2What Is the Physical Basis of Inheritance? Inheritance is the process by which traits are passed to the offspringGenes are sequences of nucleotides at specific locations on chromosomesA gene is a unit of heredity that encodes information needed to produce proteins, cells, and entire organismsThe location of a gene on a chromosome is called its locus (plural, loci)Focus on diploid organisms that utilize sexual reproduction by fusion of gametes.Chromosome – single double helix of DNA
3DNA in Eukaryotic Chromosomes Homologous chromosomes are usually not identicalMutations - Changes in nucleotide sequence in DNA of one homologous chromosomeMutations are the raw material for evolutionMutations in gametes may become a part of the genetic makeup of the speciesMutation gives rise to new alleles, alternative gene forms that may produce differences in structure or functionChromosomes occur in pairsA cell might make a mistake when it copies the DNA of one homologue but not the other. Or a carcinogen from the environment.make one homologue different genetically than the other2. May be beneficial, harmful, or neutral to the organism4. creating alleles – different hair colors, songs in birdsFor a given gene on two homologous chromosomes, both homologues may carry the same allele for the gene or may carry different allelesgene 1gene 2same allelesdifferent alleles
4The Relationship Among Genes, Alleles, and Chromosomes Both chromosomes carry the same alleleof the gene at this locus; the organism ishomozygous at this locusThis locus contains another gene forwhich the organism is homozygousEach chromosome carries a different alleleof this gene, so the organism isheterozygous at this locusa pair ofhomologouschromosomesgene locithe chromosomefrom the maleparentfrom the femaleAn organism’s two alleles may be the same or differentEach cell carries two alleles per characteristic, one on each of the two homologous chromosomesIf both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locusIf two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid)Fig. 10-1
5How Were the Principles of Inheritance Discovered? Who was Gregor Mendel?Pea plants have qualities that make it a good organism for studying inheritancePea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plant – self-fertilizationintact pea flowerflower dissected to showits reproductive structuresCarpel (female,produces eggs)Stamen (male, producespollen that contain sperm)Mendel was an Austrian monk in a monastery in the late 1800sHe discovered the common patterns of inheritance and many essential facts about genes, alleles, and the distribution of alleles in gametes and zygotes during sexual reproductionPea flowers have stamens, the male structures that produce pollen, that in turn contain the sperm (male gametes); sperm are gametes and pollen is the vehiclePea flowers have carpels, female structures housing the ovaries, which produce the eggs (female gametes)
6How Are Single Traits Inherited? The language of a genetic crossA genetic cross is the mating of pollen and eggs (from same or different parents)The parents used in a cross are part of the parental generation (known as P)The offspring of the P generation are members of the first filial generation (F1)Offspring of the F1 generation are members of the F2 generationTo see what characteristics a pea plant would inherit, Mendel used genetic crosses.
7Cross of Pea Plants True-Breeding for White or Purple Flowers pollenParentalgeneration (P)pollencross-fertilizetrue-breeding,purple-floweredplanttrue-breeding,white-floweredplantFirst-generationoffspring (F1)all purple-floweredplantsFig. 10-4
8Self-Fertilization of F1 Pea Plants with Purple Flowers First-generationoffspring (F1)self-fertilizeSecond-generationoffspring (F2)The capacity to produce white flowers had not disappeared in the F1 plants, but was only hidden.The F2 generation was allowed to self-fertilize.F3 generation – white flowers produced white flowers every time. But purple flowers produced purple or white.3/4 purple1/4 whiteFig. 10-5
9The Distribution of Alleles in Gametes homozygous parent(a) Gametes produced by a homozygous parent(b) Gametes produced by a heterozygous dominant parentAgametesheterozygous parentaEach trait is determined by pairs of genes (two alleles).Dominant alleles mask the expression of recessive alleles.Alleles separate during meiosis.These are Mendel’s hypothesis.Fig. 10-6
10Segregation of Alleles and Fusion of Gametes purple parentPPP+Pall P sperm and eggswhite parentppp+pall p sperm and eggs(a) Gametes produced by homozygous parents Fig. 10-7a
11Segregation of Alleles and Fusion of Gametes F1 offspringspermeggsP+pPporp+PPp(b) Fusion of gametes produces F1 offspringFig. 10-7b
12Segregation of Alleles and Fusion of Gametes ppp+Ppgametes from F1 Pp plantseggsF2 offspringPPPsperm(c) Fusion of gametes from the F1 generation producesFig. 10-7c
13How Are Single Traits Inherited? The hypothesis explains Mendel’s results with peasThe particular combination of the two alleles carried by an individual is called the genotypeThe physical expression of the genotype is known as the phenotype (for example, purple or white flowers)Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspringThe Punnett square method predicts offspring genotypes and phenotypes from combinations of parental gametesWe accept Mendel’s hypothesis as true based on the results.
