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 cut into three fixed lines. "Meaning" defines it in a sentence. "Detail" gives the mechanism, ratio or condition that makes it usable. "Watch for" names 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	Meaning: A unit of heredity that occupies a specific position on a chromosome. Detail: Most genes encode a protein or a functional RNA molecule. Watch for: A gene is not the same as an allele. A gene is the locus, an allele is one version of it.
Genotype	Meaning: The combination of alleles an organism carries. Detail: It is written with letter pairs, one symbol per allele. Watch for: The genotype does not fully determine appearance, because environment and other genes contribute.
Homozygous	Meaning: Carrying two identical alleles at a locus. Detail: A homozygote produces only one type of gamete for that gene. Watch for: Homozygous recessive individuals always show the recessive phenotype, but homozygous does not mean recessive.
Dominant allele	Meaning: An allele whose effect appears in the heterozygote. Detail: It is written with a capital letter by convention. Watch for: Dominant does not mean common or advantageous. Many dominant alleles are rare.
Complete dominance	Meaning: The situation in which the heterozygote looks identical to the dominant homozygote. Detail: It gives the classic three to one phenotype ratio in a monohybrid cross. Watch for: Any deviation from that ratio suggests something other than complete dominance.
Law of segregation	Meaning: Mendel's principle that the two alleles at a locus separate during gamete formation. Detail: Each gamete receives exactly one allele of each gene. Watch for: Segregation happens in meiosis one, when homologues separate, not in meiosis two.
Law of dominance	Meaning: Mendel's observation that one allele can mask another in the heterozygote. Detail: It explains why a trait can skip a generation. Watch for: It is the least general of Mendel's laws, because many genes show other patterns.
Monohybrid cross	Meaning: A cross tracking a single gene. Detail: Crossing two heterozygotes gives a one to two to one genotype ratio. Watch for: The three to one phenotype ratio appears only under complete dominance.
Dihybrid cross	Meaning: A cross tracking two genes at once. Detail: Two heterozygotes give the nine to three to three to one phenotype ratio. Watch for: That ratio breaks down if the two genes are linked or interact.
Back cross	Meaning: A cross of an offspring with one of its parents or an equivalent genotype. Detail: It is used to move a trait into a chosen genetic background. Watch for: A back cross is a test cross only when the parent is homozygous recessive.
P, F1 and F2 generations	Meaning: The parental generation and the first two generations of offspring. Detail: Mendel's ratios refer to the F2 from crossing true-breeding parents. Watch for: The F1 generation is uniform, so ratios only become informative in the F2.
Product rule	Meaning: The rule that the probability of independent events both occurring is the product of their probabilities. Detail: It is used to combine separate genes in a multi-gene cross. Watch for: It applies only when the events are genuinely independent.
Forked-line method	Meaning: A branching method for combining the outcomes of several genes. Detail: It replaces a large Punnett square with a series of simple branches. Watch for: It assumes independent assortment, so it fails for linked genes.
Pedigree	Meaning: A diagram of family relationships used to trace inheritance. Detail: Squares are males, circles are females, and shaded symbols are affected. Watch for: A pedigree shows phenotypes, so genotypes must be inferred rather than read off.
X-linked recessive pedigree	Meaning: A pattern in which affected individuals are mostly male. Detail: An affected male inherits the allele from his mother. Watch for: Father to son transmission never occurs, which rules the pattern out when it does.
Symbols in a pedigree	Meaning: The standard notation for individuals and relationships. Detail: A horizontal line joins mates and a vertical line leads to offspring. Watch for: Generations are numbered with Roman numerals and individuals within them with Arabic numerals.
Independent events across children	Meaning: The principle that each conception is independent of the ones before it. Detail: The probability for a new child does not depend on previous children. Watch for: Having three affected children does not lower the risk for the fourth.
Incomplete dominance	Meaning: A pattern in which the heterozygote shows a phenotype between the two homozygotes. Detail: A cross of two heterozygotes gives a one to two to one phenotype ratio. Watch for: The heterozygote is a blend of appearance only. The alleles themselves are not blended.
Multiple alleles	Meaning: The existence of more than two alleles of a gene in a population. Detail: Any diploid individual still carries only two of them. Watch for: More alleles in the population do not mean more alleles per individual.
