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 nucleic acid molecules. These long DNA strands are called chromosomes. The order of the individual nucleic acids along the chain contains information organisms used for growth and reproduction. The use of DNA as the information molecule is a universal property of all life on Earth. Our cellular machinery reads this genetic information allowing our bodies to synthesize the many enzymes and proteins required for life



The illustration explores 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 is a youtube video demonstrating the use of the illustration anda problem set you can use to test your understanding of these concepts.

Genetic information is carried in discrete units called genes. Each gene contains the information required to synthesize individual cellular components needed for survival. 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.

Variations 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. Having two complete copies of their genetic material makes them diploid. Matching chromosomes from each parent are called homologous chromosomes. 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 of 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.

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 an individual trait. 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 shape, seed pod color, flower position, and plant stature.

Among the significant 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.

Demonstration video:

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