PLB201 Principles of Genetics PDF

# INTRODUCTORY PRINCIPLES OF GENETICS AND EVOLUTION ## INHERITANCE The laws of heredity were worked out by Gregor Mendel, an Austrian monk in a work published in 1866. The import of his work and contributions to the development of genetics is so important that the adjective Mendelian is now used t...

# INTRODUCTORY PRINCIPLES OF GENETICS AND EVOLUTION ## INHERITANCE The laws of heredity were worked out by Gregor Mendel, an Austrian monk in a work published in 1866. The import of his work and contributions to the development of genetics is so important that the adjective Mendelian is now used to describe the kind of experiments he did and the principle he formulated. Though Mendel was not the first to carry out breeding experiments, but he was the first to analyze the results numerically and thereby discovered certain consistencies which he explained by "HEREDITARY FACTORS". He made a careful choice of plant to do his breeding experiments. The success of his experiment was determined by the crop he used, the garden pea Pisum sativum, which satisfied his requirements for the following reasons: 1. There exist many easily recognizable, distinct forms or varieties. 2. The flowers are normally self-fertilized but it is possible to remove the stamens from a flower before they are mature and to pollinate the stigma with pollen from a different variety. 3. The plants resulting from the cross fertilization are fully viable and fertile. 4. The plants are easy to cultivate. 5. The one-year life cycle is short enough to be able to collect data from several generations. Mendel obtained 34 varieties of peas and chose from them a number which showed distinct, contrasting forms referred to as the "character or trait". ### Varieties of Garden Pea Used by Mendel | Character | Trait | |---|---| | Form of ripe seed | Smooth or wrinkled | | Colour of cotyledons | Yellow or green | | Colour of testa | White or grey | | Colour of flower | White or purple | | Form of ripe pods | Inflated or constricted | | Colour of unripe pods | Green or yellow | | Position of flowers | Axial (distributed along stem) or Terminal | | Length of stem | Tall (3m) or short (less than 0.5m) | Though characters may often have more than two traits, Mendel however restricted himself to study of two traits per character. He crossed fertilized white flowered plants with purple flowered plants. He also did the RECIPROCAL CROSSING by crossing the pollens of purple flowered plants with the stigma of white flowered plants. For all the pair of traits for his experiments, he observed that the generation resulting from crosses (F1 GENERATION), the plants all showed the same trait. All the offsprings were like one of the parents and were not intermediate in appearance. Furthermore, the appearance of the F1 generation was the same regardless of which plant bore the seeds, as illustrated below. | Female Parent | Male Parent | |---|---| | Axial flowers | Terminal flowers | **Cross fertilized** **F1** **Axial flowers** **Cross fertilized** **F1** **Axial flowers** Mendel called the F1 trait the DOMINANT trait. The other he called RECESSIVE trait because it seems to recede out of sight in this generation, but reappears in subsequent generations. Mendel found the following relationships: | Dominant trait | Recessive trait | |---|---| | Smooth seed | Wrinkled seed | | Yellow cotyledons | Green cotyledons | | Grey testa | White testa | | Purple flowers | White flowers | | Inflated pods | Constricted pods | | Green unripe pods | Yellow unripe pods | | Axial flowers | Terminal flowers | | Tall stem | Short stem | *POWER POINT PRESENTATION* Plants of F1 generation were then selfed (self fertilized) and the progeny collected. The resultant F2 plants, some showed dominant trait while others showed recessive trait. For each character, Mendel counted the numbers, having from 353 F1 plants, he collected 7324 seeds. Of these, 5474 were smooth and 1850 were wrinkled. (5474: 1850 = 2.96 : 50 the ratio of smooth to wrinkled is 2.96:1. Whichever characters he investigated, he found this ratio of approximately 3 DOMINANT : 1 RECESSIVE in the F2 generation. On selfing of F2 plants, to produce the F3 generation, those F2 plants showing recessive trait had F3 offspring which also showed recessive trait. The F2 plants showing the dominant trait were of two kinds. 