2 The Work of Gregor Mendel What is Genetics? the study of heredityGregor Mendel’s PeasPollen: plant’s spermEgg Cells: plants reproductive cellsFertilization: joining of pollen + egg cells develops intos embryo in a seedBorn in 1822.His work with pea plants laid the foundation forGenetics.
3 Working with pea plants… Self-pollinating: pollen fertilizes egg cells in the SAME flower (single parent reproduction)True-breeding: offspring genetically identical to parents due to self-pollinationCross-pollination: combining reproductive cells from 2 DIFFERENT parent plantsMendel could cross- breed a purple flower with a white flower…What do you think is the color of the offspring?
4 Genes and DominanceTRAIT: specific characteristic (seed color, plant height, etc)What did Mendel do in his pea plant experiments?Studied 7 different traits each with contrasting characters (ex) Height, short or tallHe crossed the plants (with contrasting characters) and looked at their offspring
5 P = parental generation = original pair of plants F1 = first filial generation= first generationHybrids: offspring from parents with different traits
6 Tracking Generations Parental generation P mates to produce First-generation offspring F1mate to produceSecond-generation offspring F2
7 P GENERATION: purple x white flowers F1 GENERATION: all purple flowers CROSS-POLLINATION: Mendel cut the male parts of one flower (ouch!) and dusted the female parts with pollen from another flower.P GENERATION: purple x white flowersF1 GENERATION: all purple flowersHYBRID PLANTS
8 Inheritance is determined by factors that are passed down What happened in Mendel’s crosses?All the offspring only had one of the parent’s characters…the other parent’s character disappeared!!Mendel’s Conclusions:Inheritance is determined by factors that are passed downGENES: the factors that determine traitsContrasting characters are different forms of a gene called ALLELES
9 Mendel’s Principle of Dominance some alleles are dominant, some are recessiveDOMINANT ALLELE: form of trait that will always be exhibited; usually expressed in capitalsRECESSIVE ALLELE: form of trait is only exhibited when the dominant allele is NOT present(ex) Allele for tall is dominant for and the allele for short is recessive
10 What happened to the recessive allele? Mendel wanted to know if the recessive allele disappeared from the F1 plants.F1 CROSS: He self-crossed the F1 generations to make F2 offspring
11 THE F2 GENERATION… The recessive traits reappeared!! ~¼ plants had white flowers, the recessive traitSummary of Crosses:tall plants X short plants tall plantsP P F1tall plants self-pollinating ¼ short, ¾ tallF1 Cross F2
12 Explaining the F1 CrossWhy did the recessive allele reappear? At some point, the recessive allele had to separate from the dominante allele. This is called…SEGREGATION: separation of alleles occurs during formation of gametes (eggs & sperm) in anaphase II of meiosisF1 plants inherited 1 tall allele & 1 short allele from parentsWhen gametes are formed, the two alleles segregate from each other each gamete has 1 copy of each geneSo, 2 different types of gametes are formed (one w/ tall allele, one w/ short allele)
14 Probability & Punnett Squares Mendel realized that the principles of probability can explain the results of genetic crosses.PROBABILITY: likelihood an event will occur (ex) Flip coin 3x in a row, 1/8 chance it will be heads all 3 times ( ½ x ½ x ½ )The pattern in which alleles segregate is random…just like a coin flip! So which ever allele gametes receive is also random.
15 Punnett Squares What is a Punnett Square? A diagram showing the possible genetic combinations from a particular crossCan be used to predict and compare the genetic variations that will result from a crossWhat do the letters represent in a punnett square?Letters represent alleles: capital = dominant lowercase = recessiveHomozygous: has two identical alleles for a trait (ex) TT or ttHeterozygous: has two different alleles for the same trait (ex) Tt
16 Punnett Square for TT x Tt Punnett Square for YY x yy
17 Genotype vs PhenotypeGENOTYPE: the genetic makeup of an organism (ex) TTPHENOTYPE: the physical characteristics exhibitied (ex) tall plantIn the Punnett Square shownWhat is the genotype of the offspring?What is the phenotype?
