Blending inheritance is an
obsolete theory in
biology
Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, History of life, origin, evolution, and ...
from the 19th century. The theory is that the progeny
inherits any characteristic as the average of the parents' values of that characteristic. As an example of this, a crossing of a red flower variety with a white variety of the same
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
would yield pink-flowered offspring.
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
's theory of inheritance by
pangenesis, with contributions to egg or sperm from every part of the body, implied blending inheritance. His reliance on this mechanism led
Fleeming Jenkin to attack Darwin's theory of
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...
on the grounds that blending inheritance would average out any novel beneficial characteristic before selection had time to act.
Blending inheritance was discarded with the general acceptance of
particulate inheritance during the development of modern
genetics
Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinians, Augustinian ...
, after .
History
Darwin's pangenesis
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
developed his theory of
evolution
Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
by
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...
on the basis of an understanding of
uniform processes in
geology
Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
, acting over very long periods of time on inheritable variation within populations. One of those processes was competition for resources, as
Thomas Malthus
Thomas Robert Malthus (; 13/14 February 1766 – 29 December 1834) was an English economist, cleric, and scholar influential in the fields of political economy and demography.
In his 1798 book ''An Essay on the Principle of Population'', Mal ...
had indicated, leading to a struggle to survive and to reproduce. Since some individuals would by chance have traits that allowed them to leave more offspring, those traits would tend to increase in the population. Darwin assembled many lines of evidence to show that variation occurred and that
artificial selection
Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant ...
by animal and plant breeding had caused change. All of this demanded a reliable mechanism of
inheritance
Inheritance is the practice of receiving private property, titles, debts, entitlements, privileges, rights, and obligations upon the death of an individual. The rules of inheritance differ among societies and have changed over time. Offi ...
.
Pangenesis was Aristotle's attempt to provide such a mechanism of inheritance. The idea was that each part of the parent's body emitted tiny particles called
gemmules, which migrated through the body to contribute to that parent's
gamete
A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...
s, their eggs or sperms. The theory had an intuitive appeal, as characteristics of all parts of the body, such as shape of nose, width of shoulders and length of legs are inherited from both the father and the mother. However, it had some serious weaknesses. Firstly, many characteristics can change during an individual's lifetime, and are affected by the environment:
blacksmith
A blacksmith is a metalsmith who creates objects primarily from wrought iron or steel, but sometimes from #Other metals, other metals, by forging the metal, using tools to hammer, bend, and cut (cf. tinsmith). Blacksmiths produce objects such ...
s can develop strong arm muscles during their work, so the gemmules from these muscles ought to carry this acquired characteristic. That implies the
Lamarckian inheritance of acquired characteristics. Secondly, the fact that the gemmules were supposed to mix together on
fertilisation
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a zygote and initiate its development into a new individual organism or of ...
implies blending inheritance, namely that the offspring would all be intermediate between the father and the mother in every characteristic. That directly contradicts the observed facts of inheritance, not least that children are usually either male or female rather than all
intersex
Intersex people are those born with any of several sex characteristics, including chromosome patterns, gonads, or genitals that, according to the Office of the United Nations High Commissioner for Human Rights, "do not fit typical binar ...
, and that traits such as flower colour often re-emerge after a generation, even when they seem to disappear when two varieties are crossed.
Darwin was aware of both these objections, and accordingly had strong doubts about blending inheritance, as evidenced in his private correspondence. In a letter to T.H. Huxley, dated November 12, 1857, Darwin wrote:
In a letter to Alfred Wallace
Alfred Russel Wallace (8 January 1823 – 7 November 1913) was an English naturalist, explorer, geographer, anthropologist, biologist and illustrator. He independently conceived the theory of evolution through natural selection; his 1858 pap ...
, dated February 6, 1866, Darwin mentioned conducting hybridization experiments with pea plants, not unlike those done by Gregor Mendel
Gregor Johann Mendel Order of Saint Augustine, OSA (; ; ; 20 July 1822 – 6 January 1884) was an Austrian Empire, Austrian biologist, meteorologist, mathematician, Augustinians, Augustinian friar and abbot of St Thomas's Abbey, Brno, St. Thom ...
, and like him obtaining segregating (unblended) varieties, effectively disproving his theory of pangenesis with blending:
Blending inheritance was also clearly incompatible with Darwin's theory of evolution by natural selection. The engineer Fleeming Jenkin used this to attack natural selection in his 1867 review of Darwin's ''On the Origin of Species
''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life'')The book's full original title was ''On the Origin of Species by M ...
''. Jenkin noted that if inheritance were by blending, any beneficial trait that might arise in a lineage would have "blended away" long before natural selection had time to act.[ The evolutionary biologist ]Richard Dawkins
Richard Dawkins (born 26 March 1941) is a British evolutionary biology, evolutionary biologist, zoologist, science communicator and author. He is an Oxford fellow, emeritus fellow of New College, Oxford, and was Simonyi Professor for the Publ ...
commented that blending inheritance was observably wrong, as it implied that every generation would be more uniform than the one before, and that Darwin should have said as much to Jenkin. The problem was not with natural selection, but with blending, and in Dawkins's view, Darwin should have settled for saying that the mechanism of inheritance was unknown, but certainly non-blending.[
]
Replacement by Mendelian inheritance
Blending inheritance was dismissed by the eventual widespread acceptance, after his death, of Gregor Mendel
Gregor Johann Mendel Order of Saint Augustine, OSA (; ; ; 20 July 1822 – 6 January 1884) was an Austrian Empire, Austrian biologist, meteorologist, mathematician, Augustinians, Augustinian friar and abbot of St Thomas's Abbey, Brno, St. Thom ...
's theory of particulate inheritance, which he had presented in '' Experiments on Plant Hybridization'' (1865). In 1892, August Weismann
August Friedrich Leopold Weismann (; 17 January 18345 November 1914) was a German evolutionary biology, evolutionary biologist. Fellow German Ernst Mayr ranked him as the second most notable evolutionary theorist of the 19th century, after Charl ...
set out the idea of a hereditary material, which he called the germ plasm, confined to the gonads and independent of the rest of the body (the soma). In Weismann's view, the germ plasm formed the body, but the body did not influence the germ plasm, except indirectly by natural selection. This contradicted both Darwin's pangenesis and Lamarckian inheritance. Mendel's work was rediscovered in 1900 by the geneticist Hugo de Vries and others, soon confirmed that same year by experiments by William Bateson
William Bateson (8 August 1861 – 8 February 1926) was an English biologist who was the first person to use the term genetics to describe the study of heredity, and the chief populariser of the ideas of Gregor Mendel following their rediscover ...
. Mendelian inheritance with segregating, particulate allele
An allele is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule.
Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...
s came to be understood as the explanation for both discrete and continuously varying characteristics.
See also
* Aristotle's biology
Aristotle's biology is the theory of biology, grounded in systematic observation and collection of data, mainly zoology, zoological, embodied in Aristotle's books on the science. Many of his observations were made during his stay on the island ...
* Incomplete dominance
Notes
References
Sources
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{{DEFAULTSORT:Blending Inheritance
Classical genetics