MODULE No.25: Mendelian Genetics

Transcription

1 SUBJECT Paper No. and Title Module No. and Title Module Tag FSC_P13_M25

2 TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Mendel s Work: Choice of Experimental System 4. Principle of Dominance 5. Principle of Segregation 6. Principle of Independent Assortment 7. Summary

3 1. Learning Outcomes After studying this module, you shall be able to- Learn the basic factors responsible for heredity Understand the experiments conducted by Mendel on garden pea, Pisum sativum Understand the Principle of Dominance, Segregation and Independent assortment Comprehend the applications of Mendelian Principles 2. Introduction Genetic traits are delivered from generation to generations down in families in diverse patterns. Most of the people that have existed compared to or wondered why do they resemble to their grandparents? The archaeological records of human history provide evidences in form of primitive art, conserved bones and skulls, and dried seeds for the effective training of animals and plant cultivation that started 1000 of years earlier. Even despite our limited knowledge about the first identification of the existence of heredity, the domestication and cultivation is indicative of non-natural selection of varieties within populations. During the period from 8000 and 1000 B.C., camels, horses, oxen and dogs (members of the wolf species), have been domesticated which soon followed with selection of varieties and hence selective breeding. Cultivation of many plants started about 5000 B.C, including maize, wheat, rice and the date palm evidences for which have been improved in form of stored seeds in some caves in the Tehuacan Valley of Mexico. These evidences clearly prove that for 1000 of years farmers and herders have been selectively breeding varieties of floras and faunas to enhance productivity by giving upsurge to additional beneficial hybrids. The scientific basis of vertical transfer of traits and how they sometimes skip generations was first explained by the Austrian Monk Gregor Johann Mendel. Mendel's insight greatly enhanced the empathetic of patterns of genetic inheritance, and led to the development of new experimental methods. By testing with pea plant breeding, Mendel established three principles of inheritance that defined the transference of genetic traits, before anybody knew genes existed.

4 3. Mendel s Work: Choice of Experimental Model System Mendel studied the genetic inheritance in garden pea (Pisum sativum) plants. In the middle of 1856 and 1863 Mendel cultured and tested some 29,000 pea plants. His observations and findings established the presence of patterns in the garden pea that further lead to formation of basic principles that serve as the cornerstones of innovative genetics. It is important to mention that by the time Mendel concluded his observations, chromosomes, and meiosis were not discovered; yet he was righteously able to put forward the concept for presence of factors (nowadays known genes) liable for the heredity. Even though he was not the first individual to have experimented for understanding the phenomenon of heredity, Mendel s predecessors failed to go the close of success; this was attributed to the elegance in his strategy and scrutiny of experiments. This was partly also contributed by the selection of material he has used in his experiments. Garden pea (Pisum sativum) has several characteristics that helped in his research: 1. Flowers were cleistogamous (closed) and could be self-fertilised (selfed) - allows inbreeding. 2. Even though flowers were self-pollinating, these could be readily cross-fertilized to create hybrids between pure-breeding lines, carefully controlled mating and mutual crosses might rule out the outcome of one parent as opposite to the other. 3. Small generation period that allowed Mendel to develop great numbers of floras and progeny allowing computable evaluation that created vigorous outcomes which supported this interpretation. 4. In addition, garden peas have straightforward (discrete/qualitative) traits Mendel could explicitly decide between two different forms. In respect to above characteristic of experimental material, Mendel s insight contributed to his success. Prior to selecting varieties for his work, Mendel carefully analyzed the pea breeding for two years and selected pure-breeding lines that always bred true, producing the similar trait generation upon generation for e.g. a tall plant will always produce tall progeny in self-fertilized plants.

