Chapter 11 Introduction to Genetics Summary › cms › lib › VA01000195 › Centricity... · Chapter 11 Introduction to Genetics 11–1 The Work of Gregor Mendel The scientific
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11–1 The Work of GregorMendelThe scientific study of heredity is calledgenetics. Gregor Mendel used purebred peaplants in a series of experiments to under-stand inheritance.
Pea flowers have both male and femaleparts. Normally, pollen from the male partof the pea flower fertilizes the female eggcells of the same flower. This is called self-pollination. Seeds that come from self-pollination inherit all their characteristicsfrom just one parent.
To carry out his experiments, Mendelhad to prevent self-pollination. He did thisby cutting away the pollen-bearing maleparts and then dusting pollen from anotherplant on the flower. This process is calledcross-pollination. The seeds that come fromcross-pollination are the offspring of twodifferent parents.
Mendel decided to study just a fewtraits, or characteristics, of the pea plants.He studied seven traits: seed shape, seedcolor, seed coat color, pod shape, pod color,flower position, and plant height.
First, Mendel crossed two plants withdifferent characters, or forms, for the sametrait. For example, one plant was tall andthe other was short. Mendel used the seedsproduced by this cross to grow plants.These plants were hybrids. Hybrids are theoffspring of crosses between parents withdifferent traits.
To Mendel’s surprise, the hybrid plantslooked like only one of the parents. He con-cluded that each trait was controlled by onegene that occurred in two different forms.The different forms of a gene are calledalleles. Mendel formed the theory ofdominance. He concluded that some allelesare dominant, while others are recessive.Whenever a living thing inherits a domi-nant allele, that trait is visible. The effects of
a recessive allele are not seen if the domi-nant allele is present.
Mendel wanted to know what hap-pened to the recessive allele. He allowed hishybrid plants to self-pollinate. Some of theplants that were produced showed therecessive trait. The alleles responsible forthe recessive characters had not disap-peared. Before, the dominant allele hadmasked the recessive allele, so it was notvisible. Mendel concluded that the allelesfor the same trait can be separated. Hecalled this segregation. Alleles segregatewhen gametes are formed. Each gamete car-ries only one copy of each gene.
11–2 Probability and PunnettSquaresMendel used the principles of probability toexplain his results. Probability is the likeli-hood that a particular event will occur.Probability can be used to predict the out-come of genetic crosses because alleles seg-regate randomly. The gene combinationsthat might result from a genetic cross can bedetermined by drawing a Punnett square.
In a Punnett square, alleles are repre-sented by letters. A capital letter representsthe dominant allele, and a lowercase letterrepresents the recessive allele. Organismsthat have two identical alleles for a particu-lar trait are called homozygous. Homozy-gous organisms are true-breeding for aparticular trait. Organisms that have twodifferent alleles for a particular trait arecalled heterozygous. Heterozygous organ-isms are hybrid for a particular trait.
The physical traits of an organism makeup its phenotype (for example, height). Thegenetic makeup of an organism is its geno-type (for example, TT or Tt).
Summary
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One important rule of probability is thatprobabilities predict the average outcome ofa large number of events. They cannot pre-dict what will happen in a single event. Themore organisms examined, the closer thenumbers will get to the expected values.
11–3 Exploring MendelianGeneticsMendel wondered whether genes thatdetermine one trait have anything to dowith genes that determine another trait. Hewanted to know, for example, whether thegene that determines seed shape affects thegene for seed color. To answer this question,he did an experiment. He crossed plantsand recorded two traits—seed shape andseed color.
Mendel found that the gene controllingseed shape did not affect the gene control-ling seed color. Mendel concluded thatgenes can segregate independently, orundergo independent assortment, duringgamete formation.
Not all genes show simple patterns ofdominant and recessive alleles. In incom-plete dominance, one allele is not completely dominant over another. In codominance,both alleles contribute to the phenotype.Many genes have more than two alleles andare said to have multiple alleles. Polygenictraits are traits controlled by two or moregenes.
The characteristics of any organism arenot caused only by its genes. Instead, char-acteristics are determined by the interactionbetween the genes and the environment.
11–4 MeiosisAccording to Mendel, living things inherit asingle copy of each gene from each of theirparents. When gametes are formed, thesetwo copies are separated.
Gametes are made during meiosis. In acomplex process, the number of chromo-somes in each cell is cut in half. The chro-mosomes are different from one anotherand from the parent cell.
