IB Biology · Theme A Unity and diversity · Organisms
A3.1 Diversity of organisms
No two organisms are identical, and the patterns in that variation are how we name and group species. Several species concepts compete, and gradual speciation makes some boundaries a matter of judgement. Chromosome numbers and genomes vary widely, and sequencing now lets us compare them directly.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
Learn
A3.1.1 Variation is universal, and it is what classification is built on
No two individuals are identical in all their traits.
Genetic differences and environment both act on every organism.
Many traits vary independently, so patterns of variation are complex.
These patterns are the basis for naming and classifying organisms.
Students often think identical twins or clones are exact copies. In fact new mutations, gene expression and environment make them differ.
Students often classify by one obvious feature. In fact a whole set of shared traits is needed.
A3.1.2 A species as a group sharing a set of traits
Linnaeus defined a species by shared morphology: form and structure.
Organisms with the same set of traits are grouped; others are separated.
It is still the practical basis for identifying most organisms.
It gives no rule for how much difference makes a new species.
Students often think different-looking means different species. In fact males and females, or life stages, can look very different within one species.
A3.1.3 The two-part scientific name
The first part names the genus; the second distinguishes the species within it.
Panthera leo and Panthera tigris are two species in one genus, with similar traits.
The genus takes an initial capital; the species is lowercase; both are italic.
The species word means nothing alone; the same word appears in many genera.
Students often write "Homo Sapiens". In fact only the genus is capitalised: Homo sapiens.
Students often think species in one genus can interbreed freely. In fact they are separate species and normally cannot produce fertile offspring.
A3.1.4 A species as a group that can breed and produce fertile offspring
The biological species concept: members can interbreed and produce fertile offspring.
A horse and a donkey breed, but the mule is infertile, so they stay separate species.
It fails for fossils, asexual organisms and populations that never meet.
Other concepts compete: morphological, ecological, phylogenetic.
Students often think any offspring proves one species. In fact the offspring must be fertile.
Students often think geographically separate populations are separate species. In fact what matters is whether they could interbreed, which is hard to test.
A3.1.5 Speciation is gradual, so some boundaries are arbitrary
Speciation is one species splitting into two or more.
A population is one species' members in one place, able to interbreed.
Populations that stop interbreeding diverge in traits over many generations.
There is no precise moment when two populations become two species.
Students often think speciation is a single event. In fact it is gradual, with populations growing steadily more different.
Students often think every pair of populations is clearly one species or two. In fact part-way cases exist, and the call can be arbitrary.
A3.1.6 Chromosome numbers vary between species and are always even in diploid cells
Body cells are diploid: chromosomes come in homologous pairs, one from each parent.
So a diploid cell has an even number of chromosomes.
Humans have 46; chimpanzees have 48.
Numbers vary widely across plants and animals and say nothing about complexity.
Students often think more chromosomes means more complex. In fact chimpanzees have more than humans, and many plants have far more.
Students often say humans have 23 chromosomes. In fact 23 is the number of pairs; the diploid number is 46.
A3.1.7 Karyograms, and the testable idea that chromosome 2 is a fusion
A karyotype is the number and appearance of a cell's chromosomes.
A karyogram arranges them in homologous pairs, usually by decreasing length.
Pairs are matched by length, centromere position and banding pattern, not length alone.
Human chromosome 2 is probably two ancestral chromosomes fused end to end, explaining 46 versus 48.
The hypothesis is testable: it predicts telomere-like sequences and a second centromere near the middle of chromosome 2.
Students often think humans lost a chromosome pair. In fact two chromosomes fused; no DNA was lost.
Students often think events millions of years ago are untestable. In fact a hypothesis is testable if it makes checkable predictions.
A3.1.8 Within a species, genomes are mostly shared but never identical
The genome is all the genetic information of an organism.
Members of a species share most of their genome.
A single-nucleotide polymorphism is one base that differs between individuals.
Most SNPs sit in non-coding DNA or do not change the amino acid.
Students often think everyone in a species has the same genome. In fact each individual differs at many SNPs.
Students often think any base change alters a protein. In fact most SNPs have no effect on any protein.
A3.1.9 Genomes differ in size and sequence, far more between species than within
Genome size is the total amount of DNA, measured in base pairs.
It is not the number of chromosomes and not the number of genes.
Much of a large genome is non-coding or repetitive.
Differences between species are much larger than differences within one.
