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Fundamentals of Genetics Mendel’s Legacy • Mendel’s Experiments • Mendel’s Results & Conclusions Genetic Crosses • Probability • Monohybrid Crosses • Dihybrid Crosses Historical Background of Genetics • To View Video: – Move mouse cursor over slide titlelink – When hand appears, click once • ASF Video plays about 2-1/2 min Mendel’s Legacy • Gregor Mendel Mendel’s Garden Peas Mendel’s Methods • Mendel’s Experiments • Mendel’s Results & Conclusions Dominance & Recessiveness Law of Segregation Law of Independent Assortment • Chromosomes & Genes Learning Objectives • TSW … 1. Describe the steps involved in Mendel’s experiments on garden peas 2. Distinguish btw/ dominant & recessive traits 3. State 2 laws of heredity that were developed from Mendel’s work 4. Describe how Mendel’s results can be explained by scientific knowledge of genes & chromosomes Gregor Mendel mathematician natural philosopher priest & abbot The Garden Pea Pisum sativum • Annual plant • Heritable features easily observed • Monoecious: flowers have both male and female organs plants can self-pollinate or crosspollinate male stamens can easily be removed to prevent self-pollination pollen easily transferred by dusting to cross-pollinate Genetic Terminology • Heredity – transmission of characters from parents to offspring • Character – heritable feature flower color pea pod color stem length • Trait – heritable variant of a character purple flower -v- white flower green pea pod -v- yellow pea pod tall stems -v- dwarf (short) stem Pea Plant Traits • Features investigated by Mendel flower color: purple or white flower position: axial or terminal seed color: yellow or green seed shape: smooth or wrinkled pod shape: inflated or constricted pod color: green or yellow stem length: tall or short Mendel’s Methods: Flower Structure • Pollination – pollen grains from male flower part are transferred to female flower part Anther (of stamen): male flower part Stigma (of carpal): female flower part Mendel’s Methods: Pollination Techniques • Self-pollination – pollen transferred from anthers of a flower to a stigma of the same flower or a different flower on the same plant • Cross-pollination – pollen transferred from anthers of a flower on one plant to a stigma of a flower on a different plant Mendel’s Experiments: Parental Generation • P1 generation = strain of plants pure for a trait (truebreeding) “Parental” plants Developed by self-pollinating for generations to produce offspring that always have the same trait as the parents Ex: purple-flowered parents produce only purple-flowered offspring Mendel’s Experiments: Filial Generations • F1 generation = offspring (hybrids) of the P1 generation Produced by cross-pollinating 2 pure (P1) strains Ex: purple-flowered strain crosspollinated w/ white flowered strain • F2 generation = offspring of the F1 generation Produced by self-pollinating F1s Mendel’s Results Results of P1 & F1 Crosses • 7 characters, each showing 2 possible traits, were tested • 1,000s of crosses were made in each case • In every P1 cross – 1 trait disappeared in the F1 • In every F1 cross – the trait reappeared in the F2 generation Mendel’s Experiments & Conclusions • To View Video: – Move mouse cursor over slide titlelink – When hand appears, click once • ASF Video plays about 4-1/2 min Mendel’s Conclusions • Based on mathematics, each character (ex., flower color) was controlled by factors for 2 contrasting traits (exs., purple flower or white flower) • One trait-factor dominated the other in expression of the character when both were present in the same plant Mendel’s Conclusions: Dominance & Recessiveness • Dominant factor (trait) – masks or “dominates” the other trait in the F1 • Recessive factor (trait) – disappears (or “recedes”) in the F1 generation but reappears in the F2 – Mendel discovered this after carrying out monohybrid crosses for specific characters. Mendel’s Conclusions: Segregation • Law of Segregation – A pair of factors is segregated (or separated) during the formation of gametes Mendel discovered this after carrying out P1 & F1 crosses for specific characters. The two paired factors that control the expression of a trait separate during the formation of reproductive cells Mendel’s Conclusions: Independent Assortment • Law of Independent Assortment – Factors for different characters are distributed to gametes independently Mendel discovered this after carrying out P1 & F1 crosses for 2 specific characters. Plants showing the dominant trait for one character could also show the recessive trait for a different character Law of Independent Assortment • When considering two or more different