1/3 of them yielded F3 offspring which showed dominant trait while the other 2/3 yielded F3 offspring with either dominant or recessive trait, again in the ratio 3:1. **Parental generation: Axial flower X Terminal flower **F1: Axial flower *F2: self fertilized *651 axial *100 plant selfed *33 F3 plant axial - self fertilized *673 plant = axial/terminal - selfed *207 terminal *all terminal* A plant which, when selfed, has only offspring only like itself is said to be PURE BREEDING or TRUE BREEDING. There are three different types of plants in F2 generation and they occur in the following proportions: - 1/4 pure breeding for dominant trait. - 1/2 not pure breeding but showed dominant trait. - 1/4 pure breeding for recessive trait. It was from such results that Mendel worked out his first law. His most important conclusion is that inheritance units of inheritance remain as separate "PARTICLES", when they are pressed from generation to generation. They are not changed or diluted, and although their effects may be hidden, the particles themselves are passed on unchanged. This is called the Idea of PARTICULATE INHERITANCE. ### PARTICULATE INHERITANCE EXPLAINED IN MODERN TERMS Each character is controlled by a gene. Gene can exist in alternative forms called ALLELES, which controls the alternative traits in a character. The gene are "particles" which are transmitted unchanged from one generation to the next. A gene is represented by a letter, for instance "A". Two alleles, being different forms of the gene, are known by the alternative forms of the symbol, e.g. A and a. If A a gene which take account for Grey, A is the dominant allele and a, is the recessive form. Crosses involving simple trait or gene which take account for only one pair of alleles, are called MONOHYBRID CROSSED. A diploid organism will carry two alleles of each gene. If the two alleles are the same, the organism is said to be Homozygous. If it is homozygous for dominant allele, it is called HomozyGULUS DOMINANT (AA) but if homozygous for recessive it is called HomozyGULUS RECESSIVE (aa). A homozygous is a pure breeding because if it is self-crossed to a similar homozygous, all the offspring will be homozygous of the parents. If organism carries two different alleles, it is called HETEROZYGOUS (Aa), such organism is called HETEROZYGOTE. The words homozygous and heterozygous describe an individual's genetic make-up. It is GENOTYPE, whereas its outward appearance is called the PHENOTYPE. Color, form, physiology and behaviour are all aspects of phenotype. The homozygous aa will show recessive trait sized while both Aa and AA show homozygous and heterozygous dominant traits respectively. ### MENDER'S FIRST THREE POSTULATES OF INHERITANCE. Using the consistent pattern of results in the monohybrid crosses, Mendel derived the following three principles of inheritance. 1. **UNIT FACTORS IN PARIS**. - Genetic characters are controlled by unit factors that exist in paris in individual organisms. **Verification:** In monohybrid crosses involving tall and dwarf stem, a specific "unit factor" exists for each trait, because the factors occur in pairs. There are three combinations possible. Two factors for tallness, two factors for dwarfness or one factor for each trait (TT, Tt, tt) Every individual contains one of these three combination, which determines stem height. 2. **DOMINANCE | RECESSIVENESS**. - When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other, which is said to be recessive. In Monohybrid cross, the trait expressed in the F1 generation is controlled by the DOMINANT unit factor. The trait not expressed is controlled by RECESSIVE unit factor. (T is dominant over t) 3. **SEGREGATION**. - During the formation of gamates, the pair unit factors separate or segregate randomly so that each gamate receives one or the other with equal likelihood. If an individual contains a pair of like unit factors, e.g. both are specific for tall, then all gamates receive a tall unit factor. If an individual contains unlike unit factors, e.g. one for tall and one for dwarf (Tt). Then each gamate has a 50% probability of receiving either the tall or the dwarf unit factor. The postulates provider a suitable explanation for the results of the monohybrid crosses. For instance, the tall ## Plant contains identical paired unit factors, as the F1 draft plants. The gamates of tall plants all receive one tall unit factor as a result of segregation. Similarly, the gamates of the