Students often think human-to-human variation rivals human-to-chimpanzee variation. In fact humans differ at about one base in a thousand; species differ far more.
Students often equate genome size with chromosome number. In fact it is the amount of DNA.
A3.1.10 Comparing genome sizes across groups
Genome sizes can be extracted from a database for different taxonomic groups.
Some amphibians and flowering plants have genomes tens of times larger than ours.
Genome size shows no simple link to complexity.
Students often think complex organisms need the biggest genomes. In fact many plants and single-celled organisms have far larger genomes than humans.
A3.1.11 What genome sequencing is used for now, and might be soon
Whole genome sequencing reads an organism's complete base sequence.
The first human genome took over a decade; now a day and a few hundred dollars.
Current use: comparing genomes to research evolutionary relationships.
Potential use: personalised medicine, guiding risk estimates and drug choice.
Students often think sequencing still takes years. In fact speed has risen and cost has fallen by orders of magnitude.
Students often think a genome reveals every future disease. In fact it mostly shifts risk; environment and chance matter too.
A3.1.12 Why the biological species concept fails for asexual organisms and bacteria HL
Asexual organisms never breed, so the fertile-offspring test cannot be applied.
Their species are defined by morphological, ecological or DNA sequence similarity.
Horizontal gene transfer moves genes between bacteria, even of different species.
So bacterial gene pools are not sealed, as the concept assumes.
Students often think conjugation is breeding. In fact it transfers genes without reproduction, and across species.
Students often think asexual organisms have no species. In fact they do, defined by other criteria.
A3.1.13 Chromosome number is shared within a species HL
All members of a species normally share one chromosome number.
Closely related species with different numbers rarely produce fertile hybrids.
The hybrid's chromosomes cannot all form homologous pairs in meiosis, so gametes fail.
A horse (64) and a donkey (62) give a mule with 63, almost always sterile.
Students often think a mule cannot be born or cannot do mitosis. In fact it lives; meiosis is what fails.
Students often think the same chromosome number means the same species. In fact many different mammals share 2n = 48.
A3.1.14 Building a dichotomous key for local species HL
A dichotomous key offers exactly two contrasting choices at each step.
Each choice leads to a name or to another step.
Use fixed traits: leaf arrangement, number of legs, petal number.
Avoid size, colour or behaviour, which vary with age, season and condition.
Students often use any noticeable difference. In fact only fixed, unambiguous traits work.
Students often offer three or more choices per step. In fact "dichotomous" means dividing into two.
A3.1.15 Reading a habitat's species from environmental DNA HL
A DNA barcode is a short standard region that varies between species but little within them.
Animals: part of the mitochondrial COI gene; plants: chloroplast genes such as rbcL and matK.
Environmental DNA is collected from water, soil or air, shed as cells, mucus or faeces.
Barcoding eDNA surveys biodiversity rapidly, without catching or seeing organisms.
Students often think the whole genome must be sequenced. In fact a short barcode region is enough.
Students often treat an eDNA hit as a sighting. In fact DNA can persist and drift; it shows only presence in the sample.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Which statement best describes variation between organisms?
Answer and reasoning
No two individuals are identical in all their traits, and the patterns of variation are the basis for classifying organisms. — Variation is a defining feature of life. No two individuals are identical in all their traits, and because many traits vary independently the patterns of variation are complex; it is these patterns that are used to name and classify organisms.
Individuals produced by asexual reproduction share one genome and so are identical in all of their traits. — A student who equates an identical genome with an identical organism picks this. Members of a clone still differ, because of mutations arising in body cells, differences in gene expression and the effects of the environment; no two individuals are identical in all their traits.
Organisms are classified by a single distinguishing trait, so variation in the other traits is not used in naming them. — A student who sorts living things by one obvious feature picks this. The patterns of variation are complex, and classification rests on whole suites of shared traits, which is why a whale is a mammal despite living like a fish.
Organisms that look markedly different from one another must be members of two different species. — A student who treats visible difference as the definition of a species picks this. Large differences occur within a species, for example between males and females, so appearance alone cannot settle whether two organisms are the same species.
As a group that can breed to produce fertile offspring — A student who thinks there is one definition of a species and that it is the biological species concept picks this. That concept came much later; Linnaeus classified by shared traits, the original morphological concept, and competing definitions still exist.
