This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2011)

Heredity is the passing of traits to offspring (from its parent or ancestors). This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve. The study of heredity in biology is called genetics, which includes the field of epigenetics.

Overview[link]

DNA structure. Bases are in the centre, surrounded by phosphate–sugar chains in a double helix.

In humans, eye color is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of the parents.^[1] Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype.^[2]

The complete set of observable traits that make the structure and behavior of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment.^[3] As a result, many aspects of an organism's phenotype are not inherited. For example, suntanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, some people tan more easily than others, due to differences in their genotype; a striking example are people with the inherited trait of albinism, who do not tan at all and are very sensitive to sunburn.^[4]

Heritable traits are known to be passed from one generation to the next via DNA, a molecule that encodes genetic information.^[2] DNA is a long polymer composed of four types of bases. The sequence of bases along a particular DNA molecule specify the genetic information, in a manner similar to a sequence of letters spelling out a word. Before a cell divides, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. Portions of a DNA molecule that specify a single functional unit are called genes; different genes have different sequences of bases. Within cells, the long strands of DNA form condensed structures called chromosomes. The specific location of a DNA sequence within a chromosome is known as a locus. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.^[5]

However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by multiple interacting genes within and among organisms.^[6][7] Developmental biologists suggest that complex interactions in genetic networks and communication among cells can lead to heritable variations that may underlay some of the mechanics in developmental plasticity and canalization.^[8]

Recent findings have confirmed important examples of heritable changes that cannot be explained by direct agency of the DNA molecule. These phenomena are classed as epigenetic inheritance systems that are causally or independently evolving over genes. Research into modes and mechanisms of epigenetic inheritance is still in its scientific infancy, however, this area of research has attracted much recent activity as it broadens the scope of heritability and evolutionary biology in general.^[9] DNA methylation marking chromatin, self-sustaining metabolic loops, gene silencing by RNA interference, and the three dimensional conformation of proteins (such as prions) are areas where epigenetic inheritance systems have been discovered at the organismic level.^[10][11] Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effect that modifies and feeds back into the selection regime of subsequent generations. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors.^[12] Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits, group heritability, and symbiogenesis.^[13][14][15] These examples of heritability that operate above the gene are covered broadly under the title of multilevel or hierarchical selection, which has been a subject of intense debate in the history of evolutionary science.^[14][16]

Relation to theory of evolution[link]

When Charles Darwin proposed his theory of evolution in 1859, one of its major problems was the lack of an underlying mechanism for heredity. Darwin believed in a mix of blending inheritance and the inheritance of acquired traits (pangenesis). Blending inheritance would lead to uniformity across populations in only a few generations and thus would remove variation from a population on which natural selection could act. This led to Darwin adopting some Lamarckian ideas in later editions of On the Origin of Species and his later biological works. Darwin's primary approach to heredity was to outline how it appeared to work (noticing that traits that were not expressed explicitly in the parent at the time of reproduction could be inherited, that certain traits could be sex-linked, etc.) rather than suggesting mechanisms.

Darwin's initial model of heredity was adopted by, and then heavily modified by, his cousin Francis Galton, who laid the framework for the biometric school of heredity. Galton rejected the aspects of Darwin's pangenesis model, which relied on acquired traits.

The inheritance of acquired traits was shown to have little basis in the 1880s when August Weismann cut the tails off many generations of mice and found that their offspring continued to develop tails.

History[link]

The ancients had a variety of ideas about heredity: Theophrastus proposed that male flowers caused female flowers to ripen; Hippocrates speculated that "seeds" were produced by various body parts and transmitted to offspring at the time of conception; and Aristotle thought that male and female semen mixed at conception. Aeschylus, in 458 BC, proposed the male as the parent, with the female as a "nurse for the young life sown within her."^[17]

Various hereditary mechanisms were envisaged without being properly tested or quantified. These included blending inheritance and the inheritance of acquired traits. Nevertheless, people were able to develop domestic breeds of animals as well as crops through artificial selection. The inheritance of acquired traits also formed a part of early Lamarckian ideas on evolution.

During the 18th century, Dutch microscopist Antonie van Leeuwenhoek (1632–1723) discovered "animalcules" in the sperm of humans and other animals. Some scientists speculated they saw a "little man" (homunculus) inside each sperm. These scientists formed a school of thought known as the "spermists." They contended the only contributions of the female to the next generation were the womb in which the homunculus grew, and prenatal influences of the womb. An opposing school of thought, the ovists, believed that the future human was in the egg, and that sperm merely stimulated the growth of the egg. Ovists thought women carried eggs containing boy and girl children, and that the gender of the offspring was determined well before conception.

