What is Genetic Variation?
Genetic variation refers to the differences in DNA sequences among individuals within a population. It arises from mutation, sexual reproduction, and gene flow, and it is the raw material that natural selection acts upon.
Genetic variation is the diversity in gene sequences between individuals of the same species, caused mainly by mutations, meiotic recombination, and random fertilization.
- 1↓MutationRandom changes in DNA sequence (point, insertion, deletion) create new alleles.
- 2↓Crossing OverHomologous chromosomes exchange segments during meiosis I, creating new allele combinations.
- 3↓Independent AssortmentHomologous pairs line up randomly at metaphase I, shuffling maternal and paternal chromosomes.
- 4Random FertilizationAny sperm can fuse with any egg, multiplying the possible genetic combinations in offspring.
Step-by-step worked examples
A single nucleotide change turns a codon from GAG (glutamic acid) to GTG (valine), causing sickle-cell hemoglobin. What type of mutation is this and what is its effect?
This is a point (substitution) mutation — one base pair is changed. Because it changes only one amino acid, it is a missense mutation. The altered protein (hemoglobin S) folds abnormally, causing red blood cells to sickle.
Two homologous chromosomes carrying alleles Ab and aB cross over. What new chromosome combinations can result?
Before crossing over: chromosome 1 = Ab, chromosome 2 = aB. Crossing over exchanges segments between non-sister chromatids. After crossing over, recombinant chromosomes AB and ab can form alongside the original Ab and aB — four possible gamete genotypes.
A human has 23 pairs of chromosomes. Ignoring crossing over, how many chromosomally distinct gametes can one person produce through independent assortment alone?
Each of the 23 homologous pairs can independently send either the maternal or paternal chromosome to a gamete. Number of combinations = 2^23. 2^23 = 8,388,608 possible distinct gametes.
Flashcards
Quick quiz
Q1.Which of the following is NOT a source of genetic variation?
Q2.A mutation that changes one amino acid in a protein is called a…
Q3.During which phase of meiosis does crossing over occur?
Q4.What is the main evolutionary significance of genetic variation?
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Common mistakes
Mutation is always harmful. — Correct: Most mutations are neutral, some are harmful, and a few are beneficial.
Mitosis creates genetic variation like meiosis does. — Correct: Mitosis produces genetically identical daughter cells; only meiosis (plus mutation) creates variation.
Genetic variation and genetic drift are the same thing. — Correct: Variation is the diversity of alleles; drift is a random change in allele frequencies over generations.
All mutations change the amino acid sequence of a protein. — Correct: Silent mutations don't change the amino acid due to codon redundancy.
FAQ
What is genetic variation?
It is the natural variety of DNA sequences and traits found among individuals of the same species, arising from mutation and recombination.
What causes genetic variation?
Mutation, crossing over, independent assortment during meiosis, and random fertilization are the main causes.
What are examples of genetic variation?
Human blood types, eye color, and sickle-cell hemoglobin are classic examples of genetic variation caused by different alleles.
How does mutation lead to genetic variation?
Mutation changes the DNA sequence directly, creating new alleles that did not exist before in the population.