characters (genes), the traits (alleles) for each segregate into different gametes independently Flower color of each other the different characters reside on different pairs of homologous chromosomes Pod color Law of Independent Assortment • Mendel discovered this after carrying out repeated dihybrid crosses. he crossed a true-breeding plant w/ yellow, round seeds (dominants) and a true-breeding plant w/ green, wrinkled seeds (recessives) all the F1s had the dominant traits of yellow, round seeds Law of Independent Assortment he then crossed the yellow, round F1s in the F2 generation, plants appeared with: yellow, round seeds (56%); yellow, wrinkled (19%); green, round (19%); green, wrinkled (6%) Factors & Chromosomes • To View Video: – Move mouse cursor over slide titlelink – When hand appears, click once • ASF Video plays about 2-1/4 min Chromosomes & Genes • Chromosomes are DNA molecules • DNA molecules are divided into many distinct segments called genes • Genes control specific hereditary traits • Chromosomes occur in pairs • Therefore genes also occur in pairs Chromosomes & Genes • Alleles Alternate versions of the same gene 1 allele carried on each homologous chromosome Occur at the same locus • the DNA of each allele has a slightly different nucleotide sequence • results in slightly different variations of the same character Alleles Example: The gene for the character “flower color” on one homologue can have an allele (factor) for purple flowers The gene for the character “flower color” on the other homologue can have an allele (factor) for white flowers pp PP Genetic Symbology Dominant alleles are Pp symbolized w/ a capital letter Recessive alleles are symbolized w/ a the lower case letter used for the dominant allele Chromosomes & Inheritance • To View Video: – Move mouse cursor over slide titlelink – When hand appears, click once • ASF Video plays about 1 min Video Quiz: Genetics & Meiosis • To View Video: – Move mouse cursor over slide titlelink – When hand appears, click once • ASF Video plays about 2 min • 5 Questions Genetic Crosses • Genotype & Phenotype • Probability • Predicting Results of Monohybrid Crosses 6 examples • Predicting Results of Dihybrid Crosses 2 examples Learning Objectives • TSW … 1. Explain how probability is used to predict the results of genetic crosses 2. Use a Punnet square to predict the results of monohybrid & dihybrid crosses 3. Explain how a testcross is used to show the genotype of an individual w/ the dominant phenotype 4. Differentiate a monohybrid cross from a dihybrid cross Genetic Vocabulary • Phenotype outward appearance of an organism Ex: purple flowers • Genotype genetic make-up of an organism Ex: PP, Pp (– purple flowers) pp (– white flower) Genetic Vocabulary • Homozygous an organism having a pair of identical alleles for a character Ex: PP = homozygous dominant Ex: pp = homozygous recessive • Heterozygous an organism having 2 different alleles for a character Ex: Pp Probability • Probability – the likelihood that a specific event will occur – Expressed as: decimal percentage fraction Probability = # times an event is expected to occur # opportunities for an event to occur Probability Q. What is the probability that the dominant trait for purple flowers will appear in an F2 generation? Probability = # times an event occurs # opportunities for an event to occur Mendelian Experiment: Probability = 6,022 purple plants = 0.75 8,023 total plants produced Genetic Predictions Based on laws of probability • Multiplication Rule The probability that 2 or more independent events will occur together in a specific way is determined by multiplying the probability of 1 event by the probability of the other event Ex: the probability of a coin flip ending up heads is 50% or ½. The probability that a second coin flip will end up heads is also 50% or ½. Therefore, the probability that 2 coins flipped together will both end up heads is ½ X ½ = ¼ or 25% Genetic Predictions Based on laws of probability • Addition Rule The probability that any 1 of 2 or more independent events will occur is determined by adding their individual probabilities Ex: the probability that flipped coin A will come up heads while flipped coin B will come up tails is 25% or ¼. The probability that flipped coin A will come up tails while flipped coin B will come up heads is also 25% or ¼. Therefore, the probability that both events will occur is ¼ + ¼ = ½ or 50% Predicting Results of Monohybrid Crosses • Monohybrid cross – a cross btw/ individuals that involves 1 pair of contrasting traits • Punnett square – a diagram used as an aid in predicting the probability that certain traits will be inherited by offspring Monohybrid crosses