Reading Essentials for Biology Glencoe (794133), страница 32
Текст из файла (страница 32)
An individual can also study the pedigree to see if thereis a risk that he or she will pass along a genetic disease. Studying afamily pedigree gives a lot of genetic information about a family.On a pedigree, circles represent females and squares representmales. Shaded circles and squares represent those who have thetrait that is being studied. Unshaded circles and squares representindividuals who do not have the trait.
A half-shaded circle orsquare represents a carrier—someone who has a recessive allelefor a specific trait. As you have learned, an allele is one of theREADING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Making a PedigreeNameDateClassSection12.1Mendelian Inheritance of Human Traits, continueddifferent forms of a gene that exists for a genetic trait. A circleand a square connected by a horizontal line represents parents.A vertical line connects parents with their offspring.2. What symbols representmales and females in apedigree?How is a pedigree analyzed?Look at the illustration to the right. This pedigree shows howa rare, recessive allele is passed from generation to generation.Suppose individual III-1 on the pedigree wanted to knowwhether she would pass on this allele to her children. We canstudy the pedigree to answer this question.We will begin at the top.
Individuals I-1 and I-2 are bothcarriers of the recessive allele for this trait. We know this becausethey produced II-3, who shows the recessive phenotype. Wealso know that II-2 is a carrier because she has passed the alleleto later generations (IV-2 and IV-4). Because III-1 has a parent(II-2) who is a carrier of the recessive allele, III-1 has a one-intwo chance of also being a carrier of the recessive allele.The Punnett square to the right shows the mating of individuals I-1 and I-2. It also shows the probability of individuals II-2through II-5 receiving the recessive allele.________________________________________________I1II12Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.345III?1234IV12Simple Recessive HeredityMost genetic disorders are caused by recessive alleles.
Diseasessuch as cystic fibrosis, Tay-Sachs (tay saks) disease, and phenylketonuria (fen ul kee tun YOO ree uh), also known as PKU, canbe predicted by the use of a pedigree. For an offspring to inherita recessive trait, both parents must have the recessive allele.234RrRRRRrrRrrr5Simple Dominant HeredityRemember that to inherit a dominant trait, only one parentneeds to have the dominant allele for that trait.A cleft chin, a widow’s peak hairline, freely hanging earlobes,almond-shaped eyes, and thick lips are examples of dominant traits.Only one allele needs to be present for these traits to show up.Huntington’s disease is caused by a rare dominant allele. Thereis no effective treatment for Huntington’s disease, which causes abreakdown in certain parts of the brain. Because Huntington’sdisease doesn’t occur until a person is between the ages of 30 and50, many people have already had children before they develop thedisease.
A pedigree could help people with Huntington’s disease intheir family better understand their own risks for the disease andfor passing it on to future generations.READING ESSENTIALS3. Analyze Which allelescause genetic disorderssuch as PKU? (Circle yourchoice.)a. recessiveb. dominantChapter 12129NameDateClassSection12.1Mendelian Inheritance of Human Traits, continued◗ After You ReadMini Glossarycarrier: an individual who has a recessive allelefor a specific traitpedigree: visual representation of genetic inheritance used by geneticists to map genetictraits1. Read the key terms and definitions in the Mini Glossary above.
On the lines below, use thewords pedigree and carrier in a sentence.________________________________________________________________________________________________________________________________________________________________________2. Use the diagram to review what you have learned about the genetic disorders caused byrecessive alleles. In the box, fill in the name of the pattern of heredity illustrated. Then fill inthe remaining circles with other genetic disorders.Tay-SachsDisease3.
In the space below, draw each of the symbols used in a pedigree. Beside each symbol, write adefinition of the symbol.Pedigree SymbolsVisit the Glencoe Science Web site at science.glencoe.com to find yourbiology book and learn more about Mendelian inheritance of human traits.130Chapter 12READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.HeredityNameDateClassSection12.2 When Heredity FollowsDifferent RulesSC.F.1.4.7 The student knows that organisms respond to internal and external stimuli. Also covers SC.F.1.4.8, SC.F.2.4.2◗ Before You ReadThis section is about some of the ways in which inherited traits combine.
