Thursday, January 13, 2011

Alleles, genotype and phenotype

Genetics is the study of the organization, expression and transfer of heritable information. The ability for information to pass from generation to generation requires a mechanism. Living organisms use DNA. DNA is a chain, or polymer, of nucleic acids. Individual polymers of DNA can contain hundreds of millions of individual nucleic acids molecules. These long DNA strands are called chromosomes. Information is contained in the order of the individual nucleic acids that make up the DNA polymer. The use of DNA as the information molecule is a universal property of all life on Earth. Genetic information is read by our cellular machinery and allows our bodies to synthesize the many enzymes and proteins required for life

Genetic information is carried in discrete units called genes. Each gene contains the information required to synthesize individual components of an organism’s cellular machinery. The coordinated expression of many different genes is responsible for an organism’s growth and activity. Within an individual species, genes occur in set locations on chromosomes. This allows their locations to be mapped. The position of a specific gene on a chromosome is called its locus.

Variation in the order of nucleic acids in a DNA molecule allow genes to encode enough information to synthesize the huge diversity of different proteins and enzymes needed for life. In addition to differences between genes, the arrangement of nucleic acids can differ between copies of the same gene. This results in different forms of individual genes. Different forms of a gene are called alleles.

Organisms that reproduce sexually receive one complete copy of their genetic material from each parent. This is referred to as being diploid.  Matching chromosomes from each parent are called homologous chromosomes. Diploid organisms have two copies of every gene. Matching genes from each parent occur at the same location on homologous chromosomes.

A diploid organism can either have two copies of the same allele or one copy each of two different alleles. Individuals who have two copies of the same allele are said to be homozygous at that locus. Individuals who receive different alleles from each parent are said to be heterozygous at that locus. The alleles an individual has at a locus is called a genotype. The genotype of an organism is often expressed using letters. The visible expression of the genotype is called an organism’s phenotype.

Alleles are not created equal. Some alleles mask the presence others. Alleles that are masked by others are called recessive alleles. Recessive alleles are only expressed when an organism is homozygous at that locus. Alleles that are expressed regardless of the presence of other alleles are called dominant.

If one allele completely masks the presence of another at the same locus, that allele is said to exhibit complete dominance. However, dominance is not always complete. In cases of incomplete dominance, intermediate phenotypes are possible.

The model below illustrates the relationship between the presence of different alleles at a specific locus and an organism's genotype and phenotype. The organism in the model is a plant. It is diploid and the trait is flower color. Below the model is a youtube video demonstrating the use of the model.

Demonstration video:

Gene interactions can be quite complicated. The example above demonstrates a simple situation in which a single gene corresponds to an individual trait. In more complicated cases, multiple genes can influence  individual traits. This is called polygenic inheritance. In these situations the relationship between specific alleles and characteristics is not as straightforward.

In his famous pea plant studies, Mendel studied seven traits that have the characteristics needed to allow the observation of inheritance of discrete traits. The traits he studied were seed shape, seed color, flower color, seed pod shaped, seed pod color, flower position and plant stature.

Among the major contributions of Mendel's work was the understanding that information was passed from one generation to the next in discrete units rather than through blending.

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