Biology · Heredity, Variation & Evolution
Mutation, recombination and sources of variation
Changing one letter in a DNA fragment differs from exchanging sections between matching chromosomes. Both can contribute to variation, but they change different records. A third record matters too: whether the change reaches the hereditary material transmitted to descendants.
The earlier lessons on biological scales and cells provide useful background: recognise a cell, its DNA and the difference between an individual and a population. The DNA and nucleotide explanation in Chemistry of Life is optional support. You need fractions such as one half and one quarter; the worked cases explain the calculations before using them. Detailed molecular copying, Mendelian inheritance and evolutionary inference return in later chapters.
Back to the Biology mapBy the end, you can
Separate a new sequence change from rearrangement of existing variants, and explain what random mutation means with respect to usefulness.
A sequence change does not carry a purpose
A mutation is a change in DNA sequence. It can arise through copying or repair errors or through other processes affecting DNA. Some changes influence a trait or function; others have little or no detectable effect in the conditions being studied.
Our invented fragment changes from ACGTA to ACTTA: the third position changes from G to T. That establishes a sequence difference. Without knowing the fragment's genomic context or what it does, the drawing establishes no changed protein, visible trait or fitness effect.
Mutation is random with respect to what would help an organism meet a challenge. A need for a useful variant does not direct a mutation to supply that particular solution. This meaning of random does not assert equal probabilities for all positions and all types of sequence change. Environmental exposures can affect mutation processes without anticipating which change would be beneficial.
One altered DNA position
Read the diagram in words
The third base changes from G to T; the other four supplied positions match.
| Item | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Before sequence change | A | C | G | T | A |
| After sequence change | A | C | T | T | A |
Position 3: G → T
Rearrange existing alleles
Recombination can exchange DNA between homologous chromosomes. In the drawn case, one chromosome carries alleles A and B at two marked loci and its homologue carries a and b. An exchange between those loci produces chromatids carrying Ab and aB alongside unchanged AB and ab chromatids.
The marked alleles themselves have not changed in this model. What has changed is their combination along the chromosome. Exchanges within a locus and other molecular events can have more complicated effects; the simple diagram specifically places its exchange between the two loci.
Independent assortment is another source of new combinations: different homologous chromosome pairs can be distributed in different combinations during meiosis. Fertilisation combines parental contributions as well. These processes shuffle inherited material; they do not require a new mutation at every represented locus.
An exchange between loci rearranges existing alleles
Read the diagram in words
One nonsister-chromatid exchange between the loci produces Ab and aB while AB and ab remain. A, a, B and b each occur twice in the full four-chromatid inventory before and after.
| Item | Relative drawing coordinate |
|---|---|
| A/a locus | 25 |
| B/b locus | 75 |
| Exchange between the two loci | 50 |
0–100
| Chromatid identity | Before exchange: A/a locus | Before exchange: B/b locus | After exchange: A/a locus | After exchange: B/b locus |
|---|---|---|---|---|
| c1 | A | B | A | B |
| c2 | A | B | A | b |
| c3 | a | b | a | B |
| c4 | a | b | a | b |
Exchange between the two loci: c2 ↔ c3
Trace a change through the reproductive pathway
A sequence change contributes to hereditary differences among descendants only if it enters material transmitted to them. In a familiar animal example, a mutation confined to a parent's skin-cell lineage is not normally passed through that parent's eggs or sperm. A variant present in a contributing gamete can be passed on.
That skin-cell rule is not a universal rule for all reproduction. A single-celled organism can pass a copied variant to daughter organisms, and reproduction from a body fragment or cutting follows a different pathway. Ask which cells or parts actually found the next generation.
Population variation also reflects movement of inherited variants between populations through reproducing migrants or gametes. This gene flow transfers existing variants; it is different from the original sequence change that produced an allele.
Worked example
Describe what changed before predicting its effect
Compare the five positions of ACGTA and ACTTA in order: the first, second, fourth and fifth match; the third differs. The count is one substitution in this fragment. Next compare AB and Ab in the exchange model: the letters refer to alleles at two loci, not four DNA nucleotides. Mutation and recombination are therefore not interchangeable labels for a single 'letter swap'. Neither small drawing says whether a descendant will reproduce more successfully.
Check your understanding
Does the five-base substitution demonstrate a useful new trait?
Show reasoning
No. It demonstrates a sequence difference only. A trait or usefulness claim requires genomic, functional and environmental information.
Why is Ab a new combination in the pictured exchange without a new mutation at either marked locus?
Show reasoning
A and b already existed on the parental homologues. The exchange between their loci places those existing alleles together.
Does saying mutation is random mean the environment cannot influence mutation processes?
Show reasoning
No. It means usefulness does not direct the process to supply a needed variant. Environmental exposures can affect processes or probabilities without predicting future benefit.
What to carry forward
Track sequence changes, allele combinations and actual transmission separately.
References
Resources supporting this lesson’s explanations and examples.
- Talking Glossary — Genes, alleles and inheritance terminologyNational Human Genome Research InstituteScientific glossaryTalking Glossary — Genes, alleles and inheritance terminologyAlleleChromosomeGenotypePhenotypePolygenic traitMutationGenomeLocusMitochondrial DNA
- Understanding Evolution — Mutations (Reviewed and updated June 2020)University of California Museum of Paleontology · University of California, BerkeleyUniversity teaching resource
Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStax2 sources