14Determining the Outcome of a Single-Trait Cross Ppself-fertilize1Peggs1p22offspringgenotypesgenotypicratio(1:2:1)phenotypicratio(3:1)spermeggs1P21P1P124PP14PP2Genotypic Ratio = 1:2:1Phenotypic ratio = 3:1We can use this method to figure out one of the alleles of a parent to find out if the parent is heterozygous or homozygous for a trait.11sperm4PP4Pp1P1p12Pp2434purple12Pp11pp1P1pP22421p1p1pp111pp1whitepPpp2244444(a) Punnett square of a single-trait crossUsing probabilities to determine the offspring of asingle-trait crossFig. 10-8
15Traits of Pea Plants Studied by Gregor Mendel Dominant formRecessive formSeedshapesmoothwrinkledSeedcoloryellowgreenPodshapeinflatedconstrictedPodcolorgreenyellowFlowercolorpurplewhiteFlowerlocationat leafjunctionsat tips ofbranchesPlantsizetall(about6 feet)dwarf(about 8 to16 inches)Fig
16Predicting Genotypes and Phenotypes for a Cross between Parents That Are Heterozygous for Two Traits 14316SYSSYYSsYYssYYssyYSsyYSSYySsYyself-fertilizessYyssyySsyySSyysSyYsSyysSYYsSYySSyYsYsySyeggsseed shapeseed colorphenotypic ratio(9:3:3:1)smoothyellow9smooth yellowsmooth greenwrinkled yellowwrinkled greengreenwrinkled(a) Punnett square of a two-trait cross(b) Using probabilities to determine theoffspring of a two-trait crossspermFigRatio = 9:3:3:1
17How Are Multiple Traits Inherited? Mendel’s results supported his hypothesis that traits are inherited independentlyThe independent inheritance of two or more traits is called the law of independent assortmentMultiple traits are inherited independently because the alleles of one gene are distributed to gametes independently of the alleles for other genesIndependent assortment will occur when the traits being studied are controlled by genes on different pairs of homologous chromosomes
18Independent Assortment of Alleles pairs of alleles on homologouschromosomes in diploid cellsYychromosomes replicatereplicated homologouspair during metaphase ofmeiosis I, orientinglike thisor like thisSYSysysYmeiosis ISYsySysYDuring Meiosis I, lining up and separating is completely random.SYsySysYmeiosis IISSssSSssYYyyyyYYSYsySysYFigindependent assortment produces four equallylikely allele combinations during meiosis
19Do the Mendelian Rules of Inheritance Apply to All Traits? Many traits do not follow simple Mendelian rules of inheritanceNot all traits are completely controlled by a single geneNot all traits have only two possible allelesA trait may not be completely dominant to another
20Do the Mendelian Rules of Inheritance Apply to All Traits? eggsfatherC1C2C2C2C1C1motherspermIncomplete dominanceWhen the heterozygous phenotype is intermediate between the two homozygous phenotypesHair textureIn the genes studied by Mendel, one allele was dominant over the other, which was recessiveSome alleles, however, are incompletely dominant over others
21Do the Mendelian Rules of Inheritance Apply to All Traits? A single gene may have multiple allelesA species may have multiple alleles for a given characteristicHowever, each individual still carries two alleles for this characteristicThe human blood types are an example (Codominance)Blood Type(Phenotype)Genotype(alleles)Antibodies in PlasmaAAA or AOAnti-BBBB or BOAnti-AABNoneOOOAnti-A and Anti-B
22Do the Mendelian Rules of Inheritance Apply to All Traits? Polygenic inheritanceSome characteristics show a range of continuous phenotypes instead of discrete, defined phenotypesExamples of this include human height, skin color, and body build, and in wheat, grain colorPhenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple lociHeight is controlled by 180 genes.
