Chromosomal Inheritance and Genetic Mapping
Genes on Chromosomes
Linkage
Genes that are close together on the same chromosome tend to be inherited together. This is called linkage and it violates independent assortment for those loci.
Recombination Frequency
Recombination frequency estimates how often crossing over separates two loci. One map unit, or centimorgan, corresponds to approximately recombination frequency.
Mapping Logic
The farther apart two genes are, the more likely a crossover will occur between them, up to a practical limit where frequencies approach .
Mapping Terms
| Term | Meaning |
|---|---|
| Linked genes | Genes on the same chromosome inherited together more often than expected |
| Recombination frequency | Percent of offspring with new allele combinations |
| Centimorgan | Approximate map unit equal to recombination |
A recombination frequency of implies a map distance of about
Map distance is approximately equal to recombination percentage for moderate distances.
Correct answer: 12 cM
Why can recombination frequency never exceed for two loci?
Multiple crossovers can mask true distances, but observed recombination approaches .
Correct answer: Because at that point the loci behave as if they assort independently.
Linked genes are most likely to be inherited together when they are
Physical proximity lowers the chance of crossover between the loci.
Correct answer: Close together on the same chromosome
Tetrad Analysis and Testcrosses
Testcrosses and tetrad analysis are common methods for estimating linkage and gene order. They use observed offspring classes to infer the arrangement of alleles in gametes.
What kind of cross is often used to reveal linkage in a heterozygote?
Recombinant and parental gametes become visible in the offspring.
Correct answer: A testcross with a homozygous recessive individual.