Experiment 1: Colored Sweetcorn Inheritance

The ‘shrunken endosperm’ gene shown on the chromosome map is one of the most commonly used genes to create sweetcorn. The normal version of this gene ‘S’ allows the seed to build starch from sugars in the endosperm cells and make a plump kernel. Plants with the ss genotype will not make starch and accumulate sucrose sugars, making the kernel sweet. Most new sweetcorn varieties will use this shrunken ‘s’ gene combined with other genes important to the grower or consumer. In this experiment, the goal is to create colored sweetcorn for the potential consumer appeal. We will follow the results of this experiment and note where good planning was involved.

Step 1: A genotype (often called a line) of corn that was true breeding for a red kernel color was crossed to a true breeding genotype with shrunken kernels but no red color (white- or yellow-colored kernels). Even though the two parents differed by two traits, kernel color and kernel shape, the F₁ offspring produced from the cross were all red with plump kernels.

Parents: Red, Plump x White, Shrunken

F₁ offspring: all Red, Plump

Quiz

Question

Which phenotypes are dominant based on the parents and F₁ offspring in this cross?

Red color & Plump shape

Red color & Plump shape are dominant for their respective trait (color and shape, respectively). We know this because only Red, Plump seeds were produced in the test cross.

Red color & Shrunken shape

Shrunken shape seeds are not a dominant phenotype.

White color & Plump shape

White colored seeds are not the dominant phenotype.

White color & Shrunken shape

White colored seeds are not the dominant phenotype nor is the shrunken shape.

Looks Good! Red color & Plump shape are dominant for their respective trait (color and shape, respectively). We know this because only Red, Plump seeds were produced in the test cross.

Question

What is the genotype of the Red, Plump F₁ seeds?

CCSs (homozygous dominant CC and heterozygous Ss)

The test cross was homozygous dominant SSCC x homozygous recessive ccss. We shouldn't have a homozygous recessive loci in the F1 generation.

Cc Ss (heterozygous Cc and heterozygous Ss)

The test cross was homozygous dominant CCSS x homozygous recessive sscc. This means all offspring would have to be CcSs because they inherit one version of the gene from each parent.

cc Ss (homozygous cc and heterozygous Ss)

The test cross was homozygous dominant SSCC x homozygous recessive ccss. We shouldn't have a homozygous recessive loci in the F1 generation.

Looks Good! The test cross was homozygous dominant CCSS x homozygous recessive sscc. This means all offspring would have to be CcSs because they inherit one version of the gene from each parent.

Step 2: The next year these Red, Plump (CcSs) F₁ seeds were planted in the crossing nursery near the White, Shrunken (ccss) genotype. When the plants were flowering, a backcross or testcross between the F₁ progeny and White, Shrunken plants was made. This cross generates about 50% shrunken offspring. An important part of this plan was to have about 30 CcSs or ccss plants be females in these crosses to generate thousands of offspring. Why did they know this would be important? Let us follow the experiment progress for the answer.

Step 3: In the fall, the ears on the 30 plants used as females from the cross are harvested. Using Mendel’s genetic principles, what phenotypes would you expect to observe in the testcross progeny? If we use the principle of independent assortment and the information we already have, we would predict that there should be four types of seeds in the testcross progeny (Fig. 5).

Figure 5. *Seed trait inheritance in test cross progeny assuming independent assortment*. Parents were CcSs x ccss. Offspring were Red, Plump; Red, Shrunken; White, Plump; and White, Shrunken and would be expected in equal proportions (¼ for each phenotype). (Redrawn from *Linkage Part 1*, D. Lee, 2001. M. Sutter, 2023, PowerPoint v. 16.7. Redrawn with permission)

We should get offspring with the Red, Plump and White, Shrunken traits combination of traits that matches the parents. We should also get two new combinations of seed and color phenotypes: Red, Shrunken and White, Plump. Furthermore, the principle of independent assortment predicts the four combinations should be in a 1:1:1:1 ratio. If our goal is to obtain the colored shrunken combination for colored sweetcorn, we would predict about ¼ of the seeds on each ear from the backcross or testcross would have this desired trait combination.

What would we actually observe? When we harvest the ear from the first plant, peel back the husks, and examine the kernels, we could very likely observe only two types of seeds; Red, Plump and White, Shrunken seeds (Fig. 6). This means only the parental combination of traits is found among the seeds on that ear! The goal to obtain red seeds with the shrunken trait is not met from this one cross even though there might be 200-300 offspring from this one cross.

This result does not fit what we expect based on the idea of independent traits, but it is not a surprise to a maize geneticist like David Holding who knows the genetics story shared in the published maize research studies.

Figure 6. An instance of Colored vs. White tending to be passed on the phenotype of Shrunken vs. Plump. Colored (in this case purple) kernels tend to be plump and shrunken kernels tend to be white. *Image of* *shrunken2 phenotype from maizegdb.org*, Woodhouse MR, Cannon EK, Portwood JL, Harper LC, Gardiner JM, Schaeffer ML, Andorf CM. (2021) A pan-genomic approach to genome databases using maize as a model system. BMC Plant Biol 21, 385. doi: https://doi.org/10.1186/s12870-021-03173-5.

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