linked genes
Summary
Linked genes are loci close together on the same chromosome, so they tend to be inherited together instead of assorting independently. Crossing over can still separate them, and the recombination frequency stays below 50% and rises with distance.
Detail
Meaning: Linked genes are loci on the same chromosome close enough together to be inherited together more often than chance predicts. They violate Mendel's law of independent assortment, which holds for genes on different chromosomes or far apart on one. Crossing over in prophase I can separate linked alleles and produce recombinant gametes, and a crossover is more likely between genes that are farther apart.
Equation: Recombination frequency (%) = (recombinant offspring ÷ total offspring) × 100, where both counts are numbers of individuals (dimensionless) and the result is a percentage. 1% recombination = 1 map unit = 1 centimorgan.
On the MCAT: A passage gives a testcross (AaBb × aabb) with offspring counts. Independent assortment predicts 1:1:1:1; linked genes give mostly parental phenotypes and fewer recombinant ones. You may compute map distance or order three genes: the two with the largest distance between them are the outer pair.
Common confusion: Linked does not mean permanently locked; crossing over can separate the alleles. Recombination frequency rises with distance but levels off near 50%, the value for unlinked genes. Linked is also not the same as sex-linked, which means the gene lies on the X or Y chromosome.
Sources
- Campbell Biology
- Genetics: From Genes to Genomes
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.