Part II

A dominant gene may be located on one of the 21 chromosomes of hexaploid wheat by crossing euploid wheat that carries the dominant gene to a series of nullisomic wheat lines. Each genetic stock is nullisomic for a different wheat chromosome. The expected 3:1 segregation ratio will be observed for each nullisomic genetic stock, with the exception of the one genetic stock that is nullisomic for the chromosome on which the gene is located.

Sears(1953) explains:

"Ordinary dominant genes in varieties other than Chinese are located by crossing to each of the 21 nullisomics and observing the ratios obtained in the F₂ populations. With a simple dominant gene, the critical family, instead of segregating the customary 25% recessives, usually yields only 1 to 5 percent depending on the particular chromosome concerned, with 10% being about the maximum. The deviation from the usual 3:1 ratio is due to the fact that both disomics and monosomics carry the dominant allele, in double and single dose, respectively, and only the nullisomics are free to express the non-dominant or recessive characteristic. As has been previously noted, nullisomics normally appear in frequencies of only 1 to 10 percent among the offspring of monosomics."