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Mendelian Genetics

Mendelian Genetics. The term ‘Mendelian genetics’ typically relates to the outcomes of simple dominant and recessive gene pairings Shows specific ratios or patterns of inheritance within a lineage of offspring generations (e.g., F1 and F2 generations). Early ideas of heredity.

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Mendelian Genetics

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  1. Mendelian Genetics The term ‘Mendelian genetics’ typically relates to the outcomes of simple dominant and recessive gene pairings Shows specific ratios or patterns of inheritance within a lineage of offspring generations (e.g., F1 and F2 generations)

  2. Early ideas of heredity • Constancy of species – heredity occurs within the boundary of the species; not so prior to the Middle ages (Ex: giraffe and minotaur)

  3. View held thru time of Darwin • Direct transmission of traits – child is formed after hereditary material from all parts of parent’s body come together – blending occurs

  4. Gregor Mendel (1822 – 1884) • Studied garden peas • 1st to use mathematics to examine outcomes of crosses • Large # of pea varieties with at least 7 easily distinguished traits • Peas are small, easy to grow, short generation time • Peas can self-fertilize; bisexual

  5. Some definitions for tracking traits via Mendelian inheritance • Genotype/Phenotype • Gene/allele • Dominant/Recessive alleles • Homozygous/Heterozygous • P/F1/F2 generations • Genotypic ratio/Phenotypic ratio • Monohybrid cross/Dihybrid cross

  6. Mendel conducted studies in 3 stages • Self-crossed flowers to make sure white/purple flowered plants were true-breeding • Crossed true-breeding plants (white X purple) (X means “crossed with”) 3. Crossed F1 plants to see traits in future generation (F2 generation)

  7. Mendel came to understand…. • Plant progeny (offspring) did not show blending of traits • For each pair of alternative traits, 1 was not expressed in F1 generation, but re-appeared in F2 generation • Traits segregate among the progeny • Alt, traits are expressed in 3:1 ratio in F2

  8. Punnett squares allow analysis using symbols for gametes and genotypes

  9. Outcome of crossing true breeding purple-flowered and white-flowered pea plants F1 progeny: All purple flowered F2 progeny: 3 purple to 1 white Self cross each of the F2’s

  10. The Mendelian ratio • Phenotypic ratio of 3:1 yet, • Genotypic ratio of 1:2:1 When crossing heterozygous individuals of trait controlled by simple dominant/recessive alleles

  11. Mendel proposed a simple model of heredity – 5 parts: • Parents transmit “factors’ to offspring • Each individual receives 2 factors which code for the same trait • Not all factors are identical – alternative gene forms are called alleles • Alleles do not influence each other as alleles separate independently into gametes • The presence of an allele does not insure that its trait will be expressed

  12. Monohybrid Crosses genotype: total set of alleles of an individual PP = homozygous dominant Pp = heterozygous pp = homozygous recessive phenotype: outward appearance of an individual

  13. Monohybrid Crosses Principle of Segregation – Mendel’s first Law of Heredity Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.

  14. Dihybrid Crosses Dihybrid cross: examination of 2 separate traits in a single cross -for example: RR YY x rryy The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.

  15. Dihybrid cross between two heterozygous parents Instead of 4 possible outcomes, there are now 16!!

  16. Dihybrid Crosses Principle of Independent Assortment: Mendel’s 2nd Law. In a dihybrid cross, the alleles of each gene assort independently.

  17. Probability – Predicting Results Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities. When crossing Pp x Pp, the probability of producing Pp offspring is probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4) ¼ + ¼ = ½

  18. Probability – Predicting Results Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities. When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is: probability of obtaiing rr = ¼ probability of obtaining yy = ¼ probability of rr yy = ¼ x ¼ =1/16

  19. Testcross Testcross: a cross used to determine the genotype of an individual with dominant phenotype -cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) -the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

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