In experiments published in 1918 that sought to verify and expand the genetic linkage and recombination theory proposed by Morgan, Thomas Bregger studied potential genetic linkage in corn (Zea mays) for genes controlling kernel color (colored is dominant to colorless) and starch content (starchy is dominant to waxy). Bregger performed two crosses. In Cross 1, pure-breeding colored, starchy-kernel plants (C1 Wx/C1 Wx) were crossed to plants pure-breeding for colorless, waxy kernels (c1 wx/c1 wx). The F₁ of this cross were test-crossed to colorless, waxy plants. The test-cross progeny were as follows:

Phenotype                            Number
Colored, waxy                         310
Colored, starchy                      858
Colorless, waxy                       781
Colorless, starchy                    311
                                               2260

In Cross 2, plants pure-breeding for colored, waxy kernels (C1 wx/C1 wx) and colorless, starchy kernels (c1 Wx/c1 Wx) were mated, and their F₁ were test-crossed to colorless, waxy plants. The test-cross progeny were as follows:

Phenotype                           Number
Colored, waxy                        340
Colored, starchy                     115
Colorless, waxy                        92
Colorless, starchy                   298
                                               845

For each set of test-cross progeny, determine whether genetic linkage or independent assortment is more strongly supported by the data. Explain the rationale for your answer.

Key Concepts

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. This phenomenon occurs because linked genes are less likely to be separated by recombination events. In the context of Bregger's experiments, analyzing the ratios of phenotypes in the progeny can reveal whether the genes for kernel color and starch content are linked or assort independently.

Independent Assortment

Independent assortment is a principle of genetics stating that alleles for different traits segregate independently of one another during gamete formation. This means that the inheritance of one trait does not influence the inheritance of another. In Bregger's test crosses, if the observed phenotypic ratios align closely with expected ratios based on independent assortment, it suggests that the genes are not linked and assort independently.

Test Cross

A test cross involves breeding an individual with a dominant phenotype (but unknown genotype) with a homozygous recessive individual. This method helps determine the genotype of the dominant individual based on the phenotypic ratios of the offspring. In Bregger's experiments, the test crosses provided crucial data to analyze the inheritance patterns of the traits in question, allowing for conclusions about genetic linkage or independent assortment.