Heredity is the passing of traits to offspring (from its parent or ancestors). This is the process by which an offspring cell The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans, or organism In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole. An organism may either be unicellular (single-celled) or be composed of, as in humans, many trillions of cells grouped into acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause a species In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as similarity of DNA, morphology or to evolve Evolution is the change in the inherited traits of a population of organisms through successive generations. This change results from interactions between processes which introduce variation into a population, and other processes which remove it. As a result, variants with particular traits become more, or less, common. A trait is a particular. The study of heredity in biology Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy is called genetics Genetics , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the process of, which includes the field of epigenetics In biology, and specifically genetics, epigenetics is the study of inherited changes in phenotype or gene expression caused by mechanisms other than changes in the underlying DNA sequence, hence the name epi- (Greek: επί- over, above) -genetics. These changes may remain through cell divisions for the remainder of the cell's life and may also.

History

The ancients Ancient history is the study of the written past from the beginning of recorded human history in the Old World to the Early Middle Ages in Europe had a variety of ideas about heredity: Theophrastus Theophrastus , a Greek native of Eresos in Lesbos, was the successor of Aristotle in the Peripatetic school. He came to Athens at a young age, and initially studied in Plato's school. After Plato's death he attached himself to Aristotle. Aristotle bequeathed to Theophrastus his writings, and designated him as his successor at the Lyceum proposed that male flowers caused female flowers to ripen; Hippocrates Hippocrates of Cos or Hippokrates of Kos - Greek: Ἱπποκράτης; Hippokrátēs was an ancient Greek physician of the Age of Pericles (Classical Athens), and is considered one of the most outstanding figures in the history of medicine. He is referred to as the Western father of medicine in recognition of his lasting contributions to the speculated that "seeds" were produced by various body parts and transmitted to offspring at the time of conception, and Aristotle Aristotle (384 BC – 322 BC) was a Greek philosopher, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology. Together with Plato and Socrates (Plato's teacher), Aristotle is one of the most thought that male and female semen mixed at conception. Aeschylus Aeschylus was the first of the three ancient Greek tragedians whose work has survived, the others being Sophocles and Euripides, and is often recognized as the father of tragedy. His name derives from the Greek word αισχος (aischos), meaning "shame". According to Aristotle, he expanded the number of characters in plays to allow for, in 458 BC, proposed the male as the parent, with the female as a "nurse for the young life sown within her."

Various hereditary mechanisms were envisaged without being properly tested or quantified. These included blending inheritance In Darwin's time, biologists held to the theory of blending inheritance—an offspring was an average of its parent. If an individual had one short parent and one tall parent, it would always be of some interim height between that of its parents. In turn, this interim height would become a theoretical extreme that, in turn, bounded the potential and the inheritance of acquired traits. Nevertheless, people were able to develop domestic breeds of animals as well as crops through artificial selection Artificial selection describes intentional breeding for certain traits, or combination of traits. The term was utilized by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive ability (“Darwinian fitness”). As opposed to. The inheritance of acquired traits also formed a part of early Lamarckian Lamarckism is the idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring (also known as heritability of acquired characteristics or soft inheritance). It is named after the French biologist Jean-Baptiste Lamarck (1744–1829), who incorporated the action of soft inheritance into his evolutionary ideas on evolution.

In the 9th century AD, the Afro-Arab Afro-Arab refers to people of mixed Black African and genealogical Arab ancestral heritage and/or linguistically and culturally Arabized Black Africans. There are communities of Afro-Arabs in East Africa, West Africa, North Africa, the Near East and Western Europe writer Al-Jahiz Al-Jāḥiẓ (real name Abu Uthman Amr ibn Bahr al-Kinani al-Fuqaimi al-Basri) (781 – December 868/January 869) was a famous Arab scholar. One of his grandparents was a Zanj slave. He was an Arabic prose writer and author of works on Arabic literature, biology, zoology, history, early Islamic philosophy, Islamic psychology, Mu'tazili theology, considered the effects of the environment The natural environment, commonly referred to simply as the environment, encompasses all living and non-living things occurring naturally on Earth or some region thereof.[citation needed] on the likelihood of an animal to survive, and first described the struggle for existence Charles Darwin's On the Origin of Species, published on 24 November 1859, is a work of scientific literature which is considered to be the foundation of evolutionary biology. Its full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. For the sixth edition of 1872, the.^[1][2] His ideas on the struggle for existence in the Book of Animals have been summarized as follows:

