* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download genetics - New Age International
Pathogenomics wikipedia , lookup
Vectors in gene therapy wikipedia , lookup
Public health genomics wikipedia , lookup
Genetic engineering wikipedia , lookup
Epigenetics of neurodegenerative diseases wikipedia , lookup
Epigenetics of diabetes Type 2 wikipedia , lookup
Gene therapy wikipedia , lookup
Gene therapy of the human retina wikipedia , lookup
Polycomb Group Proteins and Cancer wikipedia , lookup
Gene nomenclature wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Minimal genome wikipedia , lookup
Gene desert wikipedia , lookup
Ridge (biology) wikipedia , lookup
Dominance (genetics) wikipedia , lookup
Therapeutic gene modulation wikipedia , lookup
Nutriepigenomics wikipedia , lookup
Site-specific recombinase technology wikipedia , lookup
Quantitative trait locus wikipedia , lookup
Genome evolution wikipedia , lookup
History of genetic engineering wikipedia , lookup
X-inactivation wikipedia , lookup
Biology and consumer behaviour wikipedia , lookup
Genomic imprinting wikipedia , lookup
Epigenetics of human development wikipedia , lookup
Gene expression programming wikipedia , lookup
Genome (book) wikipedia , lookup
Gene expression profiling wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
GENETICS CHAPTER 1 Mendels Experiments and Principles of Inheritance The contribution of Mendel to Genetics is called Mendelism. Gregor Johann Mendel from 1822–1884, the father of genetics who was an Austrian monk made crosses in garden pea (Pisum sativum) and conducted experiments during 1856-1863. He presented his results of the experiments before the Natural History of Society at Brunn in 1865. His paper was published in 1866 in the Annual Proceedings of the Society and distributed to libraries in Europe and America but no one appreciated and it was neglected until 1900. In 1900 the principles of genetics worked by Mendel were rediscovered by three botanists, namely Correns (Germany), De Vries (Holland) and Tshermark (Austria). Bateson confirmed Mendel’s work by a series of hybridization experiments. MENDELS EXPERIMENTS Mendel crossed varieties of edible peas (Pisum sativum). For example, he crossed a red-flowered variety with a white flowered variety. He did this by dusting the pollen of one variety (the red) on the pistils of other (the white). Of course, he prevented the white plant from pollinating itself. This he did by removing the stamens of the white flowers before the flowers had opened and shed their pollen. After pollinating the emasculated white flowers with the red pollen, he enclosed them in bags in order to prevent insects from getting to them with pollen from unknown sources. Thus he crossed the red variety and the white. The offspring of the cross was red. Mendel then self-fertilized the off-spring and he found that they produced off spring of their own in the ratio of 3 reds : 1 white. The pea plant contains a number of contrasting characters. Out of these contrasting characters, Mendel selected only seven characters. Mendel in his first experiment crossed two plants differing in one character (height) only. A plant having a tall stem was crossed with another plant having dwarf stem. Tall and dwarf are the two varieties of a single character, height. Such crosses, where parents differ in one pair of alternative characters are known as monohybrid crosses. The resulting hybrids are known as monohybrids. When the behaviour of each single character was established, Mendel crossed two plants differing in two characters, such as flower position and height of the stem. A plant having axial flower and a tall stem was crossed with a plant having terminal flower and a short stem. Such crosses, where parents differ in two pairs of alternative characters are known as dihybrid crosses. The resulting hybrids are known dihybrids. The plants involved in the above crosses are called parent plants. They form the parental generation which is marked by P. The first hybrid generation resulting from a cross between parental plants is called the first filial generation and is marked as F1. The second generation of hybrids arising from the self or cross fertilization of F1 hybrid generation is called the second filial generation and is marked as F2. 3 4 GENETICS AND PLANT BREEDING 1. Law of Dominance Each organism is formed of many characters and each character is controlled by a pair of factors or genes (T or t). Mendel’s law of dominance states that one factor in a pair may mask or prevent the expression of the other. He called the character appeared in the F1 generation or his monohybrid cross as dominant and those which did not appear in the F1 generation as recessive. A recessive factor freely expresses itself in the absence of its dominant allele. This law is formulated based on the monohybrid experiment. 