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Genetics

Genetics. Unit 8. Gregor Mendel – The Father of Genetics. Lived from 1822-1884 Austrian monk Studied pea plants Developed the Laws of Inheritance The importance of his work was not recognized until the 20 th century. Site of Gregor Mendel’s Experimental Garden.

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Genetics

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  1. Genetics Unit 8

  2. Gregor Mendel – The Father of Genetics • Lived from 1822-1884 • Austrian monk • Studied pea plants • Developed the Laws of Inheritance • The importance of his work was not recognized until the 20th century

  3. Site of Gregor Mendel’s Experimental Garden

  4. Particulate Inheritance • Mendel knew that something was passed from parent to offspring that affected physical traits  he called them “particles” • He did not know that these particles were actually chromosomes and DNA

  5. Words to Know… • Trait- any characteristic that can be passed from parent to offspring • Heredity- passing of traits from parent to offspring • Genetics- study of heredity • Gene- a sequence of DNA that codes for a protein and thus determines a trait

  6. Punnett Squares • Created by Reginald Punnett (1875-1967) • Used to help solve genetics problems

  7. Punnett Square Rules: • One parent per side of the square • Capital letters always come first: Aa = correct, aA = wrong • Keep the same letters together:AaBB = correct, ABBa = wrong

  8. Types of Crosses • Monohybrid cross - cross involving a single traitExample: seed color • Dihybrid cross - cross involving two traits Example: seed color AND wrinkled/smooth

  9. Words to Know… • Alleles- two forms of a gene(dominant & recessive) • Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) • Recessive – gene that shows up less often in a cross; represented by a lowercase letter (r)

  10. Words to Know… • Genotype - gene combination for a trait(e.g. PP, Pp, pp) • Phenotype - the physical feature resulting from a genotype(e.g. purple, white)

  11. Words to Know… • Homozygousgenotype - gene combination involving 2 dominant or 2 recessive genes (ex. RR or rr);also calledpure-bred • Heterozygousgenotype - gene combination of one dominant & one recessive allele (ex. Rr);also calledhybrid

  12. Mendel’s Experiments • Why use peas? • They can be grown in a small area • They produce many offspring very quickly • They can be self-pollinated to make pure-breds • They can be artificially cross-pollinated to make hybrids

  13. Mendel’s Experiments • Mendel studied the peas over several generations • He observed and recorded 8 traits: • Seed shape--- Round (R) or Wrinkled (r) • Seed Color---- Yellow (Y) or  Green (y) • Pod Shape--- Smooth (S) or wrinkled (s) • Pod Color---  Green (G) or Yellow (g) • Seed Coat Color---Gray (G) or White (g) • Flower position---Axial (A) or Terminal (a) • Plant Height--- Tall (T) or Short (t) • Flower color --- Purple (P) or white (p)

  14. The Results

  15. Words to Know… • Parental P1 Generation= the parental generation in a breeding experiment. • F1 generation= the first-generation offspring in a breeding experiment. (1st filial generation) • From breeding individuals from the P1 generation • F2 generation= the second-generation offspring in a breeding experiment. (2nd filial generation) • From breeding individuals from the F1 generation

  16. Following the Generations Cross 2 Pure PlantsTT x tt Results in all HybridsTt Cross 2 Hybridsget3 Tall & 1 ShortTT, Tt, tt

  17. Monohybrid Crosses

  18. P1 Monohybrid Cross • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: RoundseedsxWrinkled seeds RRxrr Genotype:Rr Phenotype:Round GenotypicRatio:All alike PhenotypicRatio: All alike r r Rr Rr R R Rr Rr

  19. F1 Monohybrid Cross • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Roundseeds xRound seeds RrxRr Genotype:RR, Rr, rr Phenotype:Round & wrinkled G.Ratio:1:2:1 (25%, 50%, 25%) P.Ratio:3:1 (75%, 25%) R r RR Rr R r Rr rr

  20. …And Now the Test Cross • Mendel then crossed a pure & a hybrid from his F2 generation • This is known as an F2 or test cross • There are two possible testcrosses:Homozygous dominant x HybridHomozygous recessive x Hybrid

  21. F2 Monohybrid Cross (1st) • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: RoundseedsxRound seeds RRxRr Genotype:RR, Rr Phenotype:Round GenotypicRatio:1:1 PhenotypicRatio: All alike R r RR Rr R R RR Rr

