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PCB 3063 General Genetics Lecture Topic 2: Mendelian Genetics

PCB 3063 General Genetics Lecture Topic 2: Mendelian Genetics. shape color. shape. color. character – detectable property of an organism. traits – different forms of a given character.

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PCB 3063 General Genetics Lecture Topic 2: Mendelian Genetics

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  1. PCB 3063 General Genetics Lecture Topic 2: Mendelian Genetics

  2. shape color shape color character – detectable property of an organism traits – different forms of a given character pure breeding – all products of self-fertilization, or selfing, will show same trait as parent. (pure for hereditary factors, homozygous) Fig. 3-1 Klug, Cummings, and Spencer, 8th ed.

  3. tall progeny x tall progeny 787 tall, 277 dwarf tall x dwarf all tall genetic cross offspring or progeny ~ 3:1 ratio

  4. Consistent Pattern • Only one of two possible traits showed up in first generation. • The trait not apparent in first generation showed up in the second generation Factor present, but hidden Fig. 3-1 Klug, Cummings, and Spencer, 8th ed.

  5. tall progeny x tall progeny 787 tall, 277 dwarf parentals (P) – pure breeding parents in the first of a series of crosses F1 – first filial generation, the offspring or progeny of the parentals tall x dwarf all tall F2

  6. GENE - Heritable agent responsible for a characteristic. Mendel called these factors. Have gene for plant height (height gene). ALLELES - Different forms of the same type of gene. Different alleles of the height gene result in the tall or dwarf traits. RECESSIVE ALLELE - The allele not expressed in the presence of the other allele. (dwarf allele) DOMINANT ALLELE - The allele that somehow masks expression of the recessive allele. (tall allele) REPRESENTING GENES AND ALLELES use first letter of recessive trait to represent gene (dwarf) lowercase for recessive allele (d for dwarf) uppercase for dominant allele (D for tall)

  7. parent a (2 alleles) x parent b (2 alleles) gamete (1 allele) gamete (1 allele) fertilization product, zygote (2 alleles) Mendel: Each plant had two alleles for each character… Clue: F1 generation plants tall, thus they had the tall allele but dwarf showed up in next generation, so dwarf allele was also present … but gametes (sex cells; eggs, pollen, sperm) must contain only one allele for each gene.

  8. tall (DD) x dwarf (dd) gametes: all D all d F1 all tall (Dd) F1 tall (Dd) x F1 tall (Dd) gametes: ½ D, ½ d ½ D, ½ d Crosses in Terms of Genes and Alleles homozygous heterozygous Dd has same appearance (phenotype) as DD, but different allelic composition (genotype) MONOHYBRID CROSS (single gene, both parents heterozygous)

  9. ¼ DD ½ Dd ¼ dd - genotypic ratio ¾ tall ¼ dwarf - phenotypic ratio To determine how gametes combine to form the F2 generation, use a probability table, called a PUNNETT SQUARE, and two simple rules of probability: Product Rule The probability that two independent events both occur is equal to the product of their individual probabilities. Sum Rule The probability of a specific outcome is equal to the sum of the probabilities of each of the mutually exclusive ways to reach that outcome.

  10. gametes from dwarf (dd) d d deduced gametic contribution of F1 tall D d observed results ½ tall ½ dwarf F1 deduced to be Dd TESTCROSS A cross between a plant with an unknown genotype and a plant that is homozygous for the recessive allele. Example: F1 tall x dwarf (dd)

  11. Mendel’s First Law of Inheritance: The Law of Segregation The two members of a gene pair segregate (separate) from each other during gamete formation • Implies that there are two alleles for each gene in each individual. • Gametes get only one allele for each gene. • Heterozygotes produce equal number of gametes with each of two alleles.

  12. Possible Crosses Involving a Single Gene With Two Possible Alleles -- all PP PP x PP PP x Pp PP x pp Pp x Pp Pp x pp pp x pp -- ½ PP, ½ Pp -- all Pp -- ¼ PP, ½ Pp, ¼ pp -- ½ Pp, ½ pp -- all pp

  13. ¾ yellow seeds, ¼ green seeds again, this ratio found regardless of plant height Additional Character – Seed Color controlled by single gene with 2 alleles: g recessive for green G dominant for yellow cross: tall, yellow seeds x dwarf, green seeds F2: ¾ tall, ¼ dwarf this ratio found if looked at all plants, regardless of seed color, or if looked just at plants with yellow seeds, or if just at plants with green seeds

  14. Mendel’s Second Law of Inheritance: Independent Assortment Pairs of alleles for one gene segregate independently from pairs of alleles for another gene (exceptions later)

  15. DIHYBRID CROSS Parentals Gametes F1 F1 x F1 tall, yellow (DD, GG) x dwarf, green (dd, gg) DG dg DdGg DdGg x DdGg

  16. Fig. 3-7 Klug, Cummings, and Spencer, 8th ed.

  17. DIHYBRID CROSS Parentals Gametes F1 F1 x F1 F2 tall, yellow (DD, GG) x dwarf, green (dd, gg) DG dg DdGg DdGg x DdGg Because these are independently assorting genes: 1/4 DD, 2/4 Dd, 1/4 dd and independently, 1/4 GG, 2/4 Gg, 1/4 gg

  18. genotypes 1/4 GG 2/4 Gg 1/4 gg 1/16 DDGG 2/16 DDGg 1/16 DDgg phenotypes 9/16 tall, yellow (D-G-) 3/16 tall, green (D-gg) 3/16 dwarf, yellow (ddG-) 1/16 dwarf, green (ddgg) 1/4 GG 2/4 Gg 1/4 gg 2/16 DdGG 4/16 DdGg 2/16 Ddgg 1/4 GG 2/4 Gg 1/4 gg 1/16 ddGG 2/16 ddGg 1/16 ddgg Branch Diagram 1/4 DD 2/4 Dd 1/4 dd

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