2. Law of Segregation (Segregation of Genes) From his experiments Mendel concluded that each parent contributed one factor for a character to the F1 hybrid. In this way the F1 hybrid has two factors for each character. When the F1 hybrid forms gametes the two factors separate from each other. There is no mixing up of factors thus emphasizing the purity of gametes. The phenomenon of separation became Mendel’s second law of principle and was later termed as the Law of Segregation. This is explained diagrammatically as follows: P Tall plant (true breeding) Factors Gametes F1 hybrid F1 gametes F2 segregation X TT (T) Dwarf plant (true breeding) tt (t) Tall Tt (Selfing) i.e. Tall Tt (T) (t) X (T) (t) 3 Tall : 1 Dwarf TT, Tt, Tt, tt 3. Law of Independent Assortment This law is based on dihybrid experiment. According to this law, the genes for each pair of characters separate independently from those of other characters during gamete formation. Example : P Round Yellow RRYY Gametes : F1 (RY) F1 gametes X Round Yellow RrYy (RY) (Ry) (rY) (ry) 1:1:1:1 Wrinkled Green rryy (ry) X Self (Round Yellow) RrYy (RY) (Ry) (rY) (ry) 5 MENDEL’S EXPERIMENTS AND PRINCIPLES F2 Checker board RY Ry rY ry RY Ry rY ry RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy 9:3:3:1 ratio Mendel applied the principles of a monohybrid cross in the dihybrid cross, the true breeding round yellow parent must be homozygous RRYY, and the wrinkled green parent rryy. Since each character is determined by two factors, in a dihybrid cross there must be four factors present in each parent. Likewise the F1 hybrid must be RrYy. Mendel found that the pair of factors for roundness will be behaving independently of the pair of factors for yellow colour of seeds. In other words, one factor for a character must be passing independently of a factor for another character. Thus in the F1 hybrids, R and r pass into different gametes. Now the probability of an R gamete formed is one-half, and of r gamete also one-half. Similar probabilities exist for Y and y gametes. It follows that the probability that R and Y will go to the same gamete is one fourth, as also of R and y, r and Y, and r and y. Therefore, gametes containing factors RY, Ry, rY and ry will form in equal proportions (1:1:1:1). The F1 hybrid producing the four types of gametes mentioned above was selfed. The results expected in the F2 progeny can be predicted by making a checker board or a Punnett Square. Gametes produced by one parent are plotted on top of the checker board, and the gametes of the other parent on the vertical side. The 16 square of the checker board are filled up by making various possible combinations of male and female gametes during fertilization. The phenotypes read out from the checker board indicate a 9:3:3:1 ratio exactly as observed by Mendel. Back Cross and Test Cross “Back cross is the cross of F1 hybrid with any one of its parents”. “Test cross is the cross of F1 hybrid with recessive parent”. Mendel verified his results by performing the test cross. He crossed the F1 hybrid heterozygous for both characters with a double recessive parent (rryy) which will produce only one type of gamete ry. The uniformity in the gametes of the recessive parent determines the differences in the types of gametes produced by the heterozygous parent. Now the hybrid RrYy produces gametes carrying RY, Ry, rY and ry with equal frequency (1:1:1:1). It follows that during fertilization if all these four types of gametes unite with ry gamete of the recessive parent, the resulting progeny will show all the four combinations of characters also in equal proportions. Indeed, Mendel observed the test cross progeny to consist of Round yellow, Round green, Wrinkled yellow and Wrinkled green plants in the ratio of 1:1:1:1. F1 : Gametes : F2 ry Round yellow RrYy (RY) (Ry) (rY) (ry) 1:1:1:1 RY Ry rY ry RrYy Rryy rrYy rryy X 1:1:1:1 Wrinkled green rryy (ry) 6 GENETICS AND PLANT BREEDING From the results of his dihybrid crosses, Mendel realized the following facts. At the time of gamete formation the segregation of alleles R and r into separate gametes occurs independently of the segregation of alleles Y and y. That is why the resulting gametes contain all possible combinations of these alleles. i.e., RY, Ry, rY, ry. In this way Mendel proved that when two characters are considered in a cross, there is independent assortment of genes for each character, and this became the Law of Independent Assortment. The Chromosomal Theory of Heredity The Mendelian laws of inheritance, formulated in 1865, still form the basis of our understanding of the transmission of heritable variation, and the inheritance test is still the basic technique for following phenotypic characters through the several generations. It is the genius of Mendel that a few, but