When Mendel did his classical studies, he was either very lucky or very astute. The essence of his thesis was that traits exhibited independent segregation during the reproductive process. The seven traits he chose to present in his study were each located on one of the seven different chromosomes in pea. The chromosome is called a linkage group and all of the genes on it are linked to one another during meiosis, the process of going from a diploid cell to a haploid sperm or egg during reproduction. Each gene is located at a characteristic location on the chromosome called a locus. One must keep clear that locus refers to the physical location on the chromosome at which a gene is located, whereas gene refers to the functional unit of nucleotides that encodes a protein or RNA.
Human cells contain 23 pairs of chromosomes. These can be observed and identified with a microscope, forming an array called a karyotype [local]. It is possible to use this technique to observe gross genetic alterations, such as those responsible for Down's Syndrome. To observe actual genes or nucleotide sequences, a microscope is inadequate and more sophisticated genetic or biochemical methods must be employed.
Does the amount of DNA in a cell (i.e. the size of the genome) correlate with the complexity or size of an organism? No. The amount of DNA in different organisms [local] can be quite different.
Do linked genes on a chromosome ever segregate? Yes. During meiosis, the lining up of the chromosomes provides an opportunity for exchange between chromosomes. In genetics, this is called crossing-over. Here is an animation [local, from U. Virginia] of crossing-over. You may think that this makes linkage imperfect and would make it difficult to carry out genetic analyses, but in fact it adds to the power of genetics. The more distant two genes (or loci) are on a chromosome, the more frequently that crossing-over occurs. The frequency of crossing over can be used to map how distant on a chromosome two genes are.
The power of genetics is to study the inheritance of traits. Traits can be due to the interaction of two different genes located at two different genetic loci, or may be due to two different genes located at the same locus. Two different genes at the same locus are termed alleles. In addition to the previously discussed dominant and recessive behaviour of alleles, it is also possible for them to exhibit codominance; this is where an individual heterozygous for both alleles shows a trait that is intermediate between individuals homozygous for either allele. Alleles include the common human blood types (A, B and O). These three alleles all code for a characteristic molecule on the surface of blood cells and all are located at the same locus on a particular human chromosome. An individual may be either homozygous for one (e.g. AA, BB or OO), or may be heterozygous and contain two different allels at the equivalent locus on two different chromosomes (e.g. AB, AO or BO). Both A and B are considered to be dominant with respect to the O type. Because the frequency of alleles is able to drift in different populations that are geographically separated, proportions of different alleles are able to drift in different directions. Consider the different proportion of the ABO blood groups found in populations originally isolated on different continents [local].
Also, it is possible for a gene's expression to be influenced by the presence of a second gene at a different location. This is not dominance, but is termed epistasis.
Finally, there are a group of concepts [local] in genetics that arose from the study of prokaryotic organisms that are not diploid.
Mutants can be produced or are naturally occurring. The products of these mutations are proteins with altered activities or no activity (nulls). A mutant that is not able to grow without the addition of a particular nutrient is termed an auxotroph. The ability to get an auxotroph to grow by adding a needed nutrient to the medium is called complementation.
Some genes are always expressed. These are termed constitutive genes or house-keeping genes.
Some genes are only produced when needed. These are termed inducible genes.
A gene that can complement or influence a gene of interest only when adjacent to it on the same chromosome is termed as cis-acting. A gene that can complement or restore activity to a gene of interest when on a different piece of DNA is termed as trans-acting.
