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An allele is one of two, or more, forms of a given gene variant. For example, the ABO blood grouping is controlled by the ABO gene, which has six common alleles. Nearly every living human's phenotype for the ABO gene is some combination of just these six alleles. An allele is one of two, or more, versions of the same gene at the same place on a chromosome. It can also refer to different sequence variations for several-hundred base-pair or more region of the genome that codes for a protein. Alleles can come in different extremes of size. At the lowest possible size an allele can be a single nucleotide polymorphism (SNP). At the higher end, it can be up to several thousand base-pairs long. Most alleles result in little or no observable change in the function of the protein the gene codes for.
The genotype of an organism is its complete set of genetic material. However, the term is often used to refer to a single gene or set of genes, such as the genotype for eye color. The genes partly determine the observable characteristics of an organism, such as hair color, height, etc. An example of a characteristic determined by a genotype is the petal color in a pea plant. The collection of all genetic possibilities for a single trait are called alleles; two alleles for petal color are purple and white.
Inbreed is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and other consequences that may arise from expression of deleterious or recessive traits resulting from incestuous sexual relationships and consanguinity.
Tay–Sachs disease is a genetic disorder that results in the destruction of nerve cells in the brain and spinal cord. The most common form is infantile Tay–Sachs disease which becomes apparent around three to six months of age, with the baby losing the ability to turn over, sit, or crawl. This is then followed by seizures, hearing loss, and inability to move, with death usually occurring by the age of three to five. Less commonly, the disease may occur in later childhood or adulthood. These forms tend to be less severe, but the juvenile form typically results in death by age 15.
Heterosis, hybrid vigor, or outbreeding enhancement is the improved or increased function of any biological quality in a hybrid offspring. An offspring is heterotic if its traits are enhanced as a result of mixing the genetic contributions of its parents. These effects can be due to Mendelian or non-Mendelian inheritance.
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.
Molecular ecology is a field of evolutionary biology that is concerned with applying molecular population genetics, molecular phylogenetics, and more recently genomics to traditional ecological questions. It is virtually synonymous with the field of "Ecological Genetics" as pioneered by Theodosius Dobzhansky, E. B. Ford, Godfrey M. Hewitt, and others. These fields are united in their attempt to study genetic-based questions "out in the field" as opposed to the laboratory. Molecular ecology is related to the field of conservation genetics.
Sandhoff disease is a lysosomal genetic, lipid storage disorder caused by the inherited deficiency to create functional beta-hexosaminidases A and B. These catabolic enzymes are needed to degrade the neuronal membrane components, ganglioside GM2, its derivative GA2, the glycolipid globoside in visceral tissues, and some oligosaccharides. Accumulation of these metabolites leads to a progressive destruction of the central nervous system and eventually to death. The rare autosomal recessive neurodegenerative disorder is clinically almost indistinguishable from Tay–Sachs disease, another genetic disorder that disrupts beta-hexosaminidases A and S. There are three subsets of Sandhoff disease based on when first symptoms appear: classic infantile, juvenile and adult late onset.
Inbreeding depression is the reduced biological fitness in a given population as a result of inbreeding, or breeding of related individuals. Population biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. In general, the higher the genetic variation or gene pool within a breeding population, the less likely it is to suffer from inbreeding depression.
Extra-pair copulation (EPC) is a promiscuous mating behaviour in monogamous species. Monogamy is the practice of having only one sexual partner at any one time, forming a long-term bond and combining efforts to raise offspring together; mating outside this pairing is extra-pair copulation. Across the animal kingdom, extra-pair copulation is common in monogamous species, and only a very few pair-bonded species are thought to be exclusively sexually monogamous. EPC in the animal kingdom has mostly been studied in birds and mammals. Possible benefits of EPC can be investigated within non-human species, such as birds.
Lethal alleles are alleles that cause the death of the organism that carries them. They are usually a result of mutations in genes that are essential for growth or development. Lethal alleles may be recessive, dominant, or conditional depending on the gene or genes involved. Lethal alleles can cause death of an organism prenatally or any time after birth, though they commonly manifest early in development.
Germline mosaicism, also called gonadal mosaicism, is a type of genetic mosaicism where more than one set of genetic information is found specifically within the gamete cells; conversely, somatic mosaicism is a type of genetic mosaicism found in somatic cells. Germline mosaicism can be present at the same time as somatic mosaicism or individually, depending on when the conditions occur. Pure germline mosaicism refers to mosaicism found exclusively in the gametes and not in any somatic cells. Germline mosaicism can be caused either by a mutation that occurs after conception, or by epigenetic regulation, alterations to DNA such as methylation that do not involve changes in the DNA coding sequence.
Patricia Adair Gowaty is an American evolutionary biologist. She received her B.A. in biology at Tulane University and her PhD in zoology at Clemson University in 1980. She is currently a Distinguished Professor at the University of California, Los Angeles.
