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.
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.
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.
CHROMOSOMAL BASIS OF INHERITANCE LECTURE 3
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?
Outline • How the Chromosome Theory of Inheritance developed - Mitosis - Meiosis • The Discovery of Sex Linkage • A critical Test for the Chromosome Theory
Rediscovery of Mendel’s Work 1900 • Hugo De Vries (Holland) • Carl Correns (Germany) • Erick von Tschermak (Austria)
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?
Genetics Took a major step forward with the notion that genes are parts of chromosomes (Chromosome Theory of Inheritance)
The Nature of Chromosomes • Each chromosome contains a single, long, folded DNA molecule
Constancy of the Numbers of Chromosomes - cell to cell within an organism - organism to organism within any species - generation to generation within species.
Mitosis Nuclear division associated with the division of somatic cells(cells of the eukaryotic body that are not destined to become sex cells).
Stages of Mitosis • Prophase • Metaphse • Anaphase • Telophase
Mitosis • produces two genetically identical cells from a single progenitor cell. • The two fundamental processes of mitosis are replicationfollowed by segregation.
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
(Thinthread) (paired) (thick) (double) (moving apart)
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
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)
The parallel behaviour of genes and chromosomes Genes are parts of chromosomes