Linked genes and Gene Mapping

Linked Genes:

• Genes that tend to be inherited together and are near each other on the same chromosome

• Results of crosses deviate from normal inheritance patterns (those seen if independently assorted)

Genetic Recombination

Production of offspring with combinations of traits that differ from those found in either parent.  This recombination occurs during crossing over

Parental Types: AB or ab

Recombinants: Ab or aB -- this combination does not match either parent

To calculate the recombinant frequency, you need to cross heterozygous parents with a homozygous recessive individual

P₁  = AABB x aabb = AaBb offspring

F₁ = AaBb x aabb = ¼ AB, ¼ Ab, ¼ aB, ¼ ab if independent assortment

Recombination Data:

Using the recombinant frequency, we can calculate map units and map the distance traits found on a chromosome

1% = 1 map unit

• cn and b recombine = 9% = 9 map units

• cn and vg = 9.5% = 9.5 map units

b and vg = 17% = 17 map units

Exceptions to recombination data:

• There is a maximum value of 50% for genes so far apart that crossing over is virtually certain

  • Despite being on the same chromosome, the genes are genetically unlinked

  • Alleles appear to sort independently, and linkage is not observed

• Linkage maps can portray the order of genes on a chromosome, but it does not accurately portray the precise location of the genes