Chapter 3 chromosomal basis of inheritance
- 2. Chapter outline • Chromosomal theory of Inheritance – chromosome behaviour and gene behaviour • Linkage - Eye colour in Drosophila and Seed colour in Maize – complete and incomplete Linkage • Crossing over, Recombination and Gene mapping – Holliday model recombination, three point test cross • Multiple alleles – self sterility in Nicotiana • Sex determination in plants – Sphaerocarpos, papaya and maize • Mutation-types, mutagenic agents and their significance - point mutation types.
- 3. T. Boveri (1902) - He was responsible for developing the chromosomal theory of inheritance. W.S. Sutton (1902) - independently recognized a parallelism (similarity) between the behaviour of chromosomes and Mendelian factors. Sutton and Boveri (1903) - independently proposed the chromosome theory of inheritance. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it chromosomal theory of inheritance. Sutton and Boveri chromosome theory
- 4. Salient features of the Chromosomal theory of inheritance Somatic cells of organisms consist of two identical sets of chromosomes. One set is received from female and the other from male. In this each two chromosomes constitute homologous pair. Chromosomes retain their structural uniqueness and individuality throughout the life cycle of an organism. Each chromosome carries specific determiners or Mendelian factors which are now termed as genes. The behaviour of chromosomes during the gamete formation (meiosis) provides evidence that genes or factors are located on chromosomes
- 5. Support for chromosomal theory of heredity • Thomas Hunt Morgan (1910) on the fruit fly Drosophila melanogaster (2n=8). • The gene for red and white eye colour are present on the X chromosome only absent in Y chromosome. • This study support that genes located on chromosomes. • Also an example for sex linkage
- 6. Comparison between gene and chromosome behaviour Comparison of chromosome and gene behaviour
- 7. Gene and chromosome behaviour during cell division The alleles of a genotype are found in the same locus of a homologous chromosome (A/a). In the S phase of meiotic interphase each chromosome replicates forming two copies of each allele (AA/aa), one on each chromatid. The homologous chromosomes segregate in anaphase I, thereby separating two different alleles (AA) and (aa). In anaphase II of meiosis, separation of sister chromatids of homologous chromosomes takes place. Therefore, each daughter cell (gamete) carries only a single allele (gene) of a character (A), (A), (a) and (a).
- 8. Number of Chromosomes Ophioglossum
- 9. Linkage • It is one of the Mendel’s deviation concept against for law of Independent assortment. • The tendency of genes to stay together during separation of chromosomes is called Linkage. Genes do not independently assort out. • It was reported in Sweet pea (Lathyrus odoratus) by Willium Bateson and Reginald C. Punnet in 1906. • Purple flower long pollen X red flower round pollen PL/PL pl/pl • F2 progenies did not exhibit in 1:1:1:1 ratio
- 10. Arrangement of linked and unlinked genes on chromosome Linked genes – close together Unlinked genes or syntenic genes – sufficiently far apart It is differed by recombination frequency value More than 50% - unlinked genes Less than 50% -linked genes
- 11. Cis-Trans arrangement of genes Coupling or Cis configuration Two dominant or recessive alleles present on same chromosome of homologous , inherit together into same gamete. Repulsion or Trans configuration Two dominant or recessive alleles present on different but same homologous chromosome, inherit apart into different gamete.
- 14. KINDS OF LINKAGE Complete linkage • The genes located very close together and do not exhibit crossing over. • Reported in male Drosophila • C.B Bridges (1919) Incomplete linkage • The genes located sufficiently apart and exhibit some crossing over. • Reported in maize • Hutchinson Linkage groups- corresponds to haploid set of chromosomes Name of organism Linkage groups Mucor 2 Drosophila 4 Sweet pea 7 Neurospora 7 Maize 10
- 16. Linkage Crossing over 1. The genes present on chromosome stay close together It leads to separation of linked genes 2. It involves same chromosome of homologous chromosome It involves exchange of segments between non-sister chromatids of homologous chromosome. 3. It reduces new gene combinations It increases variability by forming new gene combinations. lead to formation of new organism. Difference between linkage and crossing over
- 17. Structure of Synaptonemal Complex Mechanism of crossing over
- 18. Types of Crossing Over Depending upon the number of chiasmata formed crossing over may be classified into three types. 1. Single cross over: Formation of single chiasma and involves only two chromatids out of four. 2. Double cross over: Formation of two chiasmata and involves two or three or all four strands 3. Multiple cross over: Formation of more than two chiasmata and crossing over frequency is extremely low.
- 19. Importance of Crossing Over Crossing over occurs in all organisms like bacteria, yeast, fungi, higher plants and animals. Exchange of segments leads to new gene combinations which plays an important role in evolution. Studies of crossing over reveal that genes are arranged linearly on the chromosomes. Genetic maps are made based on the frequency of crossing over. Crossing over helps to understand the nature and mechanism of gene action. If a useful new combination is formed it can be used in plant breeding.
- 20. Recombination • Formation of new combination of characters in an organism called Recombinants • This process is called Recombination.
