Genetics: 250 Core Concepts, Ratios and Assumptions
A modified ratio is not a broken rule. It is a second gene telling you something.
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| Front | Back |
|---|---|
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| Inbreeding | Meaning: Mating between close relatives. Detail: It increases homozygosity throughout the genome. Watch for: It changes genotype frequencies but not allele frequencies. |
| 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. |
About this deck
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.
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.
No official exam questions are reproduced. Every card was written for this deck.Editorial reference date 2026-08-31.