5 Further, he constrained his analysis to one or only limited sets of complementary characters in each of his experiments. He also reserved precise measurable records and with his knowledge of mathematics he was able to calculate the product logically. Mendel also had his share of luck because the traits he studied for his experiments were well-ordered by genes that were non-linked, and even if they were, he completely overlooked such observations. Figure 1: Seven characteristics studied by Mendel in garden peas. The seven traits observed by Mendel in pea plants were: seed color (yellow or green); seed shape (round or wrinkled); seed coat color (gray or white); flower position (axial or terminal); stem length (short or tall); pod color (yellow or green); and pod shape (inflated or constricted). Mendel detected seven different easily recognizable characteristics (traits) in the pea plants, and each of these characteristics had two alternate forms as follows: 1. Form or shape of the ripe seeds (round or roundish, or angular and wrinkled) 2. Colour of the seed (pale yellow, bright yellow and orange-coloured, or green) 3. Colour of the seed coat (either white or grey, grey brown, leather brown, with or without violet spotting) 4. Form or shape of the ripe pods (simply inflated or deeply constricted and more or less wrinkled) 5. Colour of the unripe pod (light to dark green or vividly yellow)

6 6. Position of the flowers (axial, i.e. dispersed laterally the core stem, or terminal, this is bunched at the top of the stem) 7. Length of the stem (the long axis of six to seven feet, or a short one of 3/4 to 1 feet) The vital objective of Mendel's investigation with peas lies in several fundamental facts: characters or traits from parents pass as unmodified "units", individual "Mendelian" factors (now known as "genes") to consecutive groups rendering to conventional ratios. All individuals possess 2 groups of factors: each one received from either parent. It marks no change if any one character is inborn from male or female: they both donate in the identical way. Further, these factors may sometimes be concealed or expressed but are never lost. In general, sometimes each "unit" is delivered on independently from all other "units". For example, a pea may develop seeds that are both round and yellow, or wrinkled and yellow, or round and green, or wrinkled and green. Certain characters were manifested regularly than others whereas, some were not immediately expressed. Mendel's experimentations were scientifically documented over an era of seven years, from 1856 to 1863 and he read a paper on the succeeding results at an assembly of the People of Natural Sciences in 1865 in Burno. This paper was published the following year in the extracts of Annual Proceedings of Natural History Society. Though his ideas had been printed in 1866 but never the less they mainly went unrecognized up until 1900, which was long after his passing away when his work was "re-discovered" by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak. While Mendel's investigation was with plants, the elementary basic Principles of heredity that he revealed also apply to people and other animals because the mechanisms of heredity are fundamentally the similar for all composite life forms. Mendel s findings can be summarized in form of three basic Principles that are now well-thought-out all the angles of modern genetics.

7 1. Principle of Dominance: It says that when dominant allele exists in an organism, it marks the presence of recessive allele and the F1 progeny will express dominant allele. 2. Law of Segregation: It states that each trait is controlled by two alleles, and that each one gamete comprises of one and only one of these alleles. These alleles are the basis of genetic variability between progenies. 3. Principle of Independent Assortment: It states that the alleles for one trait separate independently of the alleles for another trait. In other words, this implies that all maternal or paternal alleles do not converge into the same gamete but rather are assorted independently of each other. This also helps to ensure genetic variability among the offsprings. To understand this further, we will look at observations that enabled Mendel to reach the conclusion for formulation of above as Principles of genetics. 4. Principle of Dominance Mendel noticed that when he crossbred two parents with unlike types of an individual trait, one of those types apparently disappeared in the hybrid (heterozygous) offspring. However, when the offsprings were hybridized with one another, the wiped out trait reappeared in the third generation, entirely unchanged in spite of being undetectable in second generation. Mendel described the type of the trait which was noticeable in the hybrids of first generation as dominant and the one that was invisible in the hybrids the recessive.

8 a) Parent 1:TT Parent 2:tt Parent 1:Tt Parent 2:Tt b) Figure 2: Principle of Dominance. a) Represents the distribution of tall and dwarf plants over the parental and progenies in F1 and F2 generations. Note that in F2 generation only, the dwarfness representing recessive character reappears. Phenotypic ratio of 3:1 is characteristic of Mendel s Principle of Dominance.

