Heredity is the passing of traits from parent to offspring. Molecules of DNA carry information that codes for various proteins. These proteins interact with the environment, causing observable patterns of life. The complex mechanisms that replicate and reproduce DNA and the organisms it creates can be recombined and mutated during the process, leading to new and various forms of life. All organisms, from the simplest bacteria to the largest eukaryotes, use DNA as the main form of heredity.
Before the role of DNA was understood, it was well known that some mechanism caused offspring to resemble the parents. Children look like their parents, livestock reproduce in predictable lines, and even plants have visible traits that pass from one generation to the next. The first scientist to fully document the passing of traits in an organism was Gregor Mendel, in the 1800s. As a friar living in a monastery, Mendel had the opportunity to breed and raise pea plants, which he observed with great care. He began to notice pattern emerging in the inheritance of certain traits, and proposed the idea that each organism carries different forms of each gene. Today, we call these genetic variants alleles, and have confirmed their existence with molecular techniques. The field of genetics has grown into a large science, with many sub-disciplines.
Around the same time, other famous scientists were trying to understand the larger picture of heredity, and how different populations of organisms can give rise to different species. These men were Charles Darwin and Alfred Wallace, who proposed the same theory of evolution, separately. They proposed that individual organisms carry information that produces certain traits. Some traits are more beneficial than others, and lead to more reproduction. These traits are passed to the offspring, and the offspring can also interbreed. In this way, certain traits can increase or decrease in a population. When mutations or barriers stop the individuals in a population from reproducing, the population becomes divided. Over time, the populations evolve into separate species. The theory of evolution has evolved into a complex study of organisms and the environments they occupy, known as ecology.
Today, many of these fields interact, as scientists study the way heredity works in organisms. Molecular techniques can be used to analyze changes created by the environment, and natural selection acting on the alleles. Or, working the other way around, the genome can be altered to see what changes occur in the organism. Either way, scientist now have a large arsenal of tools to analyze heredity, and are making serious advances in understanding the chemical and environmental forces that affect heredity. It is now even possible to change the DNA an organism inherits, and fix various mutations. As such, modern medicine has dedicated many resources to studying these mechanisms.
Examples of Heredity
Heredity in Bacteria
Bacteria are simple prokaryotic organisms. They are haploid in nature, and carry only one allele for each gene. Their genome is usually contained in a single chromosome, which exists in a ring. Bacteria reproduce through an asexual process known as binary fission. During binary fission, the DNA is copied, and the copies are segregated into new cells. The DNA in each cell exists in a double helix, one half of the helix being old DNA and the other half being newly copied DNA. In this way, each daughter bacteria is identical to the original parent.
This mode of heredity relies on mutations to change the alleles at each gene. When a mutation is beneficial, a bacteria can reproduce more. If the environment changes and the allele is no longer beneficial, the population with the allele will suffer. Sometimes, these mutations can allow bacteria to survive certain antibiotics. Even this resistance to antibiotics is a heritable trait, and once the mutation happens in a population, it is hard to get rid of. If a population of harmful bacteria infect a human and antibiotics cannot get rid of them, the infection could become lethal. Scientists study modes of heredity in bacteria to develop new strategies to fight them in the field of public health.
Heredity in Sexually-Reproducing Organisms
In sexually reproducing organisms, the mode of heredity gets more complicated. Instead of each individuals giving rise to their own offspring by simply copying the DNA, two organisms must combine their DNA to create offspring. This method is much more complex, but leads to more variation in the offspring, which can increase their chances of success in a changing world. Most sexually-reproducing organisms exists as diploids, with two alleles of each gene. In order to reproduce sexually, these organisms must produce haploid cells through the process of meiosis. Meiosis consists of two consecutive cellular divisions, in which the number of alleles is reduced to one per gene.
In some organisms, like humans, these haploid cells develop into gametes, which seek gametes of the opposite sex so fertilization can take place. Other organisms, such as ferns, have a separate life cycle as a haploid organisms, which produces many gametes. In both systems, the parents pass traits on to the offspring in a complex, multiple-allele system. The interactions of these alleles can produce different phenotypes, which add to the variety seen.