Biology - An Illustrated Guide to Science (794127), страница 13
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If cells leak across thebarrier, they will produce a rapid andpotentially fatal immune response.●maternalveinmaternalarterybloodspace(lacuna)placentalvilliuterine wallplacentaumbilical arteryumbilical cordumbilical veinStructure of the placentaThe placenta connects the fetus to themother by the u m bilical cord, whichcontains an artery and a vein.● The am n ion is a structure created bythe placenta that covers the fetus in abag of amniotic fluid. When thisbursts—the waters break—it is a signthat the birth is close.● Blood from the mother fills largespaces called lacunae, villi from thefetus penetrate. This gives a very largesurface area across which exchangetakes place while keeping the bloodfrom each individual separate.●© Diagram Visual Information Ltd.Detail of placenta74CONTINUITYKey wordscervixfetusuterusBirthFirst stage of labor2 Cervix flattens completely1 Cervix starts to flattenuterine wallpubicbonePresentation of the fetusThe head is the heaviest part of thefetu s, and toward the end ofpregnancy it will fall to rest against theinside of the cervix.
The dropping ofthe head into this position is visible asthe mother’s shape changes slightly.● During the late stages of pregnancy,the mother will also start to feelcontractions. These are musclemovements in the u teru s wall as itprepares to expel the baby.●backbonecervixrectumvagina3 Cervix partially opens4 Cervix fully opens; amnion breaksamnionLaborThe stage prior to birth is called labor.It can last from as little as an hour to afew days. Labor extending beyond 48hours is a sign of potential problems.● During labor the waters break (this isthe bursting of the amnioticmembrane), and the cervix begins toopen or dilate.●BirthWhen the baby is ready to be born,the cervix is dilated sufficiently to letthe head pass through.
The baby’shead will rotate slightly to fit moreeasily through the cervix.● After the head and shoulders havepassed through the cervix, the rest ofthe body normally follows quickly.● The last stage is the delivery of theplacenta, which typically followsmoments after the baby’s birth.●Second stage of labor5 Head rotatesThird stage of labor© Diagram Visual Information Ltd.7 Delivery of placenta6 Head is born, shoulders andrest of body follow75VariationCONTINUITYKey wordsHeight in human males as an example of continuous variationcontinuousvariationdiscontinuousvariationTypes of variationThe term “variation” is used todescribe differences betweenmembers of the same species.● There are two types of variation:continuous and discontinuous.●Bar graph of continous variation in heightContinuous variationHeight and weight are examples ofcon tin u ou s variation . This type ofvariation is produced by the action ofmultiple genes, and there are no cleargroups or classes to put individualsinto.
Individuals in a population showa complete spectrum of valuesbetween two extremes.● To investigate continuous variation,biologists assign classes to the data,e.g., all values between 60 and 64inches in height, values between 64and 68 inches etc. The frequency ofindividuals within these arbitraryclasses can be plotted.number of individuals●increasing heightDiscontinuous variation in humansDiscontinuous variationEye and hair color in mammals, abilityto roll the tongue or taste certainchemicals in humans, and flower colorin peas are examples of discon tin u ou svariation .
Discontinuous variation isproduced by the action of single or asmall number of genes.● Discontinuous variation can easily besorted into groups or classes and canbe displayed by histograms.●free earlobes andattachedear lobesfemaleandmaletonguerollingtonguelipslongandshortstemssmoothandconstrictedpodsroundandwrinkledseeds© Diagram Visual Information Ltd.Discontinuous variation in peas76CONTINUITYKey wordsdominantgenotypehomozygousphenotyperecessiveMonohybrid cross: peasMonohybridslong-stemmed(tall) plantParentalphenotypeshort-stemmed(dwarf) plantMonohybrid crossesA monohybrid cross is a cross betweentwo organisms that differ by only oneinherited characteristic controlled by asingle gene with two forms.● The gen otype is the combination ofgenes in an organism.