investigate inheritance patterns for a single character Mendel’s Crosses & Punnett Squares • To View Video: – Move mouse cursor over slide titlelink – When hand appears, click once • ASF Video plays about 6 min Monohybrid Cross: Procedure • Step 1: determine the genotypes for each mate in the cross. The genotype shows the allelic combination. Ex: Tt X Tt • Step 2: determine all the possible kinds of gametes that each mate can produce. Each gamete will have 1 allele from the genotype. Ex: T & t for one parent; T & t for the other parent Monohybrid Cross: Procedure • Step 3: construct a Punnett square. Place the gametes for one parent across the top & the gametes for the other parent along the left side. Ex: 4 boxes for a monohybrid cross. T & t across top; T & t along side • Step 4: combine the possible gametes of each parent in the 4 boxes Ex: TT in top left box; Tt in top right & bottom left boxes; tt in bottom right box Example 1: Homozygous X Homozygous • • A plant that is true-breeding for purple flower color (dominant) is crossed with a plant that is true breeding for white flower color (recessive). State the probability that any offspring will have the following flower color: a) Purple b) White ♂ ♀ PP X purple p p pp Purple: 100% white P P Pp Pp purple purple Pp Pp purple purple Example 2: Homozygous X Heterozygous • • A plant that is true-breeding for purple flower color is crossed with a purple flowered plant that carries the allele for white flower color. State the probability that any offspring will have the following flower color: a) Purple b) White ♂ ♀ PP X purple P p Pp Purple: 100% purple P P PP PP purple purple Pp Pp purple purple Example 3: Heterozygous X Heterozygous • Two purple flowered plants that carry the allele for white flower color are crossed. • State the probability that any offspring will have the following flower color: a) Purple b) White ♂ ♀ Pp X purple P p Purple: 75% Pp White: 25% purple P p PP Pp purple purple Pp pp purple white Phenotypic ratio 3:1 Genotypic ratio 1:2:1 Genetic Ratios 1 PP : 2 Pp : 1 pp • Phenotypic ratio ratio of offspring w/ dominant appearance to offspring w/ recessive appearance Ex: for offspring of 2 heterozygotes = 3:1 for monohybrid crosses • purple-flowered to white flowered individuals after a cross of Pp X Pp • Genotypic ratio ratio of homozygous dominants to heterozygotes to homozygous recessives Ex: for offspring of 2 heterozygotes = 1:2:1 for monohybrid crosses Testcross If an organism has the recessive phenotype, we know its genotype because there is only one way that the recessive form of the character can occur. Ex: White-flowered plants always have the pp ? genotype Question: If an organism has the dominant phenotype, how can we determine which of the 2 possible genotypes it has? Ex: Purple-flowered plants can have either the PP or the Pp genotypes Testcross • If you cross a homozygous recessive, pp, with an organism of the dominant phenotype, P?, there are only two possible results. If 100% of the offspring have the dominant phenotype, the dominant parent MUST BE a homozygous dominant, PP If any of the offspring have the recessive phenotype, the dominant parent MUST BE a heterozygote, Pp • Determine the genotype of a plant with the dominant phenotype for flower color (red-flowered). • Note the probability that any offspring will have the following flower color: a) Red b) White ♂ ♀ R? red r X Red: 100% rr White: 0% white R R ? Rr ?r R red red Phenotypic ratio 4:0 Genotypic ratio 4:0 r Rr red ?r R red Dominant color plant is homozygous ♂ ♀ R? red r X Red: 50% rr White: 50% white R ?r Rr ? rr red white Phenotypic ratio 1:1 Genotypic ratio 1:1 r Rr red ? rr white Dominant color plant is heterozygous Example 5: Incomplete Dominance • Hybrids have phenotypes that are a blend of the characteristics of the two parental varieties P generation: Red flower X White flowers F1 generation: All Pink flowers F2 generation: ¼ Red, ¼ White, & ½ Pink flowers Example 5a: Incomplete Dominance • • Determine the genotypic & phenotypic ratios for a cross between two true-breeding plants showing incomplete dominance for flower color. Note the probability that any offspring will have the following flower color: a) Red b) White ♀ ♂ Red: 0% RR rr White: 0% white Pink: 100% X red r r R R Rr Rr pink pink Rr Rr pink pink Phenotypic ratio 4:0 Genotypic ratio 4:0 Example 5b: Incomplete Dominance • • Determine the genotypic & phenotypic ratios for a cross between two hybrid plants showing incomplete dominance for flower color. Note the probability that any offspring will have the following flower color: a) Red b) White ♀ ♂ Red: 25% Rr Rr Pink: 50% pink White: 25% X pink R r R r RR Rr red pink Rr