As a group of organisms that can mate and have offspring — A student who remembers a species as organisms that can breed together, and drops the condition of fertile offspring, picks this. It is neither Linnaeus's concept nor a correct statement of the biological species concept.
As a group of organisms that share a set of traits — Linnaeus used the original morphological concept: a species is a group of organisms with shared traits, which distinguish them from other groups. This remains the practical basis of identification.
As organisms that look identical in every trait — A student who thinks members of a species must look the same picks this. Linnaeus grouped organisms sharing a set of traits, not identical ones; individuals within a species vary, sometimes greatly.
3 Which statement about the scientific name of the tiger, Panthera tigris, is correct?
Answer and reasoning
The second word, tigris, is on its own enough to identify the species of tiger. — A student who treats the second word as a stand-alone species name picks this. The second part only distinguishes the species within its genus; the same species word is used in other genera, so the full binomial is needed.
It should be written Panthera Tigris, because both of its words are proper nouns. — A student who capitalizes both parts as if they were English proper names picks this. The genus name is given an initial capital letter but the species name is always lowercase: Panthera tigris.
Panthera tigris and Panthera leo are varieties of one species and interbreed freely. — A student who reads a shared genus name as a shared species picks this. Tigers and lions are separate species that share similar traits; they do not interbreed in the wild and the rare captive hybrids are not a fertile breeding population.
Panthera names the genus and tigris distinguishes the species within it. — In the binomial system the first part of the name identifies the genus and the second part distinguishes the species from other species in that genus. Panthera leo, the lion, shares the genus and has similar traits.
It is the splitting of one species into two or more, usually happening gradually as populations become more different. — Speciation is the splitting of one species into two or more. It usually happens gradually rather than by a single act, with non-interbreeding populations becoming more and more different in their traits.
It occurs at the moment when a single mutation produces the first individual of a new species. — A student who models the origin of a species on the origin of a mutation picks this. Speciation is a change in populations over many generations, not the appearance of one individual.
It ends at a definite point that can be identified by testing whether the populations are separate. — A student who expects an objective boundary picks this. Because divergence is gradual, there is often no definite point, and whether two populations are regarded as separate species can be an arbitrary decision.
It is complete as soon as a population becomes geographically separated from the rest of its species. — A student who equates separation with speciation picks this. Separation may start the process, but the populations must then diverge in their traits over time; a newly separated population is still the same species.
5 Which statement about chromosome numbers is correct?
Answer and reasoning
Humans must have more chromosomes than chimpanzees, because humans are the more complex and advanced organism. — A student who reads chromosome number as a measure of complexity picks this. Humans have 46 chromosomes and chimpanzees 48; chromosome number varies between species and does not indicate complexity.
Human diploid cells have 46 chromosomes, chimpanzee diploid cells have 48, and diploid numbers are even. — Humans have 46 chromosomes and chimpanzees 48. A diploid cell has two sets of chromosomes, one from each parent, forming homologous pairs, so its chromosome number is even.
Human body cells contain 23 chromosomes in total, which is why a diploid number can be an odd number. — A student who confuses the number of pairs with the number of chromosomes picks this. Human body cells have 23 pairs, that is 46 chromosomes; because chromosomes come in pairs, diploid numbers are even.
Chromosome number determines genome size, so 48 chromosomes must contain more DNA than 46. — A student who thinks genome size is set by the number of chromosomes picks this. Genome size is the total amount of DNA; a species with fewer, longer chromosomes can have a larger genome than one with more.
The set of genes that is identical in every member of the organism's species — A student who pictures one shared blueprint per species picks this. Each individual has its own genome; members of a species share most of it, but single-nucleotide polymorphisms and other variations make every genome slightly different.
The number of chromosomes in each of the organism's diploid body cells — A student who confuses the genome with chromosome number picks this. Chromosome number is how the genome is packaged; the genome is the genetic information itself.
All of the genetic information of the organism, including non-coding DNA — The genome is all the genetic information of an organism: the whole base sequence of its DNA, coding and non-coding, in the nucleus and in mitochondria and chloroplasts. Members of a species share most of their genome.
The genes whose base sequences determine all of the proteins the organism makes — A student who thinks all DNA codes for protein picks this. Protein-coding genes are only a small part of a eukaryote genome; the genome includes all the non-coding DNA as well.
