Chapter.2. Mendelism
- 1. (T.Y.B.Sc. BOTANY PAPER –III BO.333: GENETICS AND EVOLUTION GSD ) Sir Parashurambhaucollege, Pune-30 Department of Botany Chapter. 2. MENDELISM Dr. Dhulgande G. S. Asst. prof.
- 2. Genetical Terminology: 1. Haploid - the condition of having only one set of chromosomes per cell (n) 2. Diploid - the condition of having two sets of chromosomes per cell (2n) 3. Gamete - a haploid (n) sex cell in plants and animals (egg or sperm) 4. Zygote - diploid (2n) cell resulting from the union of two gametes in sexual reproduction 5. Chromatin - the complex of DNA, RNA and proteins that makes up uncondensed eukaryotic chromosomes. 6. Chromosome - structures within the nucleus of eukaryotic cells composed of chromatin and visible at cell division (condensed chromatin). 7. Homologous chromosomes - chromosomes that are similar in morphology (shape and form) and genetic constitution. In living organisms one set comes from the father and the other from the mother.
- 3. 8. Chromatids - One of the two half of a duplicated chromosome. 9. Centromeres- specialized constricted region of a chromatid, that contains the kinetochore; sister chromatids are joined at the centromere during cell division. 10. Recombination - exchange of genetic material between chromosomes 11. Crossover- the breaking and rejoining of homologous (sister) chromatids during prophase-I (pachytene) of meiosis, resulting in recombination. LZPDD 12. Synapsis- the pairing of homologous chromosomes during prophase-I (zygotene) of meiosis. 13. Disjunction- separation of homologous chromosomes (or sister chromatids) during anaphase. 14. Genotype- the genetic make-up (the assemblage of alleles) of an individual.
- 4. 15. Phenotype- the physical or chemical expression of an organism’s gene. 16. Gene- a discrete unit of hereditary information that usually specifies a protein; a region of DNA (locus) located on a chromosome that specifies a trait (characteristic). 17. Alleles- genes governing variations of the same characteristic (trait) that occupy corresponding positions (loci) on homologous chromosomes; alternative forms of a gene. 18. Dominant allele- an allele that is always expressed when present, regardless of whether the organism is homozygous or heterozygous for that gene. 19. Recessive allele- an allele that is only expressed when the organism is homozygous for that allele and not expressed when heterozygous (when paired with a dominant allele). 20. Homozygous- possessing a pair of identical alleles for a particular locus (gene). 21. Heterozygous- possessing a pair of different alleles for a particular locus (gene).
- 5. 22. Carrier- a heterozygous individual not expressing a recessive trait but capable of passing it on to its offspring. 23. Parent generation (P)- the generation that supplies gametes to the filial generation. 24. Filial generation (F1)- the generation that receives gametes from the parental generation. 25. Hybrid- an offspring resulting from the mating between individuals of two different genetic constitutions (parents). 26. Dihybrid cross- a genetic cross that takes into account the effect of alleles at two separate loci (two different genes). 27. Monohybrid cross- a genetic cross that takes into account the effect of alleles at a single locus (single gene).
- 6. Selection of the experimental plant material: Mendel selected garden sweet pea (Pisum sativum L.) for his hybridization experiments for the following reasons: 1. The plants are annual and easy to cultivate. 2. The plants have very short life cycle. 3. The plants have many distinct, well defined and easy morphological characters. 4. The flowers are bisexual. 5. The flowers are naturally self-pollinating, but they can also be easily cross pollinated. 6. The offspring of cross pollinated plants are fertile. 7. The flowers are sufficiently large which helps in easy emasculation and artificial cross pollination. 8. The plant produce a large number of progenies (offspring’s) after each successive generations, it will help in correct conclusions.
- 7. The seven variable characteristics Mendel investigated in pea plants were •Seed texture: (Round vs wrinkled) •Seed cotyledon color :(Yellow vs green) •Flower color :(White vs purple) •Growth habit (Height): (Tall vs dwarf) •Pod shape: (Inflated vs constricted) •Pod color :(Green vs yellow) •Flower position :(Axial vs terminal)
- 9. Monohybrid Cross: Definition: “A cross between two parents possessing a pair of contrasting characters is known as monohybrid cross”. A monohybrid cross is a mating between two organisms with different variations at one genetic chromosome of interest. The character(s) being studied in a monohybrid cross are governed by two or multiple variations for a single locus. This cross is based on the first law of genetics i.e Law of Segregation. e.g. When the pure tall variety of pea plant is crossed with the pure dwarf variety of pea plant. ‘T’ is used for tall plant while‘t’ is used for dwarf.
- 10. Parents: Tall × Dwarf TT tt Gametes: T t F1 Generation: Tt --------------- Hybrid tall On selfing Tt is crossed with Tt Male Female T t T TT Tall Tt Tall t Tt Tall tt Dwarf The monohybrid phenotypic ratio is 3:1 and the genotypic is 1:2:1.
- 11. Dihybrid Cross: Definition: “A cross between two pure or homozygous parents having two pairs of contrasting characters is called dihybrid cross”. The F1 hybrids (offsprings) obtained from dihybrid cross are known as dihybrids. The phenotypic ratio is called dihybrid ratio; it will be 9:3:3:1. When pure homozygous pea plant having yellow cotyledons and round seeds with another homozygous pea plant having green cotyledons and wrinkled seeds. In this cross there are two pairs of contrasting characters are present i.e. one pair is yellow cotyledon with rounded seeds while other pair is green cotyledon with wrinkled seeds. Here, Yellow cotyledon color represented by -------------------‘Y’ Rounded seeds represented by ------------------------------‘R’ Green cotyledon color represented by ----------------------‘y’ and Wrinkled seeds represented by ------------------------------‘r’. Therefore , the parents will be as Yellow round (YYRR) and green wrinkled (yyrr).