"Animals engage in a struggle for existence; for resources, to avoid being eaten and to breed. Environmental factors influence organisms to develop new characteristics to ensure survival, thus transforming into new species. Animals that survive to breed can pass on their successful characteristics to offspring."^{[3][unreliable source?]}

In 1000 AD, the Arab physician In the history of medicine, Islamic medicine or Arabic medicine refers to medicine developed in the medieval Islamic civilization and written in Arabic, the lingua franca of the Islamic civilization. Despite these names, a significant number of scientists during this period were not Arab. Some consider the label "Arab-Islamic" as, Abu al-Qasim al-Zahrawi Abu al-Qasim Khalaf ibn al-Abbas Al-Zahrawi , (Arabic: أبو القاسم بن خلف بن العباس الزهراوي‎) also known in the West as Abulcasis, was an Andalusian Arab physician, surgeon, chemist, cosmetologist, and scientist. He is considered Islam's greatest medieval surgeon and one of the fathers of modern surgery. His (known as Albucasis in the West), wrote the first clear description of haemophilia, a hereditary genetic disorder A genetic disorder is an illness caused by abnormalities in genes or chromosomes. While some diseases, such as cancer, are due in part to a genetic disorders, they can also be caused by environmental factors. Most disorders are quite rare and affect one person in every several thousands or millions. Some types of recessive gene disorders confer an, in his Al-Tasrif The Kitab al-Tasrif (The Method of Medicine) was an influential Arabic medical encyclopedia on medicine and surgery, written near the year 1000 CE by Abu al-Qasim al-Zahrawi (Abulcasis), the "father of modern surgery". The 30-volume work includes anatomical descriptions, classifications of diseases, information on nutrition and surgery,. In this work, he wrote of an Andalusian Al-Andalus was the Arabic name given to a nation in the parts of the Iberian Peninsula and Septimania governed by Muslims (given the generic name of Moors), at various times in the period between 711 and 1492 family whose males died of bleeding after minor injuries.^[4]

During the 1700s, Dutch microscopist Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek (born on October 24, 1632 and died on August 26, 1723 – buried on August 30) was a Dutch tradesman and scientist from Delft, Netherlands. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist. He is best known for his work on the improvement of the microscope (1632–1723) discovered "animalcules" in the sperm of humans and other animals. Some scientists speculated they saw a "little man" (homunculus Homunculus is a term used, generally, in various fields of study to refer to any representation of a human being. Historically it referred specifically to the concept of a miniature though fully-formed human body, for example, in the studies of alchemy and preformationism. Currently, in scientific fields, a homunculus may refer to any scale model) 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.

Pangenesis Pangenesis was Charles Darwin's hypothetical mechanism for heredity. He presented this 'provisional hypothesis' in his 1868 work The Variation of Animals and Plants under Domestication and felt that it brought 'together a multitude of facts which are at present left disconnected by any efficient cause'. The etymology of the word comes from the was an idea that males and females formed "pangenes" in every organ. These pangenes subsequently moved through their blood to the genitals and then to the children. The concept originated with the ancient Greeks, and influenced biology until as recently as a century ago. The terms "blood relative," "bloodline," "full-blooded," and "royal blood" are relics of pangenesis. Francis Galton Sir Francis Galton FRS , cousin of Sir Douglas Galton, half-cousin of Charles Darwin, was an English Victorian polymath, anthropologist, eugenicist, tropical explorer, geographer, inventor, meteorologist, proto-geneticist, psychometrician, and statistician. He was knighted in 1909, Charles Darwin Charles Robert Darwin FRS was an English naturalist[I] who established that all species of life have descended over time from common ancestors, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection. He published his theory with compelling evidence for evolution in his 18's cousin, experimentally tested and disproved pangenesis during the 1870s.