7 From a genome size database a student extracts these values: fruit fly (Drosophila melanogaster) about 180 million base pairs; human about 3100 million; bread wheat about 17 000 million; the flowering plant Paris japonica about 150 000 million. What do the data show about genome size and organism complexity?
Answer and reasoning
Genome size varies enormously and does not increase with complexity, since two plants have genomes far larger than the human genome. — The values span nearly a thousandfold, and the two largest belong to plants rather than to the most complex animal. Genome size is the total amount of DNA, much of it non-coding, and comparisons across taxonomic groups show it does not track complexity.
Paris japonica must be the most complex organism of the four, because it has by far the largest genome of them. — A student who assumes a larger genome means a more complex organism picks this. Paris japonica is a small herbaceous plant; its huge genome consists mostly of repetitive non-coding DNA, not extra instructions for complexity.
Wheat and Paris japonica must have many more chromosomes than humans, since genome size depends on chromosome number. — A student who thinks genome size is set by chromosome number picks this. Genome size is the total amount of DNA regardless of how it is divided; both plants have fewer chromosomes than humans, packaged as much longer chromosomes.
Wheat must make about five times as many proteins as a human, because all of its extra DNA codes for protein. — A student who treats all DNA as protein-coding picks this. Most of a large eukaryote genome is non-coding or repetitive, so a fivefold larger genome does not mean a fivefold larger set of proteins.
8 Which statement about whole genome sequencing is correct?
Answer and reasoning
Sequencing a single complete human genome still takes several years and costs many hundreds of millions of dollars. — A student who remembers the figures for the Human Genome Project as if they were current picks this. The speed of sequencing has increased and its cost has fallen enormously; a human genome now takes about a day and costs hundreds of dollars.
Its cost has fallen and its speed has increased, and it is used in research into evolutionary relationships. — Whole genome sequencing has become steadily faster and cheaper. A current use is comparing the genomes of different species to research their evolutionary relationships; a potential future use is personalized medicine.
It is used routinely to tell each patient exactly which diseases they will develop and when. — A student who believes the genome determines every outcome picks this. Personalized medicine is a potential future use, and even then a genome sequence indicates risks and suitable treatments rather than fixed outcomes.
It has shown that two humans differ from each other by more than a human differs from a chimpanzee. — A student misled by how varied humans look picks this. Sequencing shows the opposite: variation between species is much larger than variation within a species.
9 A horse has 64 chromosomes in its body cells and a donkey has 62. They can mate to produce a mule. Which statement correctly explains why mules are almost always sterile? HL
Answer and reasoning
The mule's 63 chromosomes cannot all form homologous pairs in meiosis, so viable gametes are rarely produced. — The mule receives 32 chromosomes from the horse and 31 from the donkey, giving 63. Meiosis requires homologous chromosomes to pair, and with unequal parental sets many have no partner, so gametes almost never form; this is why cross-breeding between species with different chromosome numbers is unlikely to give fertile offspring.
The mule's body cells cannot complete mitosis, because an odd number of chromosomes cannot be split equally. — A student who confuses mitosis with meiosis picks this. Mitosis copies each chromosome and separates the copies, so 63 chromosomes are handled without difficulty, which is why a mule grows normally; it is meiosis that fails.
Horses and donkeys are one species because they produce offspring, so the mule is not a hybrid at all. — A student who drops the fertility condition from the biological species concept picks this. Horse and donkey are separate species precisely because their offspring is infertile, and their different chromosome numbers are a shared trait that separates them.
A mule has 63 chromosomes, a number found in no species, so it belongs to no species and so cannot breed. — A student who treats chromosome number as the definition of a species picks this. The mule is sterile because its chromosomes cannot pair in meiosis, not because of which species its number matches.
10 A student is constructing a dichotomous key to identify five species of tree growing in a local park. Which first step is the best design? HL
Answer and reasoning
1a Tree more than 10 m tall, go to 2; 1b Tree less than 10 m tall, go to 4 — A student who uses the most obvious difference between the specimens in front of them picks this. Height varies with age, so a young tree of a tall species would be sent down the wrong branch of the key.
1a Leaves arranged in opposite pairs, go to 2; 1b Leaves arranged alternately, go to 4 — Leaf arrangement is a fixed trait, present in every individual of a species regardless of age or season and easily observed, and the step offers exactly two contrasting alternatives, each leading to a further step.
