*Outline11.1 Gregor Mendel11.2 Mendels Laws11.3 Mendelian Patterns of Inheritance and Human Disease11.4 Beyond Mendelian Inheritance
Phenylketonuria: A Human Genetic DisorderPhenylalanine is an amino acid found in many foods.Phenylketonurics are people who lack the enzyme that breaks down phenylalanine.Excess phenylalanine accumulates in their bodies, causing nervous system disorders.Phenylketonuria is a recessively inherited trait, which means people have to inherit two copies of the gene to have the disorder.Food labels inform phenylketonurics which foods they should avoid.*
11.1 Gregor MendelThe Blending Concept of Inheritance:Parents of contrasting appearance produce offspring of intermediate appearance.Over time, variation would decrease as individuals became more alike in their traits.Blending was a popular concept during Mendels time.Mendels findings were in contrast with this.He formulated the particulate theory of inheritance.Mendel proposed the laws of segregation and independent assortment.Inheritance involves reshuffling of genes from generation to generation.*
Gregor MendelAustrian monkStudied science and mathematics at the University of ViennaConducted breeding experiments with the garden pea Pisum sativumCarefully gathered and documented mathematical data from his experimentsFormulated fundamental laws of heredity in the early 1860sHad no knowledge of cells or chromosomesDid not have a microscopeExperiments on the inheritance of simple traits in the garden pea disproved the blending hypothesis*
*Mendel Worked with the Garden PeaThe garden pea:Organism used in Mendels experimentsA good choice for several reasons:Easy to cultivateShort generationNormally self-pollinating, but can be cross-pollinated by handPollen was transferred from the male (anther) of one plant to the female (stigma) parts of another plant.True-breeding varieties availableSimple, objective traits
Garden Pea Anatomy*stamenanthera.Flower Structurefilamentstigmastyleovules inovarycarpelCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
*All peas are yellow whenone parent produces yellowseeds and the other parentproduces green seeds.Brushingon pollenfrom anotherplantCutting awayanthersGarden Pea AnatomyCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
11.2 Mendels LawsMendel performed cross-breeding experiments.Used true-breeding (homozygous) plantsChose varieties that differed in only one trait (monohybrid cross)Performed reciprocal crossesParental generation = PFirst filial generation offspring = F1 Second filial generation offspring = F2Formulated the law of segregation*
Monohybrid Cross Done by Mendel*Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.eggsspermOffspringP generationTtTtTTtttTTtTTTtTtttP gametesF1 gametesF2 generationF1 generationPhenotypic Ratioshort1tall3Allele KeyTtall plant=tshort plant=
Mendels LawsLaw of Segregation:Each individual has a pair of factors (alleles) for each trait.The factors (alleles) segregate (separate) during gamete (sperm & egg) formation.Each gamete contains only one factor (allele) from each pair of factors.Fertilization gives the offspring two factors for each trait.Results of the monohybrid cross: All F1 plants were tall, disproved blending hypothesis. *
Mendels LawsClassical Genetics and Mendels Cross:Each trait in a pea plant is controlled by two alleles (alternate forms of a gene).Dominant allele (capital letter) masks the expression of the recessive allele (lowercase).Alleles occur on a homologous pair of chromosomes at a particular gene locus.Homozygous = identical allelesHeterozygous = different alleles*
Homologous Chromosomes*Replicationalleles at agene locussister chromatidsb. Sister chromatids of duplicated chromosomes have same alleles for each gene.a. Homologous chromosomes have alleles for same genes at specific loci.GRStGRStGRStgrsTgrsTgrsTCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Relationship Between Observed Phenotype and F2 OffspringTraitStem lengthPod shapeSeed shapeSeed colorFlower positionFlower colorPod colorDominantCharacteristicsRecessiveTallInflatedRoundYellowAxialPurpleGreenShortConstrictedWrinkledGreenTerminalWhiteYellowDominantF2ResultsRatioRecessiveTotals:2.84:12777878822992.95:12.96:13.01:13.14:13.15:12.82:12.98:12,0011,8505,4746,02265170542814,9492072241525,010Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mendels LawsGenotype It refers to the two alleles an individual has for a specific trait.If identical, genotype is homozygous.If different, genotype is heterozygous.Phenotype It refers to the physical appearance of the individual.*
Mendels Cross Viewed by Modern GeneticsThe dominant and recessive alleles represent DNA sequences that code for proteins.The dominant allele codes for the protein associated with the normal gene function within the cell.The recessive allele represents a loss of function.During meiosis I the homologous chromosomes separate.The two alleles separate from each other.The process of meiosis explains Mendels law of segregation and why only one allele for each trait is in a gamete.