dwarf plant all receive one dwarf factor. After fertilization, all F1 plants receive one unit factor from each parent; a tall factor from one and a dwarf factor from another, reestablishing the paired relationships, but because tall is dominant to dwarf all F1 plants are tall. **TT T TT (Tall) Homozygous T Tt (Tall) Heterozygous TT (Tall) Homozygous Tt (Tall) Heterozygous When F1 plants form gamates, the postulate of Segregation demands that each gamate randomly receives either tall or dwarf unit factor. Following random fertilization event during F1 selfing, four F2 combinations result in equal frequency. (a) tall/ tall (b) tall/dwarf (c) dwarf/ tall (d) dwarf/ dwarf **Class Exercise** A combination of (a) and (d) result in tall and dwarf plants respectively. Combination of (b) and (c) both yield tall plants. Therefore, the F2 is predicted to consist of 3/4 tall and 1/4 dwarf of ratio 3:1. -- This is called Mendelian ratio. ### PUNNET SQUARE The genotypes and phenotypes resulting from the recombination of gamates during fertilization can be easily visualized by constructing a Punnet Square, named after Reginald C. Punnet who firstly used it. A typical Punnet square is illustrated below **Phenotypes: Tall x Dwarf Gross: T x t Genotype: T x Tt Gamale formation: T Gamale formation by F1: TT Tt t t (Gamates) t Tt Tt T t Tt Tt F1 cross: T x T Fertilization: TT Tt Tt Tt F2 Generation: T t TT Tt (Tall) (Tall) Tt tt (Tall) (Dwarf) Homozygous Heterozygous Homozygous From the list of items below, select all those which are: (i) Phenotypes (ii) Genotypes (iii) Homozygous (iv) Homozygous recessive (v) heterozygous, (vi) Pure breeding. (1) tall (2) AA (3) Ww (4) Aa (5) Purple flower (6) BB (7) pp (8) Tt (9) TT (10) ss (11) P ### STANDARD MONOHYBRID CROSS A monohybrid crosses are those which involve a single character which is controlled by one gene with at least two alleles. Simplest monohybrid cross involve one character with two traits, one trait being dominant to the other. It involves one gene with two alleles where one is dominant and the other one recessive. | Parent - Phenotypure | Genenoty pee | |---|---| | Grey | GG | | Albino | gg | | Fibe | Genetype | Allg. | |---|---|---| | Phenoty pe | ag | All gray. | | Gamates | G Gamates |g | Grande From Fi | GGg ggG gg gg | | Fi-interbreed | 3/4 Grey , 1/4 Albino| | | 2/3 Grey, 1/3 Albino - mixed breed. | Qut what is the phenotypre ratio in F2 generation in illustration above. What is the genotypic ratio. ### MONOHYBRID BACKCROSS / TESTCROSS In the example above, the grey F2 progeny are either homozygous dominant (GG) or heterozygous (Ag). The exact genotype is not apparent from the phenotype. The genotype of what the albino can only be gg. The way to discover an unknown genotype is by performing a further cross, the TESTCROSS. A testcross always involves crossing the unknown genotype to the homozygous recessive. This is the genotype of one of the parents in the standard monohybrid cross and so it is also known as the backcross. The outcome of two possible crosses are shown below. | Parental phenotype | Grey | Albino | |---|---|---| | Parental conotype | GG | gg | | Parental Gamates. | All G. | gg. | | | All g | Allg. | | Grey | Gg | gg | | Albino | gg | Allg. | Backcross progeny Genotype: Gg : gg If the unknown genotype is GG, all the backcross progeny will inherit a g from that parent and will show dominant trait, grey. If the unknown genotype is Gg, each of its offspring has a 1/2 chance of receiving a and g. They will inherit a g from albino parent. On the average, the offspring of the heterozygote and a homozygous recessive show a ratio of 1:1. Even if a single offspring in a testcross shows the recessive phenotype, we know that both of its parent must carry a recessive allele, therefore, the unknown genotype must be heterozygous. ### PHYSICIAL BASIS OF THE LAW OF SEGREGATION By 1916, it was already known that genes are located on a linear sequence along chromosomes. A diploid organism has two sets of chromosomes in each cell, therefore, having two copies of each gene. Position of gene in relation to other gene on the same chromosome is called GENE LOCUS. In haploid cells, each gene locus is represented only once since there is only one set of chromosomes. So, haploid organisms can not be described as either homozygous or heterozygous, dominant or recessive. Two members of haploid organism - bacteria quite straight