Q.What is 'Saltation' according to de Vries ?
Concept understanding — Morgan Fruit Fly Experiments
Morgan's Fruit Fly Experiments: A First Look
Imagine you're trying to understand how traits pass from parents to children. You could study humans, but that takes decades — one generation every 25 years. What if you could watch a hundred generations in a single year? That's exactly what Thomas Hunt Morgan realised when he picked the fruit fly, Drosophila melanogaster, for his experiments around 1910.
Why Fruit Flies?
Morgan needed an organism that was cheap, fast-breeding, and easy to handle. Fruit flies fit perfectly. They breed every 10–14 days, produce hundreds of offspring, and have just four pairs of chromosomes — simple enough to track. More importantly, they show clear, visible traits: eye colour (red or white), wing shape (normal or vestigial), body colour (grey or black). These traits became Morgan's window into how genes work.
The Breakthrough: The White-Eyed Male
Morgan started by breeding normal red-eyed flies. Then, by chance, he found a single male with white eyes — a mutant. He crossed this white-eyed male with a red-eyed female. All the offspring (the F1 generation) had red eyes. That told him red eye colour was dominant over white.
Then came the crucial step. He bred those F1 red-eyed flies with each other. In the F2 generation, he got a surprise: all the females had red eyes, but half the males had white eyes. The white-eye trait appeared only in males.
This was the first clear experimental evidence that a specific gene is located on a specific chromosome — in this case, the X chromosome. The white-eye gene is on the X chromosome, and males have only one X, so they express whatever is on it. Females have two X's, so a recessive white-eye gene can be hidden by a dominant red-eye gene on the other X.
What Morgan Proved
Before Morgan, scientists knew chromosomes existed, but no one had proven that a particular gene lived on a particular chromosome. Morgan's fruit fly experiments did exactly that. He showed that:
- Genes are physical units located on chromosomes
- The inheritance of a trait (white eyes) follows the inheritance pattern of a specific chromosome (the X)
- Sex-linked traits — those carried on the X chromosome — behave differently in males and females
This was the birth of the chromosomal theory of inheritance. It connected Mendel's abstract "factors" (genes) to real, visible structures inside cells.
Why It Matters for a Commerce/Humanities Student
You might think this is pure biology, but the principle here is about evidence linking a cause to an effect. Morgan didn't just guess that genes were on chromosomes — he designed an experiment where the pattern of inheritance forced that conclusion. This is the same logic used in economics (linking policy changes to market outcomes) or law (linking evidence to a verdict). The method matters as much as the result.
NCERT Class 12 Biology (Chapter 5, Principles of Inheritance and Variation) covers Morgan's work under "Linkage and Recombination." The textbook emphasises that Morgan's experiments with fruit flies established the concept of linkage — genes on the same chromosome tend to be inherited together — and recombination — crossing over during meiosis can break that linkage.
The Big Picture
Morgan's fruit flies gave us three things:
- Proof that genes sit on chromosomes
- A model system that scientists still use today (fruit flies are everywhere in genetics labs)
- The concept of sex linkage, which explains why some traits (like colour blindness in humans) are more common in males
For your exam, remember the core story: a white-eyed male, a careful breeding experiment, and the conclusion that the gene for eye colour lives on the X chromosome. That single experiment changed biology forever.
Morgan's fruit fly experiments are a standard part of the NCERT Class 12 Biology chapter on inheritance, and CBSE/NEET aspirants commonly search for "Morgan's experiment on Drosophila class 12" or "linkage and recombination important questions" while revising this topic. Understanding how this experiment established the chromosomal theory of inheritance is essential groundwork for questions on sex-linked inheritance in board exams and NEET.
Concept: Hugo de Vries's mutation theory and the mechanism of evolution.
De Vries, working with the evening primrose Oenothera lamarckiana, observed sudden, large, heritable variations appearing in a single generation. He termed these abrupt changes saltation (from Latin saltare, "to leap"). Unlike Darwin's gradual accumulation of small variations, de Vries proposed that new species arise discontinuously through these large mutations.
He argued that:
- Evolution proceeds by sudden jumps, not slow continuous change.