9 b) Punnet squares showing the mechanism of inheritance of dominant and recessive traits. Punnet square on left hand side shows a cross among true breeding tall and dwarf pea plants representing the cross among parental varieties. Punnet square on the right hand side shows a cross between F1 hybrids that result in reappearance of recessive character. The ratio of distribution is 3:1 in favor of dominant trait. In diploid organisms (pure breeding or hybrids), a given character is signified by two contrasting factors known alleles or allelomorphs. Among the two alleles, only one is capable to show its effect in the individual. Letter symbols can be used to denote factors. It is known as dominant or dominant allele. It is shown by capital or upper case letter of the alphabet e.g., T (tallness). The other allele which does not show its effect in the heterozygous individual is known recessive or recessive allele. An equivalent small or lower case letter is allocated to the recessive, e.g., t (dwarfness). Mendel investigated with Pisum sativum for seven characters. In every case he observed that expression of one trait of the character, for e.g., T or tallness in case of height of plant is dominant over the other trait i.e. dwarfness corresponding to the same character. It can also be demonstrated experimentally. In case of garden Pea plants, one pure or homozygous tall (height about m) and the other pure or homozygous dwarf (height about m) were considered by Mendel. Upon crossing two, the progeny called first filial or F-1generation, comprised of all tall plants (height m) even though they have also received a factor for dwarfness. Upon self-breeding F1 plants (all tall) it was further observed that individuals of F 2 generation will be both tall and dwarf and approximately corresponded to the ratio of 3: 1. It was concluded that in F 1 plants both the factors for tallness and dwarfness are present. Though, the factor for dwarfness was incapable to express in the attendance of factor for tallness. As a result, factor governing tallness is dominant over the factor responsible for dwarfness which is recessive. Mendel was ahead of his time. However, the Principle of Dominance can-not be applied to all traits universally. As now know that Mendel s discovery was confined to complete dominance, which is only one of numerous unlike kinds of dominance relationships. It is also important to observe that dominance is essentially always well-defined with respect to the phenotypic of the heterozygote.

10 5. Principle of Segregation Principle of segregation states that the two factors of a character that are present in diploid individuals keep their individuality separate or separate at the time of gametogenesis or sporogenesis and are randomly distributed to gametes. Paired condition is then restored again during fertilization. Principle of segregation can be deduced from a reciprocal monohybrid cross. Consider a cross between a pure tall pea plants (TT) and dwarf Pea plant (tt). The hybrids (Tt) or plants of first filial (F 1 ) generation are all tall though have also received the factor for dwarfness. As per the Mendel s Principle of dominance, the factor for tallness is dominant while the factors for dwarfness are recessive. If the hybrids are allowed to selfbreed, the progenies of tall pea plants in second filial or F 2 generation display both tall and dwarf phenotypes in ratio of 3:1. Moreover, self-breeding of these plants demonstrations that amongst tall plants, only 1/3 breed true, that is, yield only tall plants. The remaining 2/3 of the F 2 tall plants i.e. 50 % of the total F 2 plants were hybrid plants and produce both tall and dwarf plants in the ratio 3:1. In contrast, all the dwarf plants breed true (tt), i.e., produce only dwarf plants. It can therefore be concluded that even though phenotypic ratio of F2 generation is 3:1; genotypic ratio is 1:2:1 with 1 pure tall (TT): 2 hybrid tall (Tt): 1 dwarf (tt). Figure 3: Principle of Segregation.

