Class Note for ECOL 320 at UA
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Date Created: 02/06/15
Section 12 Mendelian Genetics Gregor Mendel Born 1822 in Heizenberg Austria son of a farmer Very bright as a student sent to gymnasium but father was crippled and family couldn t afford to keep him in school so he joined monastery to get an education and to be teacher 1843 joined Augustinian monastery at Brunn in Moravia 1847 ordained into priesthood 1849 assigned to teach in secondary school took teaching examination failed due to lack of knowledge 1851 was sent to U Vienna where got brief but extremely sound scientific education 1856 failed teaching exam again test anxiety Began experiments with peas in 185039s 1865 read paper on his results to Brunn Natural History Society 1866 paper published in Proceedings of Brunn Natural History Society A date to remember Besides studies on heredity did other kinds of natural history 1868 scientific career ended when became abbot of the monastery 1884 died Mendel Was Not the First to Try Why Did He Succeed in Deducing Laws of Heredity Where His Predecesssors Failed He was really smart Better scientific background than those before him 0 cell theory probably knew adult plant comes from egg by succession of cell divisions 0 fertilization pollen grain egg gt zygote knew from his own experiments that one pollen grain fertilized one egg 0 took math including early probability theory ready to see and understand random variation 0 took physics from Doppler saw power of quantitative data and mathematical laws What Mendel did not know Genes on chromosomes in nucleus Mitosis Meiosis Simplified problem Focused on discontinuous variation eitheror traits usually controlled by one or two genes instead of continuous variation controlled by many genes and the environment Good choice of experimental organism Worked with plan39s as did nearly all genedcists Selecmd peas because 39many diffellentphenotypes got 2 27 vaneu39es that differed in Valious phenotypic Lmits Icould do contmlled crosses or selfing selfrfem39lizau39on ma ng plant with itself crossing Pans Flall surgery Flgum n a What Mendel Did 1 From commercial seed dealers selected many pea strains differing in discrete characters Chose some differing in 7 traits 2 Subjected these to several generations of selfing Bred true eg plant green seeds grow plants self plants gt seeds all green We know and Mendel deduced that sel ng or any other form of inbreeding produces pure lines homozygous plants that produce only homozygous offspring 3 Did crosses between strains differing in one or more traits Monohybrid cross parents differ in only one trait Most of Mendel s crosses were dihybrid or trihybrid Any cross can be analyzed as monohybrid crosse by following only one trait Tvaii Seed shape Round wilnkIed True Breeding crossed to itself produces only itself Seed color Pad shape Pod calm Flower color Flower and pad position Stem length Yellow 41quot lnllaked i3 Axial on slam Swan m Canslricled 4 Yellow Tall Dwarf Cross 7 F1 Self or cross inter se l 12 round wrinkled pure lines A homozygous diploid X E gametes round wrinkled gametes round V 423 round 076 x 34 E wrinkled 024 x 14 556 100 heterozygous diploid gametes Cross 7 F1 Self or Cross Inter se l 12 round wrinkled pure lines 3 i R r round wrinkled 1 i R r round A V 423 round R 133 wrinkled r homozygous diploid gametes gametes heterozygous diploid gametes Two phenotypes produced by two different hereditary factors P0 round wrinkled pure lines A r homozygous diploid Self A gametes P1 round wrinkled 1 1 Cross gametes V F1 round Rr heterozygous diploid Self or Cross R r gametes Inter se F2 423 round R 133 wrinkled r Two phenotypes produced by two different hereditary factors F1 produces F2 with both phenotypes so must have and transmit both hereditary factors P0 round wrinkled pure lines x rr homozygous diploid Self R R A gametes P1 round wrinkled 11R 1r Cross R r gametes V F1 round Rr heterozygous diploid Self or Cross R r gametes Inter se F2 423 round RR and Rr 133 wrinkled rr Two phenotypes produced by two different hereditary factors F1 must produce F2 with both phenotypes so must have and transmit both hereditary factors If F2 has two factors all plants have to have two Inbred parents only produce one kind of gamete so have only one kind of hereditary factor P0 round wrinkled pure lines x rr homozygous diploid Self R R A gametes P1 round wrinkled 11R 1r Cross R r gametes V F1 round Rr heterozygous diploid Self or Cross R r gametes Inter se F2 423 round 076 x 34 14 RR and 12 Rr wrinkled 024 x 14 all rr 556 100 Now Mendel can explain the ratio of phenotypes in the F2 If the two kinds of F1 gametes are paired randomly in all possible combinations 14 will be RR 12 Rr and 12 rr Rr will be round as in the F1 R is dominant so Rr is round How explain F2 Mendel came up With a model 0 Mendel s first law or law of segregation Alleles segregate during formation of the gametes 12 of the gametes get one allele and 12 the other F1 gametes are 12 R and 12 r 0 Fertilization is random With respect to genotype Make Punnett square to see different combinations of egg and pollen Genotypic ratio 14 RR 12 Rr 14 rr Phenotypic ration 34 round 14 wrinkled Pollen 12 R 12 r Eggs 12 R 14 RR 14 Rr 2 F 14 rR 14 rr Mendel didn t know about meiosis or even about chromosomes so he couldn t interpret his data in those terms Walter Sutton 1902 Theodore Boveri 1903 Chromosome theory of heredity Genes are on chromosomes Different chromosomes have different sets of genes Different alleles are on different members of a pair of homologous chromosomes Alleles segregate in meiosis because homologous chromosomes segregate4 Go back and look at notes about meiosis I Iquot quot 1 fka 9 t E 355 1quotquot I H mm 12H 39 FQ 12 Iquot F a e39 P k 5 Q 3 Dihybrid Crosses Mendel gave some data for onefactor crosses but almost certainly most crosses actually had two or three factors differing and he focused on one The above cross actually had at least two traits and two genes segregating 0round and wrinked seeds R r 0yellow and green seeds Y y P1 round yellow X wrinkled green Cross F1 round yellow Self or cross inter se ratios F2 315 round yellow z 9617 916 101 wrinkled yellow z 3117 316 108 round green z 3317 316 3 wrinkled green z 1017 116 556 Why did Mendel think of the 9331 ratio instead of something else like 96 31 33 10 Updated version of this Will be put on web later today or tomorrow morning P1 round yellow wrinkled green RRYY rryy Cross RY X ry gametes F1 round RrYy Self or cross RY ry Ry rY gametes inter se F2 315 round yellow R Y z 916 101 wrinkled yellow rr Y z 3 16 108 round green R yy z 3 16 wrinkled green rr yy z 1 16 556 First note that if we analyze the cross as two onefactor crosses both give the 31 ratio in the F2 roundwrinkled alone 315 108 423 round z 34 101 32 133 wrinkled z 14 yellowgreen alone 315 101 416 yellow z 34 108 32 140 green z 14 Test to see if are segregating completely independently If they are ratio round to wrinkled should be the same in yellow and green plants and vice versa yellow green Round 315 108 Wrinkled 101 32 You do other combination Each locus shows 3414 segregation regardless of what the other locus is doing Analysis as a twofactor cross requires two steps to predict F2 1 Use Punnett square to get all possible combinations of alleles in gametes Yy 12 Y 12 y Rr 12 R 14 RY 14 Ry 2 F 14 rY 14 ry Mendel s second laW law of independent segregation different pairs of alleles segregate independently of each other 2 Use Punnett square again to get all possible combinations of gametes Eggs 14RY 14Ry 14rY 14ry 14RY RR YY RR Yy Ry YY Pollen 14 R y RR yy 14 1 Y W Yy 14 1 y W yy 116
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