1a Leaves needle-like, species A; 1b Leaves broad, species B; 1c Leaves scaly, species C — A student who wants to reach answers quickly picks this. A dichotomous key offers exactly two alternatives at each step; three choices at one step is not dichotomous, and it also cannot deal with the remaining species.
1a Grows in the damp part of the park, go to 2; 1b Grows in the dry part, go to 4 — A student who classifies organisms by one circumstantial feature picks this. Where a tree happens to be growing is not a trait of the organism, and the same species may grow in both places.
Read the ones marked not yet in Learn, then Verify.
Verify confirm before you go
14 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A gardener grows 50 plants from cuttings taken from one parent plant, so all 50 have the same genome. The plants differ from one another in height and in the number of flowers they produce. What does this show?
Answer and reasoning
The cuttings must have been taken from more than one parent plant, because the members of a clone are identical in every one of their traits. — A student who believes a clone is identical in every trait picks this. A clone shares a genome, but variation in traits still arises from the environment (light, water, soil) and from new mutations, so no two individuals are identical.
Variation occurs between all individuals, because the environment and new mutations act on organisms even when they share a genome. — No two individuals are identical in all their traits, even when they start with the same genome. Differences in the environment of each plant, and any mutations arising in its cells, produce variation in height and flowering; variation is a defining feature of life.
The plants must differ at many genes, since any difference in a trait requires some difference in base sequence. — A student who thinks every trait difference must be genetic, and every genetic difference must show, picks this. The plants share a genome; the differences here are mainly environmental, and most base differences between individuals have no effect on traits anyway.
Height and flower number are not traits used in classification, so these differences do not count as variation. — A student who thinks only classifying traits count picks this. All differences in traits between individuals are variation; classification draws on the complex patterns of that variation, not on a fixed list of traits.
2 The lion is Panthera leo and the tiger is Panthera tigris. What can be concluded from these names alone?
Answer and reasoning
Lions and tigers are two distinct species with similar traits that are classified together in the genus Panthera. — Different second words mean different species; the same first word means they are placed in the same genus. Species in the same genus share similar traits because they are closely related.
Lions and tigers are two varieties of a single species, Panthera, that interbreed freely in the wild. — A student who reads the shared first word as the species picks this. Panthera is the genus; leo and tigris are two species within it, and they do not form a single interbreeding population.
Each species is named by its second word alone, so the shared first word carries no information about them. — A student who thinks the species word stands alone picks this. The genus name is part of the species' name and tells you that the two species are closely related, with similar traits.
Lions and tigers look so different that they cannot be closely related enough to be placed in the same genus. — A student who judges relatedness by appearance alone picks this. The names themselves state that the two species are in one genus; genus membership rests on many shared traits, not on overall resemblance in colour or pattern.
3 According to the biological species concept, what is a species?
Answer and reasoning
A group of organisms that can mate and produce offspring of any kind — A student who remembers the concept without the condition of fertile offspring picks this. Horses and donkeys produce mules, but mules are infertile, so horse and donkey are separate species.
A group of organisms that can breed together and produce fertile offspring — The biological species concept defines a species as a group of organisms that can breed with one another and produce fertile offspring. Fertility of the offspring is the decisive condition.
A group of organisms that resemble one another in every visible trait — A student who defines a species by appearance picks this. Resemblance is the basis of the older morphological concept, and even there identical appearance is not required; the biological concept is about interbreeding.
A group of organisms that live in the same area and interbreed with one another — A student who reads 'can breed' as 'do breed' picks this. The concept concerns the ability to interbreed and produce fertile offspring, not whether populations actually meet; living in the same area describes a population, not a species.
4 Two populations of a bird species live on islands 2000 km apart and have never been observed to meet in the wild. In captivity, birds from the two populations breed readily and produce fertile offspring. Which conclusion is best supported?
Answer and reasoning
They are two separate species, because populations that never interbreed with each other in nature are different species by definition. — A student who reverses the logic of speciation, treating separation itself as speciation, picks this. The biological species concept asks whether organisms can interbreed and produce fertile offspring, and these birds can.
They are certainly one species, because the biological species concept is the only definition of a species that biologists accept. — A student who believes the biological species concept is the sole, official definition picks this. Competing species definitions exist, and geographically separated populations are a recognized difficulty for the biological concept, so certainty is not justified.
By the biological species concept they are one species, though applying it to populations that never meet is a known difficulty. — The populations can breed and produce fertile offspring, so by the biological species concept they belong to one species. But populations that are separated geographically are a challenge for the concept, because their capacity to interbreed cannot normally be tested in nature, and other species concepts might judge differently.