quantitatively predictable, results enabled him to postulate that an abstract entity—the element or factor, as he called it, the gene as we now designate it—was responsible for the phenotypic character he was following in his breeding experiments, that this entity was singly represented in gametes and doubly so in zygotes and somatic tissues, and that factors, when together in pairs, could exhibit dominance or recessiveness in expression. The egg and sperm of animals, despite obvious differences in size and shape, had earlier been shown to be cells; the physical continuity between generations was, therefore, a slender cellular bridge. Fertilization was shown to be the union of gametes, with the fusion of parental nuclei in the cytoplasm of the egg being the crucial event, and with each nucleus providing equal, or nearly equal, numbers of chromosomes to the newly formed zygote and hence to the developing embryo. As the embryo grew by adding to its cell numbers, the critical acts of cell division were the longitudinal replication of each chromosome, and the segregation of these longitudinal halves to the two daughter cells, thus providing both a physical basis for the qualitative and quantitative chromosomal equality of each daughter nucleus and a functional basis for the conservation of the genotype. Meiosis, a kind of cell division leading to the formation of eggs or sperm in animals and spores in most plant species, was revealed as a mechanism for halving the chromosome number; thus providing a means for counteracting the chromosome doubling that took place earlier in the fusion of parental nuclei in fertilization and making clear the patterns of haploidy and diploidy in the life cycle. Chromosomes had been demonstrated to have a physical continuity from one cell to another, and from one generation to another, and to be qualitatively different from each other in so far as they affected developmental process. By 1902, Garrod, through his study of the metabolic diseases of humans—“in born errors of metabolism”, as he called them—would forge the first link that would couple the biochemistry of disease syndromes to patterns of inheritance. With the further observation that species were characterized by a constant number of chromosomes, although the two sexes of a species may differ slightly from each other, it was becoming more and more obvious that the patterns of heredity or inheritance were somehow linked to, or mirrored in, the behaviour of chromosomes. In retrospect, therefore, the discoveries made during the last quarter century of 1900s and the first few years of the 20th century made it increasingly clear that the chromosomes were key elements whose behaviour in division and fertilization revealed a regularity that could, on the one hand, account for the transmission of heredity factors from one generation to another, whether the cells or of individual organisms, and on the other for the conservation of species reproduction, i.e., the preservation of type, generation after generation. The science of cytogenetics was thus launched with a brilliant correlation of factor (gene) and chromosome behaviour. MENDEL’S EXPERIMENTS AND PRINCIPLES 7 The basis of the theory, as stated by Sutton, is as follows: 1. In somatic cells, arising from a fertilized egg, or zygote, the chromosomes consist of similar groups, one of maternal origin inherited through the egg, the other of paternal origin and inherited through the sperm. Each somatic nucleus, therefore, contains pairs of like chromosomes, or homologues, the number of pairs being the same as the haploid set of chromosomes in a gamete. Hence, the chromosomes, like Mendel’s factors, are doubly represented in the somatic cells of an organism, and singly, or pure, in the gametes. 2. The chromosomes retain their structural individuality and their continuity throughout the life cycle of an organism. Again, the factors of Mendel retain their individuality and continuity even though the character they determine might not be expressed. The basis of genetic homogeneity and heterogeneity, dominance and recessiveness, was suggested by chromosomal behaviour. 3. In meiosis, synapsis brings together pairs of homologous chromosomes and leads to their subsequent segregation into different cells, establishing thereby a quantitative basis for both the segregation of factors and the independent assortment found when two pairs of contrasting factors are considered together. 