Chapter 3 - Review of Genetics (From The Perspective of A Plant Breeder)

Abstract

A solid grasp of the principles of genetics is essential to achieve success in plant breeding. First and foremost is a thorough understanding of Mendelian inheritance that is necessary for the visualization of genotype based on phenotype. The processes by which genes are transcribed and translated into products are also integral to the determination of whole-plant phenotypes. The ways that alleles and products of different genes interact in the determination of a phenotype give clues to the plant breeder in formulating strategies for the enhancement of that phenotype. Mendelian laws may be extrapolated and adapted to constitute the theoretical bases of quantitative inheritance and population genetics. The process of plant breeding may be described as actions that change the frequencies of alleles within populations; the frequency of desirable alleles increased and of undesirable alleles decreased. Population phenotypic variance may be absolved into constituent genetic and environmental components that further assist the breeder in determining H2 (heritability), a statistic that estimates the proportion of the phenotype that is genetic vs. environmental. The dynamics of population phenotype mean and variance under a regime of repeated selection for a certain percentage of the population may also be used to impute the relative genetic and environmental contributions to the phenotype. The genome is organized into discrete chromosomes that are endowed with inherent capabilities to self-replicate, split into two equal genomes during mitosis, and recombine chromosome segments and segregate into unequal haploid gametophytes during meiosis. The genome is usually comprised of two copies of each gene or locus, but in plants the number of chromosomes may be increased such that there may be 3 or more genomic sets, known as polyploidy. Breeding polyploids is more complicated and challenging than diploids. Cytoplasmic organelles (mitochondria, plastids) contain small functional bacteria-like genomes that include a few important genes. These corresponding phenotypes do not obey Mendel's Laws of inheritance but are transmitted to progeny via the cytoplasm of the maternal parent. The mapping of gene locations within genomes has been greatly accelerated by the use of molecular marker surrogates that are also useful selection criteria for the plant breeder.