Gene Interaction Two Or More Pairs of Alleles Located on Different Pair Of
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Gene interaction Two or more pairs of alleles located on different pair of homologous chromosome may interact in various ways to control a single trait or even the phenotypic expression of one gene is altered by the influence of other gene. Such complex interactions are called gene interactions. These interactions may be:- Intragenic interactions – The interactions between or among two or more alleles of a single gene are called intragenic or intrallelic interactions. Intergenic interactions - The interactions between or among two or more alleles located on the different loci on the same chromosome or different chromosome is called intrgenic interactions. Monogenes – Majority of heritable traits are governed by a pair of alleles that controls the expression of a complete trait. Such genes are called monogenes. Qualitative Inheritance or discontinuous inheritance – The inheritance governed by monogenes is non metric and is called qualitative Inheritance. Eg. The fruit colour is red or green; seed shape is round or wrinkled; a cattle may have horn or no horn etc. Polygenes – Many characters such as height, weight, intelligence, colour etc exhibit continuous variations which are determined by number of genes . Each gene controls only a fraction or unit of expression of a trait and all the genic pairs have cumulative effect. Such genes are called polygenes or cumulative factors. Quantitative inheritance - The inheritance of these traits controlled by polygenes is called quantitative Inheritance. INTERALLELIC OR INTERAGENIC INTERACTIONS 1. INCOMPLETE DOMINANCE OR SEMI DOMINANCE Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. Unlike complete dominance inheritance, one allele does not dominate or mask the other. Example - Flower colour in Snapdragon – In Snapdragon a cross between pure breeds of red flowered and white flowered plants of Snapdragon produces all pink flowered plants in F1. On selfing of F1 hybrid the F2 generation consists of red, pink white flowers in ratio of 1:2:1 showing that there is absence of complete dominance of one allele over the other in the same allelic pair. 2.CO- DOMINANCE - Co-dominance is a type of non-Mendelian inheritance pattern that finds the traits expressed by the alleles to be equal in the phenotype. There is neither a complete dominance or incomplete dominance of one trait over the other for that given characteristic. Co- dominance would show both alleles equally instead of a blending of the traits as is seen in incomplete dominance.In the case of co-dominance, the heterozygous individual expresses both alleles equally. There is no mixing or blending involved and each is distinct and equally shown in the phenotype of the individual. Eg. Blood alleles in man- One example of co-dominance in humans is the AB blood type. Red blood cells have antigens on them that are designed to fight off other foreign blood types, which is why only certain types of blood can be used for blood transfusions based on the recipient's own blood type. A type blood cells have one kind of antigen, while the B type blood cells have a different type. Normally, these antigens would signal that they are a foreign blood type to the body and would be attacked by the immune system. People with AB blood types have both antigens naturally in their systems, so their immune system will not attack those blood cells. This makes people with the AB blood type "universal recipients" due to the co-dominance displayed by their AB blood type. The A type does not mask the B type and vice versa. Therefore, both the A antigen and B antigen are equally expressed in a display of co-dominance. LETHAL GENES It has been observed that all genes or genetic factors are not useful to the organism. There are some genetic factors or genes, when present in any organism cause its death during early stage of development, are called lethal genes. They may even cause death of the individual either in homozygous dominant or homozygous recessive condition. Types of Lethal Genes: Lethel genes may be classified in to the following groups: 1. Recessive lethals 2. Dominant lethals 3. Conditional lethals 1. Recessive lethal: Most of the lethal genes are recessive lethals. It is expressed only when they are in homozygous condition. The survival of heterozygotes is not affected e.g., coat colour in mice and Brachyphalangy in human beings – It is the occurrence of short digits in human beings due to the absence of one in the phalanges. A marriage between two persons having brachyphalangy results in 3 types of children. (1) ¼ normal due to homozygous for normal genes (2) 2/4 with brachyphalangy, a heterozygous condition. (3) ¼ children without phalanges die due to homozygous condition. 