Heredity is the passing of traits to offspring. This is the process by which an offspring cell or
organismacquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause a speciesto evolve. The study of heredity in biologyis called genetics, which includes the field of epigenetics.
ancientshad a variety of ideas about heredity: Theophrastusproposed that male flowers caused female flowers to ripen; Hippocratesspeculated 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".
Various hereditary mechanisms were envisaged without being properly tested or quantified. These included
blending inheritanceand 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 Lamarckianideas on evolution.
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" ( 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. Pangenesiswas 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 little over 100 years ago. The terms "blood relative", "full-blooded", and "royal blood" are relics of pangenesis. Francis Galton, Charles Darwin's cousin, experimentally tested and disproved pangenesis during the 1870s.
Types of heredity
The description of a mode of biological inheritance consists of three main categories:
* 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. Additionally, 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 (
GonosomalLoci):- Sex-limitedphenotype expression (e.g. Cryptorchism):-Inheritance through the maternal line (in case of Mitochondrial DNAloci):-Inheritance through the paternal line (in case of Y-chromosomal loci)
Epistasiswith other Loci (e.g. overdominance):- Gene couplingwith 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 geneticscan also be employed.
Charles Darwin: Theory of evolution
Charles Darwinproposed 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 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 " The Origin" 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 biometricschool 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 Weismanncut 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 [cite book
last = Henig
first = Robin Marantz
title = The Monk in the Garden : The Lost and Found Genius of Gregor Mendel, the Father of Genetics
publisher = Houghton Mifflin
date = 2000
isbn = 0-395-97765-7
quote = The article, written by an obscure Moravian monk named Gregor Mendel] monk
Gregor Mendelwho 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 on The Correlation Between Relatives on the Supposition of Mendelian Inheritance.
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 synthesis. The modern synthesis bridged the gap between experimental geneticists and naturalists; and between both and palaeontologists, stating that: [ Mayr & Provine 1998] [Mayr E. 1982. "The growth of biological thought: diversity, evolution & inheritance". Harvard, Cambs. p567 et seq.]
#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 is overwhelmingly the main mechanism of change; even slight advantages are important when continued. The object of selection is the
phenotypein its surrounding environment. The role of genetic driftis 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
speciationoccurs 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 Gouldsuggests he may be an exception. [Gould S.J. "Ontogeny and phylogeny". Harvard 1977. p221-2]
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 IIera. Trofim Lysenkohowever caused a backlash of what is now called Lysenkoismin the Soviet Unionwhen he emphasised Lamarckianideas on the inheritance of acquired traits. This movement affected agricultural research and led to food shortages in the 1960s and seriously affected the USSR.
* [http://plato.stanford.edu/entries/heredity/ Stanford Encyclopedia of Philosophy entry on Heredity and Heritability]
* [http://www.britannica.com/eb/article-9111157/heredity Encyclopaedia Britannica, Heredity]
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