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An allele, or allelomorph, is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.
Inbreeding 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 recessive traits resulting from incestuous sexual relationships and consanguinity. Animals avoid inbreeding only rarely.
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 the age of 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 later in childhood, adolescence, 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. The heterotic offspring often has traits that are more than the simple addition of the parents' traits, and can be explained by Mendelian or non-Mendelian inheritance. Typical heterotic/hybrid traits of interest in agriculture are higher yield, quicker maturity, stability, drought tolerance etc.
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. Loci exhibiting heterozygote advantage are a small minority of loci. The specific case of heterozygote advantage due to a single locus is known as overdominance. Overdominance is a rare 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 subdiscipline of ecology that is concerned with applying molecular genetic techniques to 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. Molecular ecology is related to the fields of population genetics and 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.
GM2-gangliosidosis, AB variant is a rare, autosomal recessive metabolic disorder that causes progressive destruction of nerve cells in the brain and spinal cord. It has a similar pathology to Sandhoff disease and Tay–Sachs disease. The three diseases are classified together as the GM2 gangliosidoses, because each disease represents a distinct molecular point of failure in the activation of the same enzyme, beta-hexosaminidase. AB variant is caused by a failure in the gene that makes an enzyme cofactor for beta-hexosaminidase, called the GM2 activator.
Inbreeding depression is the reduced biological fitness that has the potential to result from inbreeding. The loss of genetic diversity that is seen due to inbreeding, results from small population size. 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, though inbreeding and outbreeding depression can simultaneously occur.
Extra-pair copulation (EPC) is a 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.
Hexosaminidase A (alpha polypeptide), also known as HEXA, is an enzyme that in humans is encoded by the HEXA gene, located on the 15th chromosome.
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.
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, because of relatively recent population bottlenecks and because of consanguineous marriage. These two phenomena reduce genetic diversity and raise the chance that two parents will carry a mutation in the same gene and pass on both mutations to a child.
Major histocompatibility complex (MHC) genes code for cell surface proteins that facilitate an organism's immune response to pathogens as well as its ability to avoid attacking its own cells. These genes have maintained an unusually high level of allelic diversity throughout time and throughout different populations. This means that for each MHC gene, many alleles consistently exist within the population, and many individuals are heterozygous at MHC loci.
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 accuracy of characterizing diseases as specific to one ethnicity. Jewish communities have been in the forefront of genetic screening and counseling for this disease.
Inbreeding avoidance, or the inbreeding avoidance hypothesis, is a concept in evolutionary biology that refers to the prevention of the deleterious effects of inbreeding. Animals only rarely exhibit inbreeding avoidance. 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 and natural and sexual selection, in order to prevent breeding among related individuals. 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 such mechanisms.
This glossary of genetics and evolutionary biology is a list of definitions of terms and concepts used in the study of genetics and evolutionary biology, as well as sub-disciplines and related fields, with an emphasis on classical genetics, quantitative genetics, population biology, phylogenetics, speciation, and systematics. It has been designed as a companion to Glossary of cellular and molecular biology, which contains many overlapping and related terms; other related glossaries include Glossary of biology and Glossary of ecology.
Genetic incompatibility describes the process by which mating yields offspring that are nonviable, prone to disease, or genetically defective in some way. In nature, animals can ill afford to devote costly resources for little or no reward, ergo, mating strategies have evolved to allow females to choose or otherwise determine mates which are more likely to result in viable 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.