11 When tall and dwarf plants obtained in F2 generation were crossed for checking true breeding varieties, true breeding tall (TT) and dwarf (tt) varieties were obtained in F3 generation along with 50 % hybrids (Tt). With in the hybrids, the distribution of tall versus dwarf varities was in the ratio of 3:1. The cross shown in figure 3 shows that: 1. Although F 1 plants show only one alternative or dominant trait (TT) of a character, it actually carries alleles for both the traits of the character. This is shown in F2 generation where the second alternative or recessive trait (tt) reappears. F 1 plants are thus, genetically hybrid (Tt), in the given case. 2. F 1 plants are a result of fusion of male and female gametes through fertilization. As they carry the gene complement of Tt, the fusion of gametes must have brought only one factor each (T from dominant (TT) and t from recessive (tt) parent). Male Female Offspring Gamete Gamete Cross I T t Tt Reciprocal Cross t T Tt 3. F 2 generation is produced by self-breeding of the F 1 plants. F 2 generation consists of three types of plants pure tall, hybrid tall and dwarf. This possibility can only occur when (a) The two Mendelian factors present in the F 1, plants segregate during gamete formation, (b) Gametes carry a single factor or allele for a character, 50% of one type and 50% of the second type, (c) The fusion of gametes or fertilization is random. Meanwhile, only one of the two factors transfer into a gamete, 50% of the male and female gametes created by F 1 plant retain the factor for tallness while the remaining 50% carry the factor for dwarfness.

12 Figure 4: A Monohybrid Cross in Pea Showing that Factor of Character Segregate at the time of Development of Gametes. Principle of segregation is the most fundamental principle of heredity and can be universally applied with no exception. Some scientists like Bateson call the Principle of segregation as the Belief of purity of gametes as during segregation of the two Mendelian factors of trait fallouts in gametes, only one factor out of a pair is received by each gamete. As a product gametes are always pure for a character. It is also called as law of non-mixing of alleles.

13 In his further efforts to verify and extend his results of monohybrid crosses, Mendel also crossed garden pea plants that were differing in two characters (di-hybrid cross). This enabled him to appreciate inheritance of two different genes (i.e., two pairs of alleles) at a time. It was observed that inheritance of one pair of alleles (one character) does not interfere in the inheritance of other pair of alleles (second character). Based on this, Mendel proposed a set of generalizations (postulate) which is now known as Principle of independent assortment. 6. Principle of Independent Assortment According to this principle two factors for each character assort or separate independently of the factors of other characters at the time of gamete development and get arbitrarily rearranged in the offspring generating both parental and new groups of traits. The Principle of independent assortment may be considered by means of dihybrid cross, e.g., among pure breeding Pea plants having yellow and round seeds (both dominant YYRR) and pure breeding Pea plants having green and wrinkled seeds (both recessive yyrr). When these true breeding plants are crossed, plants of the F 1 generation have all yellow and round seeds (YyRr) because yellow and round traits are respectively dominant over green and wrinkled traits. However, upon self-breeding, the outcomes second filial or F 2 generation shows four types of plants that are yellow and round, yellow and wrinkled, green and round and green and wrinkled. The results obtained by Mendel were as follows: Yellow and Round = 315/556 ~ 9/16 Yellow and Wrinkled = 101/556 ~ 3/16 Green and Round = 108/556 ~ 3/16 Green and Wrinkled = 32/556 ~ 1/16

14 Thus the phenotypic ratio of a dihybrid cross is 9: 3: 3: 1. In addition, occurrence of four varieties of plants (two more than parental types) in F 2 generation of dihybrid cross clearly indicates the fact that the factors for two characters separate autonomous of the others as if the other pair of factors is not present. This can also be proven by studying the individual characters of seed color and seed texture separately. A dihybrid cross showing the distribution of plants with above mentioned characters is shown in figure 4. Upon close examination of the outcomes, it can be observed that output is alike to monohybrid ratio. Seed Colour: Yellow (9 + 3=12): Green (3 + 1 = 4) or 3: 1 while, Seed Texture: Round (9 + 3 = 12): Wrinkled (3 + 1 = 4) or 3: 1 That the factors of the two characters assort independently, can advanced demonstrated by multiplying the different possibilities.

