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Take home. Define the following terms: monohybrid cross, self fertilisation , cross fertilisation , segregation, homozygous, heterozygous, allele, dominant, recessive, monohybrid, dihybrid cross, testcross.

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  1. Take home • Define the following terms: monohybrid cross, self fertilisation, cross fertilisation, segregation, homozygous, heterozygous, allele, dominant, recessive, monohybrid, dihybrid cross, testcross. • Consider three yellow, round peas, labeled A, B and C. Each was grown into a plant and crossed to a plant grown from a green, wrinkled pea. Exactly 100 peas issuing from each cross were sorted into phenotypic classes as follows: A: 51 yellow, round; 49 green, round B: 100 yellow, round C: 24, yellow, round; 26 yellow, wrinkled; 25 green, round; 25 green, wrinkled What were the genotypes of A, B, and C (Choose your own gene symbols)? 3. Deduce the F2 ratios of the trihybrid cross AABBCC x aabbcc.

  2. Question Assume we have two plants of genotypes “AabbCcDdEe and AaBbCcddEe” and that from the cross, we want to recover a progeny of genotype “aabbccddee” Estimate how many progeny plants one needs to grow in order to stand a reasonable chance of obtaining the desired genotype.

  3. Answer • Aa x Aa, ¼ will be aa • bb x Bb, ½ will be bb • Cc x Cc, ¼ will be cc • Dd x dd, ½ will be dd • Ee x Ee, ¼ will be ee Therefore, the overall probability or expected frequency of genotype “aabbccddee” will be 1/256.

  4. CHROMOSOMAL BASIS OF INHERITANCE LECTURE 3

  5. Key Questions • How do we know that genes are parts of chromosomes? • How is chromosome number maintained through generations? • What is the chromosomal basis of Mendel’s law of equal segregation and independent assortment?

  6. Outline • How the Chromosome Theory of Inheritance developed - Mitosis - Meiosis • The Discovery of Sex Linkage • A critical Test for the Chromosome Theory

  7. Rediscovery of Mendel’s Work 1900 • Hugo De Vries (Holland) • Carl Correns (Germany) • Erick von Tschermak (Austria)

  8. Where are the hereditary determinants (genes) located in a cell? What is the precise way in which segregation and independent assortment are achieved at the cellular level?

  9. Genetics Took a major step forward with the notion that genes are parts of chromosomes (Chromosome Theory of Inheritance)

  10. The Nature of Chromosomes • Each chromosome contains a single, long, folded DNA molecule

  11. How Did the Chromosome Theory of Inheritance Develop?

  12. Constancy of the Numbers of Chromosomes - cell to cell within an organism - organism to organism within any species - generation to generation within species.

  13. How was Chromosome Number Maintained?

  14. Transmission of Genes and Chromosomes at Mitosis and Meiosis

  15. Mitosis Nuclear division associated with the division of somatic cells(cells of the eukaryotic body that are not destined to become sex cells).

  16. Stages of Mitosis • Prophase • Metaphse • Anaphase • Telophase

  17. Mitosis • produces two genetically identical cells from a single progenitor cell. • The two fundamental processes of mitosis are replicationfollowed by segregation.

  18. Meiosis • Two successive nuclear divisions (Meiosis I & II) – four cells called the products of meiosis (haploid gametes); spermatozoa and ova in humans; meiospores (gametes) in plants • S phase before meiosis • Meiosis I & II divided into prophase, metaphase, anaphase and telophase • Prophase I is complex with sub-divisions

  19. (Thinthread) (paired) (thick) (double) (moving apart)

  20. Message • In mitosis, each chromosome replicates to form sister chromatids, which segregate into the daughter cells. • In meiosis each chromosome replicates to sister chromatids. Homologous chromosomes physically pair and segregate at the first division. Sister chromatids segregate at the second division

  21. What are the Main Differences Between Mitosis and Meiosis?

  22. The Chromosome Theory of Heredity 1902-Walter Sutton (American grad student) Theodor Boveri (German Biologist) • Genes are in pairs (so are chromosomes) • The alleles of a gene segregate equally into gametes (so do members of a pair of homologous chromosomes) • Different genes act independently (so do different chromosome pairs)

  23. The parallel behaviour of genes and chromosomes Genes are parts of chromosomes

  24. 5 Minutes Break

  25. The Discovery of Sex Linkage

  26. 1906 - L. Doncaster and G. H. Rayor, Magpie Moth (Abraxas)

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