forward. In polyploids, each locus of gene is represented 3, 4, or more times, making heredity in these organism complex. Three chromosome carrying same gone locus, are called HOMOLOGUES; they are said to be homologous. In heterozygote, Aa, one homologue carries allele A and the other "a" at the same gene locus. One of the homologue is in each allele is a copy of the original chromosome which is donated by MATERNAL HOMOLOGUE, and the partner allele is donated by PATERNAL HOMOLOGUE, and the partner Mendel assumed that the "factor" (alleles) determining each trait were present in pair in the parental, but segregated at gamete formation such that each gamete received only one of the factor, which is carried on the chromosome: a long molecule of DNA arranged on a framework of protein molecules. **Chromosome : Diagram** **Assumptions**: (a) If the two pairs are controlled by the same pair of factors, than the first time world (consist of two classes) are started with. (b) On the other hand, if two pairs of character are controlled independently, then there will be four classes at F2, because, each pair of factor will segregate independently. | Parental | Tall plant with Red flowers | X | Dwarf plant with white flowers| |---|---|---|---| | Prie Irme | Tall with red flowers. | 9 (TR).| |F2 | Tall with white flowers. | 3 (Tr).| | | Drvarf with red flowers. | 3 tr | | | Dwarf with white flowers. | 1 (tt).| | | TTRR x tter| | | TR x tr | R TR Tr | | Gamamets | TR, Tr, R, r | TR, Tr, R, r TR TIRR TTRr TERR | | | | Tr TTRy TTrr TERr TErE | | | | r TRr Trr TERr TErr | | | tr TRr Trr TErr TEr | There are four different genotypes. | | Genotype | | | |---|---|---|---| | TTRR - 1 | | ttRr - 2 | | | TTRr - 1 | 77rr-1 | | | TTrr - 2 | | ttrr - 1| | | Ttrr - 2 | | | | | Tall white 3 | | Dwarf Red 3 | | | | | | Tall Red - 4| | | | | | In inheritance of characters, sometimes a single gene can contribute more than one phenotypical effect. Such gene is said to be PLEIOTROPIC. For instance, in Drosophila melanogaster, the gene that controls white eye "w" not only affects eye color but also behaviors and pigmentation of the internal organs. ### MULTIPLE ALLELES. Over a period of time, if a change occurred in a gene up to about 1200-1500 base pairs, it could result into another allele. So that number of possible alleles at a gene locus is also very large. When there is more alleles at a locus, are present the gene is said to have multiple alleles. In D. melanogaster, for instance, a gene coding for a dehydrogenase enzyme has at least 32 alleles. An individual fly being a diploid, can carry only two. The ABO blood group system in human is controlled by a multiple alleles. Group A has only protein A, group B has protein B and AB has proteins A and B and group O has neither. Allele of I^A and I^B code for enzymes involved in the formation of protein A and B respectively. A third alleles, I^O, codes for no known enzyme. A person can have only two alleles, either both the same or two different alleles so there are six possible genotypes as shown below which correspond to only four phenotypes because I^A I^A or I^A I^O has only A protein and I^B I^B or I^B I^O has only B proteins. | Genotype | Phenotype | |---|---| | I^A I^A | Group A | | I^A I^O | Group A | | I^B I^B | Group AB | | I^B I^O | Group B | | I^O I^O | Group O | Also, in rabbit, multiple allele system determine coat colours. ### INCOMPLETE DOMINANCE AND CO-DUMINANCE Characters that Mendel investigated show complete dominance which means that heterozygote has the same phenotype as the homozygous dominant. In many cases, in dihybrid inheritance, this is not always true, neither allele is dominant to the other and heterozygote can be recognized phenotypically. In incomplete dominance, the phenotype of the heterozygote is intermediate between that of both homozygotes. The degree of dominance may be difficult to determine because it depends very much on the level of observation of the phenotype. Mendel discovered that smooth shape character. Smooth is dominant to wrinkled. Smooth homozygotes and heterozygote can not be distinguished by same observations, but by microscopic level. This at microscopic level, smooth is completely dominant to wrinkled, but at microscopic level, dominance appears to be incomplete. Co-dominance, is similar but not quite same as incomplete dominance, co-dominance alleles both made contributions so that phenotype of the