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BL 100 L6.ppt

  1. Lecture 6: Mendelian Monohybrid Inheritance (Segregation) Clara Yona Department of Biosciences Solomon Mahlangu College of Science and Education
  2. Mendelian Genetics Definitions • Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual • Allelic pair - the combination of two alleles which comprise the gene pair • Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest • Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote
  3. Defn cont… • Genotype - the specific allelic combination for a certain gene or set of genes • Phenotype - literally means "the form that is shown"; it is the outward, physical appearance of a particular trait • Dominant - the allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F1 generation from the cross of two pure lines • Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F1 generation from the cross of two pure lines and reappears in the F2 generation
  4. Defn cont… • Monohybrid cross - a cross between parents that differ at a single gene pair (usually AA x aa) • Monohybrid - the offspring of two parents that are homozygous for alternate alleles of a gene pair
  5. Animals
  6. Mechanism (1): All in the Genes • The genetic make-up of an organism is known as its genotype. • An organism’s genotype and the environment in which it lives determines its total characteristic traits i.e. its phenotype. Phenotype Genotype
  7. Mechanism (2): DNA Watson and Crick and their model of DNA DNA replication • The double-helix structure of DNA was discovered in 1953. • This showed how genetic information is transferred from one cell to another almost without error.
  8. Gregor Johann Mendel (1822-1884) "father of modern genetics"
  9. Gregory Mendel • 1st scientist to develop the fundamental principles that became the modern science of “genetics” • Mendel study was based on simple patterns of inheritance • Demonstrated that heritable properties are parceled out in separate units (independently inherited) • When doing his works, was selective growing the common pea plants (Pisum sativum) • At the time there was no knowledge of chromosomes, cell structure, fertilization, mitosis and meiosis, which were discovered after his lifetime
  10. Mendelism vs Darwinism • In 1865 Mendel proposed that invisible internal units (factors) of information account for observable traits, and that these "factors" - which later became known as genes - are passed from one generation to the next. • Mendel's work remained unnoticed, languishing in the shadow of Darwin's more sensational publication from five years earlier, until 1900, when Hugo de Vries, Erich Von Tschermak, and Carl Correns published research corroborating Mendel's mechanism of heredity.
  11. … Mendelism vs Darwinism • In 1859, Charles Darwin (1809 - 1882) hypothesized that animal populations adapt their forms over time to best exploit the environment, a process he referred to as "natural selection." As he traveled in the Galapagos Islands, he observed how the finch's beaks on each island were adapted to their food sources. He theorized that only the creatures best suited to their environment survive to reproduce. • Charles Darwin's landmark book, “The Origin of Species," was published in London. It effectively drowned out all other scientific voices, including Mendel’s, for decades.
  12. Charles Darwin
  13. • Although he was not a world renowned scientist, Mendel was the 1st to trace the characteristics of successive generations of a living thing • While his research was with plants, the basic underlying principles of heredity that he discovered also apply to people and animals • The impact of genetic theory is no longer questioned in anyone's mind • Many diseases are known to be inherited, and pedigrees are typically traced to determine the probability of passing along an hereditary circles
  14. Mendel studies • Based on seven traits of peas • These were qualitative traits and could be measured and assigned value • These characteristics were visible and were used to study the effects of reproduction
  15. The subject for his research, chose the common garden pea plants as focus of his research because: (i) they have a number of sharply contrasting characters (ii) they are easy to culture (iii) they can be grown easily in large numbers (iv) they produce many offspring per mating (v) they are capable of self-fertilization (vi) they have a number of varieties in type and (vii) their reproduction can be manipulated.
  16. SEED SHAPE Round Wrinkled
  17. POD COLOUR Green Yellow
  18. SEED COLOUR Yellow Green
  19. POD SHAPE Inflated Constricted
  20. FLOWER COLOUR Purple White
  21. FLOWER POSITION Axial (stem) Terminal (at tip)
  22. STEM LENGTH Tall Dwarf
  23. Pea- plant characters studied by Mendel Purple White Axial (stem) Terminal (at tip) Yellow Green Round/Smooth Wrinkled Inflated Constricted Green Yellow Tall Dwarf
  24. Mendel Experiments (i) Number of different traits that can be studied (ii) Be self-fertilizing with a flower structure that limits accidental contact (iii) Offspring of self-fertilized plants should be fully fertile
  25. PEA FLOWER SEX STRUCTURE Female sex structure (Pistil) Male sex structure (Stamen)
  26. Pea plant • Have both male and female reproductive organs • Mendel could open the flower buds and remove stamen before they were ripe • He used pollen from another plant and dusted the pistil to affect effective cross-pollination • As a result, they can either self-pollinate themselves or cross-pollinate with another plant • In his experiments, he was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations
  27. Mendel carried out some cross-fertilization
  28. Removed stamens from purple flower 1 2 3 Transferred pollen from stamens of white flower to carpel of purple flower 4 Planted seeds from pod Pollinated carpel matured into pod White Stamens Carpel Parents (P) Purple Offspring (F1)
  29. • Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations • With clear-cut differences it is possible to cross or self pollinate the plant and examine the characteristics of their offspring • This was the basis for his conclusions about the nature of genetic inheritance
  30. Mendelian Experiments • A cross between individuals differing in single character is a monohybrid cross or monohybrid inheritance Monohybrid test cross
  31. F2 = F1 x F1
  32. The reappearance of the recessive trait in ¼ of the F2, suggests genes come in pairs that separate in the formation of sex cells. Three genotype (YY, Yy and yy) Two phenotype (Yellow & Green) 3 Yellow 1 Green
  33. Mendel Experimental Results (i) F1 offspring showed only one of the two parental traits, and always the same trait (ii) Results were always the same regardless of which parent donated the pollen (iii) The trait not shown in the F1 reappeared in the F2 in about 25% of the offspring (iv) Traits remained unchanged when passed to offspring: they did not blend in any offspring but behaved as separate units (v) Reciprocal crosses showed each parent made an equal contribution to the offspring
  34. Conclusions from monohybrid experiment • Evidence indicated that, factors could be hidden or unexpressed, these are the recessive traits • The term phenotype refers to the outward appearance of a trait, while the term genotype is used for the genetic makeup of an organism • Male and female contributed equally to the offsprings' genetic makeup • Upper case letters are traditionally used to denote dominant traits, lower case letters for recessives
  35. Mendel’s first law Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete The reappearance of the recessive characteristic in F2 generation shows that recessive genes are not lost or modified in the F1.
  36. Principle of Segregation in Modern Genetics
  37. (a) Gamete formation/ meiosis Give rise into four daughter cells.
  38. (b) Gene segregation
  39. Thank You