Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 253

In a segregating population where genotypes PP and Pp produce the same phenotype (because of dominance), it is necessary, sometimes, to know the exact genotype of a plant. There are two procedures that are commonly used to accomplish this task.

1 Test crossDeveloped by Mendel, a test cross entails crossing the plant with the dominant allele but unknown genotype with a homozygous recessive individual (Figure 5.15). If the unknown genotype is PP, crossing it with the genotype pp will produce all Pp offspring. However, if the unknown is Pp then a test cross will produce offspring segregating 50 : 50 for Pp: pp. The test cross also supports Mendel's postulate that separate genes control purple and white flowers.

2 Progeny testUnlike a test cross, a progeny test does not include a cross with a special parent but selfing of the F2. Each F2 plant is harvested and separately bagged and then, subsequently planted. In the F3, plants that are homozygous dominant will produce progeny that is uniform for the trait, whereas plants that are heterozygous will produce a segregating progeny row.

Figure 5.15 The test cross. Crossing a homozygous dominant genotype with a homozygous recessive genotype always produces all heterozygotes (a). However, crossing a heterozygote with a homozygous recessive produces both homozygotes and heterozygotes.

Plant breeders use the progeny test for a number of purposes. In breeding methodologies in which selection is based on phenotype, a progeny test will allow a breeder to select superior plants from among a genetically mixed population. Following an environmental stress, biotic or abiotic, a breeder may use a progeny test to identify superior individuals and further ascertain if the phenotypic variation is due to genetic effects or just caused by environment factors.