Think about the plantsin a garden or some of the animals you have seen in nature. How many different combinations ofcolors are there in plant and animal life? In the Read to Learn section, highlight examples of different combinations of inherited traits.◗ Read to LearnCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Complex Patterns of InheritanceSometimes traits are not inherited through simple Mendeliangenetics. Some traits are not simply dominant or recessive. Whenneither allele of the parents is completely dominant, the phenotype of the heterozygous offspring is a mix of the two parents.This pattern of inheritance is called incomplete dominance.
Forexample, when a homozygous red snapdragon is crossed with ahomozygous white snapdragon, the offspring’s color will be a mixof the two. It will be pink.Look at the Punnett squaresRedWhitebelow. The square on the leftshows that the intermediate pinkflower happens because neitherAllallele of the pair is completelypinkdominant. The square on theright shows the F2 generation ofRedsnapdragons. Notice that when(RR)the pink flowered snapdragonsRRare crossed with each other, theratio of red to pink to whiteflowers in the F2 generation isR'1:2:1.
That means that there willRR'RR'be one red, two pink, and onewhite snapdragon in the secondR'White(R'R')generation. This followsRR'RR'Mendel’s law of segregation.All pink flowersREADING ESSENTIALSCreate a Quiz After you readthis section, create a quizbased on what you havelearned. Then be sure toanswer the quiz questions.Pink(RR')RR'RPink(RR')RRRR'RR'R'R'R'1 red: 2 pink: 1 whiteChapter 12131NameDateClassSection12.2When Heredity Follows Different Rules, continuedWhat is codominant inheritance?1. What is the term thatdescribes when both alleles show up equally?________________________________________________In codominant inheritance, both alleles show up equally.Codominant alleles cause the phenotypes of both homozygoteparents to be expressed equally in the heterozygote offspring. Forexample, when a certain variety of black chicken is crossed with awhite chicken, all of the offspring are checkered.
Some of thefeathers are black and some of the feathers are white.How do multiple alleles work?What determines the sex of an organism?________________________Humans have 23 pairs of chromosomes. Twenty-two of thesepairs of homologous chromosomes are called autosomes.Homologous autosomes look alike. The 23rd pair of chromosomes is called the sex chromosomes and is indicated by theletter X for females and the letter Y for males. If you are female,your sex chromosomes are homologous, XX. If you are male,your sex chromosomes are XY. Males make two kinds of gametes,X and Y.
Females make only X gametes. The X or Y male gametedetermines the sex of the offspring. The Punnett square on page133 illustrates how this works.________________________What are sex-linked traits?3. What is the 23rd pair ofchromosomes called?________________________Sex chromosomes also determine sex-linked traits. Sex-linkedtraits are the traits controlled by genes located on sex chromosomes. In 1910, Thomas Hunt Morgan discovered that sometraits were linked to sex chromosomes. Sex-linked traits followthe inheritance pattern of the sex chromosome on which they are132Chapter 12READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.2. Conclude Which of thefollowing is the bestdescription of codominance? (Circle yourchoice.)a.
Ratio of the trait inthe second generationis 1:2:1.b. The recessive traitshows up equally withthe dominant trait.c. Phenotypes of bothparents are expressedequally.In some populations, traits can be controlled by multiplealleles. This means there are more than two alleles for a genetictrait. We will use the pigeon population for our example.
Eachpigeon can only have two alleles for a genetic trait. There aremultiple allele combinations for some genetic traits within thepigeon population. For example, many combinations of pigeonfeather colors exist. The allele for ash-red colored feathers is dominant. The allele for wild-type blue feathers is recessive to theallele for ash-red feathers.
The allele for chocolate-brown feathersis recessive to both the ash-red and the wild-type blue alleles.Sometimes there are as many as 100 alleles for a single trait!NameDateClassSection12.2When Heredity Follows Different Rules, continuedfound. Eye color in fruit flies is an example of an X-linked trait.This means eye color in fruit flies is determined by a gene on theX chromosome.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