- 12. Parents: Yellow round × Green wrinkled YYRR yyrr Gametes: YR yr F1 generation: YyRr ------------------------ Yellow round Hybrid On selfing, Yellow round × Yellow round YyRr YyRr Gametes for F2 generation: YR, Yr, yR, yr YR Yr yR yr YR YYRR Yellow round YYRr Yellow round YyRR Yellow round YyRr Yellow round Yr YYRr Yellow round YYrr Yellow wrinkled YyRr Yellow round Yyrr Yellow wrinkled yR YyRR Yellow round YyRr Yellow round yyRR Green round yyRr Green round yr YyRr Yellow round Yyrr Yellow wrinkled yyRr Green round yyrr Green wrinkled
- 13. YR Yr yR yr YR YYRR YR YR YyRR YR Assignment
- 14. Mendel’s Laws of Heredity (Inheritance): Based on the hybridization experiments, John Gregor Mendel formulated the laws of inheritance. There are three laws: 1. Law of Dominance 2. Law of Segregation 3. Law of Independent assortment.
- 15. 1. LAW OF DOMINANCE: It is based on monohybrid cross. This law states that, “In crossing of pure homozygous organism having only one pair of contrasting character, one factor in a pair of allele may mask or prevent the expression of other one”. T t e.g. When Mendel made a monohybrid cross between tall and dwarf, then all tall plants were appeared in the F1 generation. But on selfing of F1 generation, both tall and dwarf characters appeared in F2 generation. It shows that in F1 hybrid the tall character is dominant while dwarf is recessive.
- 16. 2. LAW OF SEGREGATION or LAW OF PURITY OF GAMETES or LAW OF INCOMPLETE DOMINANCE: It is also known as law of purity of gametes. It is based on monohybrid cross. This law states that, “during gamete formation the two factors (genes) of each character segregate i.e. one factor of one character in each gamete will remains. Thus here each gamete is pure from each other”. OR It also states that, “during the formation of gametes, the two traits carried by each parent will segregate easily”. Thus, the law of segregation is based on the fact that each gamete contains only one allele. e.g. When a red flowered pea plant (RR) is crossed with white flowered pea plant (rr) then hybrid will have pink flowers (Rr). On selfing of F1, they produce three colored (Red, pink and white) flowers in F2 generation. The reappearance of pink color indicates the process of segregation.
- 17. 3. LAW OF INDEPENDENT ASSORTMENT: It is based on dihybrid cross. It states that, “when the parents differ from each other in two or more pairs of contrasting characters then the inheritance of one pair of factor is independent to that of other pair of factor”. e.g. When pure homozygous pea plant having yellow cotyledons and round seeds crossed with another homozygous pea plant having green cotyledons and wrinkled seeds. In this cross there are two pairs of contrasting characters are present i.e. one pair is yellow cotyledon with rounded seeds while other pair is green cotyledon with wrinkled seeds. Therefore , the parents will be as Yellow round (YYRR) and green wrinkled (yyrr).
- 18. BACK CROSS: “When F1 individuals are crossed with any one of the parents, then such a cross is called back cross”. In such cases there are two possibilities: (A) When F1 (Tt) is crossed to the parent with dominant phenotype i.e., homozygous tall (TT). In such a cross plants will be 100% tall. (B) When F1 plant (Tt) is crossed to the parent with pure recessive (tt) dwarf plant. In such a cross 50% plants will be tall and 50% plants will be dwarf.
- 19. Parents: Tall × Dwarf TT tt Gametes: T t F1 Generation: Tt --------------- Hybrid tall T t T TT Tall Tt Tall T TT Tall Tt Tall A) F2----- Tt × TT B) F2-----Tt × tt 100% Tall 50% Tall and 50% dwarf T t t Tt Tall tt Dwarf t Tt Tall tt Dwarf
- 20. TEST CROSS: “When F1 individuals are crossed with one of its homozygous recessive parent, then such a cross is called test cross”. e.g. When F1 plant (Tt) is crossed to the parent with pure (homozygous) recessive (tt) dwarf plant. In such a cross 50% plants will be tall and 50% plants will be dwarf.
- 21. T t t Tt Tall tt Dwarf t Tt Tall tt Dwarf Parents: Tall × Dwarf Gametes: TT tt F1------ T t F2-----Tt × tt 50% Tall and 50% dwarf
- 22. Assignments: Define: 1. Gamete - 2. Diploid- 3. Centromeres- 4. Gene- Mendel selected garden sweet pea (Pisum sativum L.) for his hybridization experiments for the following reasons: 1. The plants are --------- and easy to cultivate. 2. The plants have very --------- life cycle. 3. The plants have many distinct, ---------------- and easy morphological characters. 4. The flowers are --------------. 5. The flowers are naturally ----------pollinating, but they can also be easily cross pollinated. 6. The offspring of ------------- pollinated plants are fertile. 7. The flowers are sufficiently ---------- which helps in easy emasculation and artificial cross pollination. 8. The plant produce a -------- number of progenies (offspring’s) after each successive generations, it will help in correct conclusions.
- 23. 1. Crossing over occurs during---------------- 2. Synopsis occurs during----------------
- 25. Back cross T t T T T t t t