Reading Essentials for Biology Glencoe (794133), страница 35
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A test cross is a cross of an individual of unknowngenotype with an individual of known genotype.3. What is a test cross?________________________________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.How do you determine genotype?Let’s use Alaskan malamute dogs for our example of how todetermine genotype. There is a recessive trait that causes dwarfismin Alaskan malamutes. For a malamute to be born a dwarf, thedog would have to have two recessive alleles (homozygous recessive) for that trait.
Before breeding an Alaskan malamute, abreeder makes sure the dog does not carry the trait for dwarfism.Because the dog is normal size, the breeder knows the dog musthave either two dominant alleles (homozygous dominant), or adominant and a recessive allele (heterozygous). To perform a testcross, the breeder will mate a dwarf dog (homozygous recessive)with the unknown dog.
What are the possible genotypes ofthe offspring?A Punnett square will predict possible genotypes of the offspring (see the illustration on page 142). If the unknown dog hastwo dominant alleles for normal height (homozygous dominant),the offspring will all be heterozygous and normal height. If theunknown dog has one dominant and one recessive allele (heterozygous), half the offspring will be heterozygous and normalheight. The other half will be dwarfs because they will have tworecessive alleles (homozygous recessive).READING ESSENTIALSChapter 13141NameDateClassSectionApplied Genetics, continuedA?ⴛBHomozygous ⴛ HomozygousIf the unknown dog’sDDddgenotype is homozygousdddominant, all of theoffspring will bephenotypically dominant.D DdDdDDdDdCThe unknown dog can beeither homozygous dominant(DD) or heterozygous (Dd) forthe trait.ddHeterozygous ⴛ HomozygousIf the unknown dog’sDdddgenotype is heterozygous,ddhalf the offspring willexpress the recessivetrait and appear dwarf.D DdDdThe other half will expressthe dominant trait anddddddbe of normal size.Offspring: all dominantOffspring:1/2 dominant1/2 recessiveDdDdDdddRemember from the beginning of this section that selectivebreeding increases the frequency of desired traits in a population.In our Alaskan malamute example, the breeder would only continue to breed the dog if the results of the test cross proved thatthe dog did not carry the recessive trait.◗ After You ReadMini Glossaryinbreeding: mating closely related individuals;ensures that offspring are homozygous formost traits, but also brings out harmful,recessive traits142Chapter 13test cross: mating of one member of a species ofunknown genotype with another memberof the species of known genotype; can helpdetermine the unknown genotype of theparentREADING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.13.1NameDateClassSection13.1Applied Genetics, continued1.
Read the terms and their definitions from the Mini Glossary on page 142. Write a sentencefor each term, using the term correctly.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________2. Place the information found below the table under the correct heading in the table.Selective BreedingCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.a.b.c.d.Determining GenotypesUses test crossesProduces hybrids that are usually bigger and betterUses inbreedingWorks best if known individual is homozygous recessive3.
Write a paragraph that explains how applied genetics has helped improve human life.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Visit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about applied genetics.READING ESSENTIALSChapter 13143NameDateClassSection13.2 Recombinant DNA Technology◗ Before You ReadSC.H.3.4.2 The student knows that technological problems often create a demand for newscientific knowledge and that new technologies make it possible for scientists to extendtheir research in a way that advances science.
Also covers SC.H.3.4.3, SC.H.3.4.5, SC.H.3.4.6This section discusses the technology that allows scientists to combine the DNA of one organismwith the DNA of another organism. The result can be organisms with new characteristics. Whatwould you think of a plant that glows like a firefly or a cotton crop that produces its own insecticide?On the lines below, list some examples of useful changes to an organism that you would like to seemade through DNA technology.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnIdentifyDetails Highlight each keyterm introduced in this section.
Say the key term aloud.Then, highlight the sentencethat explains the key term.1. What advantages doesgenetic engineering haveover selective breeding?________________________________________________________________________144Chapter 13In the previous section you learned that selective breedingincreases the frequency of desired traits, or alleles, in a population. You also learned that selective breeding techniques such asinbreeding and creating hybrids take time. In many cases theoffspring have to mature before the traits become obvious. Sometimes it takes several generations before the desired trait becomescommon in the population.There is a faster and more reliable way to increase the frequencyof a desired allele in a population.
It is called genetic engineering.In genetic engineering, very small pieces or fragments of DNAare cut from one organism and placed inside another organism.When DNA is made by connecting, or recombining, fragmentsof DNA from different sources, it is called recombinant(ree KAHM buh nunt) DNA.An organism uses the recombinant DNA as if it were its own.The DNA of two different species can even be combined. Forexample, inserting a specific part of the DNA of a firefly into theDNA of a plant will cause the plant to glow.
When an organismcontains recombinant DNA from a different species, it is calleda transgenic organism. The glowing plant is an example of atransgenic organism.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Genetic EngineeringNameDateClassSection13.2Recombinant DNA Technology, continuedWhat is the process for producing atransgenic organism?Producing a transgenic organism is a three-step process.
Thefirst step is to cut the DNA fragment out of one organism. Thesecond step is to connect the DNA fragment to a carrier. The thirdstep is to insert the DNA fragment and its carrier into a neworganism. Let’s take a closer look at each step.How is a DNA fragment cut from an organism?Scientists have discovered that there are proteins called restriction enzymes that cut DNA. They are bacterial proteins that cancut both strands of the DNA molecule at a specific nucleotidesequence. There are hundreds of different restriction enzymes.Each one cuts DNA in a different place. In our example, Step 1is picking a restriction enzyme that cuts the firefly DNA strandat the sequence that codes for making the enzyme that lights upthe firefly.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.How are DNA fragments connected to a carrier?Organisms do not easily accept loose fragments of DNA fromother organisms.
For this reason, the DNA fragment needs a carrier to take it into the host cell. This is Step 2. In our example,the firefly DNA gets inserted into the carrier or vector DNA.A vector is the means by which DNA from another species canbe carried into a host cell.Biological vectors include viruses and plasmids. A plasmid is asmall ring of DNA found in a bacterial cell. The genes of a plasmid are different from those on the bacterial chromosome. In ourexample, the firefly DNA strand was inserted into a plasmid in abacterial cell in Step 2. Now, in Step 3, the plasmid vector isinserted into a plant.2.
What is a vector?________________________________________________________________________What are clones?With the firefly DNA now a part of it, the plasmid reproduceswithin the bacterial cell, making up to 500 copies of itself. Everytime the host cell divides, it copies the recombinant DNA as itcopies its own DNA. Such genetically identical copies are calledclones.
Each identical recombinant DNA molecule is called agene clone. Because the bacterial host cells in the plant will continue to copy the recombinant DNA, the plant will always havethe firefly’s DNA—and its light—within it.READING ESSENTIALSChapter 13145NameDateClassSection13.2Recombinant DNA Technology, continuedIn some experiments, scientists insert particular typesRecombinant DNABacterialof recombinant DNA intoRecombinedchromosomeplasmidhost cells.
This DNA hascode within it to make acertain type of protein.E. coliScientists then study whatPlasmidthis protein does in cells thatHumando not ordinarily produce it.growthhormoneAt other times, scientists produce mutant forms of a protein. They then study how themutation affects the functionof the protein within a cell.Technology has made gene cloning fairly simple. Scientistshave built upon gene cloning to clone an entire animal. The most3.
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