18 Probability and Segregation F2 generation from Tall F1 plants ¾ tall, ¼ short3:1 ratio of tall to short plantsPunnet squares work to predict outcomes, so Mendel’s ideas about segregation are accurate!
19 Exploring Mendelian Genetics Does the gene that determines flower color have anything to do with the gene for height?(ex) Do all tall plants have purple flowers?Mendel performed TWO-FACTOR CROSSES: crossing 2 different genes and following traits as they pass from one generation to the next
20 Two-Factor Cross: F1 Two Genes: shape of pea & color of pea The Cross: Round yellow peas x wrinkled green peasRRYY x rryyWhat are the possible alleles parent 1 can pass? RYWhat are the possible alleles parent 2 can pass? ryDraw a Punnett Square for this cross.All F1 were RrYy (round and yellow) or HYBRIDSThis cross does not answer question, but provides hybrids for next cross
21 Two-Factor Cross: F2F1 Generation = RrYyHow would these alleles segregate when F1 self-pollinated?RrYy x RrYyDo the two dominant alleles stay together?What are the possible alleles each parent can pass on?There are 4 possible combinations: Ry, RY, rY, ryDraw a Punnett Square for this cross.
22 INDEPENDENT ASSORTMENT The F1 Hybrid cross produces a 9:3:3:1 phenotype ratioMendel found that the 2 alleles (seed shape & seed color) don’t influence each other’s inheritanceThis is called the principle of Independent Assortment: genes for different traits can segregate independently during the formation of gametes
23 Independent Assortment Metaphase I:AAaaAAaaBBbbbbBBMetaphase II:AAaaAAaaBBbbbbBBGametes:BBbbbbBBAAaaAAaa1/4 AB1/4 ab1/4 Ab1/4 aB
24 Summary of Mendel’s Principles Inheritance of characteristics is determined by genes which are passed to offspringIf 2+ alleles of a trait exist, some alleles may be dominant, others may be recessiveSexually reproducing organisms have 2 copies of each gene which segregate during gamete formationAlleles for different genes segregate independently
25 Beyond Dominant and Recessive Alleles Genetics is more complicatedSome alleles are neither dominant nor recessiveMany traits are controlled by multiple alleles or multiple genes
26 Other Inheritance Patterns… Incomplete DominanceCodominanceMultiple AllelesPolygenic Traits
27 IncompleteDominanceHomozygousparent (RR)XHomozygousParent (rr)When one allele is not completely dominant; recessive allele is not totally maskedHeterozygous phenotype is in between the two homozygous phenotypes(ex) Red snapdragon flowers (RR) X snapdragon white (rr) flowers pink hybrid flowers (Rr)All F1 areheterozygousXF2 shows three phenotypes in 1:2:1 ratio
28 Incomplete Dominance X All F1 offspring heterozygous for flower color: homozygous parentXhomozygous parentAll F1 offspring heterozygous for flower color:Cross two of the F1 plants and the F2 offspring will show three phenotypes in a 1:2:1 ratio:
29 Codominance Both alleles contribute to the phenotype Heterozygous genotype expresses both phenotypes(ex) Feather colors in chickens: white feathers X black feathers speckled chicken(ex) Horse coats: red X white roan coat
30 Codominance: ABO Blood Types Alleles that controls blood type are codominantTwo alleles A & B are both exhibited when paired, a third allele (i) is recessive to othersAA or Ai = Type A BloodBB or Bi = Type B BloodAB = Type AB Bloodii = Type O Blood
31 Multiple Alleles > 2 possible alleles for a gene Individuals can still only have 2 alleles each but more than 2 alleles exist in a population(ex) coat color in rabbits lots of options due to 4 different alleles(ex) blood type is determined by multiple alleles
32 Polygenic Traits Traits controlled by the interaction of 2+ genes (ex) Fruit fly eye color (3+ different genes)(ex) Skin color in humans (4+ different genes), eye color, height, weight
33 Applying Mendel’s Principles Early 1900’s Morgan used Mendel’s principles to study fruit flies advanced study of geneticsMendel’s Principles also apply to study the inheritance of human traits and to calculate the probability of traits appearing in the next generation.