Gene Maps and Selection

This example shows how gene mapping has a practical application. Soybeans have perfect flowers that self-pollinate. Tedious flower manipulation is required to make a hybrid seed. Soybean breeders would like to find ways to make hybrid seeds in commercial quantities. They have discovered a gene that causes a lack of pollen formation and makes plants male-sterile. Therefore, when bees visit these plants they will make a cross-pollination and produce hybrid seed if they carry pollen from a nearby male-fertile plant. A hybrid seed production field could be set up if they could have all male-sterile plants in one row and male-fertile in the next. The genetic control of this trait is shown below:

                                           Male-fertile plants:  MM or Mm       Male-sterile:  mm

Can you see the problem one would have in generating seed that is all male-sterile (genotype mm)? These plants will never be true breeding because they cannot self-pollinate (they have working female parts but no pollen). The only plants that can be self-pollinated to produce mm offspring are Mm plants. Progeny from selfed Mm plants will segregate 3 fertile to 1 sterile, making it impossible to obtain a pure collection of mm seeds to plant in a 'female' row for hybrid seed production. If the mm genotype could be picked out in seeds prior to planting, this would solve the problem. Unfortunately, the male sterile trait is impossible to select for in the seed but breeders discovered a trick they could use that took advantage of their knowledge of gene maps. A second trait in soybean that is easy to select for in seeds is controlled by a gene that was closely linked to the M,m male sterile gene. This trait is green vs. yellow seed coat. The seed coat trait is controlled as follows:

                                                           GG or Gg: green       gg: yellow

The G,g locus and M,m locus have been mapped and are about two map units apart. Therefore we can take advantage of this linkage to select seeds from a cross that will tend to also be male sterile. This is how the process would work. The following cross is made:

                                            GGMM (green, male-fertile)  X  ggmm (yellow, male sterile)

The cis dihybrid (GM/gm) will be male-fertile and can be self-pollinated. The offspring depicted in Fig. 1 will be produced. Because of linkage, the GM and gm parental gametes are made 98% of the time. Therefore, seeds that are gg and yellow are almost always going to be mm and male sterile. If the breeder sorts out the F2 seeds based on color, about one fourth of the seeds will be yellow. Based on the Punnett square (Fig. 1) and the 2 map unit distance, about .24 would be the expected frequency of ggmm out of all the F2s but 96% of the yellow F2s will be male sterile (.24 out of .25). Therefore 96 out of 100 seeds planted in the 'female' row of selected yellow seeds will be male-sterile (mm). Harvesting seeds from these rows will provide a high percentage of hybrid seeds. Thus the breeder took advantage of linkage to select for a trait that was easy to detect (yellow seeds) and also obtain individuals with a trait that was impossible to select ( male-sterile). The power of this prediction potential has driven much of the recent efforts in gene mapping in crop plants, livestock and humans.

Fig. 1. Predicting the percentage of male-sterile F2s among yellow F2 seeds using a chart. Chart is in text form below for additional accessibility options. (Image by D. Lee)

  .49 big G, big M .01 big G, little M .01 little g, big M .49 little g, little m
.49  big G, big M        

.01 big G, little M

.01 big G, little M        

.01 little g, big M

    .0001 .0049

.49 little g, little m

    .0049 .2401 (selected)