Gregor Mendel: father of genetics[link]

Table showing how the genes exchange according to segregation or independent assortment during meiosis and how this translates into Mendel's laws

The idea of particulate inheritance of genes can be attributed to the Moravian^[18] monk Gregor Mendel who published his work on pea plants in 1865. However, his work was not widely known and was rediscovered in 1901. It was initially assumed the Mendelian inheritance only accounted for large (qualitative) differences, such as those seen by Mendel in his pea plants—and the idea of additive effect of (quantitative) genes was not realised until R.A. Fisher's (1918) paper, "The Correlation Between Relatives on the Supposition of Mendelian Inheritance" Mendel's overall contribution gave scientists a useful overview that traits were inheritable. As of today, his pea plant demonstration became the foundation of the study of Mendelian Traits. These traits can be traced on a single loci.^[19]

Modern development of genetics and heredity[link]

In the 1930s, work by Fisher and others resulted in a combination of Mendelian and biometric schools into the modern evolutionary synthesis. The modern synthesis bridged the gap between experimental geneticists and naturalists; and between both and palaeontologists, stating that:^[20][21]

1. All evolutionary phenomena can be explained in a way consistent with known genetic mechanisms and the observational evidence of naturalists.
2. Evolution is gradual: small genetic changes, recombination ordered by natural selection. Discontinuities amongst species (or other taxa) are explained as originating gradually through geographical separation and extinction (not saltation).
3. Selection is overwhelmingly the main mechanism of change; even slight advantages are important when continued. The object of selection is the phenotype in its surrounding environment. The role of genetic drift is equivocal; though strongly supported initially by Dobzhansky, it was downgraded later as results from ecological genetics were obtained.
4. The primacy of population thinking: the genetic diversity carried in natural populations is a key factor in evolution. The strength of natural selection in the wild was greater than expected; the effect of ecological factors such as niche occupation and the significance of barriers to gene flow are all important.
5. In palaeontology, the ability to explain historical observations by extrapolation from micro to macro-evolution is proposed. Historical contingency means explanations at different levels may exist. Gradualism does not mean constant rate of change.

The idea that speciation occurs after populations are reproductively isolated has been much debated. In plants, polyploidy must be included in any view of speciation. Formulations such as 'evolution consists primarily of changes in the frequencies of alleles between one generation and another' were proposed rather later. The traditional view is that developmental biology ('evo-devo') played little part in the synthesis, but an account of Gavin de Beer's work by Stephen Jay Gould suggests he may be an exception.^[22]

Almost all aspects of the synthesis have been challenged at times, with varying degrees of success. There is no doubt, however, that the synthesis was a great landmark in evolutionary biology. It cleared up many confusions, and was directly responsible for stimulating a great deal of research in the post-World War II era.

Trofim Lysenko however caused a backlash of what is now called Lysenkoism in the Soviet Union when he emphasised Lamarckian ideas on the inheritance of acquired traits. This movement affected agricultural research and led to food shortages in the 1960s and seriously affected the USSR.

Common genetic disorders[link]

• Down syndrome
• Huntington’s disease
• Phenylketonuria (PKU)
• Hemophilia^[19]

Types of heredity[link]

'Dominant and recessive' An allele is said to be dominant if it is always expressed in the appearance of an organism (phenotype). For example, in peas the allele for green pods, G, is dominant to that for yellow pods, g. Since the allele for green pods is dominant, pea plants with the pair of alleles GG (homozygote) or Gg (heterozygote) will have green pods. The allele for yellow pods is recessive. The effects of this allele are only seen when it is present in both chromosomes, gg (homozygote).