4. Each chromosome, or chromosome pair, plays a definite role in the development of the individual. This conclusion derived not from any specific facet of Mendel’s investigations but rather from studies of Boveri of abnormal larvae of Sea Urchins which lacked certain normal chromosomes, as well as Sutton’s own studies of the size differences among the chromosomes of the insect Brachystola. Sutton, consequently, visualized the chromosomes as the physical carriers of Mendelian factors and the segregation of a pair of homologous chromosomes and the independent assortment of nonhomologous chromosomes as the physical basis for the qualitative and quantitative aspects of Mendelian segregation. Sutton, together with Roux and Boveri, also anticipated the phenomenon of linkage when he stated that all the factors in any one chromosome must be inherited together. As the theory of gene and the concept of Mendelian factors, the heredity constitution of an organism, and eventually of a species, the notions would eventually lead to a further refinement, namely, the molecular basis of hereditary phenomena. Gene Interactions and Modified Dihybrid Ratios According to Mendel each character is controlled by a pair of factors or genes. But later discoveries proved that in many cases the expression of a single character is controlled by the interaction of more than one pair of genes. This is called “Interaction of genes” or “Factor hypothesis”. This was proposed by Bateson and Punnet. This hypothesis states that certain characters are controlled by the interaction of two or more genes. The genic interaction may occur in between genes located in the same chromosome or different chromosomes. This type of genic interaction is known as non-allelic genes interactions. The genic interaction may also occur between the two alleles of a single type of gene. This type of genic interaction is known as allelic gene interaction. However, Mendelian laws will hold good in all the type of segregation and interactions. As a matter of interaction the phenotypic ratio will alone be modified and not the genotypic ratio. Complementary Genes (9:7) Complementary genes may be defined as “two or more dominant genes occurring in different loci of the same chromosome or different chromosomes interact with one another to produce a character 8 GENETICS AND PLANT BREEDING but neither of them produces that character in the absence of the other”. The action of these independent genes are complementary. Bateson and Punnet studied the inheritance of flower colour in sweet pea, Lathyrus odoratus. There are two varieties of pea plants, one producing red flower and the other white flower. P: White flower CCaa (Ca) Red flower CcAa (Selfing) Cc Aa (CA) (Ca) (cA) (ca) Gametes : F1 Gametes : CA Ca cA ca X White flower ccAA (cA) X CcAa (CA) (Ca) (cA) (ca) CA Ca cA ca CCAA Red CCAa Red CcAA Red CcAa Red CCAa Red CCaa White CcAa Red Ccaa White CcAA Red CcAa Red ccAA White ccAa White CcAa Red Ccaa White ccAa White ccaa White 9(Red) : 7 (White) The cross between two white varieties can be explained by assuming two genes for red colour which must be present together, i.e., must act in a complementary way to each other. Thus each gene independently contributes something different but essential for synthesis of red pigment. If one of the two genes for red colour is absent, the result is a white flower. The inheritance of the colour of aleurone layer in corn also demonstrates interaction of complementary genes. The outermost layers of endosperm in the maturing corn kernels become modified into a specialized aleurone tissue, so named because the cells have rich deposits of aleurone grains. In corn the aleurone layer is coloured due to anthocyanin pigments in the cells, and is controlled by complementary effect of two genes. Supplementary Genes (9:3:4) Supplementary genes may be defined as two independent pairs of dominant genes, which interact in such a way that one dominant gene produces its effect, when the second dominant gene is added to the first, a new character is expressed. Coat Colour in Mice Inheritance of coat colour in mice was studied by Castle. There are three different varieties of mice, they are agouti (grey), black and albino (white). Agouti colour is dominant to both black and 9 MENDEL’S EXPERIMENTS AND PRINCIPLES albino. Black is dominant to albino but recessive to agouti. Albino is recessive to both agouti and black. Agouti is produced by dominant gene A in the presence of another dominant gene B. Dominant gene B alone produces black colour. Dominant gene A produces albino. The recessive condition of these genes causes albino. Castle crossed a homozygous black mice (BBaa) with a homozygous albino (bbAA). The F1 individuals are Agouti. When the F1 Agouties are inbred their progeny consists of 9 agouti, 3 black and 4 albino. Parents : Black male BBaa (Ba) X Albino female bbAA (bA) BbAa Agouti (two F1 individuals crossed) Bb Aa (BA) (Ba) (bA) (ba) X Bb Aa (BA) (Ba) (bA) (ba) Gametes : F1 generation: F1 Gametes : BA Ba bA ba BA BBAA Agouti BBAa Agouti BbAA Agouti BbAa Agouti Ba BBAa Agouti BBaa Black BbAa Agouti Bbaa Black bA BbAA Agouti BbAa Agouti bbAA Albino bbAa Albino ba BbAa Agouti Bbaa Black bbAa Albino