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Transcript
Chapter 9 FUNDAMENTALS OF GENETICS MENDEL’S LEGACY Mendel observed seven characteristics of pea plants Ex: flower color Each characteristic occurred in two contrasting traits Trait: genetically determined variant of a characteristic Ex: yellow flower color MENDEL’S LEGACY The seven characteristics: Plant height (long and short) Flower position along stem (axial and terminal) Pod color (green and yellow) Pod appearance (inflated and constricted) Seed texture (round and wrinkled) Seed color (yellow and green0 Flower color (purple and white) MENDEL’S EXPERIMENTS He initially studied each characteristic and its contrasting traits individually Began growing true-breeding plants True-breeding: pure; always produced offspring with that trait when they self-pollinate Produced true-breeding plants by self-pollinating pea plants until he had 14 MENDEL’S EXPERIMENTS He then cross-pollinated pairs of plants that were true-breeding for contrasting traits of a single characteristics Ex: he crossed a plant with purple flowers and a plant with white flowers This was called the P (parent) generation MENDEL’S EXPERIMENTS When the plants matured, he recorded the number of each type of offspring produced by each cross Called the offspring the F1 generation MENDEL’S EXPERIMENTS He then allowed the F1 generation to selfpollinate, and the next offspring generation was called the F2 generation He performed hundreds of crosses and documented every result MENDEL’S RESULTS AND CONCLUSIONS In one of his experiments, Mendel crossed a plant true-breeding for green pods with a plant that was true-breeding for yellow pods The F1 generation had all green pods He then let the F1 generation self-pollinate The F2 generation had ¾ green pods and ¼ yellow pods MENDEL’S RESULTS AND CONCLUSIONS These results made Mendel believe that each characteristic is controlled by factors A pair of factors must control each trait MENDEL’S RESULTS AND CONCLUSIONS Mendel got these results through the thousand of crosses F1 generation: one trait disappeared F2 generation: trait reappeared in a 3:1 ratio MENDEL’S RESULTS AND CONCLUSIONS Mendel hypothesized that the trait appearing in the F1 generation was controlled by a dominant factor because it masked the other trait He thought that the trait that did not appear in the F1 generation but reappeared in the F2 generation was controlled by a recessive factor SUPPORT FOR MENDEL’S CONCLUSIONS Most of Mendel’s findings agree with what biologists now know about molecular genetics Molecular genetics: the study of the structure and function of chromosomes and genes SUPPORT FOR MENDEL’S CONCLUSIONS Allele: each of two or more alternative forms of a gene Mendel’s factors are now called alleles Letters are used to represent alleles Dominant alleles: represented by a capital letter Recessive alleles: represented by a lowercase letter GENOTYPE AND PHENOTYPE Genotype: an organism’s genetic makeup Consists of the alleles that the organism inherits from its parents Ex: flower color Purple flowers: either PP or Pp White flowers: pp P is the dominant allele P is the recessive allele GENOTYPE AND PHENOTYPE Phenotype: an organism’s appearance Since PP and Pp are dominant genotypes, they will have purple flowers Since pp is a recessive genotype, they will have the recessive phenotype, which is white flowers GENOTYPE AND PHENOTYPE Homozygous: when both alleles of a pair are alike An organism may be homozygous dominant or homozygous recessive Ex: PP or pp Heterozygous: when the two alleles in the pair are different Ex: Pp PROBABILITY Probability: the likelihood that a specific event will occur May be expressed as a decimal, a percentage, or a fraction Determined by the following equation: P= # of times an event is expected to happen # of times an event could happen PROBABILITY For example, the dominant trait of yellow seed color appeared in the F2 generation 6,022 times The recessive trait of green seed color appeared 2,001 times The total number of individuals was 8,023 PROBABILITY Probability that the dominant trait will appear: 6,022 -----------8,023 = 0.75 or 75% or ¾ or 3:1 PROBABILITY The results predicted by probability are more likely to occur when there are many traits PROBABILITY Probability that the recessive trait will appear: 2,001 ----------8,023 = 0.25 or 25% or ¼ or 1:3 PREDICTING RESULTS OF MONOHYBRID CROSSES Monohybrid cross: a cross in which only one characteristic is tracked