Mendels LawsA dihybrid cross uses true-breeding plants differing in two traits.Mendel tracked each trait through two generations.It started with true-breeding plants differing in two traits.The F1 plants showed both dominant characteristics.F1 plants self-pollinated.He observed phenotypes among F2 plants.Mendel formulated the law of independent assortment.The pair of factors for one trait segregate independently of the factors for other traits.All possible combinations of factors can occur in the gametes.P generation is the parental generation in a breeding experiment.F1 generation is the first-generation offspring in a breeding experiment.F2 generation is the second-generation offspring in a breeding experiment. *
Dihybrid Cross Done by MendelCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.====TtGg9331ttggTTGGP generationP gametesF1 generationF1 gametesF2 generationspermeggsTtGgTGtgtgtGTgTGTGTtGgTgtGtgTTGGTTGgTtGGTtggTtGgTTggTTGgttGgttGGTtGgTtGGttggttGgTtggTtGgAllele KeyOffspringPhenotypic RatioYellow podgreen podshort planttall planttall plant, green podtall plant, yellow podshort plant, green podshort plant, yellow pod
Independent Assortment and Segregation during Meiosis*Parent cell has twopairs of homologouschromosomes.All possible combi -tions of chromosomesand alleles occur inthe gametes assuggested by Mendel'stwo laws.All orientations of ho-mologous chromosomesare possible at metaphase I in keepingwith the law of Independent assortment.At metaphase II, eachdaughter cell has onlyone member of eachhomologous pair inkeeping with the law ofsegregationAAAAAAAABabAbaBaaaaaaaAAbAaaaBBBBBBBBBBaBbbbAAbbBBaabbbbbAbbeitherorCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mendels LawsPunnett SquareTable listing all possible genotypes resulting from a crossAll possible sperm genotypes are lined up on one side.All possible egg genotypes are lined up on the other side.All possible zygote genotypes are placed within the squares.*
Mendel and the Laws of ProbabilityPunnett SquareIt allows us to easily calculate probability of genotypes and phenotypes among the offspring.Punnett square in next slide shows a 50% (or ) chance. The chance of E = The chance of e = An offspring will inherit:The chance of EE = = The chance of Ee = = The chance of eE = = The chance of ee = = The sum rule allows us to add the genotypes that produce the identical phenotype to find out the chance of a particular phenotype.*
Punnett Square*eggsspemPunnett squareOffspringParentsEeEeEeEeEEEeEeAllele keyPhenotypic Ratiounattached earlobes31E =e =unattached earlobesattached earlobesattached earlobesCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.ee
Mendels LawsTestcrossesIndividuals with recessive phenotype always have the homozygous recessive genotype.However, individuals with dominant phenotype have indeterminate genotype.May be homozygous dominant, orHeterozygousA testcross determines the genotype of an individual having the dominant phenotype.*
*Mendels LawsTwo-trait testcross:An individual with both dominant phenotypes is crossed with an individual with both recessive phenotypes.If the individual with the dominant phenotypes is heterozygous for both traits, the expected phenotypic ration is 1:1:1:1.
11.3 Mendelian Patterns of Inheritance and Human Disease Genetic disorders are medical conditions caused by alleles inherited from parents. Autosome is any chromosome other than a sex chromosome (X or Y).Genetic disorders caused by genes on autosomes are called autosomal disorders. Some genetic disorders are autosomal dominant.An individual with AA has the disorder.An individual with Aa has the disorder.An individual with aa does NOT have the disorder.Other genetic disorders are autosomal recessive.An individual with AA does NOT have the disorder.An individual with Aa does NOT have the disorder, but is a carrier.An individual with aa DOES have the disorder.*
Autosomal Recessive Pedigree*Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.IIIIIIIVKeyGenerationsAutosomal recessive disorders Most affected children have unaffected parents. He