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The below mentioned article provides a close view on the expansions of Mendelian Laws:- 1. Introduction to the Expansions of Mendelian Laws 2. Multiple Alleles and Complex Loci 3. Compatibility in Certain Plans 4. Variation in Dominance Relation 5. Penetrance and Expressivity 6. Modiflying Genes 7. Pleiotropic or Many-Fold Effects of a Single Gene 8. Gene Interactions.
Contents:
- Introduction to the Expansions of Mendelian Laws
- Multiple Alleles and Complex Loci
- Compatibility in Certain Plans
- Variation in Dominance Relation
- Penetrance and Expressivity
- Modifying Genes
- Pleiotropic or Many-Fold Effects of a Single Gene
- Gene Interactions
1. Introduction to the Expansions of Mendelian Laws:
Mendel’s two laws of heredity—the law of segregation and the law of independent assortment, form a base for predicting the outcome of simple crosses in the eukaryotic organisms. These laws have been explained presuming that the characters of the organisms were determined by “factors “.
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Those factors were later called genes by W. Johannsen in 1909. According to Mendel, there existed 1: 1 ratio between the traits and the factors.
But subsequent investigations have revealed that the real world of genes is more complex than is revealed by Mendelian principles of heredity and the exceptions and modifications abound.
In this article we shall see that:
(i) More than two allelic forms of a gene can exist,
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(ii) There are variations in dominance relations, and
(iii) Many phenotypes are determined not by one gene pair but by several gene pairs or by non-allelic gene pairs interacting with one another or by genes interacting with environment.
2. Multiple Alleles and Complex Loci:
In the early history of genetics, Bateson (1906) stated that Mendelian factors were free entities and the presence or absence of a particular factor or gene determined the dominance or recessiveness respectively but when it became clear that the genes were linked to chromosomes and indeed that many genes were linked to chromosomes in linear fashion, the presence and absence hypothesis ran into its severe criticism.
In the meantime, multiple allelism was also discovered. Normally, a given diploid individual can possess only two different alleles for a set of characters. L.Cuenot (1904) demonstrated that the number of alternative forms of a particular gene was not necessarily limited to two, i.e., more than two allelic forms of a gene can map at the same locus.
This is called multiple allelism. Multiple alleles obey the same rule of transmission as alleles of which there are only two kinds.
Dominance relationships among multiple alleles vary in that for some groups of alleles every homozygous and heterozygous genotype produces a different phenotype, whereas in others the alleles may be arranged in a descending series in which every allele is dominant over all alleles below it.
The human ABO blood group alleles afford a modest example of multiple allelism. There are four blood groups (phenotypes) in the ABO system in which three alleles, IA, IB and i, determine the blood groups and only two of the three alleles can be present in any one individual as shown in Table 15.1.
From this Table it is evident that in this allelic series, the alleles IA and IB determine a unique form of antigen each, and the allele i determines a failure to produce either forms of antigen. Marriages between blood groups AB (1AIbx IAIb) will produce A Blood group: AB Blood groups: B Blood groups in 1: 2: 1 ratio or genotype ratio 1IAIA: 2IAIB: 1IBIB.
Incompatibility alleles in plants form good examples of multiple allelism. In many dicots and monocots, e.g., tobacco, petunias, sweet cherries, evening primroses, Brassica, etc., one gene determines compatibility/incompatibility relations with many different allelic forms of the gene present in different plants of any one species.
In these cases incompatibility is determined by a single self-incompatibility gene S. Generally, multiple alleles (such as S1, S2, S3, S4…) are common at S-locus. In some cases two genes, each with multiple allelic series are also known. The incompatibility reaction was first explained by East and Mangelsdorf in 1925 in nicotiana sanderae.
The incompatibility reaction of the pollen is determined by its own genotype and not by the genotype of the plant on which it is produced. If the pollen grain bears a S allele that is also present in the style, then it will not grow and so fertilization will not be effected. However, if S allele is absent in the style tissue, the pollen grain produces pollen tube containing male nuclei and this tube effects fertilization.
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The number of S alleles in a series in a species can be very large-over 50 in the evening primrose and trifolium and cases with more than 100 alleles have been reported in some species.
The following crosses indicate as to how multiple alleles control compatibility in certain plants:
3. Compatibility in Certain Plans:
4. Variation in Dominance Relation:
The phenotypic ratios obtained by Mendel with garden peas have been observed in a wide range of organisms. Such ratios require that one member of an allelic pair be dominant over the other (complete dominance). But, in fact, this kind of relationship between the alleles is quite uncommon. As far as the phenotype of heterozygote is concerned there are three more dominance relations.
Incomplete Dominance:
In Mirabilis Jalapa plants, when a pureline with red flowers is crossed to a pure line with white flowers, the results obtained are shown in Fig. 15.1.
It is evident from this cross that F, have not red but pink flowers. The possible explanation in this case may be that red phenotype and its determining allele R is incompletely dominant over the white phenotype and its allele r.
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Incomplete dominance is quite common and it is identified by the intermediate appearance or phenotype of the hybrid (heterozygote) between those of the parents (homozygotes). The phenomenon of incomplete dominance was first clearly described by Kolreuter.