23Polygenic Inheritance of Skin Color in Humans FigHuman skin color is controlled by at least three genes, each with pairs of incompletely dominant alleles
24Do the Mendelian Rules of Inheritance Apply to All Traits? The environment influences the expression of genesNewborn Siamese cats demonstrate the effect of environment on phenotypeA Siamese cat has the genotype for dark fur all over its bodyHowever, the enzyme that produces the dark pigment is inactive at temperatures above 93°F (34°C)The environment in which an organism lives profoundly affects its phenotypeSkin color and heightIn the womb, the baby is warm all over inactive enzyme born all whiteAfter birth, certain areas of the body remain cooler activates the enzyme darker color
25How Are Genes Located on the Same Chromosome Inherited? Genes on the same chromosome tend to be inherited togetherMendel’s law of independent assortment works only for genes whose loci are on different pairs of homologous chromosomesCharacteristics whose genes tend to assort together are said to be linkedExample from a sweet pea (not Mendel’s edible pea)Does NOT conform to Mendel’s law of independent assortmentflower-color genepollen-shape genepurpleallele, Plongallele, Lredallele, proundallele, l
26How Are Genes Located on the Same Chromosome Inherited? Crossing over creates new combinations of linked allelesGenes on the same chromosome do not always sort togetherCrossing over, or genetic recombination, in prophase I of meiosis creates new gene combinationsflower-color genepurple allele, Plong allele, Lred allele, pround allele, lpollen-shape genesisterchromatidshomologouschromosomes(duplicated)at meiosis I(a) Replicated chromosomes in prophase of meiosis I(b) Crossing over during prophase IPpLlOne or more regions where crossing over occurs.
27Crossing Over Recombines Alleles on Homologous Chromosomes recombinedchromatidsPLplunchanged(c) Homologous chromosomes separate at anaphase Ichromosomes(d) after meiosis IIThe farther apart the two genes, the more likely crossing over will separate them. Thus causing them to seem independent assorted.Each being a different gamete with different allelesFig c, d
28How Is Sex Determined? Sex chromosomes that dictate gender Y chromosomeHow Is Sex Determined?Sex chromosomes that dictate genderFemales have two X chromosomesMales have an X chromosome and a Y chromosomeA small section of the X and Y chromosomes is homologous, allowing them to pair in prophase I and segregate during meiosisAutosomes - rest of the (non-sex) chromosomesoccur in identical pairs
29Sex Determination in Mammals eggsfemale parentfemale offspringmale offspringmale parentX1X2XmYspermGender of the offspring is determined by the sperm (father).Fig
30How Are Sex-Linked Genes Inherited? Sex-linked genes are found only on the X or only on the Y chromosomeGenes carried on one sex chromosome, but not on the other, are sex-linkedIn humans, the X chromosome is much larger than the Y and carries over 1,000 genesIn contrast, the human Y chromosome is smaller and carries only 78 genesDuring embryonic life, the action of the Y-linked gene SRY sets in motion the entire male developmental pathwayUnder normal conditions, SRY causes the male gender to be linked 100 percent to the Y chromosome
31How Are Sex-Linked Genes Inherited? Sex-linked genes are found only on the X or only on the Y chromosomesFew of the genes on the X chromosome have a specific role in female reproductionMost of the genes on the X chromosome have no counterpart on the Y chromosomeSome genes found only on the X chromosome are important to both sexes, such as genes for color vision, blood clotting, and certain structural proteins in muscles
32How Are Sex-Linked Genes Inherited? Sex-linked genes are found only on the X or only on the Y chromosomesBecause females have two X chromosomes, recessive sex-linked genes on an X chromosome may or may not be expressedBecause males, with only one X chromosome, have no second copy to mask recessive genes, they fully express all the X-linked alleles they have, whether those alleles are dominant or recessive
