Details

Each gene has two alleles for every genetic trait. The map distance between two linked genes is measured in map units or “mu”. The mu between two linked genes equals the percentage of the time crossing over occurs between the two genes. In this model, mu is measured by the percentage of crossover gametes produced by the parents. The smaller the distance between two genes, the less likely that crossing over will occur between them. The mu range is from 0 to 50. There is an upper limit of 50 because a crossover event always results in two non-crossover gametes as well as two crossover gametes. If crossing over occurred 100% of the time, 50% of the gametes produced would be crossover gametes. That resulting progeny would thus be indistinguishable from one with independently assorting genes.

Note that mu can also (and more traditionally) be measured by the percentage of recombinant (or crossover) progeny out of the whole sample. A test cross (heterozygote crossed with recessive homozygote) is very useful for this because only the crossover events from one parent are directly observable in the offspring.

Gene 1 corresponds to the shape of the organism and has alleles A and a. Gene 2 corresponds to the color and has alleles B and b. The relationship of alleles of each gene can be designated as either complete dominance or incomplete dominance. Complete dominance results in the alleles A and B being dominant over alleles a and b, respectively. The dominant phenotype of gene 1 is a square shape and the recessive is a circle. The dominant phenotype of gene 2 is blue and the recessive is green. Incomplete dominance results in heterozygotes having a phenotype that is an intermediate between the two homozygote phenotypes (not to be confused with codominance, where heterozygotes clearly express both homozygote phenotypes). In this model, incomplete dominance of gene 1 and gene 2 results in a heterozygote phenotype of a hexagon shape and teal color, respectively.

The percentages shown above and to the left of the Punnett square are the percentage each gamete is produced by each parent.

All totals are rounded to the nearest integer and all percentages to one decimal place. This may cause values to add up to more or less than 1000 or 100%.

Reference

[1] W. S. Klug, M. R. Cummings, and C. A. Spencer, Essentials of Genetics, 6th ed., Upper Saddle River, NJ: Pearson/Prentice Hall, 2007.