bbaa Albino 9:3:4 Agouti : Black : albino Duplicate Gene (15:1) When two or more genes have the same effect on a given trait, they are referred to as duplicate genes. In maize the gene for yellow endosperm is dominant over white endosperm. A pure breeding yellow endosperm plant when crossed to a white endosperm plant yields yellow endosperm in F1. On self pollination of F1 hybrids in F2 generation of 15 yellow and 1 white endosperm is obtained. The yellow endosperm results from two independent dominant genes Y1 and Y2. When anyone of these two dominant genes or both together are present, yellow endosperm is produced. When only recessive alleles are present in the homozygous condition (y1y1y2y2) it forms white endosperm. Thus the dominant genes Y1 and Y2 have an identical effect on endosperm colour and are consequently termed duplicate genes or isogenes. 10 GENETICS AND PLANT BREEDING Parent : Yellow endosperm Y1Y1y2y2 (Y1y2) Yellow endosperm Y1y1Y2y2 (Y1Y2) (Y1y2) (y1Y2) (y1y2) Gametes : F1 F1 gametes X Yellow endosperm y1y1Y2Y2 (y1Y2) (Selfing) Y1Y2 Y1y2 y1Y2 y1y2 Y1Y2 Y1Y1Y2Y2 Yellow Y1Y1Y2y2 Yellow Y1y1Y2Y2 Yellow Y1y1Y2y2 Yellow Y1y2 Y1Y1Y2y2 Yellow Y1Y1y2y2 Yellow Y1y1Y2y2 Yellow Y1y1y2y2 Yellow y1Y2 Y1y1Y2Y2 Yellow Y1y1Y2y2 Yellow y1y1Y2Y2 Yellow y1y1Y2y2 Yellow y1y2 Y1y1Y2y2 Yellow Y1y1y2y2 Yellow y1y1Y2y2 Yellow y1y1y2y2 White 15 (Yellow) : 1 (White) George H. Shull (1914) reported a case of duplicate gene in the common weeds, shepherd purse Bursa bursapastoris. There are two varieties of seeds. Normal variety produces a triangular seed case and the other mutant variety produces oval seed case. When these two varieties are crossed, the resulting F1 plants produce triangular seeds (which is thus dominant). When the F1 hybrids are selfed, in F2 plants with triangular seeds and oval seeds are produced in the ratio 15:1 instead of the normal Mendelian ratio. Epistatic Genes (13:3) Epistasis is a Greek word which means stopping or suppression. In some animals a gene at one locus on a chromosome suppresses or masks the expression of a gene at another locus. Such genes are known as inhibiting genes, since they inhibit the expression of other genes. A gene that inhibits or masks the expression of another gene (non-allelic gene) is said to be epistatic. The gene that is masked is said to be hypostatic. In poultry white birds belong to two different varieties namely white leghorns or white wyandottes. Experiments reveal that the gene for white plumage of white leghorns is dominant over the gene for coloured plumage of coloured varieties. But the gene for white plumage of white wyandottes is recessive to the gene for coloured plumage of coloured varieties. Therefore, the gene which produces white plumage in white leghorns is different from the gene for white plumage in white wyanodottes. A cross between a white leghorn and a white wyanodotte gives an F1 of white birds. When such birds are inbred, the F2 progeny segregates in the ratio of 13 white to 3 coloured birds. The experiment is explained below by postulating two gene C and I for the white leghorns. 11 MENDEL’S EXPERIMENTS AND PRINCIPLES Parents : White Leghorn CCII White CcIi F1 F1 gametes : X White Wyanodotte ccii X Inbred (CI) (Ci) (cI) (ci) (CI) (Ci) (cI) (ci) F2 CI Ci cI ci CI CCII White CCIi White CcII White CcIi White Ci CCIi White CCii Colour CcIi White Ccii Colour cI CcII White CcIi White ccII White ccIi White ci CcIi White Ccii Colour ccIi White ccii White F2 ratio : 13 White : 3 colour The F2 ratio indicates that only three out of 16 genotypes, that is CCii, Ccii, Ccii produce coloured birds. The white leghorn obviously contain a gene I, which in the dominant state inhibits or suppresses the expression of the dominant colour gene C, resulting in white plumage. The recessive alleles of the inhibitor gene (ii) produce coloured birds due to expression of gene C. In other words gene I is epistatic to gene C. This is a case of dominant epistasis because even one dominant allele of gene I is able to express itself. Lethal genes A lethal gene produces an effect which differs from the normal condition that its possessor is unable to survive. There is a mutant recessive gene which causes internal adhesions of the lungs. A child homozygous for the lethal gene might survive upto embryonic development. But at birth, when it suddenly becomes dependent upon its lungs for its oxygen supply, it would die because its lungs could not expand properly. Being recessive, it could be carried by normal parents in heterozygous condition without any ill effects. In a mice cross heterozygous yellow (Yy) XYy produces offspring 1YY dies (homozygous yellow) : 2Yy (heterozygous yellow) : 1yy homozygous non-yellow. Hence a 2 : 1 ratio is realised. The lethal gene dominant in mouse, is thus lethal, causing death in homozygous condition YY.