Sickle-cell and malaria
As a recessive gene, Sickle-cell disease is only present if homozygous, with no dominant gene to beat them out. Sickle-cell disease, originating in people living in tropical areas where malaria is prevalent, is a hereditary blood disorder characterized by rigid, sickle-shaped red blood cells.. The unusual shape and rigidity of these altered red blood cells reduces a cell's ability to effectively travel with regular blood flow, occasionally blocking veins and preventing proper blood flow. Life expectancy is shortened for people with sickle-cell disease, though modern medicine has significantly lengthened the life expectancy of someone with this disease. As detrimental as the effects of sickle-cell disease seem, it also offers an unforeseen benefit; humans with the sickle-cell gene show less severe symptoms when infected with malaria, as the abnormal shape of blood cells caused by the disease hinder the malaria parasite's ability to invade and replicate within these cells. [3] It is possible to have the sickle-cell allele, but not have the disease, for example if heterozygous.
Malaria, a mosquito-borne infectious disease of humans and other animals, is a potentially deadly disease that causes fever, fatigue, nausea, muscular pain, coughing, and, in extreme cases, coma and death. Malaria is caused by parasitic protozoans transferred through mosquito saliva into a person's circulatory system, where they travel to the liver to mature. Though eliminated in the U.S., there were an estimated 219 million documented cases of malaria in 2010 according to the World Health Organization. [4]
Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases it can cause yellow skin, seizures, coma, or death. Symptoms usually begin ten to fifteen days after being bitten by an infected mosquito. If not properly treated, people may have recurrences of the disease months later. In those who have recently survived an infection, reinfection usually causes milder symptoms. This partial resistance disappears over months to years if the person has no continuing exposure to malaria.
Mosquitoes are a group of about 3,500 species of small insects that are flies. Within Diptera they constitute the family Culicidae. The word "mosquito" is Spanish for "little fly". Mosquitoes have a slender segmented body, one pair of wings, three pairs of long hair-like legs, feathery antennae, and elongated mouthparts.
The World Health Organization (WHO) is a specialized agency of the United Nations that is concerned with international public health. It was established on 7 April 1948, and is headquartered in Geneva, Switzerland. The WHO is a member of the United Nations Development Group. Its predecessor, the Health Organization, was an agency of the League of Nations.
The correlation between sickle-cell disease and malaria is a double-edged sword. Having a sickle-cell allele does limit the life expectancy of a person, however, the presence of sickle-cell genes reduces the detrimental effects of malaria should it be contracted. Natural selection allowed for the spreading of the sickle-cell gene in areas of high numbers of mosquitoes carrying malaria; those that weren't as susceptible to malaria were much more likely to live than those that were. Because malaria is not as prevalent as it once was, the benefits of sickle-cell have since eroded, leaving behind the detrimental effects of the disease.
In biology, a mutation is the alteration of the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA.
The Extended Phenotype is a 1982 book by Richard Dawkins, in which the author introduced a biological concept of the same name. The main idea is that phenotype should not be limited to biological processes such as protein biosynthesis or tissue growth, but extended to include all effects that a gene has on its environment, inside or outside the body of the individual organism.
Plasmodium is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of Plasmodium species involve development in a blood-feeding insect host which then injects parasites into a vertebrate host during a blood meal. Parasites grow within a vertebrate body tissue before entering the bloodstream to infect red blood cells. The ensuing destruction of host red blood cells can result in disease, called malaria. During this infection, some parasites are picked up by a blood-feeding insect, continuing the life cycle.
The evolution of sexual reproduction describes how sexually reproducing animals, plants, fungi and protists could have evolved from a common ancestor that was a single celled eukaryotic species. There are a few species which have secondarily lost the ability to reproduce sexually, such as Bdelloidea, and some plants and animals that routinely reproduce asexually without entirely losing sex. The evolution of sex contains two related yet distinct themes: its origin and its maintenance.
Balancing selection refers to a number of selective processes by which multiple alleles are actively maintained in the gene pool of a population at frequencies larger than expected from genetic drift alone. This can happen by various mechanisms, in particular, when the heterozygotes for the alleles under consideration have a higher fitness than the homozygote. In this way genetic polymorphism is conserved.
A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. The specific case of heterozygote advantage due to a single locus is known as overdominance. Overdominance is a condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents, and heterozygous individuals have a higher fitness than homozygous individuals.
Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Such a gene that exhibits multiple phenotypic expression is called a pleiotropic gene. Mutation in a pleiotropic gene may have an effect on several traits simultaneously, due to the gene coding for a product used by a myriad of cells or different targets that have the same signaling function.
The gene-centered view of evolution, gene's eye view, gene selection theory, or selfish gene theory holds that adaptive evolution occurs through the differential survival of competing genes, increasing the allele frequency of those alleles whose phenotypic trait effects successfully promote their own propagation, with gene defined as "not just one single physical bit of DNA [but] all replicas of a particular bit of DNA distributed throughout the world". The proponents of this viewpoint argue that, since heritable information is passed from generation to generation almost exclusively by DNA, natural selection and evolution are best considered from the perspective of genes.
Sickle cell trait describes a condition in which a person has one abnormal allele of the hemoglobin beta gene, but does not display the severe symptoms of sickle-cell disease that occur in a person who has two copies of that allele. Those who are heterozygous for the sickle cell allele produce both normal and abnormal hemoglobin.
Any cause that reduces reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. With sufficient pressure, inherited traits that mitigate its effects—even if they would be deleterious in other circumstances—can become widely spread through a population. It is a quantitative description of the amount of change occurring in processes investigated by evolutionary biology, but the formal concept is often extended to other areas of research.
Genetics is the study of genes, and tries to explain what they are and how they work. Genes are how living organisms inherit features or traits from their ancestors; for example, children usually look like their parents because they have inherited their parents' genes. Genetics tries to identify which traits are inherited, and explain how these traits are passed from generation to generation.
Avian malaria is a parasitic disease of birds, caused by parasite species belonging to the genera Plasmodium and Hemoproteus. The disease is transmitted by a dipteran vector including mosquitoes in the case of Plasmodium parasites and biting midges for Hemoproteus. The range of symptoms and effects of the parasite on its bird hosts is very wide, from asymptomatic cases to drastic population declines due to the disease, as is the case of the Hawaiian honeycreepers. The diversity of parasites is large, as it is estimated that there are approximately as many parasites as there are species of hosts. Co-speciation and host switching events have contributed to the broad range of hosts that these parasites can infect, causing avian malaria to be a widespread global disease, found everywhere except Antarctica.
In epidemiology, a disease vector is any agent which carries and transmits an infectious pathogen into another living organism; most agents regarded as vectors are organisms, such as intermediate parasites or microbes, but it could be an inanimate medium of infection such as dust particles.
Gene–environment correlation is said to occur when exposure to environmental conditions depends on an individual's genotype.
The antagonistic pleiotropy hypothesis was first proposed by George C. Williams in 1957 as an evolutionary explanation for senescence. Pleiotropy is the phenomenon where one gene controls for more than one phenotypic trait in an organism. Antagonistic pleiotropy is when one gene controls for more than one trait, where at least one of these traits is beneficial to the organism's fitness and at least one is detrimental to the organism's fitness. The theme of G.C. William's idea about antagonistic pleiotropy was that if a gene caused both increased reproduction in early life and aging in later life, then senescence would be adaptive in evolution. For example, one study suggests that since follicular depletion in human females causes both more regular cycles in early life and loss of fertility later in life through menopause, it can be selected for by having its early benefits outweigh its late costs.
Sickle cell disease (SCD) is a group of blood disorders typically inherited from a person's parents. The most common type is known as sickle cell anaemia (SCA). It results in an abnormality in the oxygen-carrying protein haemoglobin found in red blood cells. This leads to a rigid, sickle-like shape under certain circumstances. Problems in sickle cell disease typically begin around 5 to 6 months of age. A number of health problems may develop, such as attacks of pain, anemia, swelling in the hands and feet, bacterial infections and stroke. Long-term pain may develop as people get older. The average life expectancy in the developed world is 40 to 60 years.
Human genetic resistance to malaria refers to inherited changes in the DNA of humans which increase resistance to malaria and result in increased survival of individuals with those genetic changes. The existence of these genotypes is likely due to evolutionary pressure exerted by parasites of the genus Plasmodium which cause malaria. Since malaria infects red blood cells, these genetic changes are most commonly alterations to molecules essential for red blood cell function, such as hemoglobin or other cellular proteins or enzymes of red blood cells. These alterations generally protect red blood cells from invasion by Plasmodium parasites or replication of parasites within the red blood cell.
Host–parasite coevolution is a special case of coevolution, the reciprocal adaptive genetic change of a host and a parasite through reciprocal selective pressures.
The following outline is provided as an overview of and topical guide to evolution:
