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The three areas of Genetics
1. Classical Genetics
Started with Mendel's work on inheritance of physical characteristics in
pea plants (Pisum sativum), this is primarily the study
of chromosomal theory. It is also known as "transmission genetics."
Gregor Mendel worked in the 1850's-60's on the inheritance
of physical characteristics in the edible pea (Pisum sativum).
He published his works in 1866.
- Carl Correns (Germany)
- Erich von Tschermak (Austria)
- Hugo De Vries (Holland)
...rediscovered Mendel's work in the early 1900's.
Mendel worked on DISCONTINUOUS, or QUALITATIVE CHARACTERS.
("either/or" traits, such as red vs. white
flowers; wrinkled vs. smooth peas, etc.)
Until his work, much study of inheritance had been devoted to CONTINUOUS
CHARACTERS (e.g., size, stature, brain volume etc.) which are
far more difficult to characterize genetically, as they are usually
controlled by more than one gene locus (i.e., they are polygenic).
2. Molecular Genetics
This is the study of the genetic materials (DNA and RNA) themselves,
including
- chemical structure (I hope you've all had Organic Chemistry!)
- replication
- change (mutation = any change in a gene)
- function (gene expression)
3. Evolutionary Genetics
This is the study of the mechanisms of evolutionary change on
a genetic level. It includes both
Be sure you know the meaning/significance of the
following already-familiar terms:
zygote
diploid; haploid; ploidy
prokaryote
eukaryote
gamete
gene
genotype
phenotype - characters may be
- discontinuous (qualitative) or
- continuous (quantitative)
allele
- dominant allele: one which masks the expression
of another at the same locus
- recessive allele: one whose expression is masked
by another at the same locus
The genome is the organism's basic complement of DNA
In eukaryotes, the genome is borne within chromosomes, the number and
conformation of which is species specific.
One set of chromosomes carries one copy of the genome.
Hence, diploid organisms have two sets of chromosomes, each set carrying
the same gene loci (though not necessarily the same alleles) at each
locus.
The two chromosomes in a given organism (one from mom, and one from
dad) that have exactly the same array of gene loci are said to be homologous.
Recall: When a cell divides during meiosis, the two homologs separate, and each
daughter cell (destined to become a gamete) receives only one member of each
homologous pair. This gamete is now haploid.
Here's an image of a human karyotype, to refresh our memory.
Almost all eukaryotic organisms have two copies of the genome, one from each parent, and
are thus diploid.
When considering a single gene locus in a diploid organism, recall the terms:
homozygous: carrying two of the same allele at a
given locus.
heterozygous: carrying two different alleles at
the same locus.
Recall our old pal, The Central Dogma:
DNA is transcribed into RNA
RNA is translated into protein
Recall the difference between structural and enzymatic proteins
Recall the meaning of primary, secondary, tertiary and quaternary structure
of proteins
Areas of Investigation in Genetics (from coarse-grained to fine-grained):
- Identification and isolation of phenotypic variants (mutants) in a population.
- Controlled matings between individuals of known phenotype/genotype.
- Study of biochemical pathways (and the effects of mutations on them).
- Physical analysis of chromosome structure via labeling and
microscopic examination.
- Analysis of the DNA molecule itself (via various techniques).
- genomics - the sequencing of entire genomes
- bioinformatics - the use of computers and analytical methods
applied to the study of the function of gene-encoded proteins
Old News
Genes are composed of varying segments of a strand of deoxyribonucleic acid, or DNA
located on a large, complex helix-shaped molecule known as
DeoxyriboNucleic Acid - DNA.
The information encoded on the this complex, double-helix shaped molecule exists in the form of an "alphabet" of nucleotide bases,
Adenine
Guanine
Cytosine
Thymine.
- In prokaryotes, there is usually a single, circular chromosome lacking
any associated proteins or other molecules.
- In eukaryotes, the genome is divided among multiple, linear chromosomes.
The number of chromosomes varies with
species--and it is not necessarily true that the most complex or most
derived species have the most chromosomes. It is the information encoded
on the chromosomes that makes each species unique, not the chromosome number.
The information encoded in the DNA includes instructions for making polypeptides (a.k.a. proteins): polymers of amino acids. Any given species has polypeptides that are unique to that species, and form its identity.
Side note:
While we're on the subject of different species, we are going to hear more about the
words "primitive" and "derived". So let's define them now...
- primitive - relatively unchanged from an ancestral form.
- derived - relatively changed from an ancestral form.
Genetics and Your Life
Genetics touches every aspect of your life, and all of human existence.
Control of genes is a major goal of many humans--for good or ill. The
practice of applying genetic technology to commercial use is known as
biotechnology.
Genetics and Agriculture
Homo sapiens, the world's most successful agricultural species, changes the genome of domestic crop and livestock species via selective breeding or artificial selection.
When a particularly desirable individual or strain is selected, the farmer
may plant a crop that consists of these desirable individuals, and all are
nearly genetically identical. This is a monoculture.
While monocultures can be advantageous and productive, they also introduce
pitfalls of which we must be aware:
- homozygosity at multiple, deleterious loci
- equal susceptibility to pathogens or parasites
Loss of genetic diversity can lead to ruin, both within a species and among species.
Genetics and Commercial Products
Homo sapiens also seeks to manipulate genes in an effort to improve its own quality of life:
Molecular Genetic Engineering - The manipulation of microbe DNA to
produce bacteria capable of synthesizing products useful to humans (such as
insulin, hormones, etc.)
A transgenic cell is one that has permanently acquired foreign DNA. In many cases, this foreign DNA allows the cell to synthesize products it could not otherwise make.
Genetics in Crime-solving
Genetics is also put to use when Homo sapiens are pitted against each other in crime.
DNA "fingerprinting" is
the process of comparing DNA at a crime scene with that of a suspect. (It
can't positively identify a suspect, but it can allow statistical
analysis of the likelihood that a particular suspect shares the DNA found at a crime scene.)
Genetics in Medicine
Genetics and medicine go hand in hand.
With the data still rolling in from the Human Genome Project, many goals may be met, not least of which is...
identification of mutated genes responsible for heritable disorders
modification of those genes to effect a cure
Muscular dystrophy, Huntington's disease, cystic fibrosis, and many other such disorders are already being investigated.
Genetics and Non-human Species
The study of DNA cannot be done without model organisms: non-human
species used to study genes and gene expression in the hope
that they might be similar enough to human genes and gene expression to
allow extrapolation.
Typical eukaryotic model organisms include
- Yeast (Saccharomyces spp.)
- Pink Bread Mold (Neurospora crassa)
- Black-bellied Fruit Fly (Drosophila melanogaster
- House Mouse (Mus musculus)
...and others
A host of microbes are also important players.
And so we begin a journey already begun...
