Principles of Genetics

Principles of Genetics

• The study of genes, genetic variation, and heredity in plants is known as plant genetics.
• Understanding plant genetics is crucial for crop breeding and conservation of plant genetic diversity.
• Plant genetic resources include genotypes of particular species collected from different sources and geographical origins for use in plant breeding.
• Germplasm is the sum total of hereditary material (alleles of various genes) in a crop species and its wild relatives.
• Different types of plant genetic resources exist including landraces, farmers' varieties, parental lines of hybrids, released varieties, primitive cultivars and wild/weedy relatives.
• Biodiversity is crucial for crop improvement, influencing the resilience of ecosystems and the stability of societies.
• Destroying biodiversity is akin to setting fire to the planet's lifeboat.
• Each species has a characteristic number of chromosomes in its somatic (non-reproductive) cells.
• There are different methods of assessing genotypes, both morphological and molecular.
• Morphological markers are easily observed genetically-determined traits.
• Molecular markers identify differences in the genetic code and are used for DNA fingerprinting.
• Plant DNA is segmented by enzymes called restriction endonucleases.
• DNA fragments are separated by gel electrophoresis.
• Alleles are variations of genes that may differ between chromosomes in a diploid species or among chromosomes in a polyploid species.
• Multiple alleles of a gene develop through mutations, which are structural or chemical changes in DNA.
• Reproductive cells (like sperm and egg) are haploid, with half the number of chromosomes as a diploid cell.

Genetic Variation in Plants

• Genetic variation is the result of differences between individuals in a population.
• It's influenced by allelic differences due to mutations and random assortment of alleles during meiosis (a special type of cell division that creates sperm and egg cells).
• A diploid organisms has two genomes, one from each parent.
• The total chromosomal DNA of an organism is called its genome.
• Genotype refers to the set of alleles an organism possesses at a given locus or over many loci.
• Phenotype refers to the observable characteristics of an organism.
• If different alleles exist at a given locus, that locus is called heterozygous.
• If alleles at a locus are the same, that locus is homozygous.
• Different methodologies exist that can help in the assessment of genotypes (morphological and molecular).
• Although morphological markers are simple to use, their availability is limited, mostly focusing on intensively studied cultivated varieties.
• Molecular markers use the genetic code(DNA) for identification, through the process called DNA fingerprinting.

Reproduction, Mating Systems, and Genetic Variation

• Reproduction in plants can range from complete self-pollination to complete cross-pollination.
• Individuals in self-pollinated species tend to be highly homozygous.
• In cross-pollinated species, heterozygosity is more common and preserved across generations.
• Apomixis is an asexual reproductive process where seeds develop from body cells rather than the combination of sperm and egg.
• Inbreeding may lead to a decline in plant performance known as inbreeding depression.
• A single genotype can occupy large areas in apomictic species, leading to less genetic diversity.
• Heterosis, or hybrid vigor is observed when the offspring of a mating have superior performance compared to the parent plants.

Crop Diversity

• Crop diversity is essential for sustainable food production and ensuring food security in the face of climate change and other challenges.
• Conserving and utilizing crop diversity is important as it enables species to adjust to a range of circumstances and various environmental conditions.
• Plant species exhibit great variability in sexual reproduction significantly affecting genetic constitution of species.
• Current practices are limited to using a small sample of limited crop diversity.
• The loss of plant varieties and wild relatives is irreversible, making their conservation vital.
• The global loss of varieties is problematic as it is now fewer than 200 of the 20,000 edible plants which make up the majority of our food crops globally.
• Many crop varieties are now endangered or have become extint.
• The reduction in crop biodiversity makes crop species susceptible to pests and disease, and reducing resilience to environmental changes affecting agricultural production.
• Genetic uniformity makes crop species vulnerable to epidemics, whereas diversity makes them more resilient.
• A handful of multinational companies currently supply most commercial seeds, which contributes to the decline of crop diversity.
• Artificial selection is comparable to natural selection as plant breeders choose (retain) plants with desirable traits.

Genetic Erosion

• The loss of genetic diversity in crops and wild relatives is referred to as genetic erosion.
• The modernization of agriculture, including the replacement of traditional varieties with high-yield cultivars; natural disasters; and large-scale habitat loss contribute to genetic erosion.
• Genetic uniformity contributes to a species being susceptible to epidemics.

Evolution and Natural Selection

• Evolution is a collection of processes that results in allele frequency changes over time.
• Natural selection is based on fitness—the tendency of an individual and its offspring to survive—and may be connected to a plant's longevity, fertility, ability to produce viable offspring.
• Genetic variation influencing a trait is heritable (heritability > 0), higher fitness individuals have more offspring on average, and genetic composition in the population becomes more similar to that of the more fit individuals.

Other Processes that Can Lead to Genetic Changes

• Random genetic drift is a process driven by chance.
• The founder effect is a type of genetic drift that occurs when a new population is established from a very small number of individuals from an original population.