- These mutations are random, directionless, and appear without environmental influence.
- A single saltation could produce a new species immediately, bypassing intermediate forms.
Though later work showed many of his Oenothera "mutations" were actually chromosomal aberrations (not point mutations), the concept introduced the importance of mutations to evolutionary theory.
According to de Vries, saltation refers to sudden, large, discontinuous heritable variations (mutations) that produce new species in a single step, contrasting with gradual Darwinian evolution.
De Vries proposed that evolution occurs through saltation — large, discontinuous jumps producing new species in a single generation, rather than gradual accumulation of small changes.
Hugo de Vries, working at the turn of the 20th century, challenged Darwin's gradualism with observations that seemed revolutionary at the time. To understand saltation, we need to see what puzzled him about the prevailing view.
Darwin's theory rested on slow, continuous variation — tiny changes accumulating over vast stretches of time until populations diverged into new species. But de Vries, experimenting with the evening primrose Oenothera lamarckiana, observed something startling: occasional offspring that looked dramatically different from their parents, breeding true in subsequent generations. These weren't minor tweaks; they were large, discrete morphological leaps.
De Vries's concept of saltation
Saltation (from Latin saltare, "to leap") refers to evolution by sudden, large mutations that produce new species instantaneously — in a single generation. De Vries called these dramatic variants mutants and argued they represented the true mechanism of speciation.
His key claims were:
-
Discontinuous variation is the raw material of evolution. Small, continuous variations (what Darwin emphasized) merely produce varieties within a species but cannot cross the threshold into a new species.
-
Mutations are large-effect changes. A single mutational event creates an organism so different it constitutes a new species immediately, without intermediate forms.
-
Species arise in single steps. There is no gradual transition — one generation belongs to the parent species, the next to a new one.
-
Natural selection acts only as a sieve. It eliminates unfit mutants but does not create novelty; the mutation itself does all the creative work.
De Vries's "mutations" in Oenothera turned out to be mostly chromosomal aberrations (aneuploids, translocations) rather than true gene mutations. The evening primrose has a complex, unusual genetic system that made it a misleading model organism.
Why saltation appealed (and why it faded)
The appeal was clear: saltation explained the absence of intermediates in the fossil record and bypassed the problem of how a half-formed structure could be advantageous. If wings or eyes appeared in one leap, there was no need to explain the survival value of a 5%-functional wing.
But the modern synthesis (1930s–1940s) reconciled Mendelian genetics with Darwinian gradualism. Population geneticists like Fisher, Haldane, and Wright showed mathematically that small mutations, filtered by selection over many generations, could account for all observed evolutionary patterns. Large-effect mutations are almost always deleterious; evolution proceeds through the accumulation of small genetic changes, even if the phenotypic result sometimes appears sudden (punctuated equilibrium is not saltation — it still involves many generations).
According to de Vries, saltation is evolution by large, discontinuous mutations that produce new species in a single generational leap, without gradual intermediates.
Showing the 12 most recent of 14 on this concept.
- CBSE 2025Set 57/5/11 markMCQQ.Assertion (A) : Gene pairs present on the same chromosome may be tightly linked or loosely linked. Reason (R) : Frequency of recombination between gene pairs on different chromosomes as a measure of the distance between genes can be used for 'mapping' their position on the chromosomes.
›Reveal solutionSolution
Assertion (A) is true because genes on the same chromosome can be tightly or loosely linked depending on their distance. Reason (R) is false because recombination frequency is used to map genes on the same chromosome, not different chromosomes.
Concept and Intuition
This question tests your understanding of gene linkage and gene mapping. Genes located on the same chromosome are called linked genes. They tend to be inherited together because they are physically connected. However, this inheritance is not absolute due to a process called crossing over (or recombination) during meiosis.
- Tight Linkage: If two genes are very close to each other on the same chromosome, the probability of a crossover event occurring between them is low. This means they are "tightly linked" and are very likely to be inherited together.
- Loose Linkage: If two genes are farther apart on the same chromosome, the probability of a crossover event occurring between them is higher. This means they are "loosely linked" and there's a greater chance they will be separated during meiosis, leading to recombination.