Epistasis	Meaning: An interaction in which one gene masks the effect of another. Detail: It alters the expected dihybrid ratio. Watch for: Epistasis involves two different genes, while dominance involves two alleles of one gene.
Complementary gene action	Meaning: A pattern in which two genes must both be functional for a phenotype to appear. Detail: It produces a nine to seven ratio in the F2. Watch for: It reflects two steps in one pathway, not two independent traits.
Modifier gene	Meaning: A gene that alters the expression of another gene without producing the trait itself. Detail: It explains variation in severity among individuals with the same main genotype. Watch for: A modifier is not the cause of the trait, and looking for it as such is misleading.
Polygenic trait	Meaning: A trait influenced by many genes each with a small effect. Detail: It produces a continuous range rather than distinct categories. Watch for: Polygenic means many genes affecting one trait, the reverse of pleiotropy.
Quantitative trait locus	Meaning: A region of the genome associated with variation in a continuous trait. Detail: Mapping these regions identifies where the relevant genes lie. Watch for: A mapped region usually contains many genes, so identifying the causal one takes further work.
Penetrance	Meaning: The proportion of individuals with a genotype who show the associated phenotype. Detail: Incomplete penetrance means some carriers appear unaffected. Watch for: An unaffected individual in a pedigree may still carry the allele.
Norm of reaction	Meaning: The range of phenotypes a genotype produces across environments. Detail: It shows that a genotype does not specify a single outcome. Watch for: Two genotypes can rank differently in different environments.
Sex-limited trait	Meaning: A trait expressed in only one sex although both carry the genes. Detail: Hormonal differences switch expression on or off. Watch for: The gene is not on a sex chromosome, so both sexes transmit it equally.
Maternal effect	Meaning: A phenotype determined by the mother's genotype rather than the offspring's own. Detail: Products deposited in the egg control early development. Watch for: The offspring phenotype can contradict its own genotype in the first generation.
Mitochondrial inheritance	Meaning: Transmission of mitochondrial DNA from mother to all her children. Detail: Affected mothers pass the condition to every child. Watch for: Affected fathers do not transmit it, which distinguishes it from autosomal dominance.
Genetic background	Meaning: The rest of an organism's genome, which influences how one gene is expressed. Detail: It explains why the same mutation differs between strains or families. Watch for: Results from one strain may not transfer to another with a different background.
Genomic imprinting overview	Meaning: Expression of a gene depending on the parent it came from. Detail: One copy is silenced by chemical marking during gamete formation. Watch for: The DNA sequence is unchanged, so it is an epigenetic rather than genetic difference.
Deviations from Mendelian ratios	Meaning: Departures from the expected proportions in a cross. Detail: Modified ratios point to epistasis, lethality or linkage. Watch for: The total number of classes usually still sums to sixteen, which helps identify the pattern.
Interpreting a nine to seven ratio	Meaning: A modified dihybrid ratio in the F2. Detail: It indicates complementary gene action in a two-step pathway. Watch for: Seven parts share one phenotype because any block in the pathway gives the same result.
Interpreting a fifteen to one ratio	Meaning: A modified dihybrid ratio in the F2. Detail: It indicates duplicate genes with redundant function. Watch for: Only the double homozygous recessive shows the alternative phenotype.
Chromosome	Meaning: A single long DNA molecule packaged with proteins. Detail: Packaging allows metres of DNA to fit inside a nucleus and controls access to genes. Watch for: A chromosome has one DNA molecule before replication and two identical ones after it.
Sister chromatids	Meaning: The two identical copies of a chromosome after replication. Detail: They are held together at the centromere until anaphase. Watch for: Sister chromatids are identical copies, while homologues carry different alleles.
Centromere	Meaning: The constricted region where spindle fibres attach. Detail: Its position determines the shape of the chromosome in a karyotype. Watch for: It is a structural region, not a gene-rich one.
Karyotype	Meaning: An ordered display of an individual's chromosomes. Detail: It reveals numerical and large structural abnormalities. Watch for: It cannot detect single-gene mutations, which are far too small to see.
