Reading Essentials for Biology Glencoe (794133), страница 34
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The cut stops bleeding because theblood clots. Hemophilia is an X-linked disorder that keeps bloodfrom clotting quickly. This means cuts do not stop bleeding veryrapidly. It also means that a person could have internal bleedingfrom a bruise.Hemophilia can be treated with blood transfusions and injections of the blood-clotting enzyme that is absent in people withhemophilia.
These treatments, however, are very expensive.3. What is hemophilia?________________________________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Polygenic Inheritance in HumansThink of the traits you have inherited from your parents. Manyof these were inherited through simple Mendelian patterns orthrough multiple alleles, but others were determined by polygenicinheritance. This means two or more genes may control theinheritance pattern of a trait. Usually these traits vary over a widerange. Eye color and skin color are two examples of polygenicinheritance in humans.In the early 1900s, scientists studied data collected on skincolor.
They found that when light-skinned people mate withdark-skinned people, their children have intermediate skin colors.When the children have offspring, the skin colors in the offspringrange from the light to the dark of the grandparents. Most of theoffspring, however, have an intermediate skin color. This variation indicates that three to four genes are involved in determiningskin color.4. List two examples of polygenic inheritance.________________________________________________Changes in Chromosome NumbersSometimes there are an abnormal number of cells ina set of chromosomes.
To identify an abnormal numberof chromosomes, a sample of cells is taken from an individual or a fetus (a developing mammal from nine weeksold to birth). The chromosomes are photographed andarranged in pairs by a computer. The pairs are arrangedby length and location of the centromere. The chartshowing the pairs is called a karyotype. It is very usefulin identifying unusual chromosome numbers in cells.An example of a karyotype is at right.READING ESSENTIALSChapter 12137NameDateClassSection12.3Complex Inheritance of Human Traits, continuedWhat is Down syndrome?5. What is one example ofan abnormal number ofchromosomes in humans?________________________________________________________________________Most human abnormal chromosome numbers result in embryodeath, usually even before a woman knows she is pregnant.
Downsyndrome is one example of an abnormal number of chromosomes in humans. Individuals with Down syndrome can surviveto adulthood, unlike most others born with an abnormal numberof chromosomes. Individuals with Down syndrome have at leastsome degree of mental retardation. The incidence of Down syndrome births is higher in older mothers, especially those over40 years old.What happens with abnormal numbers ofsex chromosomes?◗ After You ReadMini Glossaryblood typing: determination of ABO bloodgroup to which an individual belongskaryotype (KAHR ee uh tipe): chart of chromosome pairs arranged according to length andlocation of the centromere; used to identifyan abnormal number of chromosomes1. Read the terms and their definitions in the Mini Glossary above.
On the lines below, use theword karyotype correctly in a sentence.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________138Chapter 12READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Sometimes an X chromosome will be missing (designated XO).There may also be an extra chromosome, such as in XXX orXXY.
There may even be an extra Y chromosome (XYY). Anyindividual with a Y chromosome is a male. Any individual withouta Y chromosome is a female. What happens in these cases? Mostof these individuals lead normal lives but cannot have children.Some have varying levels of mental retardation.NameDateClassSection12.3Complex Inheritance of Human Traits, continued2. In Column 1 are some concepts you learned in this section. Column 2 gives a fact abouteach concept. Put the letter of the fact on the line next to the concept that matches it.Column 1_____1.
sickle-cell disease_____2. A, B, AB, O_____3. sex-linked traits_____4. 47 chromosomesColumn 2a. human blood typesb. an example of codominancec. abnormal number of chromosomesd. mostly located on the X chromosomeCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Visit the Glencoe Science Web site at science.glencoe.com to find yourbiology book and learn more about the complex inheritance of human traits.READING ESSENTIALSChapter 12139NameDateClassSection13.1 Applied Genetics◗ Before You ReadSC.H.3.4.2 The student knows that technological problems often create a demand fornew scientific knowledge and that new technologies make it possible for scientists toextend their research in a way that advances science. Also covers SC.H.3.4.5Skim the Read to Learn section below and highlight two important facts about applied genetics.Write those two facts in the space below.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnCreate a Quiz After youhave read this section, createa quiz based on what youhave learned.
After you havecompleted writing the quizquestions, be sure to answerthem.1. Conclude How doesselective breeding influence the frequency ofdesired traits in a population? (Circle your choice.)a. increases thefrequencyb. decreases thefrequency140Chapter 13Humans were involved in applied genetics long before theyknew what genes were. When our ancestors planted the seeds ofthe juiciest berries to grow new plants, they were using selectivebreeding techniques. When farmers breed the calves of the bestmilk-producing cow, they are using selective breeding.Selective breeding takes time. Several generations of offspringneed to be bred before the desired trait becomes common in thepopulation.
Selective breeding increases the frequency of desiredtraits, or alleles, in a population. An example of successful selective breeding is found in dairy cows. Today, the average dairy cowproduces three times more milk than the average dairy cow didfifty years ago. This means that fewer than half the number ofcows are needed to produce the same amount of milk.What is inbreeding?To make sure that breeds always have the desired traits, andto eliminate undesired traits, breeders often use inbreeding.Inbreeding is mating between closely related individuals.Inbreeding creates individuals who are homozygous for mosttraits.
Sometimes in inbreeding, harmful, recessive traits appear.That is because there is a greater chance that two closely relatedindividuals both carry a recessive allele for a harmful trait.Most people are familiar with pure breed dogs. A breed is agroup of organisms within a species that has been bred for certaincharacteristics. For example, a German shepherd is a breed. AllREADING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Selective BreedingNameDateClassSection13.1Applied Genetics, continuedpure breed German shepherds have long hair that is black andtan, a black muzzle, and they resemble a wolf.Another form of selective breeding is creating hybrids.
A hybridis the offspring of parents that have different forms of a trait. Plantsare often hybrids. To get a fragrant, long-stemmed rose, breedersmight cross a fragrant rose with a long-stemmed rose. The resultof that cross is a hybrid. Often, plant hybrids are larger andhealthier than the parent plants.2. What is a hybrid?________________________________________________________________________Determining GenotypesBreeders must carefully select the plants or animals that havethe greatest chance of passing on the desired traits. But you cannot always tell by looking at an organism if it is homozygous orheterozygous for a trait. How can breeders determine if an organism is heterozygous? They use the information that Mendeldiscovered in his experiments. Breeders perform a test cross todetermine the combination of genes, or the genotype, of anorganism.
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