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

Merge the two sets of progeny data and determine the combined recombination frequency.

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. Understanding genetic linkage is crucial for predicting the inheritance patterns of traits, as it affects the ratios of phenotypes observed in offspring.

Recombination Frequency

Recombination frequency is a measure of the likelihood that two genes will be separated during meiosis due to crossing over. It is calculated by dividing the number of recombinant offspring by the total number of offspring, often expressed as a percentage. This frequency helps in constructing genetic maps and understanding the distance between genes on a chromosome.

Test Cross

A test cross is a breeding experiment used to determine the genotype of an individual exhibiting a dominant phenotype. This is achieved by crossing the individual with a homozygous recessive individual. The phenotypic ratios of the offspring provide insights into whether the dominant individual is homozygous or heterozygous for the trait in question, aiding in genetic analysis.