33Sex-Linked Inheritance of Color Blindness eggsfemale parentfemale offspringmale offspringmale parentXCXcY(a) Normal color vision(b) Red-green color blindness(c) Expected children of a man with normal color vision (CY), anda heterozygous woman (Cc)Can’t distinguish red from greenspermColor deficiencyOne allele may be completely missing or a defective allele may cause both sets of cones to be sensitive to the same color.A person would not be able to distinguish red from green.Fig
34Family Pedigrees - show the genetic relationships among relatives How are Human Genetic Disorders Inherited?Family Pedigrees - show the genetic relationships among relatives(a) A pedigree for a dominant trait(b) A pedigree for a recessive traitHow to read pedigrees= generations= male= female= parents= offspringor= shows trait= does not show trait= known carrier (heterozygote) forrecessive trait= cannot determine the genotype fromthis pedigree?We can’t do experimental genetic crosses on humans. So geneticists study family records.Fig
35How Are Human Disorders Caused by Single Genes Inherited? Some human genetic disorders are caused by recessive allelesNew alleles produced by mutation usually code for nonfunctional proteinsAlleles coding for nonfunctional proteins are recessive to those coding for functional onesThe presence of one normal allele may generate enough functional protein to enable heterozygotes to be phenotypically indistinguishable from homozygotes with two normal alleles
36How Are Human Disorders Caused by Single Genes Inherited? Some human genetic disorders are caused by recessive allelesHeterozygous individuals are carriers of a recessive genetic trait (but otherwise have a normal phenotype)Recessive genes are more likely to occur in a homozygous combination (expressing the defective phenotype) when related individuals have childrenSickle-cell anemiaMutation in hemoglobin structureMalaria resistance in heterozygotesIt is not as likely for a non-related person to carry the same recessive alleles.
37Albinism Fig. 10-22 Results from a defect in melanin production. Recessive defectFig
38How Are Human Disorders Caused by Single Genes Inherited? Some human genetic disorders are caused by dominant allelesA dominant disease can be transmitted to offspring if at least one parent suffers from the disease and lives long enough to reproduceDominant disease alleles also arise as new mutations in the DNA of eggs or sperm of unaffected parentsDominant allele codes for an abnormal protein that interferes with the functional protein.or the new protein may carry out new, toxic reactions.or code for a protein that is overactive performing its function at inappropriate times and places in the body.
39How Are Human Disorders Caused by Single Genes Inherited? Some human genetic disorders are caused by dominant allelesHuntington disease is a dominant disorder that causes a slow, progressive deterioration of parts of the brainThe disease results in a loss of coordination, flailing movements, personality disturbances, and eventual deathThe disease becomes manifest in adulthood, ensuring its maintenance in the populationThe symptoms don’t show up until yrs of age.
40How Are Human Disorders Caused by Single Genes Inherited? Some human genetic disorders are sex-linkedThe X chromosome contains many genes that have no counterpart on the Y chromosomeBecause males have only one X chromosome, they have no other allele to exert dominance over a sex-linked (X-linked) allele causing diseaseConsequently, sex-linked diseases tend to occur in males
41How Are Human Disorders Caused by Single Genes Inherited? Some human genetic disorders are sex-linkedSex-linked disorders caused by a recessive allele have a unique pattern of inheritanceA son receives his X chromosome from his mother and passes it on only to his daughters, since the gene doesn’t exist on his Y chromosomeSex-linked genes typically skip generations because the affected male passes the trait to a phenotypically normal carrier daughter, who in turn bears affected sonsSeveral defective alleles for characteristics encoded on the X chromosome are known, including red-green color deficiency, muscular dystrophy and hemophiliaMuscular dystrophy – fatal degeneration of the muscles in young boys (use the muscles, they tear, die, and are replaced by fat)RecessiveIt is still around because the gene is very, very long making it more susceptible to mutations.
