Lesson 16 - Mendel’s Experimental System

*Lesson 16 Notes: **Tasks: -16.1: Explain the advantages of Mendel’s experimental system. -16.2: Evaluate the outcome of a monohybrid cross. -16.3: Explain Mendel’s Principle of Segregation. -16.4: Explain how meiosis contributes to genetic variation. -16.5: Evaluate the outcome of a dihybrid cross. -16.6: Explain Mendel’s Principle of Independent Assortment. -16.7: Correlate the segregation of alleles for different genes with the behavior of different homologues in meiosis. -16.8: Apply the rules of probability to genetic crosses. -16.9: Interpret data from test crosses to infer unknown genotypes. -16.10: Provide examples of patterns of inheritance that do not fit Mendel’s model. ● Gregor Mendel: ○ Was a monk ○ Founded of the field of genetics ○ Discovered common patterns of inheritance and many essential facts about genes and alleles ○ Reached important and accurate conclusions ○ Chose pea plants for his experiments: ■ Pea plant hybrids could be produced ■ Many pea varieties were available ■ Peas are small and easy to grow ● Reasonable resources and time ■ Can cross or self-fertilize ● Can manipulate parents strains ○ Took copious notes and it was only years after his death that other scientists read his notebook and analyzed it ■ Our modern understanding of genetics began ● Mendel's experimental design: ○ Monohybrid crosses ○ 1) produced true breeding pea strains for 7 different traits ■ True breeding: homozygous- both alleles → cross it with itself and allow it to fertilize it itself, the offspring will be the exact same ■ Each trait had 2 variants ■ Strain is true-breeding if an offspring produced by self fertilization exhibits the trait of interest from one generation to the next ○ 2) cross fertilized true breeding strains having different forms of a given ● ● ● ● trait ■ Also performed reciprocal crosses ■ Pollen of one plant with a form was used to fertilize the plant of another form and vice versa ■ Cross that Mendel performed→ monohybrid cross ○ 3) allowed resulted hybrid offspring to self fertilize for several generations ■ Counted the number of offspring showing each form of the trait at each generation Mendel’s seven traits: ○ 1) flower color ○ 2) seed color ○ 3) seed texture ○ 4) pod color ○ 5) pod shape ○ 6) flower position ○ 7) plant height ■ All 7 traits give the same 3:1 ration of variance when the F2 generation was counted The F1 and F2 generations: ○ Self-cross: fertilizing the plant with its own semen ○ Hybrid offspring from the first cross represent the first F1 generation ○ F1 plants: ■ Resembled one of the parents ■ One trait was dominant ■ Other was recessive ○ Then allowed them to mature and self fertilize and examine the F2 generation ○ F2 plants: ■ ¾ plants with the dominant form ■ ¼ plants with recessive form ■ Dominant: recessive was 3:1 The 3:1 is actually the 1:2:1: ○ Analysis of the F3 generation revealed the F2 Ratio wasactully: ■ 1 true breeding dominant plan ■ 2 not true breeding dominant plants ■ 1 true recessive plant Mendel's conclusions: ○ Plants did not show intermediate traits ■ No blending of traits ○ Each trait is discrete and intact as a unit ○ For each pair of traits, one was dominant and one was recessive ○ The pairs of alternative traits examined were segregated among the progeny of a particular cross ○ Alternative traits were expressed in the F2 generation in the ration of 3?4 dominate to 1/4 recessive ■ The mendelian ratio ● Mendel’s 5 element model: ○ 1) parents transmit discrete factors ■ Genes → offspring ○ 2) each individual receives one copy of a gene from each parents ■ Carried on chromosomes ■ Diploid from one set of chromosomes from each parents ○ 3) not all copies of a gene are identical ■ Allele: alternative form of a gene ■ Homozygous: 2 of the same allele ■ Heterozygous: different alleles ○ 4) alleles remain discrete ■ No blending or alteration of each other ■ Allele segregate randomly into these gametes ○ 5) presence of allele does not guarantee expression ■ Dominate allele: expressed ■ Recessive allele: hidden by dominant allele ○ Genotype: total set of alleles an individual contains ○ Phenotype: physical appearance ■ An allele’s expression ● Principle of segregation: ○ Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parents, during fertilization ○ Physical basis for alle segregation is the behavior of chromosomes during meiosis ○ Medel had no knowledge of chromosomes of meios ■ Had not yet been described ● Genetic symbols and the Punnett square: ○ Example: ■ Cross purple-flowered plant with white-flowered plant ● “P” is dominant allele (purple flowers) ○ Uppercase allele → dominant ● “p” is recessive allele (white flowers) ○ Lowercase allele → recessive ● True-breeding white-flowered plant is “pp” ● ● ● ● ● ○ Homozygous recessive ● True-breeding purple-flowered plant is “PP” ○ homozygous dominant ● “Pp” is heterozygous purple-flowered plant ○ Using these symbols, we can create a diagram to predict the results of a cross between individuals of a known genotype ■ Different female gametes → side of the square ■ Different male gametes → top of the square ■ Each potential zygote → vertical and horizontal lines on the