Nature and Nurture: TheEnvironmental Impact onPhenotype• Another departure from Mendelian genetics ariseswhen the phenotype for a character depends onenvironment as well as genotype• The norm of reaction is the phenotypic range of agenotype influenced by the environment• For example, hydrangea flowers of the samegenotype range from blue-violet to pink, dependingon soil acidity• Norms of reaction are generallybroadest for polygenic characters• Such characters are called multifactorialbecause genetic and environmentalfactors collectively influence phenotypeIntegrating a Mendelian View ofHeredity and Variation• An organism’s phenotype includes itsphysical appearance, internal anatomy,physiology, and behavior• An organism’s phenotype reflects itsoverall genotype and uniqueenvironmental historyConcept 14.4: Many human traitsfollow Mendelian patterns ofinheritance• Humans are not good subjects for geneticresearch because generation time is too long;parents produce relatively few offspring; andbreeding experiments are unacceptable• However, basic Mendelian genetics endures asthe foundation of human geneticsPedigree Analysis• A pedigree is a family tree thatdescribes the interrelationships ofparents and children across generations• Inheritance patterns of particular traitscan be traced and described usingpedigreesLE 14-14aWw ww ww WwWw wwWwwwwwWwWW wworWwNo widow’s peakThirdgeneration(two sisters)Widow’s peakSecond generation(parents plus auntsand uncles)First generation(grandparents)Dominant trait (widow’s peak)• What is the probability that individual C-1is Ww?– A. 3/4– B. 1/4– C. 2/4– D. 2/3– E. 1LE 14-14bFirst generation(grandparents) Ff FfFF or Ff ff ff Ff FfffffFfSecond generation(parents plus auntsand uncles)Thirdgeneration(two sisters)Attached earlobeFree earlobeff FForFfRecessive trait (attached earlobe)• Pedigrees can also be used to makepredictions about future offspring• We can use the multiplication andaddition rules to predict the probabilityof specific phenotypesRecessively InheritedDisorders• Many genetic disorders are inherited ina recessive manner• Recessively inherited disorders showup only in individuals homozygous forthe allele• Carriers are heterozygous individualswho carry the recessive allele but arephenotypically normalCystic Fibrosis• Cystic fibrosis is the most common lethal geneticdisease in the United States,striking one out ofevery 2,500 people of European descent• The cystic fibrosis allele results in defective orabsent chloride transport channels in plasmamembranes• Symptoms include mucus buildup in some internalorgans and abnormal absorption of nutrients in thesmall intestineSickle-Cell Disease• Sickle-cell disease affects one out of 400African-Americans• The disease is caused by the substitution ofa single amino acid in the hemoglobinprotein in red blood cells• Symptoms include physical weakness, pain,organ damage, and even paralysisDominantly InheritedDisorders• Some human disorders are due todominant alleles• One example is achondroplasia, a formof dwarfism that is lethal whenhomozygous for the dominant alleleMultifactorial Disorders• Many diseases, such as heart disease andcancer, have both genetic and environmentcomponents• Little is understood about the geneticcontribution to most multifactorial diseasesGenetic Testing andCounseling• Genetic counselors can provide information toprospective parents concerned about a familyhistory for a specific diseaseLE 15-2P GenerationGametesMeiosisYellow-roundseeds (YYRR)FertilizationGreen-wrinkledseeds (yyrr)All F1 plants produceyellow-round seeds (YyRr)MeiosisTwo equallyprobablearrangementsof chromosomesat metaphase IAnaphase ILAW OF INDEPENDENT ASSORTMENTMetaphase IIFertilization among the F1 plantsF2 GenerationGametesF1 GenerationLAW OF SEGREGATIONMorgan’s ExperimentalEvidence: Scientific Inquiry• The first solid evidence associating aspecific gene with a a specificchromosome came from Thomas HuntMorgan, an embryologist• Morgan’s experiments with fruit fliesprovided convincing evidence thatchromosomes are the location ofMendel’s heritable factorsMorgan’s Choice ofExperimental Organism• Characteristics that make fruit flies aconvenient organism for geneticstudies:– They breed at a high rate– A generation can be bred every two weeks– They have only four pairs of chromosomes• Morgan noted wild type, or normal,phenotypes that were common in the flypopulations• Traits alternative to the wild type arecalled mutant phenotypesCorrelating Behavior of a Gene’sAlleles with Behavior of aChromosome Pair• In one experiment, Morgan mated male flieswith white eyes (mutant) with female flies withred eyes (wild type)– The F1 generation all had red eyes– The F2 generation showed the 3:1 red:whiteeye ratio, but only males had white eyes• Morgan determined that the white-eye mutantallele must be located on the X chromosome• Morgan’s finding supported the chromosometheory of inheritanceLE 15-4PGenerationF1GenerationF2GenerationPGenerationF1GenerationOva(eggs)SpermF2GenerationOva(eggs)Sperm• Vermilion eyes is a sex-linked recessivecharacteristic in fruit flies. If a female havingvermilion eyes is crossed with a wild-typemale, what percentage of the F1 males willhave vermilion eyes?– A. 0%– B. 25%– C. 50%– D. 75%– E.
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