Mitosis	Meaning: Nuclear division producing two genetically identical cells. Detail: It maintains chromosome number and underlies growth and repair. Watch for: Mitosis produces two diploid cells, not four haploid ones.
Meiosis one	Meaning: The first meiotic division, in which homologues separate. Detail: It reduces the chromosome number from diploid to haploid. Watch for: Sister chromatids stay together through this division.
Crossing over	Meaning: The exchange of segments between non-sister chromatids of homologues. Detail: It creates new allele combinations on a chromosome. Watch for: It occurs between homologues, not between sister chromatids, which are identical.
Independent assortment in meiosis	Meaning: The random orientation of homologue pairs at the metaphase plate. Detail: Each pair orients independently, multiplying the possible gamete combinations. Watch for: It shuffles whole chromosomes, while crossing over shuffles within them.
Genetic linkage	Meaning: The tendency of genes close together on a chromosome to be inherited together. Detail: It violates independent assortment and distorts dihybrid ratios. Watch for: Linkage is never complete over a whole chromosome, because crossing over separates distant loci.
Map unit	Meaning: A unit of genetic distance equal to one percent recombination. Detail: It allows genes to be ordered along a chromosome. Watch for: Map distance is not physical distance, because recombination rates vary along a chromosome.
Interference	Meaning: The reduction in double crossovers caused by one crossover suppressing another nearby. Detail: It makes observed double crossovers rarer than expected. Watch for: Interference means map distances are not simply additive over long intervals.
Sex linkage	Meaning: Inheritance of genes located on a sex chromosome. Detail: It produces different results in reciprocal crosses. Watch for: Sex linkage is not the same as a sex-influenced trait, which is autosomal.
Hemizygous	Meaning: Having only one copy of a gene rather than a pair. Detail: Males are hemizygous for X-linked genes. Watch for: Hemizygous individuals cannot be heterozygous or homozygous for those genes.
Dosage compensation	Meaning: Mechanisms equalising the expression of sex-linked genes between the sexes. Detail: Without it, one sex would produce twice the product of X-linked genes. Watch for: Different species solve the problem in different ways, so mammalian rules do not generalise.
Barr body	Meaning: The condensed, inactive X chromosome visible in a female cell nucleus. Detail: Its presence confirms that inactivation has occurred. Watch for: The number of Barr bodies is one fewer than the number of X chromosomes.
Nondisjunction	Meaning: The failure of chromosomes to separate properly during cell division. Detail: It produces gametes with an extra or missing chromosome. Watch for: It can occur in either meiotic division, and the resulting gamete patterns differ.
Polyploidy	Meaning: Having more than two complete sets of chromosomes. Detail: It is common in plants and often increases size and vigour. Watch for: It is usually lethal in animals, unlike in plants.
DNA structure	Meaning: A double helix of two antiparallel strands of nucleotides. Detail: The sugar-phosphate backbone lies outside and the bases pair inside. Watch for: The two strands run in opposite directions, which constrains how replication and transcription work.
Base pairing rules	Meaning: The specific pairing of bases across the double helix. Detail: Adenine pairs with thymine and guanine pairs with cytosine. Watch for: In RNA, uracil replaces thymine as the partner of adenine.
Semiconservative replication	Meaning: Replication in which each new molecule keeps one original strand. Detail: It explains how sequence information is preserved through generations. Watch for: Neither daughter molecule is entirely new or entirely old.
Helicase	Meaning: An enzyme that unwinds the double helix. Detail: It separates the strands so each can serve as a template. Watch for: Unwinding creates strain ahead of the fork, which other enzymes must relieve.
Primer	Meaning: A short RNA sequence that provides a starting point for DNA polymerase. Detail: It supplies the free end the enzyme needs to extend. Watch for: The primer is RNA and is later removed and replaced with DNA.
Okazaki fragments	Meaning: The short pieces of DNA made on the lagging strand. Detail: They are later joined into a continuous strand. Watch for: They exist only on the lagging strand, and both strands are complete when replication finishes.
Transcription	Meaning: The synthesis of RNA from a DNA template. Detail: It copies one gene rather than the whole chromosome. Watch for: Only one strand serves as the template, and which one differs between genes.