grid Human traits: ○ Some human traits are controlled by a single gene ○ Some are these exhibit dominant and recessive inheritance ○ Pedigree analysis is used to track inheritance patterns in families ■ Benign traits ● Hair and eye color ■ Human genetic diseases A dominant pedigree- juvenile glaucoma: ○ Disease that causes degeneration of the optic nerve and blindness ○ Dominant trait appears in every generation A recessive pedigree- albinism: ○ Condition in which the pigment melanin is not produced ○ Pedigree is for form of albinism dee to a nonfunctional allele of the enzyme, tyrosinase ○ Males and females equally affected ○ Most affected individuals have unaffected parents ○ Affect parent does not usually have affected offspring ○ Affected offspring more frequently when parents related ■ Recessive mode of inheritance The dihybrid cross: ○ Dihybrid cross is a pranit cross between two different lines ■ varieties, strains, etc. ● differ in two observed traits ○ In the Mendelian sense, between the alleles of both → recessive ○ Established a series of true breeding lines ○ Created heterozygous crosses Principle of independent assortment: ○ In a dihybrid cross, the alleles of each gene assort independently ○ The segregation of different allele pairs is independent ■ Principle of independent assortment ● Independent alignment of different homologous chromosome pairs during metaphase 1 leads to the independent segregation of the different allele pairs ● Probability: ○ 1) rule of addition: ■ Probably of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities ● Two events cannot happen at the same time ■ When crossing Pp x Pp → the probability of producing Pp offspring is the probability of obtaining Pp (¼) PLUS the probability of obtaining pP (¼) ● ¼+¼=½ ● 2) rule of multiplication: ○ Probability of 2 independent events occurring simultaneously is the product of their individual probabilities ○ When crossing Pp x Pp → the probability of obtaining pp offspring is the probability of obtaining a p from the father (½) TIMES the probability of obtaining a p from the mother (½) ■ ½x½=¼ ● A testcross can reveal unknown genotypes: ○ Used to determine the genotype of an individual with a dominant phenotype ○ Cross the individual with an unknown genotype ( P?) with a homozygous recieve (pp) ○ Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent ● Extensions to Mendel: ○ Mendel's Model of inheritance assumes: ■ 1) each train is controlled by a single gene ■ 2) each gene has only 2 alleles ■ 3) a clear dominant-recessive relationship between the alleles ○ Most genes do not meet these criteria ● Polygenic inheritance: ○ Occurs when multiple genes control the phenotype of a trait ○ Phenotype is an accumulation of contributions by multiple genes ○ Traits show continuous variation ■ ex:) human height ■ Quantitative traits ● Pleiotropy: ○ Refers to an allele with more than one accent on the phenotype ● ● ● ● ○ Observed in diseases such as cystic fibrosis and sickle cell anemia ○ Multiple symptoms can be traced back to one defective allele ○ Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions ■ May be dominant to one trait but recessive to the other Multiple alleles: ○ May be more than 2 alleles for a gene in a population ○ Each individual can only have 2 alleles ■ ex:) ABO blood type in humans ● 3 alleles ○ Number of alleles possible for any gene is constrained ○ Usually more than 2 alleles exist for any gene in an outbreeding population Incomplete dominance vs. codominance: ○ Incomplete dominance: ■ Heterozygote is intermediate in phenotype between the 2 homozygotes ■ Red flowers x white flowers = pink flowers ○ Codominance: ■ Heterozygote shows shome aspect of the phenotypes of both homozygotes ■ Type AB blood ● Sugars act as recognition markers for the immune system ● The system demonstrates multiple alleles and codominance ● 3 alleles of the I gene (IA, IB, and i) ● IA and IB and dominant to i but codominant to each other ○ IA → galactosamine → A ○ IB → galactose → B ○ AB → galactose and galactosamine → AB ○ ii → none → O Environmental influence: ○ Coat color in himalayan rabbits and siamese cats ○ Allele produces an enzyme that allows pigment production only at temperatures below 33 degree celsius ■ Outside temperatures → internal temperatures ■ Proteins → denatured Epistasis: ○ Behaviour of gene products can change the raton expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment ■ ex:) genes that act in the same metabolic pathway should show some form of dependence ○ Expression of one gene can interfere with expression of another gene ______________________________________________________________________ Lesson 16: Inheritance of Genetic Information Part I Vocabulary Alleles Codominance Dihybrid cross Dominant Environmental influence Epistasis F1 generation F2 generation Genes Genetic disorders Genotype Heterozygous Homozygous Hybrid Incomplete dominance Independent assortment Mendelian ratio Monohybrid cross Multiple alleles Mutations Not true-breeding Pedigree Phenotype Pleiotropy Polygenic inheritance Principle of segregation Punnett square Recessive Rule of addition Rule of multiplication Testcross Trait True-breeding