The phen otypeis the physical characteristics of anorganism produced by the expressionof its genotype.● In certain types of pea, the height ofthe adult plant, one aspect of itsphenotype, is controlled by a singlegene with two forms: tall and short.● If an organism has two forms of thesame gene at the same time, thedom in an t form is expressed. Therecessive form is present but notvisible in the adult form of the plant.●First generation (F1)Pure (hom ozygou s) breeding plants ofthe two forms (tall and short) arecrossed.
The seeds produced by thiscross are then collected and sown toproduce the first generation or F1.● All of the plants in the F1 generationare tall. This shows that the tall gene isdominant to the short gene.●Second generation (F2)If the F1 plants are allowed to selfpollinate and the seeds produceplants, the plants produced are the F2.● The ratio of tall:short plants in the F2generation is 3:1.● The genotypes TT, Tt, tT all producetall plants because a dominant tallgene is present. Only the tt genotypeproduces a short plant. This explainswhy tall plants are three times morelikely than short plants.© Diagram Visual Information Ltd.●Parental genotypettTTGametesproducedby meiosisTTttF1 genotypeTtTtTtTtF1 phenotype:all long-stemmedF1 phenotypelong-stemmed (tall)TtF1 genotypeTtGametesproducedby meiosisTtTtF2 genotypeTTTttTttlong-stemmedlong-stemmedlong-stemmedshort-stemmed77Dihybrid cross:guinea pigsCONTINUITYKey wordsalleledominantgenotypephenotypeDihybridsThe F1 guinea pigs have two different alleles for each of the two characters (i.e., coat colorand coat length) symbolized as BbSs.
They are said to be dihybrids.Parental phenotypesBBSSParental genotypeGametesproducedby meiosisblack,short-hairedBSbbssBSspermbrown,long-hairedbsbsspermovumovumDihybrid crossesDihybrid crosses are crosses that lookat the inheritance of two independentcharacteristics, each of which iscontrolled by a single gene that existsin two forms.● The coat of guinea pigs is controlledby two genes: black/brown color andlong/short length.●First generation (F1)●F1 genotypeBbSsBbSsBbSsBbSsIn the first generation the phen otypewas entirely black short-haired guineapigs. This shows that black isdom in an t to brown and short isdominant to long hair.Second generation (F2)F1 phenotype: all black, short-hairedWhen the F1 generation was allowedto cross breed, the F2 gen otypes werein the ratio 9:3:3:1 of black, shorthaired:black, long-haired:brown, shorthaired:brown, long-haired.● The distribution of genotypes showedthat the genes were inheritedindependently, with the rules formonohybrid crosses, which look atone characteristic, operating on eachgene pair separately.●F1 phenotypes(black, short-haired)BbSsF1 genotypesGametesproducedby meiosisBSBsbSPunnett square showingpossible offspring inthe F2 generationBbSsbsBSspermovaBsbSbsBSBsbSbsBBSSBBSsBbSSBbSsBBSsBBssBbBsBbssBbSSBbBsbbSSbbSsBbSsBbssbbSsbbssBSBsbsF2 phenotyperatios9:3black,short-haired:3black,long-haired:1brown,short-hairedbrown,long-haired© Diagram Visual Information Ltd.bS78CONTINUITYKey wordsalleledominantgenotypeheterozygoushomozygousCodominanceCodominance and coat colorphenotypeDominanceGenes exist in pairs.
Where bothmembers (alleles) of the pairs are thesame, the individual is said to behom ozygou s. If the two alleles aredifferent, the individual is said to beheterozygou s.● In heterozygous individuals, one alleleis usually dom in an t and the other isrecessive. This means that when asingle dominant allele is present, thephen otype produced will be identicalto that of an individual with twodominant alleles. In effect, therecessive allele is hidden and can onlybe detected by breeding with anotherheterozygous organism.● In codominance, the differing allelesboth have an effect on the phenotype.●●The F1 phenotype is intermediate with respect to the parental phenotypes.Neither parental allele (R or W) can exert its dominance over the other.Parentalphenotype:red male,white female●Coat color in shorthornsCoat color in shorthorn cattle iscontrolled by a single pair of genesthat show codominance.● Cattle that breed true for red coatshave the gen otype RR.
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