They are one species because they can produce offspring; whether those offspring are fertile is not relevant. — A student who drops the fertility condition picks this. The conclusion happens to be one species here, but the reasoning is wrong: the fertility of the offspring is exactly what the biological species concept requires.
5 Two populations of a salamander have been separated by a river for about 10 000 years. They now differ slightly in colour, rarely interbreed where they meet, and the few hybrids are fertile. One biologist regards them as one species and another regards them as two. Why can both positions be defended?
Answer and reasoning
Speciation happens in a single act, so one of the biologists must simply have missed the event that created the new species. — A student who pictures speciation as an event at a branch point picks this. There is no single act to miss; the populations are diverging continuously, and that is precisely why the decision is open.
Whether two populations are one species or two is a fixed fact, so a better test is all that is needed to settle it for good. — A student who expects species to be sharp, objective categories picks this. Because divergence is gradual, no test can find a boundary that does not exist yet; the decision for populations at this stage is a judgement.
Populations that differ in a visible trait such as colour are different species, so the first biologist is mistaken. — A student who uses visible difference as the test of a species picks this. Slight colour differences occur within many species, and the fertile hybrids show the populations have not fully diverged.
Speciation is gradual, so populations part-way through divergence are not clearly one species or two. — Speciation is the splitting of one species into two or more, and it usually happens gradually with populations becoming more and more different. Part-way through, it can be an arbitrary decision whether two populations are regarded as the same or as different species.
6 A student reads that a dog has 78 chromosomes in its body cells, a human 46 and a chimpanzee 48. Which conclusion is justified?
Answer and reasoning
Dogs must be more complex organisms than humans, since they have the largest number of chromosomes. — A student who treats chromosome number as a scale of complexity picks this. Chromosome number is just the number of pieces into which the DNA is packaged and says nothing about complexity.
The dog genome must contain the most DNA, because its DNA is packaged into the most chromosomes. — A student who thinks genome size follows chromosome number picks this. Genome size is the total amount of DNA, and many chromosomes can be short; the dog genome is in fact smaller than the human genome.
Chromosome number varies between species, and it does not indicate how complex a species is. — Diversity in chromosome number exists among plant and animal species. The human (46) and chimpanzee (48) example shows that the number is unrelated to complexity, and the dog's 78 shows how widely it varies.
Humans have 46 pairs of chromosomes and chimpanzees 48 pairs, one set from each parent. — A student who confuses chromosomes with pairs picks this. The numbers given are total chromosomes: humans have 23 pairs (46) and chimpanzees 24 pairs (48).
7 Which set of features is used to classify the chromosomes when a karyogram is prepared?
Answer and reasoning
Length only, so that chromosomes of equal size are matched into pairs — A student who takes the size-ordered layout of a karyogram as the method picks this. Several different chromosomes are similar in length, so centromere position and banding pattern are also needed to match homologous pairs.
Number of genes and total amount of DNA in each chromosome — A student who measures chromosomes by gene number or DNA content picks this. Neither is visible under the microscope; chromosomes are classified by what can be seen: length, centromere position and banding pattern.
Base sequence differences, seen directly as bands on the chromosome — A student who thinks bands show individual base differences picks this. Bands are produced by stains binding to regions of the chromosome and are the same in every normal member of a species; they do not show single-base variation.
Length, position of the centromere, and banding pattern — Chromosomes in a karyogram are classified by their length, by the position of the centromere, which sets the relative lengths of the two arms, and by the pattern of bands produced by staining.
8 Human chromosome 2 contains telomere-like sequences near its middle and a second, inactive centromere region, and its banding pattern matches the two chimpanzee chromosomes formerly numbered 12 and 13 placed end to end. How well does this evidence support the hypothesis that chromosome 2 arose by fusion of two chromosomes in a shared primate ancestor?
Answer and reasoning
Strongly: the fusion hypothesis predicts exactly these features, and none would be expected if a chromosome pair had simply been lost. — Telomeres normally cap chromosome ends and each chromosome has one centromere, so internal telomere sequences and a second centromere are what an end-to-end fusion would leave behind, and the matching banding shows which chromosomes fused. The evidence fits the prediction and is hard to explain otherwise.
