Reading Essentials for Biology Glencoe (794133), страница 27
Текст из файла (страница 27)
What does the law ofindependent assortmentstate?________________________________________________________________________104Chapter 10To explain his results, Mendel created his second law of heredity, known as the law of independent assortment. This lawstates that genes for different traits are inherited independently ofeach other.
In a dihybrid cross you can see both of Mendel’s lawsat work. The plants in the F1 generation of the dihybrid cross hadthe genotype RrYy. When a plant with this genotype producesgametes, the alleles R and r will separate from each other. That isthe law of segregation at work. The R and r alleles will also separate from the Y and y alleles. That is the law of independentassortment at work. The alleles will then recombine in four different ways.
If the alleles for seed shape and color were inheritedtogether, only two kinds of seeds would have been produced.Instead, four different kinds of seeds were produced.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Mendel’s Dihybrid CrossesNameDateClassSection10.1Mendel’s Laws of Heredity, continuedPunnett SquaresReginald Punnett, an English biologist, came up with a wayto predict the proportions of possible genotypes in offspring. Itis called a Punnett square.
If you know the genotypes of the parents you can use a Punnett square to predict the possible genotypesof their offspring. For example, in Mendel’s original experiment,the F1 generation had the genotype Tt. That means that halfthe gametes for each plant would contain the T allele, and theother half would contain the t allele. A Punnett square can showthe possible combinations for offspring with parents with thisgenotype.A Punnett square for a single trait is two boxes tall and two boxeswide. The genotype of one parent is listed across the top. Thegenotype for the other parent is listed along the left side. Lookat the illustration to see the possible combinations.
A Punnettsquare can also be created for dihybrid crosses. It would be larger,four boxes tall and four boxes wide.Punnett SquareTtTTTTttTtttCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.ProbabilityPunnett squares show all of the possible outcomes. Becausechance is a factor in genetics, the actual results don’t always matchthe Punnett square’s probability. It is like flipping a coin. Theprobability is 50/50 that the coin will be heads or tails. Howeverif you flip a coin 100 times, you cannot guarantee that 50 times itwill be heads and 50 times it will be tails. It is the same in genetics. Even though a Punnett square may predict that one fourth ofthe offspring will have a particular genotype, in reality the amountcould be higher or lower.READING ESSENTIALSChapter 10105NameDateClassSection10.1Mendel’s Laws of Heredity, continued◗ After You ReadMini Glossaryallele (uh LEEL): alternative form of a gene foreach trait of an organismdominant: trait of an organism that is observedand that masks the recessive traitfertilization: uniting of male and female gametesgamete: male and female sex cellsgenetics: study of hereditygenotype (JEE noh tipe): allele combination ofan organismheredity: passing on of characteristics from parents to offspringheterozygous (heh tuh roh ZI gus): when thereare two different alleles for a traithomozygous (hoh moh ZI gus): when there aretwo identical alleles for a traithybrid: offspring of parents having differentforms of a traitlaw of segregation: Mendelian principle explaining that because every organism has twoalleles for every gene, it can produce different types of gametes.
During fertilization,these gametes randomly pair to producefour combinations of alleles.phenotype (FEE nuh tipe): the way an organismlooks and behavespollination: the transfer of pollen grains frommale reproductive organs to female reproductive organs of plantsrecessive: traits of an organism that can bemasked by the dominant form of the traittrait: characteristics that are passed from parentsto offspringzygote (ZI goht): cell formed when a malegamete unites with a female gamete1.
Review the terms and their definitions in the Mini Glossary above. Write a sentence using atleast two of the terms.________________________________________________________________________________________________________________________________________________________________________2. Use the pyramid diagram to help you review what you have read.
Arrange the steps Mendelused in his first experiment with pea plants. Place the letter of each step in the correct orderin the pyramid.a. Cross-pollinate P1 generation to grow monohybridsb. Observe results in F2 generation1.c. Find true breeders for a single traitd. Allow F1 generation to self-pollinate2.3.4.106Chapter 10READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.law of independent assortment: Mendelian principle stating that genes for different traitsare inherited independently of each otherNameDateClassSection10.1Mendel’s Laws of Heredity, continued3. Use the cause and effect diagram below to explain Mendel’s two laws.
Write the effect ofeach law in the correct box.CauseEffectLaw of SegregationLaw of IndependentAssortmentCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Visit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about Mendel’s laws of heredity.READING ESSENTIALSChapter 10107NameDateClassSection10.2 MeiosisSC.F.2.4.1 The student understands the mechanisms of asexual and sexual reproduction and knows the differentgenetic advantages and disadvantages of asexual and sexual reproduction. Also covers SC.F.2.4.2◗ Before You ReadIn this section you will read about a particular type of cell division, meiosis, that creates many genecombinations. Genes determine the individual traits of all living things.
On the lines below, list threedifferent traits or characteristics that you can see in a rose.__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnIdentify FactsUse one color to highlighteach fact about diploid cells.Use another color to highlighteach fact about haploid cells.1.
What do you call a cellwith two of each kind ofchromosome?________________________2. What do you call a cellwith one of each kind ofchromosome?________________________108Chapter 10All living things have thousands of genes. Genes determineindividual traits. The genes do not just float around in a cell.They are lined up on chromosomes.
A typical chromosome canhave thousands of genes.If you took a cell from one of Mendel’s pea plants, you wouldsee that it has 14 chromosomes, or seven pairs. In the body cells ofmost living things, chromosomes come in pairs. One of the chromosomes in each pair comes from the male parent. The other chromosome comes from the female parent. A cell that has two of eachkind of chromosome is called a diploid cell. Using microscopes,scientists can now see the paired chromosomes and know that anallele for each trait is located on each paired chromosome.How are diploid cells different from haploid cells?A diploid cell is a body cell. Organisms also produce a differentkind of cell called a haploid cell. This cell contains only one ofeach kind of chromosome.
A gamete, or sex cell, is a haploid cell.Remember, Mendel concluded that parents give one allele ofeach trait to their offspring. That happens because the gameteis a haploid cell containing half the number of chromosomes asa body cell.Every living thing has a set number of chromosomes. Forexample, every dog has 78 chromosomes, every human has 46chromosomes, and every tomato plant has 24 chromosomes.
Asyou can see, the number of chromosomes is not related to howcomplex an organism is. When the organism reproduces, it onlyREADING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Genes, Chromosomes, and NumbersNameDateClassSection10.2Meiosis, continuedpasses on half the number of chromosomes. Earlier in this sectionyou learned that genes are lined up on a chromosome.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
