Biology - An Illustrated Guide to Science (794127), страница 14
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Cattle thatbreed true for white coats have thegenotype WW.● If an RR individual is crossed with aWW individual, the F1 will be RW. Bothgenes are expressed, giving a roancoat with red and white hairs in it.● Crossing the RW individuals producesan F2 with the ratio1 red:2 roan:1 white. The red andwhite coats are homozygous, while theroans are heterozygous.© Diagram Visual Information Ltd.●WWRRParental genotypeGametesproducedby meiosisRRWspermspermWovumovumRWRWF1 genotypeRWRWF1 phenotype: all roanShorthorns with two identical alleles (i.e., RR or WW) have coats with solid red or white,respectively.● Their progeny (the F1 individuals) have dissimilar alleles (i.e., RW), and have a phenotypethat is intermediate with respect to the parental phenotypes.● Their coat has white hairs intermingled with the colored hairs, giving a roan appearance.● Mating between F1 individuals can bring the parental alleles together in certain offspring,and so the solid coat color reappears.●F1 phenotype:roan male,roan femaleRWF1 genotypeGametesproducedby meiosisRWWRRspermspermovumWWRWroanredF2 phenotype:1 red: 2 roan: 1 whiteovumRWRRF2 genotypeWroanwhite79Karyotype preparationCONTINUITYKey wordsKaryotype preparationColchicine is addedto stop cell divisionat metaphase.A blood sampleis removedfrom a donor.The sample issuspended insaline; red bloodcells settle out.chromosomediploidkaryotypemetaphaseWater is addedso that redblood cellsswell and burst.KaryotypesThe karyotype is a picture of thephysical form of the chrom osom esfound in normal body cells in aspecies.● Karyotypes usually show pairs ofchromosomes corresponding to thediploid number for the species.
Thediploid number is the number ofchromosomes in a normal cell. Whereorganisms have more than two sets ofchromosomes (e.g., wheat plants havesix), the karyotype is morecomplicated.●The cells are observedunder a microscope.ColchicineAfter a sample of m etaphase cells(usually blood cells) has beenextracted from an organism, it ispoisoned with colchicine. Colchicineinterferes with cell division, causing itto stop at metaphase, when all of thechromosomes are visible.● The poisoned cells are broken openand placed on a slide—often with adye to stain the chromosomes to makethem easier to see.●The cells arespread ontoa slide.Sorting the karyotypeA microscope with a cameraattachment is used to photograph aselection of cells that show thechromosomes.● The photograph is cut up so that thechromosomes spread randomly withina dividing cell can be arranged in pairson a piece of paper.
Only one cell isused to create the karyotype, butphotographs of other cells can provideuseful extra information if some of thechromosomes are overlapping andobscuring each other.The chromosomes arephotographed.Individualchromosomesare cut out.Chromosomes are arranged indiminishing order of size.© Diagram Visual Information Ltd.●80CONTINUITYKey wordscentromerechromatidchromosomekaryotypeHuman chromosomesHuman karyotypeFemale karyotypeChromosomesChrom osom es are structures made ofDNA and protein.
They are normallyinvisible in the cell but shorten andthicken during cell division to becomevisible as small X-shaped bodies.● The “limbs” of the X are calledchrom atids. The point where theyjoin, the center of the X, is called thecen trom ere.●Human karyotypeA karyotype is a picture of thechromosomes as they appear duringcell division. Karyotypes are normallyarranged to show similarchromosomes in groups or series.● The human karyotype shows 22 pairsof chromosomes and one “pair” thatconsists of two chromosomes that arethe same in females (XX) but slightlydifferent in males (XY).● The “pair” of chromosomes that aredifferent in males and females arecalled the sex chromosomes and dealwith the inheritance of gender inhumans.
All of the chromosomes thatare not sex chromosomes are calledautosomes.1326137814159410161751112181920●21XXSex chromosomesMale karyotype12613© Diagram Visual Information Ltd.22714381591621410171118X121922YSex chromosomes52081Human sex inheritanceCONTINUITYKey wordsSex inheritancemalechromosomegametegenotypephenotypefemaleSexual characteristicsParentalphenotypeThere are many characteristics thatdefine “maleness.” Some arestructures that are not present infemales, e.g., penis, testes. Some areslight differences in structures thatexist in both genders, e.g., facial hair.● Such a large package of characteristicscannot be controlled by a single gene.However, all of these characteristicsare inherited as a package that impliesthey are linked together in some way.●Parental genotypeXYX and Y sexchromosomesXXX sexchromosomesGametes producedby meiosisXYXXspermspermovumovumThe sex chromosomesOne pair of chrom osom es is differentin males and females.