Weakly: the simpler explanation is that humans lost one whole chromosome pair, which requires no fused chromosome. — A student who explains 46 versus 48 by subtraction picks this. Losing a whole chromosome pair would delete hundreds of genes and be lethal, and it would not explain telomere sequences and a spare centromere in the middle of chromosome 2.
Not at all: the fusion happened millions of years ago, so no observation made today can test what actually occurred. — A student who thinks only repeatable experiments count as tests picks this. A hypothesis about the past is testable if it makes predictions about present evidence, and this one does.
Weakly: the human genome contains less DNA than the chimpanzee genome, so DNA was lost rather than rearranged. — A student who thinks fewer chromosomes must mean less DNA picks this. Chromosome number does not set genome size; fusion repackages the same DNA into one chromosome, and the internal telomeres and second centromere show that is what happened.
9 A student claims: 'The fusion that formed human chromosome 2 happened too long ago to be tested scientifically.' Which response is correct?
Answer and reasoning
The student is right: a hypothesis about a past event cannot be tested because the event itself cannot be repeated. — A student who equates testing with repeating an event in an experiment picks this. Testing means checking predictions; the fusion predicts features of chromosome 2 that can be examined today.
The hypothesis is testable, because it predicts observable features of chromosome 2 that might have been absent. — A testable hypothesis is one whose predictions can be checked and could fail. Fusion predicts internal telomere sequences, a second centromere region and matching banding; had these been absent, the hypothesis would have been rejected. Its age is irrelevant.
The hypothesis is testable only because most scientists have now accepted that the fusion took place. — A student who confuses 'testable' with 'accepted' or 'proven' picks this. Testability depends on whether the claim makes checkable predictions, not on how many scientists believe it.
The student is right: loss of a chromosome pair is an equally good explanation that no evidence can rule out. — A student who thinks humans lost a chromosome pair picks this. That alternative makes its own predictions, and it fails them: loss would not leave telomere sequences and a spare centromere in the middle of a chromosome.
10 Two unrelated people have their genomes sequenced. At one position on chromosome 7, in a region between genes, one person has T and the other has C. What is the best description of this difference?
Answer and reasoning
A mutation that must alter a protein, so the two people will differ in some visible trait — A student who thinks every base difference changes a protein picks this. The position is between genes, so it is not translated; most of the millions of SNPs between two people have no effect on the phenotype.
A sequencing error, because any two members of one species must have the same genome — A student who believes all humans share one genome picks this. Members of a species share most of their genome, but any two people differ at millions of positions; this is normal within-species diversity.
A variant that fixes which diseases each of the two people will develop later in life — A student who reads the genome deterministically picks this. A single non-coding base difference does not fix anyone's medical future; even variants that matter usually change risk rather than determine outcomes.
A single-nucleotide polymorphism that is unlikely to affect either person's traits — A single-base difference between individuals of a species is a single-nucleotide polymorphism. Between genes it is not translated, so it is unlikely to affect either person's phenotype; SNPs like this are the main source of genome diversity within a species.
11 A hospital proposes to sequence the whole genome of every patient diagnosed with cancer. Which is the most accurate description of what this could achieve?
Answer and reasoning
Each patient's exact future diseases could be read straight from the sequence, so no further diagnosis would be needed. — A student who reads the genome as a fixed script picks this. For most diseases the genome affects risk rather than determining the outcome, and a sequence cannot replace diagnosis.
The plan is unrealistic, because each genome would take several years to sequence and cost many millions of dollars. — A student who still carries the Human Genome Project's timescale and cost picks this. Sequencing has become fast and cheap enough, about a day and a few hundred dollars per genome, for such use to be realistic.
Treatment could be matched to the particular mutations in each patient's tumour, a form of personalized medicine. — Personalized medicine, a potential future use of whole genome sequencing, uses an individual's sequence to guide care, for example choosing drugs that target the specific mutations found in a tumour or that suit the patient's genotype.
The sequences would all be the same, because every member of a species shares a single genome. — A student who thinks a species has one shared genome picks this. Individuals' genomes differ at millions of positions, and tumour cells carry additional mutations, which is exactly what sequencing is meant to find.
12 Genes for antibiotic resistance carried on a plasmid in Escherichia coli have been found with an identical base sequence in Klebsiella pneumoniae, a bacterium of a different genus. Why does this create difficulty for the biological species concept? HL
Answer and reasoning
It shows the two bacteria have been breeding together, so they must be members of a single species. — A student who treats plasmid transfer as breeding picks this. Horizontal gene transfer is not reproduction and produces no offspring, so it does not make the two bacteria one species; it shows that genes cross species boundaries.
