Crop Improvement. Sexual and Asexual Reproduction. Long term survival requires reproduction. Even the longest-lived organisms are less than 10,000 years old. Cellular machinery wears out, or gets clogged with waste products. Environmental conditions change
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Long term survival requires reproduction. Even the longest-lived organisms are less than 10,000 years old.
Cellular machinery wears out, or gets clogged with waste products.
Environmental conditions change
Plants often reproduce asexually, through cuttings or runners or buds (e.g. potatoes). The resulting plants are clones: they are genetically identical to the parent.
Used to preserve good combinations of traits.
Sexual reproduction is also found in plants, and in all animals. Sexual reproduction means combining genes from two different parents, resulting in new combinations of genes. Each parent contributes a randomly-chosen half of their genes to the offspring.
This can be a good thing, because some new combinations will survive better than the old ones.
It can also be bad: lack of uniformity in the offspring.
Diploid organism generates haploid gametes using the process of meiosis. The gametes combine during the process of fertilization to form a new diploid organism.
In animals, the haploid phase is just one cell generation, the gametes, which immediately do fertilization to produce a diploid zygote, the first cell of the new individual.
In plants, the haploid phase is several cell generations at least.
Lower plants are mostly haploid
Higher plants are haploid for only a few cell generations
The diploid plant is called the sporophyte, and the haploid plant is called the gametophyte.
The science of genetics is devoted to understanding the patterns of how traits are inherited during sexual reproduction. It was founded by Gregor Mendel in the 1850's, using pea plants. Despite the obvious differences, humans and peas have very similar inheritance patterns.
The fundamental observation of genetics: within a species, there are a fixed number of genes, and each gene has a fixed location on one of the chromosomes.
This allows genes to be mapped: a gene's neighbors are always the same.
Most species of higher organism have about 25,000 different genes distributed onto 10-30 different chromosomes.
Alleles. Many genes have several variant forms, which are called alleles.
For example, a gene the produces color in the flower might have a purple allele and a white allele. These alleles are designated P and p.
Differences in alleles are what makes each human different from all others
True-breeding lines. If you cross close relatives with each other for many generations, eventually all the offspring look alike.
Mendel started with several true-breeding lines, which differed from each other in 7 distinctive characteristics
Dominant and recessive. If a heterozygote is identical to one parent, the allele from that parent is dominant. The allele from the other parent is recessive. That is, the heterozygote looks like the dominant parent.
This is why we say purple is dominant to white, and give purple the capital letter P.
Phenotype and genotype. Phenotype is the physical appearance, and genotype is the genetic constitution.
The heterozygote in the previous paragraph has the same phenotype as the homozygous dominant parent (i.e. purple flowers), but a different genotype (the heterozygote is Pp and the parent is PP).
Left: elite inbred B73;
Right: elite inbred Mo17
Center: B73 x Mo17 hybrid
Polygenic traits. Many traits are controlled by many genes, each of which contributes a small amount to the phenotype. Grain yield is a good example: lots of genes contribute to this.
Such traits respond well to selection. In the simplest sense, selection means using the best seeds to start the next generation. If this is done consistently, the crop slowly improves over many generations.
Genetic research has led to an understanding of what happens during selection. This allows much faster and more effective selection than just saving the best seeds.
This is often called “conventional breeding” or “traditional plant breeding”. It has been the main way crops have been improved for a long time.
Diploid vs. tetraploid daylilies
Diploid vs. octoploid strawberries
Interspecies hybrid of
Drosera (sundew), a
Blue rose, with blue
pigment gene from
Dead volunteer corn
in a glyphosate-resistant
The first step in genetic engineering is molecular cloning.
Molecular cloning means taking a gene, a piece of DNA, out of the genome and growing it in bacteria. The bacteria (usually E. coli) produce large amounts of this particular gene.
The cloned gene can then be used for further research, or to produce large amounts of protein, or to be inserted into cells of another species (to confer a useful trait).
The basic tools:
1. plasmid vector: small circle of DNA that grows inside the bacteria. It carries the gene being cloned
2. Restriction enzymes: cut the DNA at specific spots, allowing the isolation of specific genes.
3. DNA ligase, an enzyme that attached pieces of DNA together.
4. transformation. Putting the DNA back into living cells and having it function.
1. Cut genomic DNA with a restriction enzyme.
2. Cut plasmid vector with the same restriction enzyme.
3. Mix the two DNAs together and join them with DNA ligase.
4. Put the recombinant DNA back into E. coli by transformation.
5. Grow lots of the E. coli containing your gene.
The real trick, however, is to find the gene that confers your desired trait.
Diversity of maize from Mexico
Diversity of potatoes from Peru