Heredity is the passing of traits to offspring (from its parent or ancestors). This is the process by which an offspring
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
or
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause a
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
to
evolveIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
. The study of heredity in
biologyBiology is the natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy...
is called
geneticsGenetics, , 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...
, which includes the field of
epigeneticsIn biology, the term epigenetics refers to changes in phenotype or gene expression caused by mechanisms other than changes in the underlying DNA sequence, hence the name epi- -genetics. These changes may remain through cell divisions for the remainder of the cell's life and may also last for...
.
History
The ancients had a variety of ideas about heredity:
TheophrastusTheophrastus , a Greek native of Eressos in Lesbos, was the successor of Aristotle in the Peripatetic school. His interests were wide-ranging, extending from biology and physics to ethics and metaphysics. His two surviving botanical works, Enquiry into Plants and On the Causes of Plants, were an...
proposed that male flowers caused female flowers to ripen;
HippocratesHippocrates of Cos or Hippokrates of Kos - Greek: ; Hippokrátēs was an ancient Greek physician of the Age of Pericles, and was considered one of the most outstanding figures in the history of medicine...
speculated that "seeds" were produced by various body parts and transmitted to offspring at the time of conception, and
AristotleAristotle was a Greek philosopher, a student of Plato and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology.Together with Plato and Socrates , Aristotle is one of...
thought that male and female semen mixed at conception.
AeschylusAeschylus was an ancient Greek playwright. He is often recognized as the father of tragedy, and is the earliest of the three Greek tragedians whose plays survive, the others being Sophocles and Euripides...
, in
458 BC-Greece:* Pleistoanax succeeds his father Pleistarchus as king of Sparta.* Pericles continues Ephialtes' democratising activities by making the archonship a paid office and the lower class of Athenian citizens eligible to hold the office....
, 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 inheritanceIn 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...
and the inheritance of acquired traits. Nevertheless, people were able to develop domestic breeds of animals as well as crops through
artificial selectionArtificial 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...
. The inheritance of acquired traits also formed a part of early Lamarckian ideas on evolution.
In the 9th century AD, the
Afro-ArabAfro-Arab can refer to people of mixed Black African and genealogical Arab ancestral heritage and/or linguistically and culturally Arabized Black Africans...
writer
Al-JahizAl-Jāḥiẓ was a famous Afro-Arab scholar of East African descent, the grandson of a Negro slave...
considered the effects of the
environmentThe natural environment, commonly referred to simply as the environment, is a term that encompasses all living and non-living things occurring naturally on Earth or some region thereof....
on the likelihood of an animal to survive, and first described the struggle for existence. His ideas on the struggle for existence in the
Book of Animals have been summarized as follows:
In 1000 AD, the Arab physician, Abu al-Qasim al-Zahrawi (known as Albucasis in the West), wrote the first clear description of
haemophiliaHaemophilia is a group of hereditary genetic disorders that impair the body's ability to control blood clotting or coagulation, which is used to stop bleeding when a blood vessel is broken...
, a hereditary
genetic disorderA 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...
, in his
Al-TasrifThe Kitab al-Tasrif was an influential Arabic medical encyclopedia on medicine and surgery, written near the year 1000 CE by Abu al-Qasim al-Zahrawi , the "father of modern surgery"...
. In this work, he wrote of an
AndalusianAl-Andalus was the Arabic name given to the parts of the Iberian Peninsula and Septimania governed by Arab and North African Muslims , at various times in the period between 711 and 1492....
family whose males died of bleeding after minor injuries.
During the 1700s, Dutch microscopist Antoine van Leeuwenhoek (1632–1723) discovered "animalcules" in the sperm of humans and other animals. Some scientists speculated they saw a "little man" (
homunculusA homunculus is, most generally, any representation of a human being. It is often used to illustrate the functioning of a system...
