Construction plans of organisms are encoded in segments of deoxyribonucleic acid (DNA) called genes, and thus, inherited from one generation to the next. In eukaryotes, DNA is organised as linear chromosomes containing long stretches of double-stranded DNA (dsDNA) tightly packed together with special proteins called histones.
Since the completion of the Human Genome project, the estimated number of genes in the human genome has been repeatedly revised downward. In current studies, the size of the human genome is estimated as less than billion base pairs and about 20,000-25,000 protein-coding genes.

By the late 1960s, the almost ubiquitary genetic code was enciphered, i.e. the correlation between the sequences of nucleotides and amino acids in proteins. Here, one amino acid is encoded by three consecutive nucleotides, the so-called "codon".




Gene expression includes transcription, translation, determinating the phenotype (trait) together with environmental influences. Each step involved in the expression of proteins and in the determination of phenotypes is target of complex regulation mechanisms.

Variable expressivity describes the plasticity by which a certain genotype can result in different phenotypes under the influence of variable environmental conditions. Penetrance on the other hand is the mere manifestation of genetic traits.
Think of penetrance as a light switch that can only be on or off, and expressivity as a dimmer on that light switch.




For further reading enjoy The Nature of DNA or watch the amazing video by Drew Berry.

Compared to prokaryotic systems (eg. Bacteria), the eukaryote genome consists of different amounts of "functionless" or "junk" DNA. This DNA does not encode for proteins but is essential for regulation of gene expression (B-chromosomes, introns, pseudogenes, retropseudogenes, jumping genes, micro- and minisatellites, retroviruses, centromeres, telomeres, enhancer, etc.) Thus, only 1.5% of the human genome actually code for protein or functional RNA.