Plant Bible

                                                                        Linkage in Sweet Pea Bateson and Punnet (1906) studied linkage in sweet pea ( Lathyrus odoratus ). In sweet pea, blue flower colour is dominant (BB) over red flower (bb). Long pollen grain is dominant (LL) over round pollen grain(ll). They crossed blue flowered, long pollen sweet pea with red flowered, round pollen plant. In these plants, the genes for flower colour and pollen shape are located in the same chromosome. That is, they are linked. The linked genotypes of the parents are written as follows: Blue flowered long pollen grained plant - BL/BL Red flowered round pollen grained plant-bl/bl The F1 plants were blue flowered with long pollen. The genotype is written as BL/bl. The F1 plant (BbLI) was test crossed with double recessive plant (bl/bl). In the F2 generation, the plants appeared in the ratio 7:1:1:7 instead of the Mendelian test cross ratio 1:1:1:1. Blue long-7 Blue round-1 Red long-

                                                                            Linkage in Maize   Hutchison experimentally proved the existence of linkage in maize ( Zea mays ). In maize, coloured seed (CC) is dominant and colourless seed is recessive (cc). Full seed (endosperm) is dominant (FF) and shrunken seed is recessive (ff). A cross was made between coloured full seeded plant with colourless shrunken seeded plant. The genes for seed colour a shape are linked. i.e. C and F are linked; c and f are linked. T linked genotypes of the parents are written as follows. Colourless shrunken seeded plant cf/cf , Coloured full seeded plant CF/CF All the F1 plants are coloured and full and the genotype written as CF/cf A test cross is made by crossing the F1 plant (CF/cf) with double recessive plant (cf/cf). In the F2 generation, plants were obtained in the ratio of 48.2: 1.8: 1.8:48.2. Coloured full CF/cf - 48.2% Coloured shrunken Cf/cf - 1.8% Colourless full cF/cf - - 1.8% Col

                                                                           Linkage in Drosophila In Drosophila, grey colour(G) is dominant over black colour (g) and long wing (L) is dominant over vestigial wing (l). The genes for these two characters (colour of the body and length of the wings) are linked together in the same chromosome. The linked genotypes of the parents are written as follows: Grey long-GL/GL Black vestigial-gl/gl When a fly with grey body and long wings (GGLL) is crossed with another fly having black body and vestigial wings (ggll), all the F, individuals are having grey body and long wings (GgLI). When F, male hybrid is test crossed with the double recessive parent, the F, contains only two types of individuals in equal numbers (1:1 ratio) instead of the expected 1:1:1:1 dihybrid test cross ratio. In the F2 generation, all the flies are like their parents. There are no new combinations. This is because the gene G is linked with L in one chromosome and

                                                                           Coupling and Repulsion                   Bateson and Punnet in 1906 proposed the hypothesis of coupling and repulsion to explain the later proposed linkage.  Coupling: The tendency of two dominant alleles or recessive alleles coming from the same parent tend to enter the same gamete and Coupling inherit together is called coupling. Experimental Proof for Coupling:                       W. Bateson and RC. Punnet in 1906 reported the phenomenon of coupling and repulsion (linkage) in sweet peas. A sweet pea with blue flower (B) and long pollen grains (L) is crossed with one having red flower (b) and round pollen grains (l). In this experiment, the blue colour B is dominant over red colour b and the long pollen L is dominant over pollen I grains. So all the F, offspring has blue flowers and long pollen grains. When they are test crossed (a heterozygous blue long (BbLI) with recessive parent (bbll) offspring appear i

                                                ALLELIC RELATIONSHIP   Dominant and Recessive Alleles:   Whenever one of a pair of alleles can come to phenotypic expression only in a homozygous genotype, we call that allele a recessive factor. The allele which can phenotypically express itself in the heterozygote as well as in the homozygote is called a dominant factor. Upper- and lower-case letters are commonly used to designate dominant and recessive alleles respectively. Usually, the genetic symbol corresponds to the first letter in the name of the abnormal (or mutant) trait.   (a) Carriers:   Recessive alleles (such as the one for albinism) are often deleterious to those who possess them in duplicate (homozygous recessive genotype). A heterozygote may appear just as normal as the homozygous dominant genotype. A heterozygous individual who possesses a deleterious recessive allele hidden from phenotypic expression by the dominant normal allele is called a carrier. Most of the

                                                                Modifications of root Tap root modification:   Storage roots: 1. Conical Root These are cone like, broad at the base and gradually tapering towards the apex. Example: Daucus carota. 2. Fusiform root: These roots are swollen in the middle and tapering towards both ends. Example: Raphanus sativus 3. Napiform root: It is very broad and suddenly tapers like a tail at the apex.   Example: Beta vulgaris Breathing root: Some mangrove plants like Avicennia, Rhizophora, Bruguiera develop special kinds of roots (Negatively geotropic) for respiration because the soil becomes saturated with water and aeration is very poor. They have a large number of breathing pores or pneumatophores for exchange of gases. Adventitious root modification:   Storage roots: 1. Tuberous root: These roots are swollen without any definite shape. Tuberous roots are produced singly and not in clusters. Example: Ipomoea bat