The frequency of recombination (the percentage of recombinant offspring) is directly proportional to the physical distance between linked genes on a chromosome. This relationship is fundamental to gene mapping, where geneticists use recombination frequencies to determine the relative positions of genes and construct genetic maps. It's crucial to remember that this principle applies to genes on the same chromosome. Genes on different chromosomes assort independently, and their recombination frequency is typically 50%, which doesn't reflect a physical distance on a chromosome.
Step-by-step Evaluation
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Analyze Assertion (A): Gene pairs present on the same chromosome may be tightly linked or loosely linked.
- This statement is true. As explained above, the degree of linkage between genes on the same chromosome depends on the physical distance separating them. Genes that are close together are tightly linked, meaning they have a low recombination frequency and are often inherited together. Genes that are farther apart are loosely linked, meaning they have a higher recombination frequency due to a greater chance of crossing over between them.
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Analyze Reason (R): Frequency of recombination between gene pairs on different chromosomes as a measure of the distance between genes can be used for 'mapping' their position on the chromosomes.
- This statement is false. The core principle of gene mapping is that the frequency of recombination is a measure of the distance between genes on the same chromosome. Genes located on different chromosomes assort independently. For independently assorting genes (on different chromosomes), the recombination frequency is always 50% (due to random alignment and segregation of homologous chromosomes), which does not provide information about their physical distance or allow for mapping their positions relative to each other on a chromosome. Recombination frequency as a measure of distance is specifically for linked genes.
-
Conclusion:
- Assertion (A) is true.
- Reason (R) is false.
✓Final answerAssertion (A) is true, but Reason (R) is false.
- CBSE 2025Set F1 markMCQQ.What is the number of linkage groups in human being?(a) 46(b) 22(c) 92(d) 23
›Reveal solutionSolution
Linkage groups = haploid chromosome number; in humans that is 23.
All the genes located on one chromosome are inherited together and form a linkage group. Therefore the number of linkage groups in a species equals its haploid (n) number of chromosomes — the number of different chromosome types.
Humans have 46 chromosomes (2n) = 23 pairs, so the haploid number is 23. Hence there are 23 linkage groups (22 autosomal + 1 for the sex chromosomes).
✓Final answer(D) 23.
- CBSE 2024Set ANNUAL1 markMCQQ.Lack of independent assortment of two genes 'A' and 'B' in fruit fly Drosophila is due to(a) Repulsion(b) Recombination(c) Linkage(d) Crossing over
›Reveal solutionSolution
Genes located close together on the same chromosome show linkage, deviating from independent assortment.
Morgan's classic experiments with the fruit fly Drosophila melanogaster demonstrated that when two genes are located on the same chromosome, they tend to be inherited together (rather than assorting independently, as predicted by Mendel's Law of Independent Assortment for genes on different chromosomes) — a phenomenon he termed linkage. Genes on the same chromosome that show a strong tendency to stay together during inheritance are said to be "tightly linked," while genes farther apart on the same chromosome show more crossing over and behave more like independently assorting genes ("loosely linked"). Recombination (and crossing over, the physical process that produces it) is what breaks up linkage, generating new (recombinant) combinations of alleles; repulsion is a specific linkage phase where dominant and recessive alleles of two genes are on opposite homologous chromosomes.
✓Final answer(c) Linkage
- CBSE 2023Set ANNUAL1 markQ.What is crossing over?
›Reveal solutionSolution
Crossing over reshuffles alleles between homologous chromosomes during meiosis, and is a key source of genetic recombination/variation.
Crossing over occurs during the pachytene stage of prophase I of meiosis, after homologous chromosomes have paired up (synapsis) to form a bivalent/tetrad. At specific points called chiasmata, non-sister chromatids of the two homologous chromosomes break and rejoin, exchanging corresponding segments of DNA. This process shuffles the combination of alleles present on each chromosome, generating new (recombinant) allele combinations in the gametes that were not present in either parent chromosome — one of the principal sources of genetic variation.
✓Final answerCrossing over is the exchange of segments of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis, producing new (recombinant) combinations of alleles.
