What is an Allele?
An allele is a specific variation of a gene. In organisms like bacteria that have a single strand of DNA, there is only one allele per gene. However, in sexually reproducing organisms, each parent contributes one allele for every gene, resulting in two alleles per gene in the offspring. These alleles are located at the same position, or locus, on corresponding chromosomes in different individuals. This arrangement allows for significant genetic variation within a population, as different combinations of alleles can lead to a variety of traits.
Alleles and Genetic Variation
The Role of Mutations in Allele Formation
Mutations are key to the creation of new alleles. Certain regions of the genome are more prone to mutations than others. For instance, the ends of chromosomes, known as telomeres, are particularly susceptible to damage due to interactions with the surrounding cytosol and cellular membranes. When damage occurs, the cell’s DNA repair mechanisms, such as those involving DNA polymerase, attempt to correct it. However, errors can sometimes slip through, resulting in mutations that may lead to new alleles.
How DNA Polymerase Works
DNA polymerase is a crucial enzyme in the DNA repair process. It uses free-floating nucleotides to “rebuild” damaged DNA strands. After another enzyme, helicase, unwinds the DNA, DNA polymerase reads one strand and adds complementary nucleotides to form a new, paired strand. The base pairing in DNA is highly specific: guanine (G) pairs with cytosine (C), and thymine (T) pairs with adenine (A). However, errors in this process can lead to incorrect base pairing, resulting in mutations. Other enzymes are designed to “proofread” the DNA and correct these mistakes, but if a mutation is not caught, it can be passed on when the cell divides.
The Importance of Alleles in Evolution and Disease
Beneficial and Deleterious Mutations
Mutations can lead to the creation of both beneficial and harmful alleles. In sexually reproducing organisms, a beneficial mutation is most valuable if it occurs early in development or in the production of gametes (sperm or eggs). Such a mutation can be passed on to future generations and spread throughout a population. On the other hand, deleterious mutations can result in non-functional alleles, which may lead to diseases such as cancer. For example, mutations in tumor suppressor genes can result in uncontrolled cell growth, as the non-functional allele fails to regulate the size, shape, and division of cells.
Examples of Alleles in Action
Flower Color in Peas: A Classic Example
One of the most famous examples of how alleles work comes from Gregor Mendel’s experiments with pea plants. Mendel studied various traits in peas, including flower color. The plants he studied produced either purple or white flowers, depending on the alleles they inherited. The gene responsible for flower color encodes an enzyme that produces the purple pigment. A plant with one functional allele and one non-functional allele will still produce purple flowers because the functional allele is dominant. However, if a plant inherits two non-functional alleles, it will produce white flowers, as there is no functional allele to produce the pigment.
Mendel’s Laws of Segregation and Independent Assortment
Mendel’s experiments led to the formulation of two key genetic principles: the Law of Segregation and the Law of Independent Assortment. The Law of Segregation states that the two alleles for a gene are separated during the formation of gametes (sperm or eggs), ensuring that each gamete carries only one allele for each gene. This law explains why recessive alleles can be passed on to offspring even if they are not expressed in the parent organism.
The Law of Independent Assortment states that alleles of different genes are distributed independently of each other during gamete formation. This principle is crucial because it accounts for the genetic diversity seen in sexually reproducing organisms. Thanks to this independent assortment, offspring can inherit a wide variety of trait combinations, even from parents that appear phenotypically similar.
The Broader Implications of Allele Diversity
The variety of alleles within a population is essential for adaptation and survival. While some alleles may confer advantages in certain environments, others may be neutral or even harmful. However, the presence of multiple alleles allows populations to adapt to changing conditions over time. For example, a non-functional allele might be beneficial if it results in a trait that increases an organism’s chances of survival or reproduction, such as a flower color that attracts more pollinators.
Related Biological Terms
- Polymerase: An enzyme that links monomers, such as nucleotides, into polymers like DNA strands.
- Mutation: A change in the DNA sequence that occurs due to errors during replication or as a result of environmental factors.
- Genotype: The set of alleles present at a specific locus in an organism’s DNA, which determines potential traits.
- Phenotype: The observable characteristics of an organism, which are influenced by its genotype.