Types of heredity

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 In the fields of genetics and evolutionary computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map. Gene mapping is the process of determining the locus for a particular

• Monogenetic (also called "simple") – one locus In the fields of genetics and evolutionary computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map. Gene mapping is the process of determining the locus for a particular
• Oligogenetic – few loci
• Polygenetic Inheritance of quantitative traits or polygenic inheritance refers to the inheritance of a phenotypic characteristic that varies in degree and can be attributed to the interactions between two or more genes and their environment. Though not necessarily genes themselves, quantitative trait loci are stretches of DNA that are closely linked to the – many loci

2. Involved chromosomes A chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek χρῶμα

• Autosomal An autosome is a chromosome that is not a sex chromosome – that is to say there are an equal number of copies of the chromosome in males and females. For example, in humans, there are twenty-two pairs of autosomes, and, in addition, there are the X and Y sex chromosomes – 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 X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals . It is a part of the XY sex-determination system and X0 sex-determination system. The X chromosome was named for its unique properties by early researchers, which resulted in the naming of its counterpart Y chromosome, for the next letter in (the more common case)
  • Y-chromosomal – loci are situated on the Y chromosome The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs. DNA in the Y chromosome is passed from father to son, thus tracking many surnames. Y-DNA analysis is
• Mitochondrial In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 10 micrometers (μm) in diameter. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of – loci are situated on the mitochondrial DNA Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the energy from food into a form that cells can use. Most other DNA present in eukaryotic organisms is found in the cell nucleus

3. Correlation genotype The genotype is the genetic constitution of a cell, an organism, or an individual usually with reference to a specific character under consideration. For instance, the human albino gene has two recessive alleles, recessive a and recessive a. It is a generally accepted theory that inherited genotype, transmitted epigenetic factors, and non-–phenotype A phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior . Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and the interactions between the two

• Dominant
• Intermediate (also called "codominant")
• Recessive Dominance in genetics is a non-linear relationship between different forms of a gene and the resultant phenotype. A canonical example in a diploid species like humans is the simple 2-allele model of eye-color, where the brown (B) allele is dominant over the blue (b) allele. Both BB homozygotes and Bb heterozygotes express the brown-eye phenotype,

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

4. Coincidental and environmental interactions

• Penetrance Penetrance in genetics is the proportion of individuals carrying a particular variation of a gene that also express an associated trait (phenotype). In medical genetics, the penetrance of a disease-causing mutation is the proportion of individuals with the mutation who exhibit clinical symptoms. For example, if a mutation in the gene responsible
  • Complete
  • Incomplete (percentual number)
• Expressivity Expressivity is a term used in genetics that refers to variations of a phenotype in individuals carrying a particular genotype. The term can be used to qualitatively or quantitatively characterize the extent of the phenotype variation given a particular genotype. The term is equivalent to the severity of a condition in clinical medicine. For
  • Invariable
  • Variable
• Heritability Heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. Phenotypic variation among individuals may be due to genetic and/or environmental factors. Heritability analyses estimate the relative contributions of differences in genetic and non-genetic factors to the total (in polygenetic and sometimes also in oligogenetic modes of inheritance)
• Maternal or paternal imprinting Genomic imprinting is a genetic phenomenon by which certain genes are expressed in a parent-of-origin-specific manner. It is an inheritance process independent of the classical Mendelian inheritance. Imprinted genes are either expressed only from the allele inherited from the mother , or in other instances from the allele inherited from the father phenomena (also see epigenetics In biology, and specifically genetics, epigenetics is the study of inherited changes in phenotype or gene expression caused by mechanisms other than changes in the underlying DNA sequence, hence the name epi- (Greek: επί- over, above) -genetics. These changes may remain through cell divisions for the remainder of the cell's life and may also)

5. Sex-linked interactions

• Sex-linked inheritance (gonosomal loci)
• Sex-limited phenotype expression (e.g., cryptorchism Cryptorchidism is the absence of one or both testes from the scrotum. This usually represents failure of the testis to move, or "descend," during fetal development from an abdominal position, through the inguinal canal, into the ipsilateral scrotum. About 3% of full-term and 30% of premature infant boys are born with at least one)
• Inheritance through the maternal line (in case of mitochondrial DNA Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the energy from food into a form that cells can use. Most other DNA present in eukaryotic organisms is found in the cell nucleus loci)
• Inheritance through the paternal line (in case of Y-chromosomal The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs. DNA in the Y chromosome is passed from father to son, thus tracking many surnames. Y-DNA analysis is loci)

6. Locus–locus interactions

• Epistasis Epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is said to be epistatic, while the phenotype altered or suppressed is said to be hypostatic. Epistasis can be contrasted with dominance, which is an interaction 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 primarily achieved through statistical analysis of pedigree data. In case the involved loci are known, methods of molecular genetics can also be employed.

Charles Darwin: theory of evolution

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 could be inherited which were not expressed explicitly in the parent at the time of reproduction, 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.

Gregor Mendel: father of modern genetics

The idea of particulate inheritance of genes can be attributed to the Moravian^[5] 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."

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