These are thesex chromosomes and contain thepackage of genes that determinewhether an individual is male orfemale.● Females have two X chromosomeswhile males have one X and a shorterY chromosome.●Sex inheritanceA male will produce gam etes thatcontain one chromosome from eachhomologous pair found in normalbody cells. This means that half of thegametes will have a single Xchromosome and half will have thecorresponding Y chromosome.● Females, with two X chromosomes,only produce gametes with X.● If a sperm carrying an X chromosomejoins with an egg, it produce XX—afemale cell. A sperm containing a Ychromosome would produce XY—amale.F1 genotypeXXXXfemalefemaleXYXYF1 phenotypemalemale© Diagram Visual Information Ltd.●82CONTINUITYKey wordsfactor VIIIHuman sex linkage:hemophiliaSex linkagemalefemaleThe sex chromosomesFemales have two X chromosomeswhile males have one X and a shorterY chromosome.
Genes on thesechromosomes are described as sexlinked.● Traits such as colorblindness andhemophilia are sex linked.●Hemophilia●Hemophilia is a condition in whichblood does not clot. This leads toserious bleeding following even smallcuts. Hemophilia is caused by a lack ofa blood chemical called factor VIII.The gene that codes for factor VIII ison the X chromosome (Hx).The Y chromosome, which is shorter,is missing the corresponding part.ParentalphenotypeParental genotypeHXYHXhXnormal malecarrier femaleGametes producedby meiosisHXspermYspermHXhXovumovumCarriers and sufferersA woman with a faulty gene on one Xchromosome ( h X) will produce factorVIII. She will not suffer from the illnessbut could pass it on to her offspringthrough the damaged gene on one Xchromosome.● The shorter Y chromosome has nospace for the factor VIII gene.
If a malehas the defective gene on his single Xchromosome, he suffers from thedisease.● Males get an X chromosome fromtheir mother and a Y chromosomefrom the father. A father cannot passon the faulty factor VIII gene becausehe does not pass on an Xchromosome.© Diagram Visual Information Ltd.●F1 genotypeHXHXHXhXHXYhXYnormal femalecarrier femalenormal malehemophiliac maleF1 phenotypefemalefemalemalemale83AmniocentesisCONTINUITYKey wordsAmniocentesisamnionDown syndromefetusSample of amniotic fluid, including cellssyringeplacentauterine wallThe amniotic fluid●fetusamnioticfluidThe growing fetu s is protected by fluidproduced by a membrane called theam n ion .
This amniotic fluid containsfetal cells floating freely around in it.Amniocentesis●umbilicalcordAmniotic fluidcentrifugedamnionsupernatant(soluble liquid)cervixvaginacellsCells grown innutrient solutionAround the 16th week of pregnancy, afine needle is inserted into theamniotic sac and a small sample offluid containing some fetal cells iswithdrawn. The technician carryingout this procedure uses ultrasoundimaging to direct the needle—if itwere to scratch the fetus, it couldcause potentially serious complicationsfor both mother and child. For thisreason, the benefits to be gained fromthe procedure must be weighedcarefully against the risks.Analysis of fluidThe amniotic fluid sample iscentrifuged to separate out thefloating cells.
These are cultured andcan be tested for a variety ofabnormalities. One of the commonesttests is for Down syn drom e.● In Down syndrome the cells of thefetus have 47 chromosomes instead of46. The extra chromosome is foundwith the 21st chromosome. As thebaby develops, a number of problemsarise. He or she will grow slowly andfail to reach full adult developmenteither physically or mentally.culture dishChromosomal analysisCultured cellsDown syndrome karyotypeBiochemicalanalysis© Diagram Visual Information Ltd.●84CONTINUITYKey wordsalleledominantgenegenotypeInheritance of bloodgroupsInheritance of blood groupsI AI A x iOiOParental phenotypeMultiple allelesAlleles are different forms of the samegen e.
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