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Transcript
Mendelian Genetics How pea plants and humans mix it up Early Ideas About Heredity People learned that it took two parents to make a baby Many early beliefs about how characteristics are transmitted. Blending theory Early Research Mendel was not the first to perform experiments with pea plants British farmers (stem height) Performed almost the same experiments Obtained the same results Over 200 years before T A Knight (flower color) Same experiments Same results 1790’s Gregor Mendel Born in 1822 in the Chech republic Joined an Augustinian order in 1843 Flunked out of college, but made some great friends who showed him the value of good data Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring The Garden Pea Plant Self-pollinating True breeding (different alleles not normally introduced) Can be experimentally crosspollinated F1 Results of One Monohybrid Cross F2 Results of Monohybrid Cross Mendel’s Monohybrid Cross Results F2 plants showed dominant-torecessive ratio that averaged 3:1 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf Mendel’s Theory of Segregation An individual inherits a unit of information (allele) about a characteristic from each parent During gamete formation, the alleles segregate from each other Now we know about Alleles, alternative forms of a gene Punnett Square of a Monohybrid Cross Female gametes A Male gametes a A AA Aa a Aa aa Dominant phenotype can arise 3 ways, recessive only one Test Cross Individual that shows dominant phenotype is crossed with individual with recessive phenotype Examining offspring allows you to determine the genotype of the dominant individual Punnett Squares of Test Crosses Homozygous recessive a a Homozygous recessive a a A Aa Aa A Aa Aa a aa aa A Aa Aa Two phenotypes All dominant phenotype A Dihybrid Cross - F1 Results purple flowers, tall TRUEBREEDING PARENTS: AABB GAMETES: AB x AB white flowers, dwarf aabb ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall 16 Allele Combinations in F2 1/4 AB 1/4 Ab 1/4 aB 1/4 ab 1/4 AB 1/4 Ab 1/4 aB 1/4 ab 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible 1/4 AB 1/4 Ab 1/4 aB 1/4 ab 1/4 AB 1/4 Ab 1/4 aB 1/4 ab 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb Phenotypic Ratios in F2 AaBb X AaBb Four Phenotypes: Tall, purple-flowered (9/16) Tall, white-flowered (3/16) Dwarf, purple-flowered (3/16) Dwarf, white-flowered (1/16) Independent Assortment Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis