Classical Genetics

Classical genetics is the branch of

genetics

based solely on visible results of reproductive acts. It is the oldest discipline in the field of genetics, going back to the experiments on

Mendelian inheritance

by

Gregor Mendel

who made it possible to identify the basic mechanisms of heredity. Subsequently, these mechanisms have been studied and explained at the molecular level.

Classical genetics consists of the techniques and methodologies of genetics that were in use before the advent of molecular biology. A key discovery of classical genetics in

eukaryote

s was genetic linkage. The observation that some

gene

s 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

chromosome

s. 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 enzyme

s, 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.

Basic definitions

At the base of classical genetics is the concept of a

gene

, the hereditary factor tied to a particular simple feature (or character).

The set of genes for one or more characters possessed by an individual is the genotype. A diploid individual often has two alleles for the determination of a character.

Overview

Classical genetics is the part of

genetics

that is solely about the method in which genetic traits are transmitted via the acts of reproduction. Genetics is, generally, the study of genes,

genetic variation

, and

heredity

. The process by which characteristics are passed down from parents to their offspring is called heredity. In the sense of classical genetics, variation is known as the lack of resemblance in related individuals and can be categorized as discontinuous or continuous. Genes are a fundamental part of DNA that is aligned linearly on a eukaryotic chromosome. Chemical information that is transported and encoded by each gene is referred to as a trait. Many organisms possess two genes for each individual trait that is present within that particular individual. These paired genes that control the same trait is classified as an allele. In an individual, the allelic genes that are expressed can be either homozygous, meaning the same, or heterozygous, meaning different. Many pairs of alleles have differing effects that are portrayed in an offspring's

phenotype

and genotype. The phenotype is a general term that defines an individual's visible, physical traits. The genotype of an offspring is known as its genetic makeup. The alleles of genes can either be dominant or recessive. A dominant allele needs only one copy to be expressed while a recessive allele needs two copies (homozygous) in a diploid organism to be expressed. Dominant and recessive alleles help to determine the offspring's genotypes, and therefore phenotypes.

History

Classical genetics is often referred to as the oldest form of genetics, and began with

Gregor Mendel's

experiments that formulated and defined a fundamental biological concept known as

Mendelian inheritance

. Mendelian inheritance is the process in which genes and traits are passed from a set of parents to their offspring. These inherited traits are passed down mechanistically with one gene from one parent and the second gene from another parent in sexually reproducing organisms. This creates the pair of genes in diploid organisms. Gregor Mendel started his experimentation and study of

inheritance

with phenotypes of garden peas and continued the experiments with plants. He focused on the patterns of the traits that were being passed down from one generation to the next generation. This was assessed by

test-crossing

two peas of different colors and observing the resulting phenotypes. After determining how the traits were likely inherited, he began to expand the amount of traits observed and tested and eventually expanded his experimentation by increasing the number of different organisms he tested.

About 150 years ago, Gregor Mendel published his first experiments with the test crossing of ''Pisum
* Genotype
*

Phenotype

*

Thomas Hunt Morgan

References

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