The description of a mode of biological inheritance consists of three main categories:

1. Number of involved loci

• Monogenetic (also called "simple") – one locus
• Oligogenetic – few loci
• Polygenetic – many loci

2. Involved chromosomes

• Autosomal – loci are not situated on a sex chromosome
• Gonosomal – loci are situated on a sex chromosome
  • X-chromosomal – loci are situated on the X-chromosome (the more common case)
  • Y-chromosomal – loci are situated on the Y-chromosome
• Mitochondrial – loci are situated on the mitochondrial DNA

3. Correlation genotype–phenotype

• Dominant
• Intermediate (also called "codominant")
• Recessive

These three categories are part of every exact description of a mode of inheritance in the above order. In addition, more specifications may be added as follows:

4. Coincidental and environmental interactions

• Penetrance
  • Complete
  • Incomplete (percentual number)
• Expressivity
  • Invariable
  • Variable
• Heritability (in polygenetic and sometimes also in oligogenetic modes of inheritance)
• Maternal or paternal imprinting phenomena (also see epigenetics)

5. Sex-linked interactions

• Sex-linked inheritance (gonosomal loci)
• Sex-limited phenotype expression (e.g., cryptorchism)
• Inheritance through the maternal line (in case of mitochondrial DNA loci)
• Inheritance through the paternal line (in case of Y-chromosomal loci)

6. Locus–locus interactions

• Epistasis with other loci (e.g., overdominance)
• Gene coupling with other loci (also see crossing over)
• Homozygotous lethal factors
• Semi-lethal factors

Determination and description of a mode of inheritance is achieved primarily through statistical analysis of pedigree data. In case the involved loci are known, methods of molecular genetics can also be employed.

See also[link]

• Genetics
• Genetic disorder
• Hard inheritance
• Heritability

Notes and references[link]

External links[link]

Look up heredity in Wiktionary, the free dictionary.

• Stanford Encyclopedia of Philosophy entry on Heredity and Heritability

Gregor Mendel
Born	Gregor Johann Mendel (1822-07-20)July 20, 1822 Heinzendorf bei Odrau, Austrian Empire
Died	January 6, 1884(1884-01-06) (aged 61) Brno (Brünn), Austria-Hungary
Nationality	Empire of Austria-Hungary
Fields	Genetics
Institutions	Abbey of St. Thomas in Brno
Alma mater	University of Olomouc University of Vienna
Known for	Discovering genetics

Gregor Johann Mendel (July 20, 1822^[1] – January 6, 1884) was an Austrian^[2] scientist and Augustinian friar who gained posthumous fame as the founder of the new science of genetics. Mendel demonstrated that the inheritance of certain traits in pea plants follows particular patterns, now referred to as the laws of Mendelian inheritance. Although the significance of Mendel's work was not recognized until the turn of the 20th century, the independent rediscovery of these laws formed the foundation of the modern science of genetics.^[3]

Biography[link]

Mendel was born into an ethnic German family in Heinzendorf bei Odrau, Austrian Silesia, Austrian Empire (now Hynčice, Czech Republic), and was baptized two days later as Johann. He was the son of Anton and Rosine (Schwirtlich) Mendel, and had one older sister (Veronica) and one younger (Theresia). They lived and worked on a farm which had been owned by the Mendel family for at least 130 years.^[4] During his childhood, Mendel worked as a gardener, studied beekeeping, and as a young man attended Gymnasium (school) in Opava. From 1840 to 1843, he studied practical and theoretical philosophy as well as physics at the University of Olomouc Faculty of Philosophy, taking a year off because of illness.

When Mendel entered the Faculty of Philosophy, the Department of Natural History and Agriculture was headed by Johann Karl Nestler, who conducted extensive research of hereditary traits of plants and animals, especially sheep. In 1843 Mendel began his training as a priest. Upon recommendation of his physics teacher Friedrich Franz,^{[citation needed]} he entered the Augustinian Abbey of St Thomas in Brno in 1843. Born Johann Mendel, he took the name Gregor upon entering religious life. In 1851 he was sent to the University of Vienna to study under the sponsorship of Abbot C. F. Napp. At Vienna, his professor of physics was Christian Doppler.^[5] Mendel returned to his abbey in 1853 as a teacher, principally of physics, and by 1867, he had replaced Napp as abbot of the monastery.^[6]

Besides his work on plant breeding while at St Thomas's Abbey, Mendel also bred bees in a bee house that was built for him, using bee hives that he designed.^[7] He also studied astronomy and meteorology,^[6] founding the 'Austrian Meteorological Society' in 1865.^[5] The majority of his published works were related to meteorology.^[5]

Experiments on plant hybridization[link]

Gregor Mendel, who is known as the "father of modern genetics", was inspired by both his professors at the University of Olomouc (i.e. Friedrich Franz & Johann Karl Nestler) and his colleagues at the monastery (e.g., Franz Diebl) to study variation in plants, and he conducted his study in the monastery's 2 hectares (4.9 acres) experimental garden,^[8] which was originally planted by Napp in 1830.^[6] Between 1856 and 1863 Mendel cultivated and tested some 29,000 pea plants (i.e., Pisum sativum). This study showed that one in four pea plants had purebred recessive alleles, two out of four were hybrid and one out of four were purebred dominant. His experiments led him to make two generalizations, the Law of Segregation and the Law of Independent Assortment, which later became known as Mendel's Laws of Inheritance.

