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Mendelian Genetics

Mendelian Genetics. Chapter 9 Patterns of Inheritance. Gregor Mendel. 1850s, Mendel studied inheritance in pea plants by breeding them Peas are good because they have distinctive traits, male and female parts, and are easy to manipulate

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Mendelian Genetics

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  1. Mendelian Genetics Chapter 9 Patterns of Inheritance

  2. Gregor Mendel • 1850s, Mendel studied inheritance in pea plants by breeding them • Peas are good because they have distinctive traits, male and female parts, and are easy to manipulate • He crossed true breeding with other true breeding as well as with hybrids

  3. Gregor Mendel • P = parental generation • F1 = First filial generation (P X P) • F2 = Second filial (F1 X F1) • True breeding (homozygous) = plants that only breed to produce one phenotype • Hybrid (heterozygous) = results of crosses between two plants that breed true for different phenotypes for the same trait

  4. Gregor Mendel • Genotype = genetic makeup of an organism • Sequence of the gene • Phenotype = expressed genotype • Visible or measurable traits – based upon genotype

  5. Gregor Mendel • He crossed a true breeding purple-flowered plant with a true breeding white-flowered plant • All of the F1 generation was purple

  6. Gregor Mendel • He then crossed two F1’s with each other • The results (F2’s) were roughly 3 purples to 1 whites • White trait was absent in F1’s but reappeared in the F2’s

  7. Gregor Mendel • The reappearance of the white trait meant that it was not altered in the F1, just “hidden” • Mendel did the same crosses, and got similar results, with other traits of the pea plant

  8. Gregor Mendel

  9. Gregor Mendel • Four ideas spawn from Mendel’s work: • Alleles account for different traits • Organisms inherit two alleles, one from each parent (may be same or different) • If alleles are different, then one is dominant over the other • The two alleles segregate (law of segregation) during gamete formation

  10. Punnett Square • Predicts the statistical results of a cross

  11. Punnett Square • A test cross, breeding a homozygous recessive with a dominant phenotype, but unknown genotype, can determine the identity of the unknown allele

  12. Punnett Square • One Factor Crosses = crosses with only one trait • Monohybrid crosses = crossing one heterozygote with another • Two Factor Crosses = crosses with two traits • Dihybrid crosses = crossing one organism that is heterozygous for both traits with another that is heterozygous for both traits

  13. Punnett Square • In monohybrid crosses, he noticed that genotypic and phenotypic ratios could be predicted • Genotypic ratio = 1:2:1 • Phenotypic ratio = 3:1

  14. Punnett Square • Example of a dihybrid cross: • Crossing one plant that is heterozygous for seed shape and yellow seed coat (RrYy) with another plant that is also heterozygous for both traits (RrYy) • RrYy X RrYy • Dihybrid results point to the law of independent assortment

  15. Independent Assortment • In our example (RrYy X RrYy), both parents can produce four possible gametes • RY • Ry • rY • Ry • Since both parents have four possible gametes, there are 16 possible combinations for fertilization

  16. Independent Assortment

  17. Independent Assortment • These possible combinations produce a genotypic ratio of 1:2:1:2:4:2:1:2:1 • It also produces a phenotypic ratio of 9:3:3:1 • The law of independent assortment simply means that during gamete formation, allelic pairs that code for different traits assort independent of each other

  18. Dominance • Mendel was very lucky to have studied peas • They follow simple dominance laws • Not all cells follow the simple laws of dominance vs. recessiveness

  19. Variations • Incomplete dominance = neither allele is shown in its dominant form; an intermediate phenotype is shown • Genotypic and phenotypic ratios are the same (1:2:1)

  20. Variations • Codominance = heterozygote expresses a phenotype that is distinct from and not intermediate between those of the two homozygotes Ex. Human AB blood type

  21. Variations • Codominance

  22. Variations • Multiple alleles – gene exists as more than two alleles in the population • Rabbit coat color gene has 4 alleles: C, c, cch & ch • 5 phenotypes • 10 genotypes

  23. Variations • Pleiotropy – one gene affects more than one phenotypic characteristic • Ex: sickle-cell disease affects much more than just the overall conformation of the hemoglobin protein

  24. Variations • Epistasis – one gene at one locus affects the phenotype of another gene at another locus • Ex: mice coat color depends on two genes • One, the epistatic gene, determines whether or not pigment will be deposited in hair • Presence (C) is dominant to absence (c) • The second determines whether the pigment to be deposited is black (B) or brown (b) • An individual that is cc has a white (albino) coat regardless of the genotype of the second gene

  25. Variations – epistasis • A cross between two black mice that are heterozygous (BbCc) will follow the law of independent assortment • However, unlike the 9:3:3:1 offspring ratio of an normal Mendelian experiment, the ratio is 9 black, 3 brown, and 4 white

  26. Variations • Polygenic – one phenotype determined by the combined effect of more than one gene • Ex: human skin color, eye color, height

  27. Variations

  28. Variations • Multifactorial Traits – determined by the combined effect of one or more genes plus the environment • Ex: heart disease, body weight, intelligence

  29. Pedigrees • Autosomal Recessive Traits: • located on non-sex chromosomes • affects males and females • parents must be carriers or affected • affected individuals are homozygous recessive Ex: Albinism, Cystic fibrosis, Phenylketonuria, Sickle cell disease

  30. Autosomal Recessive

  31. Pedigrees • Autosomal Dominant Traits: • located on non-sex chromosomes • affects males and females • at least one parent is affected • does not skip generations • affected individuals are homozygous dominant or heterozygous Ex. Achondroplasia, Huntington disease, Lactose intolerance, Polydactyly

  32. Autosomal Dominant

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