Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 251

The term genotype is used to describe the totality of the genes of an individual. Because the totality of an individual's genes is not known, the term, in practice, is usually used to describe a very small subset of genes of interest in a breeding program or research. Conventionally, a genotype is written with an uppercase letter (H, G) indicating the dominant allele (expressed over the alternative allele), while a lower case letter (h, g) indicates the recessive allele. A plant that has two identical alleles for genes is homozygous at that locus (e.g. AA, aa, GG, gg) and is called a homozygote. If it has different alleles for a gene, it is heterozygous at these loci (e.g. Aa, Gg) and is called a heterozygote. Certain plant breeding methods are designed to produce products that are homozygous (breed true – most or all of the loci are homozygous) whereas others (e.g. hybrids) depend on heterozygosity for success.

The term phenotype refers to the observable effect of a genotype (the genetic makeup of an individual). Because genes are expressed in an environment, a phenotype is the result of the interaction between a genotype and its environment (i.e. phenotype = genotype + environment, or symbolically, P = G + E). In Chapter 23, a more complete form of this equation will be introduced as P = G + E + GE + error, where GE represents the interaction between the environment and the genotype. This interaction effect helps plant breeders in the cultivar release decision making process (see Chapter 23).