Mendel did read his paper, Versuche über Pflanzenhybriden (Experiments on Plant Hybridization), at two meetings of the Natural History Society of Brünn in Moravia in 1865. It was received favorably and generated reports in several local newspapers.^[9] When Mendel's paper was published in 1866 in Verhandlungen des naturforschenden Vereins Brünn,^[10] it was seen as essentially about hybridization rather than inheritance and had little impact and was cited about three times over the next thirty-five years. (Notably, Charles Darwin was unaware of Mendel's paper, according to Jacob Bronowski's The Ascent of Man.) His paper was criticized at the time, but is now considered a seminal work.

Life after the pea experiments[link]

After completing his work with peas, Mendel turned to experimenting with honeybees to extend his work to animals. He produced a hybrid strain (so vicious they were destroyed, but failed to generate a clear picture of their heredity because of the difficulties in controlling mating behaviours of queen bees.^{[dubious – discuss]}) He also described novel plant species, and these are denoted with the botanical author abbreviation "Mendel".

After he was elevated as abbot in 1868, his scientific work largely ended, as Mendel became consumed with his increased administrative responsibilities, especially a dispute with the civil government over their attempt to impose special taxes on religious institutions.^[11] Mendel died on January 6, 1884, at age 61, in Brno, Moravia, Austria-Hungary (now Czech Republic), from chronic nephritis. Czech composer Leoš Janáček played the organ at his funeral. After his death, the succeeding abbot burned all papers in Mendel's collection, to mark an end to the disputes over taxation.^[12]

Rediscovery of Mendel's work[link]

Dominant and recessive phenotypes. (1) Parental generation. (2) F1 generation. (3) F2 generation.

Mendel's work was rejected at first, and was not widely accepted until after he died. During his own lifetime, most biologists held the idea that all characteristics were passed to the next generation through blending inheritance, in which the traits from each parent are averaged together. Instances of this phenomenon are now explained by the action of multiple genes with quantitative effects. Charles Darwin tried unsuccessfully to explain inheritance through a theory of pangenesis. It was not until the early 20th century that the importance of Mendel's ideas was realized.

By 1900, research aimed at finding a successful theory of discontinuous inheritance rather than blending inheritance led to independent duplication of his work by Hugo de Vries and Carl Correns, and the rediscovery of Mendel's writings and laws. Both acknowledged Mendel's priority, and it is thought probable that de Vries did not understand the results he had found until after reading Mendel.^[3] Though Erich von Tschermak was originally also credited with rediscovery, this is no longer accepted because he did not understand Mendel's laws.^[13] Though de Vries later lost interest in Mendelism, other biologists started to establish genetics as a science.^[3]

Mendel's results were quickly replicated, and genetic linkage quickly worked out. Biologists flocked to the theory; even though it was not yet applicable to many phenomena, it sought to give a genotypic understanding of heredity which they felt was lacking in previous studies of heredity which focused on phenotypic approaches. Most prominent of these latter approaches was the biometric school of Karl Pearson and W.F.R. Weldon, which was based heavily on statistical studies of phenotype variation. The strongest opposition to this school came from William Bateson, who perhaps did the most in the early days of publicising the benefits of Mendel's theory (the word "genetics", and much of the discipline's other terminology, originated with Bateson). This debate between the biometricians and the Mendelians was extremely vigorous in the first two decades of the twentieth century, with the biometricians claiming statistical and mathematical rigor, whereas the Mendelians claimed a better understanding of biology.

In the end, the two approaches were combined, especially by work conducted by R. A. Fisher as early as 1918. The combination, in the 1930s and 1940s, of Mendelian genetics with Darwin's theory of natural selection resulted in the modern synthesis of evolutionary biology.