15 Figure 5: Principle of independent assortment. A dihybrid crosses between yellow, smooth seeded pea varieties and green and wrinkled seeded garden pea plants. All plants in F1 are Yellow and smooth seeded. However, upon selfing F1 progeny, four different phenotypic varieties were derived that were distributed in phenotypic ratio 9:3:3:1. At the genotype level, these four varieties were of nine types yielding a genotypic ratio of 1:2:1:2:4:2:1:2:1. As all four characters namely smooth and wrinkled textures as well as yellow and green color of seeds are observed in F2 generation, it proves that expression of none of these characters have any influence over expression of other. This is in according to the Principle of independent assortment.

16 It is well known that the Principle or law of independent assortment is appropriate to only those factors or genes which are mostly found on various chromosomes or distantly on the similar chromosome. A single chromosome may bear hundreds of genes; all of which are inherited together except when crossing over takes place during meiosis. This occurrence of inheritance of a number of genes or factors because of their occurrence with each other on the same chromosomes is called linkage. Mendel himself found that white flowered Pea plants always form white seeds while, red flowered plants always generate grey seeds though he was unable to explain the results. Mendel brief his findings in form of three basic principles of heredity without having any knowledge of mitosis, meiosis or chromosomes. Regardless of its significance, Mendel s work was overruled at first, and was not broadly acknowledged even after he died. During his own time, most biologists thought the impression that all characteristics were transfer to the next generation by blending inheritance, in which the traits from each parent are be around together. Instances of this occurrence are now described by the action of multiple genes with measurable effects such as skin color in humans. Validity of his work was also established with understanding of meiosis, where paternal and maternal chromosomes separate into different gametes with distinguished traits of a character. Further investigation and extension of Mendel s work on other organisms revealed that there were exceptions to his results, thus his Principles/laws have their limitations, for e.g. the independent assortment is held valid only for the non-linked genes while exception to principle of dominance were observed in case of co-dominance. Also, for genes found on the X chromosome, expression of the trait can be linked to the sex of the offspring. Mendel didn t mention any overlap of characters as the traits he has considered were controlled by non-linked genes i.e. they were controlled by alleles on different chromosomes or were located far off on same chromosome. None the less, Mendel s work led the foundation for genetics, which is defined as the branch of biology that deal with the detailed study of heredity and variation. Mendel himself is famous as Father of Genetics.

17 Gene interaction is the impact of alleles and non-alleles on the normal phenotypic expression of genes. It may be inter allelic (intragenic) or non-allelic (inter-genic). In the intragenic interaction the two alleles (existent on the similar gene locus on the two homologous chromosomes) of a gene relate in such a manner as to produce a phenotypic expression varies from typical dominant-recessive phenotype, e.g., incomplete dominance, co-dominance, multiple alleles. In inter-genic or non-allelic contact, more than two self-determining genes present on same or dissimilar chromosomes interact to create a different expression, e.g., epistasis, duplicate genes, complementary genes, supplementary genes, lethal genes, inhibitory genes, etc. We will study about these in further details in the next module. 7. Summary Mendel studied the genetic inheritance in garden pea (Pisum sativum) plants by cultivating about 29,000 pea plants. His observations and findings established the presence of patterns in the garden pea that further lead to formation of basic principles that serve as the cornerstones of new genetics. Mendel briefs his findings in form of three basic principles of heredity without having any knowledge of mitosis, meiosis or chromosomes. His findings remained obscure for more nearly three decades. Principle of Dominance states that when dominant allele is present in an organism, it will mask the existence of recessive and the F1 progeny will express dominant gene. Law of Segregation states that each trait is controlled by two alleles, and that each gamete contains one and only one of these alleles. These alleles are a basis of genetic variability between offspring. Principle of Independent Assortment means that the alleles for one trait separate independently of the alleles for another trait. In additional, this implies that all maternal or paternal alleles do not converge into the same gamete but rather are separated independently of each other. This is useful to confirm genetic variability between offspring.