heterozygote showed features of both traits. Co-dominance gave an expected 1:2: 1 phenotypic ratio in F2 generations. (Hb^A, HBS) *Hb^S (abnormal)*. ### LETHAL ALLELES There are certain gene loci whose gene product is essential for life. Any allele which fails to produce it will be lethal in the homozygous state, and phenotype of the heterozygote may be affected. In the frame, they D. melanogaster, the dominant allele Gy caused early wrong when heterozygous (Gy/ay) but is lethal when homozygous (Gy/6y). Homozygon flies do not survive to the adult stage. The homozygon for the recessive, wild type allele (cy/cy) have normal, straight wings. Genes that are lethal when homozygous but not when heterozygous are called RECESSIVE LETHAL genes.. | Parental genotype | Cy/cy X Cy/cy | |---|---| | Platnotype | Curly x Curly | | Parental gomater | 1/2 Cy, 1/2 cy | 1/2 Cy, 1/2 Cy | | Wimartes of progeny | Cy x Cy/cy | Cy/cy, 1/2 Cy/Cy | 1 wild (Cy/cy) |1 lethal cy/cy | | | Statistical consideranturi of Mondhybord orass | F1 will show only one genotype and 1 phenotype. F2 ........... 3 genotypes but 2 phenotypes. ### DIHYBRID INHERITANCE In Mendek's second experiments, he crossed pea plants differing simultaneously in two characters in order to find out whether or not the characters has any influence on each other's inheritance. A cross involving two loci is called DIHYBRID CROSS. A dihybrid cross is also called a two-factor cross where the inheritance of two pairs of contrasted characters is studied. One of Mendel's experiments concerned two seed characters, Shape and color of the cotyledons. He crossed pure plant of smooth yellow seed with pure breeding plants from wrinkled and white green seed. The F1 and F2 progeny were shown below | Parental phenotypre | Smooth, Yellow | x | Wrinkled, green | |---|---|---|---| | F, | all smooth and yellow | | F2 | seed coton | Yellow | Green | Total | Ratio | | | shape | | | | | | | smooth | 315 | 108 | 423 | 3.18 | | | wrinkled | 101 | 32 | 133 | 3:1 | | | | | | | | | | Total | 416 | 140 | 566 | | | Ratio | 3:1 | Mendel two contrasting characters for Dihybrid cross: - Yellow pod with round seed. - Green cross two wrinkled seeds **Parent: YYRR x yyrr** **Gameter Yr yr** **F1 = YyRr. (Yeluw pod round seeds).** If factor for pod color is the same as seed structure, then inheritance will follow as in monohybrid experiment i.e. 3:1 If separate factors are responsible for each character, pair then F2 will give not only the parental genotype, but also classes that will combine both parents. **Gamates: YR, Yr, yR, yr** | | YR | Yr| yR| yr| |---|---|---|---|---| | YR | YYRR* | YYRr* | YYRr* | YyRr* | | Yr |YYRr* | YYrr* | YyRr* | Yyrr* | | yR | YYRR* | YyRr* | yyRR | yyRr | | yr | YyRr* | Yyrr* | yyRr | yyrr | *Yeluw pod round seed = 9 = 9/16 *Yeluw pod wrinkled seed = 3 = 3/16 *Green pod round seeds = 3 = 3/16 *Green pod wrinkled seed = 1 = 1/16 *9:3:3:1* The numbers which Mendel observed in the result of his dihybrid cross agreed with murders that are expected if the inheritance of seed shape has no effect on inheritance of color and vice versa. He was able to formulate his second law, called the law of Independent Assortment that states that the segregation of one pair of alleles is independent of the segregation of the second pair and any combination of allele can occur in the Zygotes . Because the alleles at one gene locus segregate at random with respect to alleles at another gene locus, the expected phenotypic ratio in the F2 progeny 9 (both dominant) 3 (1 dominant and 1 recessive) 3 (1 dominant and 1 recessive) 1 (both recessive fruits). Qust. What fraction of 556 progeny is expected to be smooth and yellow? Green and wrinkled? wrinkled and yellow? How many will be smooth and green? wrinkled and green? wrinkled and yellow? A plant with genotype AaBb is selfed, what is P(AA); P(Bb); P(AABb):? ### DIHYBRID TESTCROSS/BACK CROSS. The genotypes YYRR, YYRR, YYRr and YyRr all have the same phenotype, smooth and yellow. In order to discover the genotype of a plant grown from smooth, yellow seed, a testcross can be done. A testcross is made to an homozygous recessive in case of dihybrid cross, (gym), as shown below. Parent of gamater 1/4 YR 1/4 Yr 1/4 yR 1/4 yr | All r | YyRr | Yu | Yyor | ¼ yy Rr | ayyor. | |-------|--------|--------|--------|--------|--------| | || Smoth, yellow || Smoth green ||grem, womited || wrinkle, greas || The ratio is 