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Introduction to Genetics

Introduction to Genetics. Sexual Reproduction : Zygote is a product of both parents The chromosomes of each gamete (egg and sperm) bring hereditary material from each parent Offspring is both similar and different from both parents. Phenotypes and Genotypes.

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Introduction to Genetics

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  1. Introduction to Genetics

  2. Sexual Reproduction: • Zygote is a product of both parents • The chromosomes of each gamete (egg and sperm) bring hereditary material from each parent • Offspring is both similar and different from both parents

  3. Phenotypes and Genotypes • Phenotype: the way an organism looks or appears • Genotype: the actual gene combination an organism has (may not be visually apparent)

  4. Development of Genetic Theories

  5. Gregor Mendel (Mid 1800s) • Studied pea plants. • How were traits passed on from one generation to the next • Discovered basic laws of genetics • Crossed plants that were purebred for different traits • Purebred: organisms that produced organisms identical to themselves • Studied one trait at a time

  6. Mendel’s First Experiment Monohybrid Cross: • Parents differ by a single trait • Offspring of this type of cross are called hybrids • Ex: Mendel crossed tall and short pea plants

  7. Parent Generation (P1) • 1st generation (F1): • All tall offspring • What happened to the short? • 2nd generation (F2): • ¾ tall and ¼ short • The short trait reappeared!!

  8. He repeated his tests for 7 other traits in pea plants and got same result. • One trait disappears in F1 generation • But reappears in ¼ of F2 generation WHY???

  9. Bill Nye: • Greatest Discoveries: Law of Inheritance • http://www.youtube.com/watch?v=0vAAf4g5iF8&safe=active • Online Activity • http://www.hhmi.org/coolscience/resources/SPT--FullRecord.php?ResourceId=73

  10. Mendel’s Rules for Inheritance Rule of Unit “Factors” Characteristics are determined by factors that are passed from one generation to the next • Each organism has 2 “factors” for each trait • These “factors” can exist in alternate forms

  11. We now call these factors genes These genes can exist in different forms or alleles Organisms inherit one allele from each parent (on homologous chromosomes)

  12. Mendel’s Rules for Inheritance Rule of Dominance • Some alleles are dominant and some recessive • EX: • TT and tt pea plants are crossed • All offspring are: • Genotypically Tt (hybrid) • Phenotypically Tall • Trait that phenotypically appears is dominant (Tall) • Trait that disappears is recessive (short)

  13. Dominance in Human Traits

  14. Mendel’s Rules for Inheritance Law of Segregation How are different forms of a gene distributed to offspring? During gamete formation (meiosis), alleles for each gene segregate or separate independently from each other Each gamete only carries one allele for each gene.

  15. Homozygous: Individuals that have two of the same alleles for a trait (ex: BB or bb) • Heterozygous: “hybrid” individuals that have two different alleles for the same trait. (ex: Bb)

  16. Punnet Squares • Illustrates the law of segregation of traits • Shows all possible combination of gametes • Way of finding expected proportions of traits in offspring • (Need a large sample size to really get expected ratio)

  17. Monohybrid Cross: • Ex: Parents differ by single trait for height • TT x tt • Tt x Tt • Tt x tt • Are the parents in each cross homozygous/heterozygous? • What are the phenotypes for each parent? • Let’s cross

  18. What are the outcomes? • What are the genotypes and phenotypes of the offspring?

  19. Dihybrid Cross: • Cross involving two different traits • Ex: Round yellow seeds x wrinkled green seeds • Round = R, Wrinkled = r • Yellow = Y, Green = y • Let’s try a cross! RRYY x rryy

  20. P1 Generation: RRYY x rryy • F1 Generation: All round yellow (RrYy) • 100% heterozygous hybrids for both traits • Phenotype is 100% yellow round • Round and yellow traits must be dominant!

  21. F2 Generation: • Cross “dihybrids” from the first generation • RrYy x RrYy • Create a Punnet Square: • What goes at top of each column and row? • What are the possible combination of these genes in the gametes?

  22. Phenotype Outcome: 9 : 3 : 3 : 1 Yellow Round Green Round Yellow Wrinkled Green Wrinkled 9 3 3 1 • Do you get a 9:3:3:1 phenotype ratio?

  23. Law of Independent Assortment Alleles for different traits are inherited independently of each other. During gamete formation (meiosis) chromosomes are separating independent of each other • Ex: • Round/wrinkled and yellow/green traits are inherited independantly of each other • Note: • Only true if the genes for traits are located on different chromosomes

  24. Mendelian Genetics Summary Law of Unit Factors: • Traits have 2 factors (or alleles) that can be inherited from parents Law of Dominance: • One factor is dominant and the other recessive Law of Segregation: • During gamete formation the factors segregate randomly into eggs and sperm and then recombine in offspring Law of Independent Assortment: • Different traits sort themselves independently of each other • Phenotype (how it looks) vs. Genotype (what genes/alleles it has) • Homozygous (BB or bb) vs. Heterozygous (Bb) • Monohybrid Cross (hybrid for one trait) get 3:1 phenotype ratio • Dihybrid Cross (hybrids for 2 traits) get 9:3:3:1 phenotype ratio • Punnet Square: know how to set them up!

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