Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 15.3 Genes Are Linked, but Assort Independently When Far Apart on a Chromosome
Genes located near each other on the same chromosome tend to be inherited together—a phenomenon known as genetic linkage. However, crossing over during meiosis can break this linkage, leading to recombinant offspring that carry new combinations of alleles.
Linked Genes and Inheritance Patterns
- A linked gene is one of a group of genes located close together on the same chromosome
- Linked genes tend to be inherited together in genetic crosses
- Mendel’s law of independent assortment doesn’t apply when genes are on the same chromosome
- Thomas Hunt Morgan’s Drosophila experiments revealed gene linkage between body color and wing size
- Deviations from Mendelian ratios in testcrosses suggested the two genes were inherited as a unit
Genetic Recombination: Parental and Recombinant Types
- Offspring with phenotypes matching either parent are parental types
- Offspring with new trait combinations are recombinants
- A 50% recombination frequency suggests genes are on different chromosomes (unlinked)
- In Morgan’s testcross, 83% of offspring were parental types; 17% were recombinants → genes are linked
- Crossing over during prophase I of meiosis causes recombination of linked genes
Crossing Over and Recombinant Chromosomes
- Crossing over exchanges DNA between nonsister chromatids of homologous chromosomes
- Generates recombinant chromosomes and breaks linkage between genes
- The farther apart two genes are on a chromosome, the more likely crossing over will occur between them
- Recombination frequency is used to estimate gene distance
Mapping Genes Using Recombination Data
- Alfred Sturtevant created the first linkage map, based on recombination frequencies
- One map unit = 1% recombination frequency
- Example map:
- b to cn = 9%
- cn to vg = 9.5%
- b to vg = 17% (slightly less than 9 + 9.5 due to double crossovers)
- Linkage maps show gene order, but not precise physical distances
Limitations and Modern Applications
- Linkage maps are approximate, not based on actual base-pair distances
- Crossing over is not uniform across chromosomes
- Modern genetics uses cytogenetic maps and genome sequencing for more accurate gene positioning
- Genes far apart on the same chromosome may appear unlinked due to frequent crossovers
- Today, whole-genome sequencing gives the most precise genetic maps
Genetic Variation and Evolution
- Independent assortment, crossing over, and random fertilization generate genetic diversity
- Recombinant chromosomes provide new allele combinations for natural selection to act on
- Genetic variation is the raw material for evolution
- Even linked genes can be reshuffled through crossing over, increasing evolutionary flexibility
In a Nutshell
Genes located close together on the same chromosome are linked and tend to be inherited together. However, crossing over during meiosis can separate linked genes, producing recombinants. Recombination frequencies allow geneticists to construct linkage maps and understand chromosome structure and inheritance.