AP Syllabus focus:
‘Genes located on the same chromosome are genetically linked, and recombination frequencies estimate map distances between them.’
Genetic linkage explains why some traits are inherited together more often than expected by chance. Gene mapping uses recombination data from meiosis to infer the relative order of genes and estimate distances along chromosomes.
Genetic linkage: the core idea
Linked genes vs. independent assortment
Genes on the same chromosome can be transmitted together because they share a physical DNA molecule. When genes are far apart, crossing over can separate them, making them behave more independently.
Genetic linkage: The tendency of alleles of different genes located on the same chromosome to be inherited together more often than expected by independent assortment.
Practice Questions
FAQ
Interference means one crossover can reduce the probability of another nearby crossover.
This can change the expected frequencies of double crossovers, making long-distance estimates less reliable unless interference is accounted for.
With three genes, comparing single- and double-recombinant classes helps distinguish between possible orders.
The least frequent classes often correspond to double crossovers, which can reveal which gene lies in the middle.
Yes. Some species show sex-specific recombination rates (including cases where recombination is reduced or absent in one sex).
As a result, genetic maps may differ depending on which sex produces the gametes measured.
Genetic distance (cM) reflects recombination likelihood; physical distance is measured in base pairs.
Because recombination hotspots and cold spots exist, the relationship between cM and base pairs is not constant across the genome.
They may use alternative markers and approaches, such as:
Increasing sample size to detect rare recombinants
Using tightly linked molecular markers nearer the locus
Complementing genetic data with physical mapping (e.g., sequencing-based methods)