2. Dominant lethal: There are some lethal genes which reduce viability even in heterozygotes, are said as dominant lethals. e.g., epiloia gene in human beings. This cause mental defects, abnormal skin growth and tumors in heterozygotes, therefore, they die before reaching adulthood. The dominant lethals may be produced in every generation through mutation. Eg. Sickle shaped Anemia in man- This disease is an autosomal hereditary disorder in which erythrocytes become sickle shaped under oxygen deficiency due to the formation of abnormal haemoglobin by a gene Hbs. The gene for normal haemoglobin is represented as Hba. Homozygous individuals ( HbsHbs) die of fatal Anemia before reaching maturity whereas heterozygous individuals also called carriers have both Hbs and Hba. They show mild anemia because only a few RBC become sickle shaped. 3. Conditional lethal: The lethal genes require a definite or specific condition for their lethal action are said as conditional lethals. Many mutants of barley, maize, Neurospora, Drosophila and many other organisms are termed as temperature sensitive mutations. Each of them needs a definite, generally high temperature to express their lethal action. A chlorophyll mutant of barley allows normal chlorophyll development at a temperature of 19°C or above, but it develops albina or abnormal white seedlings at temperature below 8°C. Temperature is not only responsible to bring out conditional lethals. Some conditional lethals require light, nutrition etc. NON ALLELIC OR INTERGENIC INTERACTIONS COMPLEMENTARY GENES – These are two pairs of non allelic genes present on separate gene loci, interact to express for one phenotypic character but neither of them is able to express this character independently in the absence of other. Eg. Flower colour in sweet pea- In sweet pea flower is governed by interaction of two non- allelic genes WW and CC which are dominant over their recessive alleles ww and cc respectively. Independently both genes produce white coloured flowers. A cross between two white coloured varieties pure for gene W and C respectively produce purple coloured flowered in F1 hybrids. When these F1 plants were self pollinated , both purple and white flowered plants appeared in F2 generation in the ratio of 9:7. SUPPLIMENTARY GENES - Supplementary genes are two independent pairs of genes interacting in such a manner that one dominant factor produces its effect whether the other is present or not, while the second gene can produce its effect only in the presence of the first. Eg.- In lablab a dominant a dominant gene B express for khaki seed coat in homozygous or heterozygous condition while it’s recessive allele b express for buff colour in homozygous recessive condition. Another dominant gene L when interact with the dominant gene B produces chocolate colour. Features of Selaginella Observe the external features of the plant, and arrangement, types and shape of leaf, ligule, rhizophore and roots. 1. Plant body is sporophytic and the sporophyte is evergreen and perennial. 2. Most of the species are prostrate but Selaginella trachyphylla is sub-erect and S.erythropus is erect 3. Size of the sporophyte ranges from few centimetres to several feet in different species. 4. Plant body is differentiated into stem, leaves, rhizophore and roots. 5. Two sub-genera, namely Heterophyllum and Homoeophyllum, have been recognized in the genus Selaginella on the basis of characters of stem and leaves. 6. In sub-genus Heterophyllum the stem is prostrate, dorsiventral with lateral branching. It contains two types of leaves. But in case of sub-genus Homoeophyllum the stem is somewhat erect showing dichotomous type of branching and all the leaves are of only one type. 7. Leaves are simple, small, thin and ovate to lanceolate in shape 8. Each leaf contains a midrib. 9. In most of the species, leaves are of two types, i.e., smaller and larger 10. Leaves are arranged on the stem in four longitudinal rows. 11. Two rows of smaller leaves are present on the dorsal surface of stem while remaining two rows of larger leaves on the ventral surface. 12. A pair of leaves comprises of a small leaf on the dorsal surface and a large leaf on the ventral surface of the stem. 13. At the base of upper or adaxial surface of each leaf is present a thin membranous finger- like structure called ligule 14. Each ligule (Fig. 212) consists of a basal hemispherical glossopodium made up of large thin-walled cells and surrounded by a glossopodial sheath. Above the glossopodium is the body of ligule made up of many small and large cells. 15. At the place of branching in stem, arises a long, unbranched, leafless structure towards the lower side. This is known as rhizophore16. Rhizophore becomes branched at its tip and forms many adventitious roots. Pteridophyta Before the flowering plants, the landscape was dominated with plants that looked like ferns for hundreds of millions of years.