts after transcription, so the gene is still transcribed.
Post-transcriptional regulation — what is it?	Control of gene expression after RNA is made. A gene can be transcribed heavily and still produce little protein.
Differential gene expression — what is it?	The expression of different gene sets in different cell types. Cells differ in which genes are active, not in which genes they contain.
Differentiation — what is it?	The process by which a cell acquires its specialised structure and function. Differentiation is usually stable but not always irreversible.
Homeotic genes — what is it?	Genes that specify which structures develop in each body region. They control identity, not whether a structure forms at all.
Population in genetics — what is it?	A group of individuals of one species that interbreed. A population is defined by interbreeding, not by an arbitrary geographic boundary.
Hardy-Weinberg equilibrium — what is it?	The state in which allele and genotype frequencies stay constant across generations. It is a baseline, not a description of any real population.
Hardy-Weinberg genotype equation — what is it?	The relationship giving expected genotype frequencies from allele frequencies. The heterozygote frequency requires the factor of two, which is easily dropped.
Natural selection — what is it?	Differential survival and reproduction based on heritable variation. Selection acts on phenotypes, so alleles with no phenotypic effect are invisible to it.
Disruptive selection — what is it?	Selection favouring both extremes over the intermediate. It increases variation, unlike the other two modes.
Genetic drift — what is it?	Random change in allele frequencies from sampling between generations. Drift is undirected, so it can fix a harmful allele as easily as a beneficial one.
Gene flow — what is it?	The movement of alleles between populations through migration. It can introduce new alleles as well as change frequencies of existing ones.
Inbreeding — what is it?	Mating between close relatives. It changes genotype frequencies but not allele frequencies.
Polymerase chain reaction — what is it?	A method that amplifies a specific DNA sequence. It requires primers flanking the target, so the surrounding sequence must be known.
Restriction enzyme — what is it?	An enzyme that cuts DNA at a specific recognition sequence. Different enzymes recognise different sequences, so cutting patterns are not interchangeable.
Recombinant DNA — what is it?	DNA combining sequences from different sources. It depends on the near universality of the genetic code.
Genetic counselling — what is it?	Advising individuals and families about inherited conditions. The role is to inform choices, not to direct them.

Frequently asked

What is in each section of the deck?

Mendelian inheritance has 40 cards, beyond Mendel 40, chromosomes and linkage 40, molecular genetics 50, gene regulation and development 40, and population and applied genetics 40, for 250 in total. Every card carries section and subtopic tags, so you can drill only linkage or only regulation.

What level is this pitched at?

Introductory university genetics, which also covers most of an advanced secondary course. It assumes you know what a cell and a chromosome are, but it explains everything built on top of that, from segregation through Hardy-Weinberg equilibrium and gene editing.

Does it include genetics problems?

It includes the reasoning that problems require rather than the problems themselves. Cards state what each modified ratio indicates, how to judge a mode of inheritance from a pedigree, and which assumption breaks in each case. Working through crosses on paper is still necessary.

Can I import the whole deck on the free plan?

Yes. Importing a saved deck runs no new AI generation and does not use your AI allowance, so the free plan imports all 250 cards. You can study, edit and delete them afterwards.

Will importing it twice create duplicates?

No. Cards you already have are skipped and only cards added in a revision come through. Including re-imports after deleting it, one official deck can be imported three times per account.

Can I use it on the web and in the mobile app?

Yes. The deck is added to your account rather than to a device, so the same cards and the same progress are there on the web, on iOS and on Android.

Can I edit the cards after importing?

Yes. Imported cards are yours: you can edit both sides, delete cards you do not need, change tags, and move cards to another deck.