The offspring are called monohybrids Biologists use a Punnett square to aid them in predicting the probable distribution of inherited traits in the offspring EXAMPLE 1: HOMOZYGOUS X HOMOZYGOUS PP and pp All offspring are Pp 100% probability that the offspring will have the genotype Pp and thus the phenotype purple flower color EXAMPLE 2: HOMOZYGOUS AND HETEROZYGOUS BB and Bb Offspring are BB and Bb The probability of an offspring having BB genotype is 2/4 or 50% The probability of an offspring having Bb genotype is 2/4 or 50% The probability of an offspring have the dominant black coat is 4/4 or 100% EXAMPLE 3: HETEROZYGOUS X HETEROZYGOUS Bb and Bb The probability of an offspring having a BB genotype is ¼ or 25% The probability of an offspring having a Bb genotype is 2/4 or 50% The probability of an offspring having a bb genotype is ¼ or 25% EXAMPLE 3: HETEROZYGOUS X HETEROZYGOUS ¾ or 75% of the offspring resulting from this cross are predicted to have a black coat ¼ or 25% of the offspring are predicted to have a brown coat (recessive phenotype) EXAMPLE 3: HETEROZYGOUS X HETEROZYGOUS Genotypic ratio: the ratio of the genotypes that appear in offspring The probable genotypic ratio of the monohybrid cross represented is 1BB: 2 Bb: 1 bb Phenotypic ratio: the ratio of the offspring’s phenotypes The probable phenotypic ratio of the cross is 3 black : 1 brown EXAMPLE 4: TESTCROSS In guinea pigs, both BB and Bb result in a black coat How would you determine whether a black guinea pig is homozygous (BB) or heterozygous (Bb)? Perform a testcross Testcross: an individual of unknown genotype is crossed with a homozygous recessive individual Can determine the genotype of any individual whose phenotype expresses the dominant trait EXAMPLE 4: TESTCROSS If the black guinea pig of unknown genotype is homozygous black, all offspring will be black If the individual with the unknown genotype is heterozygous black, about half will be black EXAMPLE 5: INCOMPLETE DOMINANCE In Mendel’s pea-plant crosses, one allele was completely dominant over another Called complete dominance In complete dominance, heterozygous plants and homozygous dominant plants are indistinguishable in phenotype Ex: PP and Pp produce purple flowered plants EXAMPLE 5: INCOMPLETE DOMINANCE Sometimes, the F1 offspring will have a phenotype in between that of the parents Called incomplete dominance Incomplete dominance occurs when the phenotype of a heterozygote is between the phenotypes determined by the dominant and recessive traits EXAMPLE 5: INCOMPLETE DOMINANCE Both the allele for red flowers (R) and the allele for white flowers (r) influence the phenotype Neither allele is completely dominant When red flowers are crossed with white flowers, all of the F1 offspring have pink flowers EXAMPLE 5: INCOMPLETE DOMINANCE 100% of the offspring have the Rr genotype The probable genotypic ratio is 1RR: 2 Rr: 1 rr Since neither allele is completely dominant, the phenotypic ratio is 1 red: 2 pink: 1 white EXAMPLE 6: CODOMINANCE Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring Neither allele is dominant or recessive, nor do the alleles blend in the phenotype Example: blood types Determined by two alleles PREDICTING RESULTS OF DIHYBRID CROSSES Dihybrid cross: a cross in which two characteristics are tracked The offspring are called dihybrids HOMOZYGOUS X HOMOZYGOUS Ex: Predict the results of a cross between a pea plant that is homozygous for round, yellow seeds and one that is homozygous for wrinkled, green seeds Round seeds (R) is dominant over wrinkled seeds (r) Yellow seeds (Y) is dominant over green seeds (y) HOMOZYGOUS X HOMOZYGOUS The Punnett Square used to predict the results of a cross between a parent of the genotype RRYY and a parent of the genotype rryy will contain 16 boxes Alleles are carried by the male and female gametes HOMOZYGOUS X HOMOZYGOUS The genotype of all of the offspring of this cross will be heterozygous for both traits: RrYy All have round, yellow seed phenotypes HETEROZYGOUS X HETEROZYGOUS Cross two pea plants heterozygous for round, yellow seeds Offspring are likely to have nine different genotypes HETEROZYGOUS X HETEROZYGOUS These 9 genotypes will result in pea plants that have the following four phenotypes: 9/16 that have round, yellow seeds (genotypes RRYY, RRYy, RrYY, and RrYy) 3/16 that have round, green seeds (genotypes Rryy and Rryy) 3/16 that have wrinkled, yellow seeds (genotypes rrYY and rrYy) 1/16 that have wrinkled, green seeds (genotype rryy)