Controversy[link]

Mendel's experimental results have later been the object of considerable dispute.^[12] Fisher analyzed the results of the F2 (second filial) ratio and found them to be implausibly close to the exact ratio of 3 to 1.^[14] Only a few would accuse Mendel of scientific malpractice or call it a scientific fraud—reproduction of his experiments has demonstrated the validity of his hypothesis—however, the results have continued to be a mystery for many, though it is often cited as an example of confirmation bias. This might arise if he detected an approximate 3 to 1 ratio early in his experiments with a small sample size, and continued collecting more data until the results conformed more nearly to an exact ratio. It is sometimes suggested that he may have censored his results, and that his seven traits each occur on a separate chromosome pair, an extremely unlikely occurrence if they were chosen at random. In fact, the genes Mendel studied occurred in only four linkage groups, and only one gene pair (out of 21 possible) is close enough to show deviation from independent assortment; this is not a pair that Mendel studied. Some recent researchers have suggested that Fisher's criticisms of Mendel's work may have been exaggerated.^[15][16]

Gallery[link]

• 

  Gregor Johann Mendel – memorial plaque on facade of Palacký University Faculty of Philosophy in Olomouc, where Mendel studied in 1840–1843

• 

  Gregor Mendel, frontispiece from "Mendel's principles of heredity: A Defence"

• 

  The Augustinian Abbey of St Thomas, Brno

• 

  Bust of Mendel at Mendel University of Agriculture and Forestry Brno, Czech Republic

See also[link]

• List of Roman Catholic cleric–scientists
• Mendel Museum of Genetics
• Mendel Polar Station in Antarctica
• Mendel University Brno
• Mendelian error

References[link]

Bibliography[link]

External links[link]

Wikimedia Commons has media related to: Gregor Mendel

• Works by or about Gregor Mendel in libraries (WorldCat catalog)
• Mendel's Paper in English
• Mendel Museum of Genetics
• Mendel in Darwin's Shadow, by David Allen at Macroevolution.net
• Biography, bibliography and access to digital sources in the Virtual Laboratory of the Max Planck Institute for the History of Science
• 1913 Catholic Encyclopedia entry, "Mendel, Mendelism"
• Online Mendelian Inheritance in Man
• Augustinian Abbey of St. Thomas at Brno
• A photographic tour of St. Thomas' Abbey, Brno, Czech Republic
• Johann Gregor Mendel: Why his discoveries were ignored for 35 (72) years (German)
• This has the basics of Mendel and is more appropriate in style for a GCSE student
• Masaryk University to rebuild Mendel’s greenhouse | Brno Now
• "Gregor Mendel (1822–1884)". http://www.accessexcellence.org/RC/AB/BC/Gregor_Mendel.html. Retrieved 2008-01-22. 
• "Biography of Gregor Mendel". Archived from the original on 2008-01-17. http://web.archive.org/web/20080117233318/http://mendel.imp.ac.at/mendeljsp/biography/biography.jsp. Retrieved 2008-01-22. 
• Gregor Mendel Primary Sources

Robert Plomin (born 1948 in Chicago, Illinois) is an American psychologist best known for his work in twin studies and behavior genetics. Plomin has made two of the most important discoveries in that field. First, he has shown the importance of non-shared environment, a term that he coined to refer to the environmental reasons why children growing up in the same family are so different. Second, he has shown that many environmental measures in psychology show genetic influence and that genetic factors can mediate associations between environmental measures and developmental outcomes.

He earned a B.A. in Psychology from DePaul University in 1970 and a Ph.D. in Psychology in 1974 from the University of Texas, Austin. He then worked at the Institute for Behavioural Genetics at the University of Colorado at Boulder. From 1986 until 1994 he worked at Pennsylvania State University studying elderly twins reared apart and twins reared together to study aging and is currently at King's College London, Institute of Psychiatry. He has been president of the Behavior Genetics Association, which in 2002 awarded him the Dobzhansky Memorial Award for a Lifetime of Outstanding Scholarship in Behavior Genetics. He was awarded the William James Fellow Award by the Association for Psychological Science in 2004.

Plomin is currently conducting the Twins Early Development Study of all twins born in England from 1994 to 1996, focusing on developmental delays in early childhood and their association with behavioural problems.^[1]

In 1994 he was one of 52 signatories on "Mainstream Science on Intelligence,^[2]" an editorial written by Linda Gottfredson and published in the Wall Street Journal, which declared the consensus of the signing scholars on issues related to intelligence research following the publication of the book The Bell Curve.

His most recent books are Behavioral Genetics in the Postgenomic Era (Washington, DC: APA Books, 2003) and Behavioral Genetics (4th edition, New York: Worth Publishers, 2001).

References[link]

External links[link]

• Official website
• William James Fellow Award citation

This article about a psychologist is a stub. You can help Wikipedia by expanding it. v t e