) inside each
spermA sperm, from the ancient Greek word σπέρμα and and more commonly known as a sperm cell, is the haploid cell that is the male gamete. It joins an ovum to form a zygote. A zygote is a single cell, with a complete set of chromosomes, that normally develops into an embryo.Sperm cells contribute...
. 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.
PangenesisPangenesis 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...
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", "full-blooded", and "royal blood" are relics of pangenesis.
Francis GaltonSir Francis Galton FRS , cousin of Sir Douglas Galton, was an English Victorian polymath, anthropologist, eugenicist, tropical explorer, geographer, inventor, meteorologist, proto-geneticist, psychometrician, and statistician. He was knighted in 1909.Galton had a prolific intellect, and produced...
,
Charles DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
's cousin, experimentally tested and disproved pangenesis during the 1870's.
Types of heredity
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...
- 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...
- Oligogenetic – few loci
- Polygenetic – many loci
- 2. Involved chromosome
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...
s
- 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 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 more common case)
- Y-chromosomal – loci are situated on the Y chromosome
The Y chromosome is the sex-determining chromosome in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development, thus determining sex. The human Y chromosome is composed of about 60 million base pairs.- Overview :...
- Mitochondrial – loci are situated on the mitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within cells that convert the energy from food into a form that cells can use...
- 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...
–phenotypeA phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and possible interactions...
- Dominant
- Intermediate
Intermediate means "occurring between two extremes, or in the middle of a range" and may refer to:* Reactive intermediate, a short-lived, unstable molecule in a chemical reaction* Intermediate goods, goods used to produce other goods...
(also called "codominant")
- Recessive
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 is a term used in genetics to describe the proportion of individuals carrying a particular variation of a gene that also express an associated trait . In medical genetics, the penetrance of a disease causing mutation is the proportion of individuals with the mutation who exhibit...
- Complete
- Incomplete (percentual number)
- Expressivity
The term "Expressivity" is used in geneticsthat refering to variations of a phenotypein individuals carrying a particular genotype.The term can be used to qualitatively orquantitatively characterize the extent of the...
- Heritability
In genetics, Heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. Variation among individuals may be due to genetic and/or environmental factors...
(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...
phenomena (also see epigeneticsIn biology, the term epigenetics refers to changes in phenotype or gene expression caused by mechanisms other than changes in the underlying DNA sequence, hence the name epi- -genetics. These changes may remain through cell divisions for the remainder of the cell's life and may also last for...
)
- 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
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within cells that convert the energy from food into a form that cells can use...
loci)
- Inheritance through the paternal line (in case of Y-chromosomal
The Y chromosome is the sex-determining chromosome in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development, thus determining sex. The human Y chromosome is composed of about 60 million base pairs.- Overview :...
loci)
- 6. Locus–locus interactions
- Epistasis
Epistasis is the interaction between genes. Epistasis takes place when 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...
with other loci (e.g., overdominanceOverdominance is a condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents.- Overdominance in human genetics :...
)
- Gene coupling with other loci (also see crossing over
Chromosomal crossover is the exchange of genetic material between homologous chromosomes. This often occurs during prophase 1 of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed until near the end of prophase 1...
)
- 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 geneticsMolecular genetics is the field of biology that studies the structure and function of genes at a molecular level. The field studies how the genes are transferred from generation to generation. Molecular genetics employs the methods of genetics and molecular biology. It is so-called to...
can also be employed.
Charles Darwin: theory of evolution
Charles DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
proposed a theory of evolution in 1859 and 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
traitTrait may refer to:* Trait, a characteristic or property of some object.* Biological traits, which involve genes and characteristics of organisms.* Trait theory, an approach to the psychological study of personality.In computing...
s (
pangenesisPangenesis 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...
). 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
sexIn biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...
-linked, etc.) rather than suggesting mechanisms.
Darwin's initial model of heredity was adopted by, and then heavily modified by, his cousin
Francis GaltonSir Francis Galton FRS , cousin of Sir Douglas Galton, was an English Victorian polymath, anthropologist, eugenicist, tropical explorer, geographer, inventor, meteorologist, proto-geneticist, psychometrician, and statistician. He was knighted in 1909.Galton had a prolific intellect, and produced...