- 21. The widely accepted model is Holliday’s hybrid DNA model. It was first proposed by Robin Holliday(1964) 1. Homologous DNA molecules are paired side by side with their duplicated copies of DNAs 2. One strand of both DNAs cut in one place by the enzyme endonuclease. 3. The cut strands cross and join the homologous strands forming the Holliday structure or Holliday junction. 4. The Holliday junction migrates away from the original site, a process called branch migration, as a result heteroduplex region is formed. 5. DNA strands may cut along through the vertical (V) line or horizontal (H) line. 6. The vertical cut will result in heteroduplexes with recombinants. 7. The horizontal cut will result in heteroduplex with non recombinants. Holliday model showing Recombination
- 22. RF = Total number of recombinants Total number of offsprings = 6+6 x 100 44+6+6+44 = 12 x 100 100 RF = 12%
- 23. RF = Number of recombinants x 100 Number of off springs = 1+1 x 100 49+49+49+49+1+1+1+1 = 2 x 100 200 RF = 1% Recombination frequency observation
- 24. Genetic mapping The diagrammatic representation of position of genes and related distances between the adjacent genes is called genetic mapping. It is directly proportional to the frequency of recombination between them. It is also called as linkage map. The concept of gene mapping was first developed by Morgan’s student Alfred H Sturtevant in 1913. Map distance: The unit of distance in a genetic map is called a map unit (m.u). One map unit is equivalent to one percent of crossing over. One map unit is also called a centimorgan (cM) in honour of T.H. Morgan. 100 centimorgan is equal to one Morgan (M). Genetic maps can be constructed from a series of test crosses for pairs of genes called two point crosses. But this is not efficient because double cross over is missed.
- 25. Three point test cross A more efficient mapping technique is to construct based on the results of three- point test cross.
- 26. Three point test cross A more efficient mapping technique is to construct based on the results of three- point test cross. The analysis of a three-point cross
- 28. Gene order showing double recombinant Gene mapping
- 29. Uses of genetic mapping � It is used to determine gene order, identify the locus of a gene and calculate the distances between genes. � They are useful in predicting results of dihybrid and trihybrid crosses. � It allows the geneticists to understand the overall genetic complexity of particular
- 30. Three or more allelic forms of a gene occupy the same locus in a homologous chromosomes called multiple alleles. Multiple alleles Check your Grasp There may be multiple alleles within the population, but individuals have only two of those alleles. Why? Characteristics of multiple alleles • It occupy the same locus in the homologous chromosome, no crossing over occurs within the alleles of a series. • Multiple alleles are always responsible for the same character. • The wild type alleles of a series exhibit dominant character whereas mutant type will influence dominance or an intermediate phenotypic effect. • When any two of the mutant multiple alleles are crossed the phenotype is always mutant type and not the wild type
- 31. Self-sterility in Nicotiana East (1925) observed multiple alleles in Nicotiana which are responsible for self-incompatibility or self-sterility.
- 32. Sex determination in plants C.E. Allen (1917). It is a complex process determined by genes, the environment and hormones. About 94% of all flowering plants sexually monomorphic. Some 6% of flowering plants dimorphic. Sex determination in Silene latifolia (Melandrium album) 1. Y chromosome determines maleness 2. X specifies femaleness 3. X and Y show different segments
- 33. Sex determination in Papaya Sex determination in papaya Recently researchers in Hawaii discovered sex chromosomes in Papaya (Carica papaya, 2n=36).
- 34. Sex Determination in Sphaerocarpos • First described among bryophytes • Sphaerocarpos donnellii which has heteromorphic chromosomes. • The male gametophyte as well as the female gametophyte is an haploid organism with 8 chromosome (n=8). • The diploid sporophyte is always heterogametic.
- 35. Sex determination in maize Zea mays (maize) is an example for monoecious Two types of inflorescence. • Tassel- staminate florets • Ear- pistillate florets Sex determination in Maize (Superscript (+) denotes dominant character)
- 36. Mutation • Hugo de Vries (1901) • He has studying on the plant, evening primrose (Oenothera lamarkiana) and proposed ‘Mutation theory’. • Two broad types of mutation • Point mutation • Chromosomal mutations • Mutagens or mutagenic agents • Mutagenesis The production of mutants through exposure of mutagens. • The organism is said to be mutagenized.
- 40. Mutagenic agents Muller (1927) was the first to find out physical mutagen in Drosophila. Physical mutagens: Temperature, Radiation Sharbati Sonora Castor Aruna Chemical mutagens Mustard gas, nitrous acid, ethyl and methyl methane sulphonate (EMS and MMS), ethyl urethane, magnous salt, formaldehyde, eosin and enthrosine Comutagens The compounds which are not having own mutagenic properties but can enhance the effects of known mutagens are called comutagens. Example: Ascorbic acid increase the damage caused by hydrogen peroxide.
- 42. Allopolyploidy • Two or more basic sets of chromosomes derived from two different species is called allopolyploidy. • Allopolyploids are formed between closely related species only. • Example: Raphanobrassica • G.D. Karpechenko (1927) a Russian geneticist, crossed the radish (Raphanus sativus, 2n=18) and cabbage (Brassica oleracea, 2n=18) to produce F1 hybrid which was sterile.
- 43. Triticale, the successful first man made cereal. (i) Tetraploidy: Crosses between diploid wheat and rye. (ii) Hexaploidy: Crosses between tetraploid wheat and rye. (iii) Octoploidy: Crosses between hexaploid wheat and rye. Colchicine, an alkaloid extracted from Colchicum autumnale
- 44. Activity: Solve this When two plants (A and B) belonging to the same genus but different species are crossed, the F1 hybrid is viable and has more ornate flowers. Unfortunately, this hybrid is sterile and can only be propagated by vegetative cuttings. Explain the sterility of the hybrid and what would have to occur for the sterility of this hybrid to be reversed.
- 45. Significance of Ploidy • Many polyploids are more vigorous and more adaptable than diploids. • Many ornamental plants are autotetraploids and have larger flower and longer flowering duration than diploids. • Autopolyploids usually have increase in fresh weight due to more water content. • Aneuploids are useful to determine the phenotypic effects of loss or gain of different chromosomes. • Many angiosperms are allopolyploids and they play a role in an evolution of plants.
- 46. Structural changes in chromosome (Structural chromosomal aberration)