Lecture 3: Mendelian Genetics and Monohybrid Crosses

Lecture 3: Mendelian Genetics and Monohybrid Crosses CK-12 alamy.com  Summary of previous lecture Chromosomes, chromosome territories and karyotypes Bivalent and tetrad Synapsis and synaptonemal complex Mitosis and meiosis Cross-overs and...

Lecture 3: Mendelian Genetics and Monohybrid Crosses CK-12 alamy.com  Summary of previous lecture Chromosomes, chromosome territories and karyotypes Bivalent and tetrad Synapsis and synaptonemal complex Mitosis and meiosis Cross-overs and chiasmata Mitotic spindle, cohesion and shugoshin Ushiki et al., 2008 Historical Views of Genetics Historical Views of Genetics Preformationism Homunculus Germ-plasm Theory  Blending Inheritance Sex cells contain essences of the parents. The union of these cells blended the respective essences. Red Flowers x White Flowers Pink Flowers Gregor Mendel Born in 1822 in Heinzendorf, Austria (now Czech Republic). Originally named Johann Mendel. Took the name Gregor upon entering religious life. Joined Augustinian Abbey of St. Thomas in Brno (now Czech Republic) in 1843. Gregor Mendel 1822-1884 Between 1856 and 1863, he experimented with pea plants, growing a total of about 29,000. Published his paper on his results in 1866. In 1867, he became the Abbott and stopped experimenting. Died in 1884. Posthumously became known as the "Father  Definitions A gene determines a trait. Alleles are alternate forms of a gene. Pollen and eggs (or any sex cells, sperm and egg) have 1 determinant. The adult organism has 2 alleles. Locus is a specific place on a chromosome occupied by an allele (plural is loci). If the two alleles are the same, the organism is said to be homozygous (homo = same; zygous refers to the zygote = the organism). If the two alleles are different, the organism is said to be heterozygous  What did Mendel do that was different? Used clear cut traits. Traits were true breeding. Worked quantitatively. Exceptionally bright Mendel’s Crosses  Types of crosses Monohybrid cross: cross involving a single characteristic (e.g. sha Dihybrid cross: cross involving two characteristics (e.g. shape To cross different varieties of peas, Mendel removed the anthers from flowers to prevent self-fertilization… …and dusted the stigma with pollen from a different plant. To cross different varieties of peas, Mendel removed the anthers from flowers to prevent self-fertilization… …and dusted the stigma with pollen from a different plant. The pollen fertilized ova, which developed into seeds. The seeds grew into plants. True Breeding True Breeding Mendel crossed two homozygous varieties of peas. All the F1 seeds were round. Mendel allowed plants grown from these seeds to self-fertilize.  Mendels F1 Observations hybrids are uniformly round regardless of the way the cross was performed (male round x female wrinkled and vice versa). Wrinkled F2 offspring appear when F1 hybrids self-fertilize in the ratio of 3:1 round to wrinkled. When F2 plants were self-fertilized to give an F3 generation, all the wrinkled bred true whereas 1/3rd of the round bred true and 2/3rds gave rise to  Mendel’s Conclusions Heredity was particulate - not blending. Each plant has 2 determinants (round-wrinkled or purple- white) or alleles for each trait. In a heterozygote, one allele may be expressed (dominant) and the other is hidden (recessive)  Definitions In a cross between 2 different true breeding strains, the trait expressed in the F1 is said to be dominant. The trait which is not expressed is recessive. An F1, having two different alleles would produce two different types of gametes (sex cells). One type would have the dominant allele and the other would have the recessive allele and these two types would be  Mendel’s First Law The Law of Segregation During gamete formation, alleles separate or segregate from each other. Half of the gametes carry one member of the pair and the other half carry the  Definitions Genotype: the genetic constitution of an organism. For example: RR or Rr Genotypic ratio: the ratio of the genotypes that result from a cross. For example: 1:2:1 ratio (1RR:2Rr:1rr) Phenotype: the appearance of an organism. For example: tall plants or dwarf, round seeds or wrinkled. The phenotype doesn’t always indicate the genotype (RR and Rr) Phenotypic ratio: the ratio of the Mendel’s purple and white flower experiments Type of cross? Phenotype and genotype of F1 generation? What color is dominant? What color is recessive? CK-12 Phenotype and genotype of F2 Mendel’s purple and white flower experiments Monohybrid cross = one trait = color F1 generation has purple phenotype F1 genotype is heterozygous Purple appears to be dominant over white White flowers in F2 CK-12 generation are  Types of crosses Backcross: crossing F1 back to either parental type. Reciprocal cross: reverse male and female phenotypes Test cross: one parent is homozygous recessive Test cross Why do we care about Mendel’s work?  Summary Genes and alleles Monohybrid cross Genotype: heterozygous and homozygous Pathwayz Phenotype Genotypic and phenotypic ratio Mendel’s first law Back cross, reciprocal cross and test cross Disorders in humans