, 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 WeismannFriedrich Leopold August Weismann was a German evolutionary biologist. Ernst Mayr ranked him the second most notable evolutionary theorist of the 19th century, after Charles Darwin....
cut the
tailThe tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals and birds...
s off many generations of
miceA mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. The American white-footed mouse and the deer mouse also sometimes live in houses. In some places, certain kinds of field mice are also common...
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
MoraviaMoravia is a historical region in central Europe in the east of the Czech Republic, one of the former Czech lands. It takes its name from the Morava River which rises in the northwest of the region.-Geography:...
n monk
Gregor MendelGregor Johann Mendel was an Augustinian priest and scientist, and is often called the father of genetics for his study of the inheritance of certain traits in pea plants. Mendel showed that the inheritance of these traits follows particular laws, which were later named after him...
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. FisherSir Ronald Aylmer Fisher, FRS was an English statistician, evolutionary biologist, eugenicist and geneticist. He was described by Anders Hald as "a genius who almost single-handedly created the foundations for modern statistical science," and Richard Dawkins described him as "the greatest of...
's (1918) paper on
The Correlation Between Relatives on the Supposition of Mendelian InheritanceThe Correlation Between Relatives on the Supposition of Mendelian Inheritance is a scientific paper by R.A. Fisher which was published in the Philosophical Transactions of the Royal Society of Edinburgh in 1918,...
.
Modern development of genetics and heredity
In the 1930s, work by Fisher and others resulted in a combination of Mendelian and biometric schools into the
modern evolutionary synthesisThe modern evolutionary synthesis is a union of ideas from several biological specialties which forms a logical account of evolution. This synthesis has been accepted by nearly all working biologists...
. The modern synthesis bridged the gap between experimental geneticists and naturalists; and between both and palaeontologists, stating that:
- All evolutionary phenomena can be explained in a way consistent with known genetic mechanisms and the observational evidence of naturalists.
- 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).
- Selection
Natural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
is overwhelmingly the main mechanism of change; even slight advantages are important when continued. The object of selection is the phenotypeA phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and possible interactions...
in its surrounding environment. The role of genetic driftGenetic drift or allelic drift is the change in the relative frequency with which a gene variant occurs in a population due to random sampling and chance: the alleles in offspring are a random sample of those in the parents, and chance has a role in determining whether a given individual survives...
is equivocal; though strongly supported initially by Dobzhansky, it was downgraded later as results from ecological genetics were obtained.
- 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.
- 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
speciationSpeciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
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 allelesAllele frequency is the proportion of all copies of a gene that is made up of a particular gene variant . In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place in a population. It can be expressed for example as a...
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 BeerSir Gavin Rylands de Beer FRS was a British evolutionary embryologist. He was Director of the British Museum , President of the Linnean Society, and received the Royal Society's Darwin Medal for his studies on evolution.-Biography:...
's work by
Stephen Jay GouldStephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation. Gould spent most of his career teaching at Harvard University and working at the American Museum...
suggests he may be an exception.
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 IIWorld War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...
era.
Trofim LysenkoTrofim Denisovich Lysenko was a Ukrainian agronomist who was director of Soviet biology under Joseph Stalin...
however caused a backlash of what is now called
LysenkoismLysenkoism was a set of repressive political and social campaigns in science and agriculture by the powerful Stalinist director of the Soviet Lenin All-Union Institute of Agricultural Sciences, Trofim Denisovich Lysenko and his followers, which began in the late 1920s and formally ended in...
in the
Soviet UnionThe Union of Soviet Socialist Republics was a constitutionally socialist state that existed in Eurasia from 1922 to 1991. The name is a translation of the , tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik, abbreviated СССР, SSSR. The common short name is Soviet Union, from , Sovetskiy Soyuz...
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.
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