Bacteria do not reproduce sexually, and so they cannot be placed in any species at all. — A student who applies only the biological species concept picks this. Bacteria are placed in species using other criteria such as DNA sequence similarity; the difficulty is that genes cross between those species.
Both bacteria have a single chromosome, so by chromosome number they are the same species. — A student who uses chromosome number as a test of species identity picks this. Most bacteria have one chromosome; a shared number does not make two organisms one species, and it is irrelevant to the difficulty raised here.
Genes are being transferred between species, so bacterial gene pools are not closed. — The biological species concept assumes genes are exchanged only within a species. Horizontal gene transfer moves genes between bacteria of different species and genera, so the concept does not work well for bacteria.
13 Water samples from a pond were filtered, the DNA in them was extracted, and a barcode region was sequenced. Sequences matching the reference barcode of the great crested newt were found, although no newts were seen during the survey. Which is the best interpretation? HL
Answer and reasoning
A living newt was certainly present at the exact spot and time of sampling, since eDNA occurs only where the organism is. — A student who equates DNA with the organism picks this. Environmental DNA persists for days or weeks and can be moved by water or carried by other animals, so a detection does not fix where or when a newt was present.
The result is unreliable, because only a short region of DNA was sequenced, rather than the complete genome of the newt species. — A student who thinks identification needs the whole genome picks this. A barcode is a short standardized region chosen because it differs between species but not within them; it is the whole basis of the method, not a weakness.
Newts probably use the pond, because eDNA barcoding is a sensitive method, though the DNA could have been carried in. — Barcoding environmental DNA allows the biodiversity of a habitat to be surveyed rapidly and detects species that are not seen. A match is good evidence that newts have been in the pond, with the caveat that shed DNA can persist and be transported.
The match could be to any amphibian, because different species share most of their genome and cannot be told apart. — A student who underestimates variation between species picks this. Barcode regions differ far more between species than within a species, which is what lets a sequence be assigned to one species.
14 Five chromosomes from one metaphase spread are described. V: long, centromere near the middle, dark bands near both ends. W: long, centromere near the middle, dark bands near both ends. X: long, centromere near the middle, one dark band near one end only. Y: long, centromere near one end, dark bands near both ends. Z: short, centromere near the middle, dark bands near both ends. Which two chromosomes should be placed together as a homologous pair in the karyogram?
Answer and reasoning
W and X, because homologous chromosomes are simply the two chromosomes that share the same length — A student who takes the size-ordered layout of a karyogram as the method picks this. Length is only one of three features: W and X are the same length, but W has dark bands at both ends and X has one band at one end only, so their banding patterns differ and they are not homologous.
W and Y, because their length and banding match and centromere position is not used to match homologues — A student who pictures every chromosome as a symmetrical X with a central centromere picks this. Centromere position varies between chromosomes and is a classifying feature: W has its centromere near the middle and Y near one end, so they are different chromosomes despite matching in length and bands.
W and Z, because chromosomes with the same bands carry the same genes and so must be homologous — A student who thinks each band is a gene picks this. Bands are regions of stain uptake, not genes, and matching bands are used together with length and centromere position. Z is short and W is long, so they are different chromosomes, whatever their banding looks like.
V and W, because they match one another in length, centromere position and banding pattern — Homologous chromosomes are matched on all three features together: length, position of the centromere and banding pattern. V and W are the only two described that agree on all three. Each of X, Y and Z matches W on two features but differs on the third, so none of them can be its homologue.
That was your twenty minutes. Real practice on A3.1 is past-paper questions marked against the mark scheme.
What the exam asks of A3.1
Paper 1A asks you to apply binomial rules, spot a correctly matched karyogram pair, or pick the species concept a scenario uses. Paper 1B may give genome-size or chromosome-number data and ask what it does and does not show, or supply a karyogram to interpret. Paper 2 uses *outline* for a species concept, *discuss* for its limitations, and *explain* for why speciation makes boundaries arbitrary. At HL, expect *evaluate* on the chromosome 2 fusion evidence, *explain* on hybrid sterility, and *outline* on eDNA barcoding; state the prediction, the observation, and what it shows.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
DP Science Cafe