ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Thirteen Outline THE FACES OF VARIATION Variation in Appearance Of humans fig 13.1 Variation among dogs fig 13.2 Sources of Variation Differences in diet during development Variation in environment, color of arctic mammal coat fig 13.3 Similarities within families fig 13.4 EARLY IDEAS ABOUT HEREDITY: THE ROAD TO MENDEL Early Genetic Concepts Heredity occurs within species Cannot create bizarre creatures by cross breeding Common animals are not combinations of breeding Traits are transmitted directly Once thought body parts transmitted in sex cells Male and female traits blended in offspring Resultant paradox If no variation enters from outside species If variation blended with each generation In time, would result in little species variation Koelreuter Experiments Hybridized tobacco plants Offspring appeared different from either parent Crosses of hybrids resulted in further variation Offspring resembled parents or grandparents Parental traits not blended Traits masked for a generation, reappeared in next Alternative forms segregating among offspring T. A. Knight Experiments Crossed true-breeding peas, purple and white flowers fig 13.5 All offspring of first cross had purple flowers Offspring of next cross had both color flowers Purple flowers predominated over white flowers Early, Pre-Mendel Genetic Concepts Some forms of inherited traits masked in one generation Forms of a trait segregate among offspring Some forms represented more frequently than others MENDEL AND THE GARDEN PEA Carried Out First Quantitative Studies fig 13.6 Used Garden Pea Familiar to Earlier Investigators Expected segregation among offspring, via early studies Many true-breeding traits, studied only seven fig 13.7 Small plants, easy to grow, short generation time Male and female parts within flower fig 13.8 Self-fertilized male and female from same flower Cross-fertilized female with other flower's pollen (male) MENDEL'S EXPERIMENTAL DESIGN Allowed Several Generations of Self-Fertilization Progeny produced only a single form of a trait Assured that forms of traits were transmitted regularly Conducted Crosses Between Alternate Forms of a Trait fig 13.9 Removed male parts from a flower with white flowers Fertilized with pollen from plant with purple flowers Performed reciprocal crosses white flower pollen on purple flower plant Allowed Self-Fertilization of Hybrids Allowed segregation of alternate forms of traits Counted number of offspring of each type per generation Quantification of results most important to studies WHAT MENDEL FOUND First Filial (F1) Progeny Resembled One of Parents fig 13.10 Trait expressed in F1 called dominant Trait masked in F1 called recessive All seven traits had dominant and recessive forms Planted F1 Seeds To Produce F2 (Second Filial) Generation Determined proportion of dominant to recessive Three fourths of plants exhibited dominant form One fourth of plants exhibited masked, recessive form Dominant:recessive ratio was close to 3:1 for all seven traits Subsequent Generations Recessive individuals bred true One third of dominant individuals bred true Two thirds of dominant individuals produced 3:1 progeny 3:1 ratio really 1:2:1 ratio, separating dominant genotypes HOW MENDEL INTERPRETED HIS RESULTS Understood Four Things About Nature of Heredity Alternatives of traits are inherited intact One form did not appear in F1, but reappeared in F2 Pairs of alternative forms segregated among progeny Characteristic Mendelian Ratio of segregation is 3:1 fig 13.11 Mendel's Model Parents transmit factors that provide information about traits Each individual contains two factors for each trait May code for same form or alternative forms Diploid set of chromosomes in individuals Haploid chromosomes randomly distributed in gametes Not all copies of a factor are identical Alternate forms of factor called alleles Individual is homozygous when both alleles are the same Individual is heterozygous when alleles are different Position of gene on DNA is called its locus Alleles from each parent do not influence one another They remain discrete and "uncontaminated " They do not blend with one another They further segregate randomly when forming progeny Presence of a factor does not insure its expression Heterozygote dominant expressed, recessive unexpressed Genotype is the totality of the genes (blueprint) Phenotype is the expression of the genes (outcome) Complete Dominance of One Allele Over Another fig 13.12 Exhibited in all seven traits studied by Mendel Exhibited by many human traits tbl 13.1 The F1 Generation Use letter of recessive to name allele Dominant trait is upper case letter (W) purple Recessive trait is lower case letter (w) white Designation of alleles in individuals True-breeding white flower = ww True-breeding purple flower = WW Heterozygous purple flower = Ww Mendel's first cross = ww x WW fig 13.13 Each parent can produce gametes of only its kind Purple gametes contain only W allele White gametes contain only w allele Resulting progeny all