Reading Essentials for Biology Glencoe (794133), страница 33
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X-linked traits are passed to both males andfemales. Y-linked traits are passed only to male offspring becausethe genes for these traits are on the Y chromosome.Look at the Punnett square again. You will notice that any traiton a Y chromosome could only pass to a male offspring, since themale offspring are the only ones to receive a Y chromosome.XYXXXXYXXXXYWhat is polygenic inheritance?Some traits, such as skin color and height in humans, vary overa wide range. This is because the traits are controlled by manygenes rather than by just one gene. Polygenic inheritance is theinheritance pattern of a trait that is controlled by two or moregenes.
The genes may be on the same or different chromosomes.Environmental InfluencesCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.It is important to know that the genetic makeup of an organism at fertilization determines only the organism’s potential todevelop and function. Many factors can influence how the gene isexpressed, or whether the gene is expressed at all. There areinternal and external influences.4. Apply Height in humansis an example of (Circleyour choice.)a.
polygenic inheritance.b. external environmental influences.c. internal environmental influences.How does external environment affect organisms?Nutrition, light, chemicals, infectious agents such as bacteria,fungi, parasites, and viruses, and other factors can all influencethe ways genes are expressed. The arctic fox, for example, hasgray-brown fur in warm temperatures. When temperatures fall,the fur becomes white.
In this case, temperature is the externalfactor that affects the phenotype of fur color.How does internal environment affect organisms?The internal environments of males and females are differentbecause of hormones and structural differences. Horn size inmountain sheep is expressed differently in males and females. Inmales, the horns are much heavier and more coiled than thehorns of females.The age of an organism can also affect the way genes function.The internal environment of an organism changes with age, butit is not clearly understood how these changes affect the functionof genes.READING ESSENTIALSChapter 12133NameDateClassSection12.2When Heredity Follows Different Rules, continued◗ After You ReadMini Glossaryautosomes: pairs of homologous chromosomescodominant alleles: pattern where phenotypesof both homozygous parent’s alleles appearequallypolygenic inheritance: inheritance pattern of atrait controlled by two or more genes;genes may be on the same or differentchromosomesincomplete dominance: neither allele of the parent is completely dominant but combineand display a new traitsex chromosomes: in humans, the 23rd pair ofchromosomes, determine the sex of an individual and carry sex-linked traitsmultiple alleles: presence of more than two alleles for a genetic traitsex-linked traits: traits controlled by geneslocated on the sex chromosomes1.
Read the terms and definitions in the Mini Glossary above. Circle two terms that are relatedto each other. On the lines below, tell how these terms are related.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.2. Fill in the partially completed outline to help you review what you have read.I. Sex of an individualA.
Determined byB. Females are represented by the letterC. Males are represented by the letterII. Sex-linked traitsA. First discovered byB. Genes are located onVisit the Glencoe Science Web site at science.glencoe.com to find yourbiology book and learn more about when heredity follows different rules.134Chapter 12READING ESSENTIALSNameDateClassSection12.3 Complex Inheritance ofHuman TraitsSC.F.2.4.2 The student knows that every cell contains a “blueprint” coded in DNA molecules that specify howproteins are assembled to regulate cells.◗ Before You ReadBefore you read about family gene pools, think about your own family.
Then, on the lines below,make a list of the ways the members of your family resemble each other.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnCodominance in HumansCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.You will remember that in codominance the phenotypes ofboth homozygote parents are expressed equally in the heterozygote offspring.
One example of this in humans is a group ofinherited red blood cell disorders called sickle-cell anemia.What is sickle-cell anemia?Sickle-cell anemia is a major health problem in the UnitedStates and in Africa. It is most common in African Americans andin white Americans whose families came from countries aroundthe Mediterranean Sea.In a person who is homozygous for the sickle-cell allele, abnormal sickle-shaped red blood cells are produced. Normal red bloodcells are disc shaped. Sickle-shaped cells occur in the body’s narrowcapillaries.
They slow blood flow, block small vessels, and causetissue damage and pain. Because of the short life span of the sicklecells, individuals with this disease have several related disorders.Individuals who are heterozygous for the sickle-cell alleleproduce both normal and sickle-shaped red blood cells. This isan example of codominance. These individuals produce enoughnormal red blood cells that they do not have the serious healthproblems of those individuals who are homozygous for the allele.Individuals who are heterozygous for the allele can lead relativelynormal lives.READING ESSENTIALSCheck for UnderstandingAs you read this section,be sure to reread any partsyou don’t understand.
Highlight each sentence as youreread it.1. Analyze Which termdescribes when the phenotypes of both parentsare expressed equally?(Circle your choice.)a. multiple allelesb. codominanceChapter 12135NameDateClassSection12.3Complex Inheritance of Human Traits, continuedMultiple Alleles Govern Blood TypePossible GenotypeCombinationsPhenotypesA and AAA and BABA and OAB and OBO and OOThe ABO blood group is a good example of a single gene that has multiple alleles in humans. Humanblood types are determined by the presence orabsence of certain molecules on the surface of redblood cells. Refer to the chart at the left to study thegene combination of blood types.Why is blood typing important?2.
Conclude If a mother hastype O blood and a fatherhas type A blood, theirchild can have which ofthe following blood types?(Circle ALL that apply.)a. Ab. Oc. AOBlood typing determines the ABO blood group to which anindividual belongs. It is necessary to know the blood type of aperson before a blood transfusion can be given. If the wrongblood type is given, the red blood cells could clump together andcause death.Blood typing can also be helpful in cases of disputed parentage.For example, if a child has type AB blood and his or her motherhas type A blood, a man with type O blood could not be thefather. Blood tests cannot prove that a man is the father, only thathe could be. DNA tests are needed to determine actual parenthood.Genes that are carried on the sex chromosomes determine manyhuman traits. Most of these genes are located on the X chromosome.
You will remember that males pass an X chromosome toeach daughter and a Y chromosome to each son. Females pass anX chromosome to both daughters and sons. If a recessive X-linkedallele is passed to a son, the recessive phenotype will be expressedbecause there are no X-linked alleles on the Y chromosomereceived from the male.Two traits that are determined by X-linked recessive inheritancein humans are red-green color blindness and the blood disorder,hemophilia.
X-linked dominant and Y-linked human disordersare rare.What is red-green color blindness?People who have red-green color blindness are unable to tellthe difference between these two colors. Color blindness is causedby the inheritance of a recessive allele on the X chromosome.One problem people with red-green color blindness have is theinability to identify red and green traffic lights by color.136Chapter 12READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Sex-Linked Traits in HumansNameDateClassSection12.3Complex Inheritance of Human Traits, continuedWhat is hemophilia?If you have ever cut yourself, you may have noticed that cutsusually stop bleeding quickly.
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