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Chapter 14 – Mendelian Genetics

Chapter 14 – Mendelian Genetics. Blending vs. Particulate Theory of Inheritance. Over time, populations do not become uniform Often traits that seemed to have “disappeared” would reappear in subsequent generations. Traits of offspring were a “blend” of the parental traits.

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Chapter 14 – Mendelian Genetics

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  1. Chapter 14 – Mendelian Genetics

  2. Blending vs. Particulate Theory of Inheritance Over time, populations do not become uniform Often traits that seemed to have “disappeared” would reappear in subsequent generations Traits of offspring were a “blend” of the parental traits. Infer  over time, populations begin to look uniform and look alike Mendel observed that genes retain their separate identities His observations lead to what’s now accepted as the Particulate Theory of inheritance

  3. Vocabulary Review Genotype = Genetic make-up of characteristics represented by 2 alleles in diploid organisms Homozygous = When both alleles of the genotype are the same Heterozygous = When both alleles of the genotype are different (Hybrid) Trait: Characteristic of an organism Ex. Plant Height Allele: Different forms of a gene responsible for different traits Phenotype = expressed trait, based on the genotype Ex. T = Tall t = short TT or Tt TT tt tt Tall Tt Short

  4. Single-Factor Crosses P = Parental Generation F1 = 1st Filial Generation Memorize these ratios! F2 = 2nd Filial Generation Genotypic Ratio of monohybrid cross 1:2:1 Phenoytypic Ratio of monohybrid cross: 3:1

  5. Two Factor Crosses (2 traits) Y = yellow, y = green R = Round, r = wrinkled X Dihybrid Cross RrYy RrYy Memorize these ratios!

  6. Test Crosses If a plant has a dominant phenotype, (for example yellow seeds) and we are unsure of its genotype (YY, or Yy), you can determine it’s genotype by crossing it with another with a recessive phenotype (green seeds) with the genotype yy. Yy If F1 = 100% Yellow Then P must be = YY X YY or Yy? yy Yy yy If F1 = 50% Yellow, 50% green Then P = Yy (hybrid)

  7. Pedigree A = tongue roller a = can not roll tongue Aa ? aa ? ? ? ? aa aa AA Aa Can you figure out the rest of the genotypes on your own? male Mating couple female Shaded = trait being followed Children/Siblings

  8. Other Pedigree Symbols Examples of connected symbols: • Fraternal twins • Identical twins

  9. Unique Degrees of Dominance(exceptions to the rule) Incomplete Dominance: Dominant trait “blends” when combined with a recessive allele Notice how the genotypes are written… X CRCR CRCW CWCW

  10. Unique Degrees of Dominance What’s expressed when we’re together? Co-Dominance: When there are multiple alleles that are dominant and are of equal strength - then both dominant alleles will be expressed when combined - a dominant allele will always mask a recessive allele. Notice how the genotypes are written… I’m the dominant allele for Type A blood! I B Don’t count me out just ‘cuz I’m recesssive. I’m type O allele! Example: Blood Type I’m the dominant allele for Type B blood ! IB IB or IB i = type B blood I A i i = type O blood IA IA or IA i = type A blood IA IB = type AB blood i

  11. Unique Gene Interactions Pleiotrophy: The ability of a gene to affect an organism in many phenoytypic ways Ex. Sickle Cell Anemia Blood Clumping Physical Weakness or Brain Damage

  12. Unique Gene Interactions Polygenic Inheritance: when multiple genes have an added effect on a single phenotype (Opposite of Pleiotrophy) – ex. Skin color, height Notice the range in genotypes… aabbcc AABBCC

  13. Unique Gene Interactions Epistasis: when a gene at one locus alters the expression of a gene at another locus Alleles for Fur Color: B = Black Fur b = brown fur C = Color c = albino BBCC, BBCc, BbCC, BbCc  bbCC, bbCc  BBcc, Bbcc, bbcc  Since cc genotype is albino, the alleles for fur color (B or b) are not expressed

  14. Genetics is cool! But wait…sample probability problems to come!

  15. Sample Problem Brown eyes are dominant over blue eyes. Parent A has blue eyes, while Parent B is heterozygous for brown eyes. What is the probability that they will have a child with blue eyes? Parent A (blue eyes) = bb Parent B (brown eyes) = Bb Process: Blue-eyed Child has to be bb Probability of parent A donating one “b” allele= 1 Probability of parent B donating the other “b” allele = ½ 1 X ½ = ½ (50% probability) How do you calculate probability? (Alternatives to Punnett Squares) Rule of Multiplication Check your work! 50% probability for blue eyes

  16. Sample Problem #2: In a cross between AaBbCc x Aabbcc, what is the probability that at least two of the three recessive traits is present in the offspring? Steps for solving: Write out the genotypic possibilities Use rule of X (multiply probabilities of each genotypic combination) Use rule of + Rule of Multiplication and Addition AAbbcc Aabbcc aaBbcc aabbCc aabbcc  AA (1/2) x bb (1/2) x cc (1/2) = 1/8  Aa (1/2) x bb (1/2) x cc (1/2) = 1/8  Aa (1/4) x Bb (1/2) x cc (1/2) = 1/16  Aa (1/4) x bb (1/2) x Cc (1/2) = 1/16  Aa (1/4) x bb (1/2) x cc (1/2) = 1/16 Sum of the fractions: 6/16 = 3/8

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