  22. F2 Monohybrid Cross (2nd) • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: WrinkledseedsxRound seeds rrxRr R r Genotype:Rr, rr Phenotype:Round & Wrinkled G. Ratio:1:1 P.Ratio: 1:1 Rr rr r r Rr rr

  23. bC b__?_ bc Test Cross • A mating between an individual of unknown genotype and a homozygous recessive individual. • Example:bbC_?_ x bbcc • BB = brown eyes • Bb = brown eyes • bb = blue eyes • CC = curly hair • Cc = curly hair • cc = straight hair

  24. bC b___ C bC b___ c bc bbCc bbCc or bc bbCc bbcc Test Cross • Possible results:

  25. Mendel’s Laws

  26. Law of Dominance • Dominant traits mask recessive traits • In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. • All the offspring will be heterozygous and express only the dominant trait. • RR x rryields all Rr (round seeds)

  27. Law of Segregation • Only one allele is passed on from each parent • During the formation ofgametes (eggs or sperm), thetwoalleles responsible for a trait separate from each other. • Alleles for a trait are then "recombined" atfertilization, producing the genotype for the traits of the offspring.

  28. Law of Independent Assortment • Alleles for different traits are inherited independently from one another • Example: You can get your hair color from Mom and your eye color from Dad

  29. How many types of gametes can be produced? • Formula = 2n • n = # of heterozygous alleles • 1. RrYy n=2  4 different gamete types • 2. AaBbCCDd n=3  8 different gamete types • 3. MmNnOoPPQQRrssTtQq n=6  64 different gamete types

  30. Incomplete Dominance and Codominance

  31. Incomplete Dominance • F1 hybrid offspring will have a “blended” appearance between the two phenotypes • Example: Snapdragons (flower) • Red (RR) x white (rr) = Pink (Rr)

  32. Codominance • Two different alleles are completely expressed at once in heterozygous individuals • Example: Calico cat expresses orange AND black genes • Example 2: Human blood type Four types: • 1. type A = IAIA or IAi • 2. type B = IBIB or IBi • 3. type AB = IAIB • 4. type O = ii

  33. Codominance • Question:If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? • boy-type O (ii) X girl-type AB (IAIB)

  34. IA i IAIB IB i ii Codominance • Answer: Parents: genotypes = IAi and IBi phenotypes = A and B

  35. What does this symbol mean? • RR’ • It is just another way of marking a trait • For example: Instead of R = red and r = white, you have R = red and R’ = white • It is usually used with non-Mendelian genetics  What is this?

  36. Sex-linked Traits • Traits (genes) located on the sex chromosomes • Sex chromosomes are X and Y • XX genotype for females • XY genotype for males • Many sex-linked traits carried on X chromosome • Therefore, males are the most affected  no “back-up copy”

  37. fruit fly eye color XX chromosome - female Xy chromosome - male Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes

  38. Xr Xr XR Y Sex-linked Trait Problem • Example: Eye color in fruit flies • (red-eyed male) x (white-eyed female)XRY x XrXr • Remember: the Y chromosome in males does not carry traits. • RR = red eyed • Rr = red eyed • rr = white eyed • XY = male • XX = female

  39. Xr Xr XR Xr XR Xr XR Y Xr Y Xr Y Sex-linked Trait Solution: 50% red eyedfemale 50% white eyed male

  40. What is a carrier? • A carrier is an individual who has a recessive trait that they can pass on to their off-spring, but don’t express • They “carry” it to the next generation • For sex-linked traits, women are usually carriers

  41. Human Genetics

  42. How do we study human genetics? • There are some major problems scientists must overcome to study human genetics • It is not ethical to do a test cross on a human being • Humans have long life spans, long gestational period, and a small number of offspring  hard to trace a trait over several generations

  43. How do we study human genetics? • Population sampling = study how often a trait is found in a small, randomly selected group • Problem: It is really hard to get a truly random sample • Pedigree = a graphic record of a trait over several generations within a family; use family/historical records, photographs, medical records, etc. • Only trace ONE trait per pedigree

  44. What do the shapes in a pedigree mean?

  45. Connecting lines indicate relationships in a family • P1 • F1 • F2

  46. Pedigree Example • The parents in the P1 generation MUST have been carriers • Neither of them show the trait, but they had a child who did • Therefore, they CARRIED the trait to the next generation

  47. Genetic Disorder vs Chromosomal Disorder • A genetic disorder is caused by a defect in a single gene • A chromosomal disorder is caused by an entire extra or deleted chromosome

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