Promoter	Meaning: A DNA sequence where transcription begins. Detail: It positions RNA polymerase at the correct start site. Watch for: The promoter is not transcribed into the RNA product.
Transfer RNA	Meaning: The RNA that brings amino acids to the ribosome. Detail: Its anticodon pairs with the codon on the messenger RNA. Watch for: Each transfer RNA carries one specific amino acid, matched by a dedicated enzyme.
Codon	Meaning: A group of three bases specifying one amino acid or a stop. Detail: The reading frame determines how the sequence is divided into codons. Watch for: Codons are read in the messenger RNA, not directly from DNA.
Start codon	Meaning: The codon that begins translation. Detail: It sets the reading frame and specifies methionine. Watch for: It also codes for methionine at internal positions, so not every occurrence is a start.
Degeneracy of the genetic code	Meaning: The property that most amino acids are specified by several codons. Detail: It buffers the effect of some point mutations. Watch for: Degenerate does not mean ambiguous. Each codon still specifies only one amino acid.
Universality of the genetic code	Meaning: The near-identical use of the same codon assignments across life. Detail: It allows genes to be expressed across species in biotechnology. Watch for: A few exceptions exist, notably in mitochondria.
RNA processing	Meaning: The modification of a eukaryotic transcript before it leaves the nucleus. Detail: It includes capping, tailing and splicing. Watch for: Bacteria do not process transcripts this way, and translation can begin before transcription ends.
Exon	Meaning: A sequence retained in the mature transcript. Detail: Exons are joined together during splicing. Watch for: Not every exon is coding, since untranslated regions are also exonic.
Point mutation	Meaning: A change affecting a single base pair. Detail: Its effect depends on where in the codon it falls. Watch for: Many point mutations have no effect at all because of code degeneracy.
Missense mutation	Meaning: A base change that substitutes one amino acid for another. Detail: Its effect depends on how different the new amino acid is and where it lies. Watch for: A missense change can be harmless, so it should not be assumed damaging.
Frameshift mutation	Meaning: An insertion or deletion that shifts the reading frame. Detail: Every codon downstream is altered. Watch for: Inserting or deleting a multiple of three bases does not shift the frame.
Mutagen	Meaning: An agent that increases the rate of mutation. Detail: Radiation and certain chemicals damage or alter DNA. Watch for: Mutagens raise the rate of random changes rather than directing specific ones.
Mismatch repair	Meaning: A system that corrects base pairing errors after replication. Detail: It identifies the newly made strand and replaces the incorrect base. Watch for: Loss of this system causes a strong increase in mutation rate.
Telomere	Meaning: A repetitive sequence capping the end of a linear chromosome. Detail: It protects the end from degradation and from being treated as a break. Watch for: Telomeres carry no genes, so their loss does not remove coding sequence directly.
Gene regulation	Meaning: The control of when and how much a gene is expressed. Detail: It allows one genome to produce many cell types and respond to conditions. Watch for: Regulation acts at several stages, not only at transcription.
Inducible gene	Meaning: A gene switched on in response to a signal. Detail: It saves resources by producing a product only when needed. Watch for: Induction usually means removing a block rather than adding an activator.
Operon	Meaning: A cluster of bacterial genes transcribed from a single promoter. Detail: It coordinates the expression of enzymes in one pathway. Watch for: Operons are characteristic of bacteria and are rare in eukaryotes.
Repressor protein	Meaning: A protein that binds the operator and prevents transcription. Detail: Its binding is controlled by a small molecule. Watch for: A repressor blocks transcription. It does not degrade the product.
Catabolite repression	Meaning: The suppression of alternative sugar operons when glucose is available. Detail: It ensures the preferred energy source is used first. Watch for: It reduces expression indirectly, by removing an activator rather than adding a repressor.
Corepressor and inducer	Meaning: Small molecules that control repressor binding. Detail: A corepressor activates a repressor while an inducer inactivates it. Watch for: The two act in opposite directions on the same kind of protein.
Eukaryotic promoter	Meaning: The region where transcription of a eukaryotic gene begins. Detail: General transcription factors assemble there before polymerase binds. Watch for: A eukaryotic promoter alone gives only a low basal level of transcription.
