An allele is the specific variation of a gene. Bacteria, because they have only one DNA ring, have one allele per gene per organism. In sexually reproducing organisms, each parent gives one allele for each gene, giving offspring two alleles per gene. Because alleles are only specific gene variants, different alleles are found at the same locations on the chromosomes of different individuals. This is important because it gives organisms an incredibly varied variety in the functions of their various allele while being able to reproduce. This creates variety caused by mutations in specific genes that results in a large number of alleles for any trait in a given population.
Some areas of the genome are more protected against mutation than other areas. For example, the ends of chromosomes often break and chemically change due to interactions with the cytosol and the surrounding membranes with which it can come into contact. This breakage or damage requires DNA repair. While enzymes that repair DNA are extremely efficient, they sometimes make mistakes.
The repair of DNA molecules is carried out using a variety of enzymes, one of the most important being DNA polymerase. DNA polymerase uses floating nucleic acid bases to “rebuild” DNA, one nucleic acid at a time. After the DNA is unwound by another enzyme, helicase, DNA polymerase works on each strand of the double-stranded DNA molecule. By “reading” a chain and adding nucleic acid bases, a new chain is created that can be attached to the first chain. DNA bases have counterparts that always go together. Guanine (G) is the base pair of cytosine (C). Thymine (T) is always the base pair of adenine (A).
Sometimes, the polymerase makes a mistake and the wrong base pairs come together. Other enzymes are designed to “verify” DNA after it has been synthesized to find these errors. The enzyme runs through the DNA, looking for lumps that mean that two base pairs are not properly attached. If all of these mechanisms fail to catch the mutation, it will replicate the next time the cell divides. In bacteria, this can give rise to entire colonies that have novel mutations and can be easily studied. In sexual reproductive organisms, a beneficial mutation is only valuable if it occurs early in gamete development or production. A mutation in a single skin cell, for example, will not be able to help the body greatly. The cell may give rise to a few thousand “good” skin cells, but compared to the trillions in your body, they wouldn’t matter. However, in early development, or gamete production, mutations of a gene in different alleles can be transmitted to entire organisms, which can then reproduce the allele for their full benefit.
Other mutations, known as deleterious mutations, disrupt cellular function. These mutations cause the appearance of non-functional alleles, as is the case with cancers. Some cancers are caused by mutations in tumor suppressor genes, which regulate the size, shape, and growth of individual cells. A non-functional allele in this gene means that the cell will continue to grow and divide, regardless of the signals it receives. As part of a functional body of trillions of cells, this can cause a tremendous amount of damage if cancer cells are in a sensitive or vital area.
Basics of Mendelian Inheritance
In the mid-1800s, a European monk named Gregor Mendel was busy dedicating his life to developing an understanding of how traits are passed from one generation of organisms to the next. For centuries, farmers had been raising animals and plants strategically, with the intention of producing offspring with valuable traits based on the traits of parent organisms. Because the exact means by which hereditary information was passed from parent to child was unknown, these were inaccurate efforts at best.
Mendel focused his work on pea plants, which made sense because the plants’ generation times are short, and there were no ethical concerns at stake, as there might have been with animals. His most important finding initially was that if he raised plants that had distinctly different characteristics, they did not mix in the offspring, but appeared whole or did not appear at all. Furthermore, some traits that were evident in one generation but were not evident in the next could resurface in later generations.
Flower color in peas
Multiple genes in peas
Flower color in peas
The founder of the field of genetics, Gregor Mendel, was a friar who studied peas. One of the traits he studied was the color of the flowers. Mendel’s peas produced two different colors of flowers, purple and white. Although I didn’t know it at the time, these two colors represented the interactions of different alleles in the plant genomes. Plants reproduce sexually, which means they receive two alleles for each trait.
The flower’s color trait is determined by a gene that creates an enzyme responsible for creating the pigment we see as purple. Plants that received even one functional allele produce purple flowers, while plants that receive two non-functional alleles produce white flowers. Because a functional allele can completely mask the effects of the non-functional allele, the former is said to be the dominant allele, while the non-functional allele is the recessive allele.
The interactions between these alleles produce significant variability in the flowers. Although the recessive allele may be masked by the dominant allele, it does not mean that the dominant allele is better for the plant. It could be true that white flowers attract more pollinators and are therefore more successful inbreeding. If this were true, the allele frequency of the non-working allele would increase in the population, even if it is not working. Sometimes the most adaptive function of an enzyme is to not have the enzyme working at all.
Multiple genes in peas
One of the things that Mendel was most interested in was the enormous variety that he could obtain by crossing two identical plants. Below is a table of the various traits that Mendel observed. He noted that while each of these traits had only two shapes, the different alleles could be combined in a huge variety of patterns and shapes. What Mendel was beginning to describe were the laws of segregation and independent assortment.
Segregation and independent assortment laws address how cells divide their DNA to prepare haploid cells such as sperm and egg. Although both alleles for a given trait start in the same diploid cell, at the end of meiosis they will separate into separate eggs or sperm.
This, the segregation law means that while a recessive allele can be masked in the expression of an organism, it has the same possibility of being transmitted to offspring as a dominant allele. Also important is the law of independent assortment, which says that alleles of the same gene will be classified independently of alleles of other genes. This is important because it gives rise to the enormous complexity of life. From the same pea plant parents, thanks to these laws, you could receive offspring with any combination of traits listed in the table above, even if the parents looked the same.
Allele definition, Mendelian and Example