- THE FIRST MILLENIUM
1933 – USA
‘‘The Mechanism of Mendelian Heredity’ (1915), ‘The Theory of the Gene’ (1926)’
Starting with Mendel’s laws of segregation and independent assortment, Morgan investigated why there are far fewer chromosomes – the long thread-like structures present in the nucleus of every living cell, which grow and divide during cell splitting, – than there are ‘units of heredity’. Morgan could not see how these few chromosomes could account for all the changes that occur from one generation to the next.
Mendel’s ‘factors of heredity’ had been renamed ‘genes’ in 1909 by the Dane Wilhelm Johannsen.
When the organism forms its reproductive cells (gametes), the genes segregate and pass to different gametes.
Since it had been separately established that chromosomes play an important part in inheritance, then groups of genes had to be present on a single chromosome.
If all the genes were arranged along chromosomes, and all chromosomes were transmitted intact from one generation to the next, then many characteristics would be inherited together. This implicitly invalidates Mendel’s law of independent assortment, which dictated that hereditary traits caused by genes would occur in all possible mathematical combinations in a series of descendants, independent of each other.
Experimental evidence often seemed to back-up the mathematical forecasts for characteristics present in descendants that Mendel had suggested; Morgan felt that the law of independent assortment could not accurately model the process of arriving at the end result.
He began his experiments with the fruit fly, which has just four pairs of chromosomes, in 1908.
He observed a mutant white-eyed male fly, which he extracted for breeding with ordinary red-eyed females. Over subsequent generations of interbred offspring, the white-eyed trait returned in some descendants, all of which turned out to be males. Clearly, certain genetic traits were not occurring independently of each other but were being passed on in groups.
Rather than invalidating Mendel’s law of independent assortment, a simple adjustment was required to unite it with Hunt’s belief in chromosomes to produce his thesis.
He suggested that the law of independent assortment did apply – but only to genes found on different chromosomes. For those on the same chromosome, linked traits would be passed on; usually a sex-related factor with other specific features (such as, the male sex and the white-eyed characteristic).
The results of his work convinced Morgan that genes were arranged on chromosomes in a linear manner and could be mapped. Further testing showed that, as chromosomes actually break apart and re-form during the production of sperm and egg cells, linked traits could occasionally be broken during the exchange of genes (recombination) that occurred between pairs of chromosomes during the process of cell division. He hypothesised that the nearer on the chromosome the genes were located to each other, the less likely the linkages were to be broken. Thus by measuring the occurrence of breakages he could work out the position of the genes along the chromosome.
In 1911 he produced the first chromosome map showing the position of five genes linked to gender characteristics.
In 1933 Hunt Morgan received the Nobel Prize for Physiology.
1865 – Austria
‘Law of Segregation: In sexually reproducing organisms, two units of heredity control each trait. Only one of such units can be represented in a single sexually reproductive cell’
‘Law of Independent Assortment: Each of a pair of contrasted traits may be combined with either of another pair’
These laws laid the foundation for the science of genetics.
The biologist Lamarck (1744-1829) had proposed a theory of inheritance of acquired characteristics and had suggested that inherited characteristics are influenced by environment. Mendel planted an atypical variety of an oriental plant next to a typical variety – the offspring retained the essential traits of their parents, which meant that the characteristics that were inherited were not influenced by the environment. This simple test led Mendel to embark on the path that would lead to the discovery of the laws of heredity.
Mendel’s aim was to discover ” a generally applicable law of the formation and development of hybrids “. He addressed this by studying the effect of cross-breeding on seven pairs of contrasting characteristics of Pisum sativum, a strain of pea.
His work on peas indicated that features of the plant; seed shape, seed colour, pod shape, pod colour, flower colour, flower position and stem length; were passed on from one generation to the next by some physical element. He realised that each characteristic of a plant was inherited independently, and that the ratios of plants exhibiting each trait could be statistically predicted.
A common assumption in Mendel’s time was that when two alternative features were combined, an average of these features would occur. For example, a tall plant and a short one would result in medium height offspring. For seven years Mendel kept an exact record of the inherited characteristics of 28,000 pea plants, taking great pains to avoid accidental cross-fertilization; then he applied mathematics to the results. These quantitative data allowed him to see statistical patterns and ratios that had eluded his predecessors.
From his analysis he found that certain characteristics of plants are due to factors passed intact from generation to generation.
Mendel observed that individual plants of the first generation of hybrids (crossbred plants) usually showed the traits of only one parent. The crossing of yellow seeded plants with green seeded ones gave rise to yellow seeds; the crossing of tall stemmed ones with short-stemmed varieties gave rise to tall-stemmed plants.
The factors determining a trait are passed on to the offspring during reproduction.
Mendel worked out that the factors for each trait are grouped together in pairs and that the offspring receives one part of a pair from each parent.
Contrary to the popular belief of the time, these factors do not merge. Any individual pea always exhibits one trait or the other, never a mixture of the two possible expressions of the trait; only one trait from each pair of factors donated by the parents would be expressed in the offspring, although there are four possible combinations of factors.
This is now described as Mendel’s law of segregation.
An offspring inherits from its parents either one trait or the other, but not both.
He decided that some factors were ‘dominant’ and some were ‘recessive’ and was able to conclude that certain expressed traits, such as yellow seeds or tall stems, were the dominant ones and that other traits, such as shortness of stem and green seeds, were recessive. It appeared that the dominant factors consumed or destroyed the recessive factors – but this could not be the case, as the second generation of hybrids exhibited both the dominant and recessive traits of their ‘grandparents’. Across a series of generations of descendants, plants did not average out to a medium, but instead inherited the original features (for example, either tallness or shortness) in consistent proportions, a ratio of 3:1, according to the dominant factor.
The 3:1 ratio would apply because the dominant factor would feature whenever it was present.
He also noted that the different pairs of factors making up the characteristics of the pea plant ( such as the pair causing flower colour, the pair causing seed shape and so on ), when crossed, occurred in all possible mathematical combinations. This convinced him that the elements regulating the different features acted independently of each other, so the inheritance of one particular colour of flower was not influenced, for example, by the inheritance of pea shape.
This is now described as Mendel’s law of independent assortment.
He first articulated his results in 1865 and in 1866, which was shortly after Darwin’s ‘Origin of Species’ appeared, published them in an article ‘Versuche über Pflanzen-Hybriden’ (Experiments with plant hybrids).
No one before him had attempted to use mathematics and statistics as a means of understanding and predicting biological processes and during his lifetime and for some time after, his results were largely ignored.
Around the time of Mendel’s death, scientists using ever improving optics to study the minute architecture of cells coined the term ‘chromosome’ to describe the long, stringy bodies in the cell nucleus.
|The seven traits studied in peas|
|TRAIT||DOMINANT TRAIT||RECESSIVE TRAIT|
|Type of seed surface||smooth||wrinkled|
|Colour of seed albumen||yellow||green|
|Colour of seed coat||grey||white|
|Form of ripe pod||inflated||constricted|
|Colour of unripe pod||green||yellow|
|Position of flowers on stem||axial||terminal|
|Length of stem||tall||short|
‘There is doubt as to the probity of this Jesuit scholar, some claiming that his data was falsified whilst others argue that it is accurate’
Pilgrim, I. (1984) The Too-Good-to-be-True Paradox and Gregor Mendel. Journal of Heredity,#75, pp 501-2. Cited in Brake,M.L. & Hook, N. Different Engines – How science drives fiction and fiction drives science
1953 – UK
‘The self reproducing genetic molecule DNA has the form of a double helix’
The structure explains how DNA stores information and replicates itself.
The helical strands of DNA (deoxyribonucleic acid) consist of chains of alternating sugar and phosphate groups. Four types of base – adenine (A), cytosine (C), guanine (G) and thymine (T) – form the rungs of the DNA ladder, which can only be linked by hydrogen bonds in four combinations: A-T, C-G, T-A, G-C.
The DNA code is based on the order of these four bases and is carried from one generation to the next. The sequence of base pairs along the length of the strands is not the same in DNAs of different organisms. It is this difference in the sequence that makes one gene different from another.
1996 – Scotland
‘A mammal can be cloned from adult tissues’
Clones are genetically identical individuals produced from the same parent by non-sexual reproduction.
Wilmut and his team at the Roslin Institute near Edinburgh, Scotland, took the nuclei of somatic cells from the tissues of mammary glands of a mature sheep. They took eggs from another sheep, removed their nuclei, which contain DNA, and fused the somatic nuclei with the gamete cells by passing electric pulses through them. The process replaced the DNA of the egg with the genetic material from the mammary tissue. The cloned eggs were placed in a culture dish where they grew into embryos. The researchers cloned 277 eggs, of which 29 grew into embryos. These were transplanted into 13 ewes, acting as surrogate mothers. Five months later one lamb was born. The lamb, Dolly, had no father and its genes came entirely from the udder of a ewe. Dolly the cloned sheep died in 2003.
The mammal cloning experiment has been repeated successfully on other species of mammals. These experiments show that cloning humans is possible, but it has major theological, ethical, moral and social implications.
1952 – London, England
‘Description of the basic helical structure of the DNA molecule’
Her work is used, unaccredited, in Watson & Crick’s Nobel Prize-winning paper, from information ‘secretly’ leaked from Maurice Hugh Frederick Wilkins.
Through her work on X-ray diffraction, she realised that the ‘backbone’ of the DNA molecule was on the outside.
By 1952, Franklin had taken the clearest pictures of the molecules to date, which provided evidence of a helical, or spiral structure.
Watson & Crick would eventually articulate a ‘double-helix’ construction.