Transcription factor	Meaning: A protein that binds DNA and influences transcription. Detail: It recognises specific sequences through a DNA-binding domain. Watch for: Transcription factors regulate transcription. They do not synthesise RNA themselves.
Chromatin remodelling	Meaning: The repositioning of nucleosomes to change access to DNA. Detail: It opens or closes regions for transcription. Watch for: Remodelling changes packaging, not sequence.
Euchromatin and heterochromatin	Meaning: Loosely and tightly packed chromatin. Detail: Genes in tightly packed regions are largely silenced. Watch for: The distinction is dynamic, and regions can switch between states.
Genomic imprinting	Meaning: Expression of a gene depending on which parent contributed it. Detail: One copy is silenced by methylation established during gamete formation. Watch for: The silenced copy differs between the sexes, so imprints are reset each generation.
RNA interference	Meaning: Regulation of gene expression by small RNA molecules. Detail: Small RNAs guide complexes that degrade or block messenger RNA. Watch for: It acts after transcription, so the gene is still transcribed.
Post-transcriptional regulation	Meaning: Control of gene expression after RNA is made. Detail: It includes splicing choices, transcript stability and export. Watch for: A gene can be transcribed heavily and still produce little protein.
Differential gene expression	Meaning: The expression of different gene sets in different cell types. Detail: It is how one genome produces many specialised cells. Watch for: Cells differ in which genes are active, not in which genes they contain.
Differentiation	Meaning: The process by which a cell acquires its specialised structure and function. Detail: It follows determination and involves large changes in gene expression. Watch for: Differentiation is usually stable but not always irreversible.
Homeotic genes	Meaning: Genes that specify which structures develop in each body region. Detail: Mutations transform one body part into another. Watch for: They control identity, not whether a structure forms at all.
Population in genetics	Meaning: A group of individuals of one species that interbreed. Detail: It is the unit within which allele frequencies are measured. Watch for: A population is defined by interbreeding, not by an arbitrary geographic boundary.
Hardy-Weinberg equilibrium	Meaning: The state in which allele and genotype frequencies stay constant across generations. Detail: It provides a null model against which real populations are compared. Watch for: It is a baseline, not a description of any real population.
Hardy-Weinberg genotype equation	Meaning: The relationship giving expected genotype frequencies from allele frequencies. Detail: The three genotype frequencies together sum to one. Watch for: The heterozygote frequency requires the factor of two, which is easily dropped.
Natural selection	Meaning: Differential survival and reproduction based on heritable variation. Detail: It shifts allele frequencies in a consistent direction. Watch for: Selection acts on phenotypes, so alleles with no phenotypic effect are invisible to it.
Disruptive selection	Meaning: Selection favouring both extremes over the intermediate. Detail: It can produce two distinct groups within a population. Watch for: It increases variation, unlike the other two modes.
Genetic drift	Meaning: Random change in allele frequencies from sampling between generations. Detail: Its effect is strongest in small populations. Watch for: Drift is undirected, so it can fix a harmful allele as easily as a beneficial one.
Gene flow	Meaning: The movement of alleles between populations through migration. Detail: It makes populations more genetically similar. Watch for: It can introduce new alleles as well as change frequencies of existing ones.
Inbreeding	Meaning: Mating between close relatives. Detail: It increases homozygosity throughout the genome. Watch for: It changes genotype frequencies but not allele frequencies.
Polymerase chain reaction	Meaning: A method that amplifies a specific DNA sequence. Detail: Repeated cycles of heating and cooling double the target each round. Watch for: It requires primers flanking the target, so the surrounding sequence must be known.
Restriction enzyme	Meaning: An enzyme that cuts DNA at a specific recognition sequence. Detail: It produces reproducible fragments used in cloning and analysis. Watch for: Different enzymes recognise different sequences, so cutting patterns are not interchangeable.
Recombinant DNA	Meaning: DNA combining sequences from different sources. Detail: It allows a gene from one organism to be expressed in another. Watch for: It depends on the near universality of the genetic code.
Genetic counselling	Meaning: Advising individuals and families about inherited conditions. Detail: It combines pedigree analysis, risk estimation and support for decisions. Watch for: 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 spends no AI credits, 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.