Ww, W dominant, all purple The F2 Generation All are heterozygous, purple, cross = Ww x Ww Alleles segregate randomly in gametes, either W or w Construct Punnett square to determine progeny of cross fig 13.14 Square predicts 3:1 phenotypic ratio Further Generations Three kinds of F2 individuals Pure-breeding white flowers (ww) Heterozygous purple flowers (Ww) Pure-breeding purple flowers (WW) Closer examination of 3:1 ratio indicates 1:2:1 genotypic ratio fig 13.13 THE TESTCROSS Used to Determine Genotype of Dominant Phenotype Observing phenotype insufficient, WW and Ww appear same Cross unknown to organism of known lineage fig 13.15 Homozygous dominant (WW) produces dominant phenotype (Ww or WW) Heterozygous (Ww) produces all possible genotypes of offspring (WW, Ww, ww) Homozygous recessive as known (ww) All Ww offspring indicates WW unknown Half Ww, half ww offspring indicates Ww unknown Experimental cross with homozygous recessive called a testcross Mendel's First Law of Heredity: Law of Segregation Explained segregation without cellular knowledge Behavior of alternative alleles Alternative forms encoded by discrete alleles Alternative alleles separate in gametes formation Each gamete has equal possibility to get either allele INDEPENDENT ASSORTMENT Mendel Questioned Effect of Traits Upon One Another Establish pure-breeding lines differing in two traits Cross contrasting pairs of traits Results in F1 generation of identical dihybrids Dihybrids are individuals heterozygous for two genes Allow dihybrids to self-fertilize 1/4 chance for a single trait to occur 1/4 x 1/4 = 1/16 for any pair to occur Predicts 9:3:3:1 ratio fig 13.16 Mendel's Second Law of Heredity: Law of Independent Assortment Genes located on different chromosomes assort independent of one another Mendel picked traits on different chromosomes FROM GENOTYPE TO PHENOTYPE: HOW GENES INTERACT Complex Genetic Patterns Multiple alleles: more than two alleles Gene interaction: many genes act sequentially or jointly Epistasis: one gene modifies expression of other gene Continuous variation: multiple genes act jointly fig 13.17 Pleiotropy: gene has more than one effect on phenotype Incomplete dominance: alternative alleles not dominant or recessive fig 13.18 Environmental effects: modify gene products fig 13.3 Modified Mendelian Ratios Difficult to determine phenotypic classes Example corn seed coat pigment CHROMOSOMES: THE VEHICLES OF MENDELIAN INHERITANCE Many Organelles Segregate in Meiosis Sutton's Explanation Hereditary material resides in nucleus Chromosomes segregate in meiosis Two copies of each chromosome in adult forms Homologous chromosomes assort in meiosis SEX LINKAGE Proof of Chromosomal Theory of Inheritance Discovery of mutant, white-eyed male fruit fly fig 13.19 Crossed with wild type red-eyed female All progeny had red eyes, concluded red eye color dominant Cross of F1 generation 3:1 ratio red to white eyes All recessive white eye flies were male Testcross F1 to white-eyed male 1:1:1:1 ratio Eye color and sex equally represented Explanation: eye color gene related to sex chromosome Eye Color Gene Located on Sex Chromosome in Fruit Flies fig 13.20 Two kinds of sex chromosomes, X and Y XX = female, XY = male Eye color gene located on the X chromosome Sex linked trait CROSSING OVER More Independently Assorting Factors Than Chromosomes Janssen's X configuration of chromosomes during meiosis fig 12.6 Mechanism for exchange of genetic material: Stern fig 13.21 Physical change in chromosomes Observed corresponding change in genetic traits Crossing over can occur at anywhere along chromosome Independent assortment more likely if genes are far apart fig 13.22 Genetic Maps fig 13.23 Distance between genes = frequency of crossing over Map unit, centimorgan = 1% recombination Monitor recombination among three or more genes Wild type is most frequent allele of a locus Syntenic alleles located on same chromosome Linked genes do not assort independently Three-point cross occurs with three linked genes Human genetic maps used to determine genetic disorders fig 13.24 MULTIPLE ALLELES Most Genes Possess More Than Two Possible Alleles ABO Blood Groups Three alleles affect cell surface antigens Gene designated I Allele B codes for galactose Allele A codes for galactosamine Allele O codes for neither sugar A and B are codominant and can be expressed together A and B are both dominant over O Four phenotypes produced from three alleles Type A: genotype AA or AO Type B: genotype BB or BO Type AB: genotype AB Type O: genotype OO Blood may agglutinate due to presence of antigens fig 13.25 Type A recognizes type B blood with B antigens Type A recognizes type AB blood with A and B antigens Type A does not recognize type O blood, no antigens Type AB does not recognize either A or B as foreign The Rh Blood Group Associated with presence of Rh cell surface markers Rh+ possess marker, most adult humans Rh- lacks marker, fewer in number Rh- is homozygous recessive condition Blood may agglutinate due to presence of antigens Rh- mother, Rh+ child (Rh+ father) Rh+ blood crosses placenta into mother's blood Induces production of anti-Rh antibodies in mother's blood In later pregnancy, Rh antibodies can cross back Cause next baby's blood to clump: erythroblastosis fetalis HUMAN CHROMOSOMES Morphology of Human Chromosomes 46 chromosomes in 23 pairs Divided into seven groups fig 11.6 Sex Chromosomes 22 pairs of autosomes, 2 sex chromosomes XY is normal male Y has few active genes, counterparts to X alleles Genes for maleness present on Y Male possesses at least one Y XX is normal female Female possesses no Y chromosome One X inactivated in form of Barr body Other X active and expressed, activity of X is random in each cell HUMAN ABNORMALITIES DUE TO ALTERATIONS IN CHROMOSOME NUMBER Primary Nondisjunction Caused by failure of chromosomes to separate in meiosis Can result in severe abnormalities Down Syndrome Monosomics possess one less copy of an autosome Trisomics possess one extra copy of an autosome Most do not survive Down syndrome results from extra chromosome 21 fig 13.26 Affects physical and mental development Arises from primary nondisjunction during meiosis More likely to occur in pregnancy of older women fig 13.27 Nondisjunction Involving the Sex Chromosomes The X chromosome fig 13.28 Produces XX gamete and O gamete XX plus normal X results in XXX individual Two Barr bodies, one active X Sterile, but otherwise normal female XX plus normal Y results in XXY individual Kleinfelter syndrome Sterile male with female characteristics O plus normal Y results in inviable YO individual O plus normal X results in XO individual Turner syndrome Sterile female with characteristic appearance The Y Chromosome Produces YY gametes YY plus normal X results in XYY individual Fertile males with normal appearance Greater numbers of individuals in penal institutions HUMAN GENETIC DISORDERS tbl 13.1 Variant Alleles May Be Produced by Mutations Detrimental alleles are generally rare in populations Can become more populous in isolated communities Are frequently homozygous recessive diseases Are maintained in populations in heterozygous carriers Genetic disorder: detrimental gene at high frequency in population Cystic Fibrosis fig 13.29 Most common genetic disorder in Caucasians 1 in 20 carry single copy of defective gene 1 in 1800 are homozygous recessive, exhibit disease Affected individuals secrete clogging mucus Defect in transport of chloride ions across membranes Sickle-Cell Anemia Improper transport of oxygen due to defective hemoglobin Results from alteration in single amino acid Red blood cells become stiff and sickle-shaped fig 13.30 Blood cells clog blood vessels, are unable to enter small vessels Disorder of homozygotes but heterozygotes slightly affected Most common disorder among those of African descent Tay-Sachs Disease fig 13.31 Causes fatal brain deterioration in children Allele codes for nonfunctional form of enzyme Cannot degrade gangliosides in brain cell lysosomes Lysosomes swell and burst, killing brain cells Highest occurrence in Jewish populations 1 in 28 in specific population carry defective gene 1 in 3600 of same population exhibit disease 1 in 300,000 of overall population exhibit disease Phenylketonuria Abbreviated PKU Affected individuals unable to break down phenylalanine Converted to other chemicals that accumulate in blood Interfere with development of brain cells in infants Can be treated by controlling amino acid intake In US, 1 in 15,000 are homozygous recessive Hemophilia Loss of activity in blood clotting factors Disorder due to recessive condition Most clotting proteins located on autosomes Two (VII and IX) located on X chromosome More prominent in males since they possess only one X If X defective, no proteins made Y lacks comparable allele Most common form has defective IX fig 13.32 Called Royal hemophilia, prominent in family of Queen Victoria fig 13.33 Carried into royal families of Europe fig 13.34 Huntington's Disease fig 13.35 Hereditary condition caused by dominant allele Causes progressive deterioration of brain cells Maintained in population, 1 in 10,000 affected Symptoms develop after reproductive activity Allele transmitted prior to its expression GENETIC COUNSELING In Absence of Cures Seek to Not Produce Children With Disorders Genetic counseling Identify parents at risk Assess genetic state of early embryos High risk of Down syndrome in older women fig 13.27 Prenatal Diagnosis of Disorders Amniocentesis fig 13.36 Sample amniotic fluid during fourth month Observe fetus and position via ultrasound fig 13.37 Fetal cells grown in culture Cells examined for major chromosomal damage Chorionic villi sampling Sample placental tissue Can be performed earlier than amniocentesis at eight weeks Tests for genetic disorders Enzyme activity tests Association with genetic markers Cut DNA with restriction enzymes Observe restriction fragment-length polymorphisms, RFLPs fig 13.38 Identify heterozygotes Genetic Therapy May recommend termination of pregnancy if severe Treatable disorders (PKU) controlled by special diets