The medical genetics of Jews have been studied to identify and prevent some rare genetic diseases that, while still rare, are more common than average among people of Jewish descent. There are several autosomal recessive genetic disorders that are more common than average in ethnically Jewish populations, particularly Ashkenazi Jews. This is due to population bottlenecks that occurred relatively recently in the past as well as a practice of consanguineous marriage. These two phenomena lead to a decrease in genetic diversity and a higher likelihood that two parents will carry a mutation in the same gene and pass on both mutations to a child.
Bateman's principle, in evolutionary biology, is that in most species, variability in reproductive success is greater in males than in females. It was first proposed by Angus John Bateman (1919–1996), an English geneticist. Bateman suggested that, since males are capable of producing millions of sperm cells with little effort, while females invest much higher levels of energy in order to nurture a relatively small number of eggs, the female plays a significantly larger role in their offspring's reproductive success. Bateman's paradigm thus views females as the limiting factor of parental investment, over which males will compete in order to copulate successfully.
The major histocompatibility complex in sexual selection concerns how major histocompatibility complex (MHC) molecules allow for immune system surveillance of the population of protein molecules in a host's cells. In 1976, Yamazaki et al. demonstrated a sexual selection mate choice by male mice for females of a different MHC.
For preventing Tay–Sachs disease, three main approaches have been used to prevent or reduce the incidence of Tay–Sachs disease in those who are at high risk:
Advances in knowledge about Tay–Sachs disease have stimulated debate about the proper scope of genetic testing, and the correctness of having diseases characterised as specific to one ethnicity. Jewish communities have been in the forefront of genetic screening and counselling for the disorder.
Inbreeding avoidance, or the inbreeding avoidance hypothesis, is a concept in evolutionary biology that refers to the prevention of the deleterious effects of inbreeding. The inbreeding avoidance hypothesis posits that certain mechanisms develop within a species, or within a given population of a species, as a result of assortative mating, natural and sexual selection in order to prevent breeding among related individuals in that species or population. Although inbreeding may impose certain evolutionary costs, inbreeding avoidance, which limits the number of potential mates for a given individual, can inflict opportunity costs. Therefore, a balance exists between inbreeding and inbreeding avoidance. This balance determines whether inbreeding mechanisms develop and the specific nature of said mechanisms.
Exposure of zebrafish to a chemical environmental agent, analogous to that caused by anthropogenic pollution, amplified the effects of inbreeding on key reproductive traits. Embryo viability was significantly reduced in inbred exposed fish and there was a tendency for inbred males to sire fewer offspring.
References
- ↑ Levin, BR (1967). "The effect of reproductive compensation on the long term maintenance of the Rh polymorphism: The Rh crossroad revisited". American Journal of Human Genetics. 19 (3 Pt 1): 288–302. PMC 1706281 . PMID 4961094.
- ↑ Ober, C; Hyslop, T & Hauck, WW (1999). "Inbreeding effects on fertility in humans: evidence for reproductive compensation". American Journal of Human Genetics. 64 (1): 225–231. doi:10.1086/302198. PMC 1377721 . PMID 9915962.
- ↑ Overall, AD; Ahmad, M & Nichols, RA (2002). "The effect of reproductive compensation on recessive disorders within consanguineous human populations". Heredity. 88 (6): 474–479. doi: 10.1038/sj.hdy.6800090 . PMID 12180090.
- ↑ Hastings, IM (2001). "Reproductive compensation and human genetic disease". Genetics Research. 77 (3): 277–283. doi: 10.1017/S0016672301004992 . PMID 11486510.
- ↑ Gowaty, PA (2008). "Reproductive Compensation". Journal of Evolutionary Biology. 21 (5): 1189–200. doi: 10.1111/j.1420-9101.2008.01559.x . PMID 18564347.
- ↑ Gowaty PA, Anderson WW, Bluhm CK, Drickamer LC, Kim YK, Moore AJ (2007). "The hypothesis of reproductive compensation and its assumptions about mate preferences and offspring viability". Proceedings of the National Academy of Sciences. 104 (38): 15023–15027. doi: 10.1073/pnas.0706622104 . PMC 1986606 . PMID 17848509.
- ↑ Koeslag JH, Schach SR (1984). "Tay-Sachs disease and the role of reproductive compensation in the maintenance of ethnic variations in the incidence of autosomal recessive disease". Annals of Human Genetics. 48 (Pt 3): 275–281. doi:10.1111/j.1469-1809.1984.tb01025.x. PMID 6465844. S2CID 23470984.
- ↑ Koeslag JH, Schach SR (1985). "On the perpetuation of relic genes having an inviable homozygote". Annals of Human Genetics. 49 (Pt 4): 291–302. doi:10.1111/j.1469-1809.1985.tb01705.x. PMID 4073837. S2CID 32086409.