42Hemophilia Among the Royal Families of Europe unaffected maleunaffected femaleEdwardDuke of KentVictoriaPrincess ofSaxe-CoburgAlbertPrinceof Saxe-Coburg-GothaQueenof EnglandLouis IVGrand Duke ofHesse-DarmstadtAlicePrincessof HesseMaryElizabethAlexandraTsarinaFrederickErnestIreneOlgaTatianaMariaAnastasiaAlexisTsarevitchEdward VIIKing ofEnglandof DenmarkLeopoldDukeof AlbanyHelenWaldeck-PyrmontHenryPrince ofBattenburgBeatricepresent Britishroyal family(unaffected)AlexanderAlfonsoXIIof SpainMauriceCrownJuandiedininfancyMarieJaimeGonzalocarrierdaughterandhemophiliacgrandsonseveralunaffectedchidrencarrier femalehemophiliac maleNicholas IIof Russia?FigRecessiveDeficiency of a protein needed for blood clottingBruise easily and may bleed extensively from minor injuries.
43How Do Errors in Chromosome Number Affect Humans? The incorrect separation of chromosomes or chromatids in meiosis is known as nondisjunctionNondisjunction causes gametes to have too many and too few chromosomesMost embryos that arise from fusion of gametes with abnormal chromosome numbers spontaneously abort, but some survive to birth and beyondAn individual needs to have at least one X to survive.
44How Do Errors in Chromosome Number Affect Humans? Some genetic disorders are caused by abnormal numbers of sex chromosomes (continued)Turner syndrome (XO) occurs in females with only one X chromosomeAt puberty, hormone deficiencies prevent XO females from menstruating or developing secondary sexual characteristicsHormone treatment promotes physical development, but because affected women lack mature eggs, they remain infertileMore susceptible to recessive disorders such as red-green color blindness and hemophiliaAdditional symptoms include short stature, folds of skin around the neck, and increased risk of cardiovascular disease, kidney defects, and hearing loss1 in 3000 females
45How Do Errors in Chromosome Number Affect Humans? Some genetic disorders are caused by abnormal numbers of sex chromosomesTrisomy X (XXX) results in a fertile “normal” woman with an extra X chromosomeMost affected women show no abnormal symptomsThere is an increased chance of learning disabilities and a tendency toward tallnessBy some unknown mechanism that prevents an extra X chromosome from being included in their eggs, women with trisomy X bear normal XX and XY children1 in 1000 females
46How Do Errors in Chromosome Number Affect Humans? Some genetic disorders are caused by abnormal numbers of sex chromosomesMen with Klinefelter syndrome (XXY) have an extra X chromosomeMost afflicted males show no symptoms, although some may show mixed secondary sexual characteristics, including partial breast development, broadening of the hips, and small testesXXY men are often infertile because of low sperm count but are not impotent1 in 1000Don’t find out until they are trying to have children.
47How Do Errors in Chromosome Number Affect Humans? Some genetic disorders are caused by abnormal numbers of sex chromosomesMales with Jacob syndrome (XYY) have an extra Y chromosomeHave high levels of testosterone, tend to develop severe acne, and may be exceptionally tall, more susceptible to learning disabilities1 in 1000 males
48How Do Errors in Chromosome Number Affect Humans? Some genetic disorders are caused by abnormal numbers of autosomesNondisjunction of autosomes can occur during meiosis in the father or mother, resulting in eggs or sperm that are missing an autosome or that have two copies of an autosomeEmbryos with one or three copies of an autosome (trisomy) usually spontaneously abort; however, a small fraction of embryos with three copies of chromosomes 13, 18, or 21 survive to birthThe frequency of nondisjunction increases with the age of the parentsFrequency increases especially with the age of the mother.
49How Do Errors in Chromosome Number Affect Humans? Some genetic disorders are caused by abnormal numbers of autosomesIn trisomy 21 (Down syndrome), afflicted individuals have three copies of chromosome 211 in 800 birthsSymptoms: weak muscle tone, small mouth with normal sized tongue, distinctively shaped eyelidsheart malformations, low resistance to infectious diseases, and varying degrees of mental retardation.