1:1:1:1 which is obtained when a double heterozygote is crossed to a double homozygous recessive. Qhout what would be genotype of progeny from the following crosses. (a) AAABb Xaabb (b) Ttzz x ttzz (c) agHh x ggHH. ### LINKED GENES (LINKAGE). After consideration of crosses that involve two characters that are contrasting (Dihybrid cross), the emergence of a 9:3:3:1 ratio in the F2 generation depends on two types of gametes that are behaving independently. If we consider an F1 cross Aa Bb x Aa Bb. (Gamater AB, Ab, ab, and ab) then 9:3:3:1 ratio in the F2 generation will only appear if each F1 organism produced AB, Ab, ab and ab in equal number. Since the publication of Mendel, a large number of exceptions to the 9:3:3:1 ratio have been described. At mating during assortment of genes, some gene seems to segregate if they were somehow joined together or linked together. Further investigation showed "each gene was part of the same chromosomes as they were indeed transmitted as a single unit". The phenomenon where genes that are associated together are linked on a single chromosome. The loci of the genes are located together is referred to as LINKAGE. The reason that genes do not normally become unlinked. They transimmited together is that the loci will always remain on the same chromosomes unless a very rare event separates them. e.g. translocation, Linked genes do not normally become unlinked. In an individual that is heterozygous, at two gene loci (the double heterozygote) has the different allele at both loci. If the gene loci are linked, the allele may be linked in two different ways as shown below. (a) A **--** B + a **--** b Two dominant alleles on one homologue and two recessive alleles on the other. (b) A **--** b + a **--** B one dominant and one recessive allele on each homologue. ### CROSSING OVER. During meiosis, chromatids from homologous chromosomes exchange materials in the process known as crossing over. The consequence is that an individual does not always passes on to its offspring exactly identical chromosomes to those which it received from both parents. During interphase proceeding meiosis, each chromosome replicates itself and becomes two chromatids, which are identical to each other and are called sister chromatids. During prophase I, the chromosomes pair off, gene, for gene, with its homologue, while they are closely associated, non-sister chromatids exchange segments by breaking meioterm name. This exchange of genetic material is called CROSSING OVER, because genes from each homologue have crossed over to the other. | | | | | |---|---|---|---| | Y **--** R | | Y **--** R | | | y **--** r | Prophone | y **--** r | | | | | replicate | | | | | | | H**--** K | X | H**--** K | | | h **--** k | | h **--** k | | | | | | Crossing over | What could be recombinant chromosomes if the parental genotypes are: YyRr x yyrr? **Amphore 1** The chromosomes R and y are the same as that of F1 and they are called PARENTAL CHROMOSOMES/ NON RECOMBINANT chromosomes or simply NON RECOMBINANTS. The chromosomes r and y do not have the same allele combination as in the parental generation, they are called NON-PARENTAL CHROMOSOMES and are a result of CROSSING OVER between Y and R loci. In heterozygote (crossing over product), They carry new combination of alleles (homologus but recombinant). These chromosomes are called RECOMBINANT CHROMOSOMES. The degree of crossing over between any 2 loci in a single chromosome is proportional to the distance between them. Thus, the percentage of recombination gamates varies based on the loci under consideration. Crossing over is currently viewed as a process that occurs during meiosis. This exchange of genetic material provides for an enormous potential variation in gametes formed by any individual. This type of variation in combination with assortment among offspring, in addition to providing individual diversity, results from make progress in protecting and advancing organismic evolution as a paramount importance to the process of organic evolution. ### RELATIONSHIP BETWEEN LINKAGE AND CROSSING OVER. If 2 genes located on the same chromosome are selected randomly, it is more likely that crossing over will occur between them. Since, the intergenic distance for two genes will be far less in complete linkage, the result will be a very low percentage of crossing over among gametes formed and therefore, a very high ==End of OCR for page 16==