Several examples of incomplete dominance may be found in animals also. One such example is found in Andalusian fowls where black feather colour has been found to be incompletely dominant over the splashed white feather colour. When black feathered fowl is crossed with a white feathered hen, the F1, individuals are blue feathered. When inbreeding is allowed in F1, hybrids, F2 generation is represented by black feathered, blue feathered and splashed white feathered individuals in 1: 2: 1 ratio as illustrated by fig 15.2.
Overdominance:
Another type of dominance relation is overdominance in which the phenotype of heterozyote is more extreme than that of either parent. The amount of florescent eye pigment in heterozygous white eyed Drosophila exceeds that found in either parent.
Co-dominance:
There is yet another type of dominance relation, the co-dominance. In the case of co-dominance the heterozygote shows the phenotype of both homozygotes or parents rather than expressing an internmediate phenotype as in incomplete dominance. In a sense, then, co-dominance is no dominance at all.
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AB blood group in man is an example of co-dominance in which both the alleles lA and IB are equally expressed, i, e., IAIB individuals have a phenotype that is essentially a combination of those shown by individuals with A and B blood groups, at least in terms of surface antigens of red blood cells specified by the alleles involved.
M-N blood group system also presents a good example of co-dominance. The three blood groups and their genotypes are given in Table 15.2.
Blood groups are characterised by the presence of specific immunological antigens on the surface of red blood cells. Individuals with genotype L have both antigens.
5. Penetrance and Expressivity:
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Some genes are little influenced by the other genes or the environment and are generally governed by one or more genes with large easily detectable effects. These genes are called oligogenes. The traits determined by oligogenes show distinct classes and are known as qualitative characters.
On the other hand many characters are affected by the genetic background and by the environment. These characters are governed by polygenes.
The characters which are affected by polygenes are referred to as quantitative characters. Since the quantitative characters do not show distinct classes, they have to be studied by measurements. A gene does not determine a phenotype by acting alone, it does so in conjunction with other genes and with environment.
Generally, the oligogenes express themselves in all the individuals that carry them and their expression is fairly uniform.
The ability of a gene to express itself in an individual that carries it is called penetrance.
An organism may be of genotype aa or A– but may not express the phenotype associated with its genotype—because of modifying effect of the environment or because of modifiers. Such genes are said to have incomplete penetrance. Penetrance is defined as the percentage of individuals with a given genotype which exhibit the phenotype specified by that genotype.
For example, if only 10 per cent of the seedlings of lima bean carrying a gene for chlorophyll deficiency exhibit chlorophyll deficiency, this gene has penetrance of only 10 per cent. The gene which expresses itself in every individual that carries it is said to have complete penetrance.
Expressivity, on the other hand, refers to the degree or extent to which a given genotype is expressed phenotypically in an individual. Some genes show variable degrees of expression in different individuals. Such genes are said to have variable expressivity which may be slight, intermediate or severe.
A gene which expresses itself uniformly in all the individuals carrying it has uniform or complete expressivity while those genes not expressed phenotypically in all the individuals uniformly are said to have variable or incomplete expressivity. The gene causing chlorophyll deficiency in lima bean (Phaseolus lunatus) seedlings exhibits variable expressivity in addition to incomplete penetrance.
In some seedlings the cotyledonary leaves have no chlorophyll, in some the chlorophyll is absent only in the tips, while in some others the chlorophyll deficiency may be observed in the margins of the leaves. Thus a single gene with variable expressivity may exhibit a number of phenotypes as if more than one gene was involved.
Some genes express themselves in a specific environment such as a particular temperature. The characters whose development depends upon a specific environment are referred to as threshold characters, as for example, a mutant gene in barely (hordeum vulgare) which produces albino seedlings at temperature below 8°C and normal green ones at the temperature above 19°C.
The incomplete penetrance of some genes may be due to a threshold requirement which may be helpful in identification of desirable types. For example, disease resistant and disease susceptible plants cannot be distinguished unless they are exposed to disease.
6. Modiflying Genes:
There are many genes which seem to have little or no effect of their own but increase or decrease the expression of other major genes. These are called modifiers or modifying genes since they modify the effects of other genes.
7. Pleiotropic or Many-Fold Effects of a Single Gene:
Mendel for convenience of explanation of his results said that single independent factor controlled a single trait only, but actually it is not always so. Now several such genes are known which govern many traits at a time.
In Drosophila, the recessive gene in homozygous state produces not only the vestigial wings but also it affects several other traits such as tiny wing-like balancer behind the wings, certain bristles, structure of reproductive organs, less production of eggs and decrease in the longevity.
In Jowar (sorghum), the gene that controls the length of internodes also determines breadth and length of leaves and length of inflorescence (penicle). In cotton, the height, boll size, number of ovules per locule and viability of seeds are governed by a single gene.
This type of gene controlling several characters at a time is termed as pleiotropic gene and the effect is referred to as pleiotropic effect. The pleiotropic genes sometimes show one major effect and several secondary effects.
8. Gene Interactions:
Most of the qualitative characters are determined by oligogenes or major genes. Nonallelic genes may not function independently in determining phenotypic characters in an organism. The effect of a gene depends not only on its own function but also on the functions of other genes and also on the environment.
Interaction between gene products may occur to produce new phenotypes without modifying typical Mendelian ratios, or it may cause modification of Mendelian ratios by one-gene product interfering with phenotypic expression of another non-allelic gene or genes.
The genetic interactions are discussed under the following heads:
(i) Gene-Gene Interactions.
(ii) Gene-Environment Interactions.
