11.1 The Work of Mendel

Transcription

1 11.1 The Work of Mendel Originally prepared by Kim B. Foglia Revised and adapted by Nhan A. Pham

2 Objectives Describe Mendel s classic garden pea experiment. Summarize Mendel s conclusion about inheritance. Explain the principle of dominance. Explain Mendel s laws of segregation and independent assortment.

3 Bozeman on Mendelian Genetics

4 Gregor Mendel Gregor Mendel ( ), an Augustinian monk, documented inheritance in garden peas used very large sample used quantitative analysis

5 Mendel s Experiment let plants selfpollinate produce true breeding parents (P) cross-pollinate to produce the first generation F 1 (F = filial, son ) allowed offspring of F 1 to self-pollinate to produce F 2 Pollen transferred from white flower to stigma of purple flower P all purple flowers result F 1 self-pollinate anthers removed F 2

6 P Looking closer at Mendel s work true-breeding purple-flower peas X true-breeding white-flower peas F 1 generation (hybrids) 100% purple-flower peas Where did the white flowers go? 100% F 2 generation 75% purple-flower peas self-pollinate 25% white-flower peas White flowers came back! 3:1

7 What did Mendel s findings mean? Traits come in alternative versions called alleles (purple vs. white flower color) à for each trait, an organism inherits 2 alleles from 2 sets of chromosomes, 1 from each parent homologous chromosomes (similar, not exact) purple-flower allele and white-flower allele are two DNA variations at flower-color locus

8 What did Mendel s findings mean? purple and white flower colors are separate traits that do NOT blend I ll speak for both of us! purple masks white dominant allele allele produces functional protein (visible as trait) recessive allele makes a malfunctioning protein wild type allele producing functional protein mutant allele producing malfunctioning protein homologous chromosomes

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10 Genotype vs. Phenotype phenotype description of an organism s trait (physical appearance) genotype an organism s genetic makeup (genes)

11 Punnett Squares F 1 generation (hybrids) female / eggs P p Pp x Pp male / sperm P PP p Pp PP Pp pp pp Phenotype and genotype can have different ratios Pp Pp % genotype 25% 50% 25% % phenotype 75% 25% 1:2:1 3:1

12 Mendel s 1 st Law of Heredity Law of Segregation during meiosis, alleles segregate (homologous chromosomes separate) each allele for a trait is packaged into a separate gamete PP pp P P p p Pp P p

13 Law of Segregation During which stage of meiosis do homologous chromosomes segregate? Whoa! And Mendel didn t even know DNA or genes existed! Metaphase 1

14 Monohybrid Cross Some of Mendel s experiments followed the inheritance of a single character/trait Ex: flower color, seed color monohybrid crosses

15 Dihybrid Cross Other of Mendel s experiments followed the inheritance of TWO different characters Ex: seed color and seed shape dihybrid crosses

16 Dihybrid Cross P true-breeding yellow, round peas Y = yellow R = round YYRR x yyrr true-breeding green, wrinkled peas y = green r = wrinkled F 1 generation (hybrids) yellow, round peas YyRr 100% self-pollinate 9:3:3:1 F 2 generation 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas

17 What s going on here? If genes are on different chromosomes how do they assort in the gametes? Together or independently? YyRr Is it this? Or this? YyRr Which system explains the data? YR yr YR Yr yr yr

18 Is this the way it works? YyRr YR yr x YyRr YR yr YYRR YyRr NOT YyRr yyrr YyRr or YR yr YR Well, that s right! Yr YyRr yr yr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled

19 Dihybrid Cross YyRr x YyRr YR Yr yr yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRr Yyrr yr YyRR YyRr yyrr yyrr yr YyRr Yyrr yyrr yyrr YyRr YR yr BINGO! or YR Yr YyRr yr yr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled

20 Mendel s 2 nd Law of Heredity yellow green round wrinkled Law of Independent Assortment different loci (genes) separate into gametes independently. types of gametes produced in equal amounts (YR = Yr = yr = yr) only true for genes on separate chromosomes or on same chromosome but so far apart that crossing over happens frequently YyRr Yr Yr yr yr YR YR yr yr 1 : 1 : 1 : 1

21 Law of Independent Assortment During which stage of meiosis do alleles assort independently? Remember Mendel didn t even know DNA or genes existed! Metaphase 1 Exception genes on same chromosome and close together rarely crossover separately will usually be inherited together linked

22 The chromosomal basis of Mendel s laws Trace the genetic events through meiosis, gamete formation and fertilization to offspring

23 Review: Mendel s Laws of Heredity Law of segregation monohybrid cross (single trait) each allele segregates into separate gametes established by metaphase 1 Law of independent assortment dihybrid (or more) cross (2 or more traits) genes on separate chromosomes assort into gametes independently established by metaphase 1 Exception linked genes metaphase1

24 Mendel chose peas wisely Pea plants are good for genetic research for a variety or reasons available in many varieties with distinct heritable features with different variations (flower color, seed color, seed shape, etc.) Mendel had strict control over which plants mated with which each pea plant has male and female structures pea plants can self-fertilize Mendel could also cross-pollinate plants: moving pollen from one plant to another

25 Mendel chose peas luckily Pea plants are good for genetic research relatively simple genetically most characters are controlled by a single gene with each gene having only 2 alleles one completely dominant over the other