The below mentioned article provides a study note on the gene action and interaction.
Gene Action and Interaction:
The impetus of the rediscovery of Mendel’s work caused hundreds and thousands of genetical experiments to be performed and the word ‘Mendelism’ or the term ‘Mendelian genetics’ today does not mean merely what Mendel discovered but includes many post-Mendelian modifications based on the gene concept. Mendel was extremely lucky to have found rather simple inheritance in all the cases that he studied.
But, today, sufficient post-Mendelian evidences have accumulated which show that inheritance is not always as simple as Mendel found it to be Very often, as in the many cases cited below, inheritance of characters cannot be explained on the basis of single independent units showing dominance in allelomorphic pairs, So at the first sight, these inheritances may appear non-Mendelian.
Factor Hypothesis and the Gene Concept:
To explain this apparent anomaly, Bateson presented his factor hypothesis as a supplement to original Mendelism. This stated that characters are not always caused by ‘units’ but by factors of a fractional nature which may combine in different ways to cause the different characters. At that time the concept of particulate genes and the true structure of chromosomes were not very clear.
Clarification of these facts has greatly changed the original concept of factors. It is now becoming more and more clear that genes or factors behave as unit particles which remain quite separate and yet they may have combined effect on a character. Thus, although these are not cases of ‘unit characters’ yet they are being controlled by units of genes. It has already been stated that genes are complex molecules which are able to cause a number of physico-chemical activities which develop different characters.
The behaviour of the genes is being studied very closely and this new science of physiological genetics has advanced rapidly. With the modern concept of gene action, it is now quite easy to visualise that different gene actions may interact with one another, a number of genes may combine to form a character, a single gene may affect different characters, and so on. Thus, Mendel’s idea of ‘unit characters’ has to be modified. Moreover, the particulate concept of the ‘gene’ has induced many geneticists to use the word ‘factor’ as a synonym of it which was not its original meaning.
Presence or Absence Hypothesis—Explanation of Dominance:
In this connection, mention may be made of the Mendelian concept of dominance. After Mendel had clarified his idea about dominance some attempts were made to explain why this takes place. Bateson and Punnett tried to explain this by their Presence or absence hypothesis which was, later on, further elaborated by others. This hypothesis supposes that alleles do not mean that two determiners are present.
There is only one determiner whose presence is dominance and whose absence causes the recessive characters. Thus, the recessive determiner does not exist. That means that in the mono- hybrid ratio the true-breeding dominant tall has two doses of the determiner (say, TT) and is called duplex, the segregating dominant tall has one dose of the determiner (Tx instead of Td, x signifying absence), and is called simplex while the true breeding recessive dwarf has no determiner at all (xx or nil instead of dd) and should be called nulliplex.
The above explanation is HO longer popular among geneticists as it does not explain many other experimental results (e.g., that of multiple alleles, discussed later in connection with mutations) and is, therefore, untenable. Modern explanation of dominance is different and rather more complicated but it is always based on the assumption that there are actually two different genes representing an allelomorphic pair.
Some cases of dominance may be explained rather simply by supposing that a character is caused by certain reactions and that while the dominant enables these reactions to be carried on to the fullest extent, the recessive gene limits or checks the reactions so that the character is not fully developed or is developed differently, or, in some cases, the recessive gene may have no effect at all.
Xenia:
When, in course of hybridisation, the pollen changes the appearance of the hybrid seed still on the mother plant, the phenomenon is known as xenia. This is apparent in the case of endosperm characters. Formerly, it was thought to be a direct effect of the male on the female exerted through the pollen and the term ‘xenia’ was used by Focke as early as 1881 in this sense.
While the zygote is formed by the union of a male and a female gamete so that it is diploid with two sets of genomes, the endosperm mother cell in Angiosperms is formed by the fusion of three nuclei—first two embryo-sac nuclei and then a male gamete. So the endosperm mother cell is triploid having three sets of genomes.
The endosperm will then have three genes of an allelomorphic pair which may be symbolically represented as Aaa, Ttt, Ppp, etc. It is found that, in most cases where strict dominance is observed, if only one of these three genes is a dominant gene then the endosperm shows the dominant character inspite of the simultaneous presence of two recessive genes.
The second important point in xenia is that endosperm characters are evident one generation earlier. In ordinary hybridisation the hybrid seeds and ordinary self-fertilised seeds on the mother plant all look alike. The hybrid character becomes apparent only when the hybrid seed grows into the F1 plant.
On the other hand, in the case of xenia, the hybrid seed become at once distinct from the self-fertilised seeds because of their endosperm characters. This is because the endosperm develops fully while it is still located on its mother plant, i.e., on the plant representing the previous generation.
The phenomenon can be clearly seen when a maize plant with white grains is crossed with one with purple grains (Fig. 853). If some of the flowers in a female inflorescence of a pure breeding white grained maize plant are intentionally or accidentally pollinated with pollens from a pure breeding purple grained maize plant, the mature maize cob will show the hybrid grains as purple (since purple is dominant over white) while the self-fertilised grains will remain white.
Thus, this purple F1 character will be seen on the mother parent itself as an expression of xenia. Similarly the F1 plant, when fully grown and self-fertilised, will show the F2 segregation of endosperm colour on the cobs that will still be on the F1 plants. The proportion of purple to white grains on such a cob will be 3: 1 (of course, allowing statistical limits of variations), i.e., in the F2 ratio.
Similar endosperm characters are common specially in the different cereal crops which are mainly endosperm crops. Starchy (dominant) and glutinous (recessive) grains of rice, as also the starchy (dominant) and sugary (recessive) grains of maize, are similar pairs of endosperm characters which show xenia.
Metaxenia is another phenomenon sometimes included within xenia. This means the effect of the pollen (or the post-pollination effect induced by the growing endosperm and embryo) on the maternal tissue enclosing the seed. This effect is specially visible in the development of apples.
This type of development, however, is better explained differently than in the case of xenia.
Heterosis or Hybrid Vigour:
Ever since the art of hybridisation became known it was also clear that the F1 hybrid plant is much more vigorous than either parent. F1 shows the limit of such vigour and usually it decreases in the future generations. Shull (1914) proposed the name heterosis for this hybrid vigour.
Jones (1917) suggested that the reason of this vigour is that the maximum number of dominant genes comes together in the F1 when they counteract any ill effect of the recessive genes. East (1936) pointed out that because 6f linkage it is not possible to get all the dominants in any pure line form and the maximum association of the dominants is possible only in the heterozygous form of the F1 Hence the increase in vigour in hybrids as seen in the F1 hybrid rice plant in Fig. 854.
If the cause of increased vitality of hybrids is heterozygosis, it follows that the homozygous condition means weakness. A large number of crop-plants (wheat, rice, pulses, tomato, tobacco, etc.) are naturally self-pollinated and self-pollination always leads to homozygosity. But it is seen that these crops do not show so much loss of vigour as inbreeding does in case of maize.
The explanation may be that while harmful and lethal mutant genes are constantly arising in plants, natural inbreeding results in homozygosity bringing out the harmful characters and their eventual elimination in course of natural selection. But, in normally cross-bred plants, these harmful genes remain in a heterozygous recessive condition and are not eliminated causing weakness of the strain.
In recent years, the phenomenon of heterosis has been utilised commercially in the use of hybrid maize grains (‘hybrid corn’) directly for seed.