rr pink white Phenotypic ratio 1:2:1 Genotypic ratio 1:2:1 Codominance Phenotype Genotype • Hybrids show the phenotypes of both parental varieties in separate, distinguishable ways P generation: Type A RBC antigen X Type B RBC antigen F1: All Type AB RBC antigen Example 6a: Codominance • A woman who is homozygous for type A blood marries a man who is homozygous for type B blood. • State the probability that any child they produce will have the following blood types: a) A b) B c) AB ♂ A A B B I I X I I ♀ IB B I 100% probability for AB blood type IA A I A B I I A B I I Type AB Type AB A B I I A B I I Type AB Type AB Example 6b: Codominance • A woman who is heterozygous for blood type (AB) marries a man who is heterozygous for blood type (AB) . • State the probability that any child they produce will have the following blood types: a) A b) B c) AB ♂ A B A B I I X I I ♀ IA B I 50% probability for AB blood type 25% probability each for blood types A & B IA B I A A I I A B I I Type A Type AB A B I I Type AB B B I I Type B Mendel’s Experiments • Mendel deduced the law independent assortment from repeated observations of P & F1 dihybrid crosses. the appearance of dominant and recessive traits according to principles of probability led to this conclusion Mendel’s Experiments • Experimental Model With “dependent” assortment, traits will be inherited together F1 gametes will be either YR or yr Phenotypic ratio will be 3:1 Hypothesis for independent assortment, offspring will show 4 different combinations of traits F1 gametes will be YR, Yr, yR, & yr Phenotypic ratio will be 9:3:3:1 Mendel’s Experiments • The Dihybrid Cross Mendel crossed a true-breeding Dihybrid crosses plant w/ yellow, round seeds (both investigate dominant traits) and a truebreeding plant w/ green, wrinkled inheritance patterns seeds (bothcharacters recessive traits) for two all the F1s had the dominant traits: yellow, round seeds Mendel’s Experiments he then crossed the yellow, round F1s in the F2 generation, plants appeared with: yellow, round seeds (56%); yellow, wrinkled (19%); green, round (19%); green, wrinkled (6%) Dihybrid Crosses: Procedure • Step 1: determine the genotype for each mate in the cross. The genotype shows the allelic combination. Ex: TtAa X TtAa • Step 2: determine all the possible kinds of gametes that each mate can produce. Each gamete will have 1 allele from each character. Ex: for both parents – TA, Ta, tA, ta Dihybrid Crosses: Procedure • Step 3: construct a Punnett square. Place the gametes for one parent across the top & the gametes for the other parent along the left side. Ex: 16 boxes for a dihybrid cross. TA, Ta, At, ta across top & along side • Step 4: combine the possible gametes of each parent in the 16 boxes Ex: 1 TTAA; 2 TTAa; 2 TtAA; 1 TTaa; 2 Ttaa; 4 TtAa; 1 ttAA; 2 ttAa; 1 ttaa Punnett’s Contribution: Cross Predictions Dihybrid • To View Video: – Move mouse cursor over slide titlelink – When hand appears, click once • ASF Video plays about 5 min Example 1: 100% Round & Yellow Homozygous X Homozygous 100% heterozygous for both characters • A plant that is true-breeding for round, yellow seeds is crossed w/ a plant that is true-breeding for wrinkled, green seeds. • State the probability that any offspring will have the following seed phenotypes: a) Round & yellow c) Round & green b) Wrinkled & green d) Wrinkled & yellow Note: 9 : 3 : 3 : 12: Example phenotypic ratio Heterozygous X Heterozygous • Two plants w/ round, yellow seeds that are carrying alleles for wrinkled & green are crossed. • State the probability that any offspring will have the following flower color: a) Round & yellow c) Round & green b) Wrinkled & green d) Wrinkled & yellow Common Genetic Ratios • Phenotypic ratios Monohybrid heterozygote crosses: 3:1 Dihybrid heterozygote crosses: 9:3:3:1 • Genotypic ratios Monohybrid heterozygote crosses: 1:2:1 Dihybrid heterozygote crosses: not commonly done 1:2:2:1:4:1:2:2:1 Dominance/Recessiveness Relationships • Range complete dominance – degrees of incomplete dominance – codominance • Reflection of mechanisms by which specific alleles are expressed in phenotype & do not involve the ability of one allele to subdue another at the level of DNA • Do not determine or correlate with the relative abundance of alleles in a population Effect of Environment on Phenotype hydrangea in acidic soil hydrangea in alkaline soil Effect of Environment on Phenotype Introduction to DNA • To View Video: – Move mouse cursor over slide titlelink – When hand appears, click once • ASF Video plays about 1 min Chromosomes, Genes, & DNA • To View Video: – Move mouse cursor over slide titlelink – When hand appears, click once • ASF Video plays about 4-3/4 min