Classical genetics consists of the technique and methodologies of genetics that predate the advent of molecular biology. A key discovery of classical genetics in eukaryotes was genetic linkage. The observation that some genes do not segregate independently at meiosis broke the laws of Mendelian inheritance, and provided science with a way to map characteristics to a location on the chromosomes. Linkage maps are still used today, especially in breeding for plant improvement.
After the discovery of the genetic code and such tools of cloning as restriction enzymes, the avenues of investigation open to geneticists were greatly broadened. Some classical genetic ideas have been supplanted with the mechanistic understanding brought by molecular discoveries, but many remain intact and in use. Classical genetics is often contrasted with reverse genetics, and aspects of molecular biology are sometimes referred to as molecular genetics.
A somatic cell (Greek: σὠμα/soma = body) is any biological cell forming the body of an organism; that is, in a multicellular organism, any cell other than a gamete, germ cell, gametocyte or undifferentiated stem cell. By contrast, gametes are cells that fuse during sexual reproduction, for organisms that reproduce sexually; Germ cells are cells that give rise to gametes; Stem cells are cells that can divide through mitosis and differentiate into diverse specialized cell types. For example, in mammals, somatic cells make up all the internal organs, skin, bones, blood and connective tissue. By contrast, mammalian germ cells give rise to spermatozoa and ova which fuse during fertilization to produce a cell called a zygote, which develops into an embryo. Somatic cells are diploid.
The word "somatic" is derived from the Greek word sōma, meaning "body".
The Y chromosome is one of two sex-determining chromosomes (allosomes) in mammals, including humans (the other is the X chromosome). In mammals, the Y chromosome contains the gene SRY, which triggers testicle development if present. The human Y chromosome is composed of about 50 million base pairs. DNA in the Y chromosome is passed from father to son, and Y-DNA analysis may thus be used in genealogical research. With a 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest evolving parts of the human genome.