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Mendelian Genetics

Mendelian Genetics. Chapter 14. Mendel’s Big Ideas. The Law of Segregation The 2 alleles of a gene separate (segregate) during gamete formation, so that a sperm or egg only carries 1 allele of each pair Explains 3:1 ratio found in hybrid crosses The Law of Independent Assortment

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Mendelian Genetics

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  1. Mendelian Genetics Chapter 14

  2. Mendel’s Big Ideas • The Law of Segregation • The 2 alleles of a gene separate (segregate) during gamete formation, so that a sperm or egg only carries 1 allele of each pair • Explains 3:1 ratio found in hybrid crosses • The Law of Independent Assortment • Each pair of alleles segregates into gametes independently of other pairs • Explains 9:3:3:1 ratio found in hybrid crosses

  3. Vocabulary • Character – heritable feature • Trait – variant of a character (heritable feature) • True Breed – Whatever traits the parent has are expressed in ALL subsequent populations • For example, self-pollinating a purple flowered plant produces a generation of only purple flowered plants. • Typically, need to do this for at least two (2) generations to ensure that the parent is a true-breed • Hybrid – Mating (crossing) of two (2) true-breeding varieties of true-breeds

  4. Vocabulary (Page 2) • P Generation – Parent generation • Two (2) true-breeding parents being crossed • F1 – First filial (child or son) generation • Hybrids • F2 – Second filial generation • Each member of F1 self-pollinates • Hybrids again • 3:1 ratio and • 9:3:3:1 ratio when 2 characters are considered

  5. More Vocab • Dominant trait • One that will mask the recessive trait if found together • Recessive trait • Trait that will be masked if found with the dominant trait

  6. What Mendel found (stretched the truth about) • Only looked at “all-or-nothing” traits • Sometimes called binary traits – “yes” vs. “no” • Mendel took true breeding for 1 trait, and pollinated it with a true breed for another trait • What is the difference between trait & character? • Purple flowered + White flowered • F1 = All colored purple (all expressed only one trait) • F2 = 3:1 ratio of one trait to the other

  7. ___ Generation What type of plants? ____________ ___ Generation What type of plants? ____________ ___ Generation What type of plants? ____________

  8. Mendel’s Model 1. Alternate versions of genes account for variations in inherited characteristics • The alternative versions are called alleles • One plant had the allele for purple flower color while the other had the allele for white flower color 2. For each character, an organism inherits two (2) alleles, one from each parent • The two alleles may be the same or different • If the 2 alleles are the same = Homozygous • Different = Heterozygous

  9. Mendel’s Model (Page 2) 3. If the alleles differ (heterozygous), then the dominant allele determines the organism’s appearance • Heterozygous individuals display dominant trait • Homozygous Dominant display _________ trait • Homozygous Recessive display _________ trait

  10. Mondel’s Model (Page 3) 4. Law of Segregation • 2 alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes • We already know that homologous chromosomes assort independently during meiosis into gametes • But Mendel did not know about chromosomes

  11. Questions • What is the difference between a gene and a character? • What is the difference between a gene and an allele? • What is the difference between a character and a trait?

  12. Important Vocab. • Phenotype - appearance • Characters • Traits are different types of the character • IF character is eye color, trait is brown, blue, etc • Genotype - genetic makeup • Genes • Alleles are different types of genes • Gene for eye color, alleles = brown (dominant), blue (recessive), etc.

  13. Punnett Squares • One parent at top, other on the left • Here we are crossing homozygous dominant (HD) in the form of [AA] with Heterozygote (Ht) as [Aa]

  14. Cross all 6 combinations

  15. Pattern Recognition • What happens when you have 1 parent who is homozygous Dominant (HD)? • What must the parental genotypes be to get 100% recessive phenotype offspring? • What must the parental genotypes be to get 1:1 phenotypic ratio of offspring? • What about 3:1 phenotypic ratio of offspring?

  16. TestCross • A dominant phenotype crossed with a recessive phenotype • We know the genotype of recessive phenotype (hr) • We do NOT know the genotype of the dominant phenotype (could be HD or Ht) • So we turn the lights down, put on some Maxwell, and let nature take its course... • If all the offspring are dominant phenotype, then? • If the offspring are 1:1, then?

  17. Monohybrid vs. Dihybrid • Monohybrid Cross – Take pure breeds for 1 character and cross (AA xaa) • We got these • Dihybrid Cross – Take pure breeds for 2 characters and cross (AABB xaabb) • 2 Characters like seed color + seed shape

  18. Repeat but use Probability • Compute P(green & wrinkled) from the table • Now do so from Punnett Squares for each character • Compute P(Yellow & Round) from table • Now do so from Punnett Squares for each character

  19. Law of Independent Assortment • States that each pair of alleles segregates independently of other pairs of alleles during gamete formation • This rule really only pertains to genes (allele pairs) on different chromosomes • If on the same chromosome = linked genes • Cannot do Mendelian genetics with linked genes, so we save this for next chapter (CH 15)

  20. Probability Laws • Multiplication rule – probability of a compound event is equal to the product of the individual probabilities of the independent events • P(flip coin twice / Both times heads) = ½ * ½ = ¼ • Addition rule – If an event that can occur in two or more independent, mutually exclusive ways, its probability is the sum of the individual probabilities • P(roll dice / Get “1” or “2”) = [1/6 + 1/6] = 2/6 = 1/3

  21. Probability Basics • Probabilities range from 0 to 1 • 0 = Not gonna happen 1 = gonna happen • P(event) = Probability of event occurring • Sum of probabilities of all possible outcomes = 1 • P(heads) + P(Tails) = 1

  22. Multiplication Rule (AND Rule) • If I flip a coin twice, what is the P(heads on first flip AND second)? • P(heads) = ½ • P(heads on 1st flip) = ½ P(heads on 1st flip) = ½ • P(Heads on flip 1 AND 2) = ½ * ½ = ¼ • Only caveat: Are the flips independent of each other? • If NOT, cannot use the multiplication rule

  23. Addition Rule (OR rule) • When there are 2 or more ways independent & mutually exclusive ways to get some result, you can add up the individual probabilities for the compound result • What is the probability of rolling a “4” or higher on a die? = P(4) or P(5) or P(6) = 1/6 + 1/6 + 1/6 = 3/6 = 1/2

  24. Bringing Laws Together • What is the probability that if you flip a coin twice, it will only land heads once? • P(flip coin 2x / Heads once & Tails once) = • P(Heads 1st AND Tails 2nd) OR P(Tails1st AND Heads 2nd) = • [½ * ½] + [½ * ½ ] = ¼ + ¼ = ½

  25. Problems • In a dihybrid cross, what is the possibility of heterozygous in one trait and homozygous recessive in the other, given both heterozygous parents? • In a trihybrid cross, what is the probability of getting exactly 2 recessive phenotypes, if one parent is heterozygous for all 3 genes and the other is homozygous recessive for 2 genes, and hetero for the 3rd?

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