- CBSE 2023Set ANNUAL1 markMCQQ.Crossing over is a characteristic of which stage?(a) leptotene(b) zygotene(c) pachytene(d) diakinesis
›Reveal solutionSolution
Crossing over occurs at the pachytene stage.
During prophase-I of meiosis, homologous chromosomes pair (synapsis) at zygotene, and at pachytene the paired homologues (bivalents) undergo crossing over — reciprocal exchange of segments between non-sister chromatids, mediated by recombination nodules. The points of exchange (chiasmata) become visible later at diplotene. Thus the actual crossing over is a feature of the pachytene stage.
✓Final answer(c) pachytene.
- CBSE 2021Set D1 markMCQQ.An exception of Mendel's law is(a) Dominance(b) Purity of gamete(c) Linkage(d) Independent assortment
›Reveal solutionSolution
Linkage violates Mendel's law of independent assortment, so the answer is (C).
Mendel's law of independent assortment states that alleles of different genes assort independently during gamete formation. This holds true only when the genes are located on different chromosomes.
When two or more genes lie close together on the same chromosome, they tend to be inherited together and do not assort independently — this phenomenon is called linkage. Linkage was discovered by Morgan and is a well-known exception to Mendel's law of independent assortment. Dominance and purity of gametes are Mendelian principles, and independent assortment is the law itself.
✓Final answer(C) Linkage.
- CBSE 2021Set D1 markMCQQ.In which stage does crossing-over take place ?(a) Leptotene(b) Cytokinesis(c) Pachytene(d) Diakinesis
›Reveal solutionSolution
Crossing-over takes place in the pachytene stage of meiosis I.
During prophase I of meiosis the homologous chromosomes pair (synapsis) at zygotene. In the next stage, pachytene, the paired homologues (bivalents) develop recombination nodules and the enzyme recombinase brings about crossing-over — the reciprocal exchange of genetic material between non-sister chromatids. The points of exchange appear later as chiasmata at diplotene. Leptotene is the earliest condensation stage, cytokinesis is cytoplasmic division, and diakinesis is terminalisation of chiasmata — none of these is when crossing-over occurs.
✓Final answer(C) Pachytene.
- CBSE 2019Set 57/3/11 markQ.What is 'Saltation' according to de Vries ?
›Reveal solutionSolution
De Vries proposed that evolution occurs through saltation — large, discontinuous jumps producing new species in a single generation, rather than gradual accumulation of small changes.
Hugo de Vries, working at the turn of the 20th century, challenged Darwin's gradualism with observations that seemed revolutionary at the time. To understand saltation, we need to see what puzzled him about the prevailing view.
Darwin's theory rested on slow, continuous variation — tiny changes accumulating over vast stretches of time until populations diverged into new species. But de Vries, experimenting with the evening primrose Oenothera lamarckiana, observed something startling: occasional offspring that looked dramatically different from their parents, breeding true in subsequent generations. These weren't minor tweaks; they were large, discrete morphological leaps.
De Vries's concept of saltation
Saltation (from Latin saltare, "to leap") refers to evolution by sudden, large mutations that produce new species instantaneously — in a single generation. De Vries called these dramatic variants mutants and argued they represented the true mechanism of speciation.
His key claims were:
-
Discontinuous variation is the raw material of evolution. Small, continuous variations (what Darwin emphasized) merely produce varieties within a species but cannot cross the threshold into a new species.
-
Mutations are large-effect changes. A single mutational event creates an organism so different it constitutes a new species immediately, without intermediate forms.
-
Species arise in single steps. There is no gradual transition — one generation belongs to the parent species, the next to a new one.
-
Natural selection acts only as a sieve. It eliminates unfit mutants but does not create novelty; the mutation itself does all the creative work.
Watch outDe Vries's "mutations" in Oenothera turned out to be mostly chromosomal aberrations (aneuploids, translocations) rather than true gene mutations. The evening primrose has a complex, unusual genetic system that made it a misleading model organism.
Why saltation appealed (and why it faded)
The appeal was clear: saltation explained the absence of intermediates in the fossil record and bypassed the problem of how a half-formed structure could be advantageous. If wings or eyes appeared in one leap, there was no need to explain the survival value of a 5%-functional wing.
