. The heat/cold part is largely symmetrical, though influenced on the father's side by other factors, but the form part is not. Scientists in
Antiquity 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 fluids 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". Ancient understandings of heredity transitioned to two debated doctrines in the 18th century. The Doctrine of Epigenesis and the Doctrine of Preformation were two distinct views of the understanding of heredity. The Doctrine of Epigenesis, originated by
Aristotle, claimed that the modifications of the parent's traits are passed off to an embryo during its lifetime, as the embryo continually develops. The foundation of this doctrine was based on the theory of
inheritance of acquired traits. In direct opposition, the Doctrine of Preformation claimed that "like generates like" where the germ would evolve to yield offspring similar to the parents. The Preformationist view believed procreation was an act of revealing what had been created long before. However, this was disputed by the creation of the
cell theory in the 19th century, which proved that all cells are created from preexisting cells. Various hereditary mechanisms, including
blending inheritance were also envisaged without being properly tested or quantified, and were later disputed. 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. An early research initiative emerged in 1878 when
Alpheus Hyatt led an investigation to study the laws of heredity through compiling data on family phenotypes (nose size, ear shape, etc.) and expression of pathological conditions and abnormal characteristics, particularly with respect to the age of appearance. One of the projects aims was to tabulate data to better understand why certain traits are consistently expressed while others are highly irregular.
Gregor Mendel: father of genetics and how this translates into Mendel's laws The idea of particulate inheritance of genes can be attributed to the
Moravian monk
Gregor Mendel who published his work on pea plants in 1865. However, his work was not widely known and was rediscovered in 1900. It was initially assumed that
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. His pea plant demonstration became the foundation of the study of Mendelian Traits. These traits can be traced on a single locus.
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: • 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
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. • 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. 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. 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. There is growing evidence that there is transgenerational inheritance of epigenetic changes in humans and other animals.
Common genetic disorders :*
Fragile X syndrome :*
Sickle cell disease :*
Phenylketonuria (PKU) :*
Haemophilia ==Types==