But the modern synthesis (1930s–1940s) reconciled Mendelian genetics with Darwinian gradualism. Population geneticists like Fisher, Haldane, and Wright showed mathematically that small mutations, filtered by selection over many generations, could account for all observed evolutionary patterns. Large-effect mutations are almost always deleterious; evolution proceeds through the accumulation of small genetic changes, even if the phenotypic result sometimes appears sudden (punctuated equilibrium is not saltation — it still involves many generations).
✓Final answerAccording to de Vries, saltation is evolution by large, discontinuous mutations that produce new species in a single generational leap, without gradual intermediates.
-
- CBSE 2019Set HE1 markQ.Fill in the blank: Drosophila melanogaster work was done by ______.
›Reveal solutionSolution
Detailed genetic work on Drosophila melanogaster was carried out by Thomas Hunt Morgan, establishing the chromosomal theory of inheritance.
Thomas Hunt Morgan chose Drosophila melanogaster (the common fruit fly) as his experimental organism because it could be grown on simple synthetic medium in the laboratory, completed its life cycle in about two weeks, had a short reproductive cycle producing hundreds of offspring in one mating, showed clear sex differences between males and females, and had many easily visible hereditary variations (e.g., body colour, eye colour, wing shape). Using Drosophila, Morgan and his group demonstrated that genes are located on chromosomes, worked out the linkage between genes located on the same chromosome, and studied crossing over — establishing the chromosomal theory of inheritance, work for which Morgan later won the Nobel Prize (1933).
✓Final answerThomas Hunt Morgan.
- CBSE 2019Set ANNUAL1 markMCQQ.Morgan's experiments were on(a) housefly(b) fruit fly(c) sand fly(d) mosquito
›Reveal solutionSolution
Morgan's foundational genetics experiments used the fruit fly, Drosophila melanogaster.
Thomas Hunt Morgan and his colleagues used the fruit fly, Drosophila melanogaster, as their experimental organism because it is easy to grow in the laboratory on simple synthetic medium, completes its life cycle in about two weeks, produces a large number of offspring, shows clear-cut hereditary variations (e.g., red vs white eye colour), and has just four pairs of chromosomes that are easy to distinguish morphologically. Through breeding experiments (notably the white-eye/red-eye X-linked trait), Morgan provided the first solid experimental evidence that genes are located on chromosomes, and went on to establish the principles of linkage and recombination.
Housefly (a), sand fly (c) and mosquito (d) were not Morgan's experimental subjects.
✓Final answer(b) Fruit fly.
- CBSE 2019Set ANNUAL1 markQ.Name the scientist who coined the term 'linkage'.
›Reveal solutionSolution
T. H. Morgan, working on Drosophila, coined the term 'linkage' to describe genes located on the same chromosome that tend to be inherited together.
While Bateson and Punnett first observed (in sweet pea, 1906) that some gene combinations did not assort independently as Mendel's laws predicted, it was Thomas Hunt Morgan, through his extensive breeding experiments on the fruit fly Drosophila melanogaster, who explained this phenomenon and coined the term 'linkage' for genes situated on the same chromosome that tend to be inherited together, and 'recombination' for the generation of non-parental gene combinations via crossing over.
✓Final answerThomas Hunt (T. H.) Morgan.
- CBSE 2019Set ANNUAL1 markQ.Genetic exchange takes place in ____ phase.
›Reveal solutionSolution
Crossing over — the exchange of chromosome segments between non-sister chromatids of homologous chromosomes — occurs at the pachytene sub-stage of meiotic prophase I, and is the physical basis of genetic recombination.
Meiotic prophase I is divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
-
In zygotene, homologous chromosomes pair up (synapsis) to form bivalents/tetrads.
-
In pachytene, recombination nodules appear along the paired chromosomes; at these points, non-sister chromatids break and rejoin, physically exchanging segments of genetic material between the maternal and paternal homologues.
-
This crossing over creates new combinations of alleles on each chromatid, which is later visible as chiasmata in diplotene, and is a major source of genetic variation in the gametes produced.
✓Final answerPachytene stage (of meiotic prophase I).
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