There are two stages in meiosis. Duringthe first stage, the DNA in special cells inthe reproductive organs is copied. The cellsthen divide. Two cells are formed. Thesecells are different from each other and dif-ferent from the parent cell. In the secondstage of meiosis, the cells divide again. Thistime, their DNA is not copied first. Fourdaughter cells are produced. Each cell con-tains half the number of chromosomes ofthe original parent cell.
In male animals, the gametes producedby meiosis are called sperm. Some plantsalso have sperm cells. In females, meiosisproduces one large reproductive cell andthree smaller cells. In animals, the largerreproductive cell is called an egg. In someplants, it is called an egg cell. The threesmaller cells produced during meiosis arecalled polar bodies. They do not participatein reproduction.
Meiosis is very different from mitosis.Mitosis makes two cells that are exactlyalike. The cells are also exactly like the par-ent cell. Meiosis, however, produces fourcells. Each of the cells has only half thenumber of chromosomes of the parent cell.The cells are genetically different from oneanother.
11–5 Linkage and Gene MapsSome genes are almost always inheritedtogether. These genes belong to the samelinkage group. A chromosome is a group oflinked genes. It is actually the chromosomesthat assort independently during gameteformation, not single genes.
The location of genes can be mapped toa chromosome. The rate of crossover eventsis used to find the distance between geneson a chromosome. The farther apart twogenes are, the more likely they will be sepa-rated by a crossover event.
Name______________________________ Class __________________ Date ______________
11. Is the following sentence true or false? Homozygous organisms are true-breeding
for a particular trait.
12. Is the following sentence true or false? Plants with the same phenotype always
have the same genotype.
Probability and Segregation (page 269)
13. Circle the letter of each sentence that is true about probability and segregation.
a. In an F1 cross between two hybrid tall pea plants (Tt), 1⁄2 of the F2 plants will havetwo alleles for tallness (TT).
b. The F2 ratio of tall plants to short plants produced in a cross between two hybrid tallpea plants (Tt) is 3 tall plants for every 1 short plant.
c. Mendel observed that about 3⁄4 of the F2 offspring showed the dominant trait.
d. Segregation occurs according to Mendel’s model.
14. In Mendel’s model of segregation, what was the ratio of tall plants to short plants in
the F2 generation?
Probabilities Predict Averages (page 269)
15. Is the following sentence true or false? Probabilities predict the precise outcome of an
individual event.
16. How can you be sure of getting the expected 50 : 50 ratio from flipping a coin?
17. The the number of offspring from a genetic cross, the closer theresulting numbers will get to expected values.
18. Is the following sentence true or false? The ratios of an F1 generation are more likely tomatch Mendelian predicted ratios if the F1 generation contains hundreds or thousands
of individuals.
Reading Skill PracticeTaking notes helps the reader focus on the main ideas and the vocabulary of thereading. Take notes while rereading Section 11–2. Note the main ideas and thehighlighted, boldface terms in the order in which they are presented. You may copythe ideas word for word or summarize them using your own words. Do your workon a separate sheet of paper.
Name______________________________ Class __________________ Date ______________
13. Circle the letter of each sentence that is true about meiosis.
a. During meiosis I, homologous chromosomes separate.
b. The two daughter cells produced by meiosis I still have the two complete sets ofchromosomes, as does a diploid cell.
c. During anaphase II, the paired chromatids separate.
d. After meiosis II, the four daughter cells contain the diploid number of chromosomes.
Gamete Formation (page 278)
Match the products of meiosis with the descriptions.
Product of Meiosis
14. eggs
15. sperm
16. polar bodies
Comparing Mitosis and Meiosis (page 278)
17. Circle the letter of each sentence that is true about mitosis and meiosis.
a. Mitosis produces four genetically different haploid cells.
b. Meiosis produces two genetically identical diploid cells.
c. Mitosis begins with a diploid cell.
d. Meiosis begins with a diploid cell.
Description
a. Haploid gametes produced in males
b. Haploid gametes produced in females
c. Cells produced in females that do not participate in reproduction
Reading Skill PracticeYou can often increase your understanding of what you’ve read by makingcomparisons. A compare-and-contrast table helps you to do this. On a separate sheetof paper, make a table to compare the processes of mitosis and meiosis. For moreinformation about compare-and-contrast tables, see Organizing Information inAppendix A in your textbook.
Name______________________________ Class __________________ Date ______________
Matching In the space provided, write the letter of the definition that best matches each term.
_____ 7. phenotype
_____ 8. gamete
_____ 9. genetics
_____ 10. probability
_____ 11. haploid
_____ 12. gene map
_____ 13. gene
_____ 14. multiple alleles
_____ 15. trait
Completion Fill in the blanks with terms from Chapter 11.
16. The process in which two genes segregate independently is called
_______________________________.
17. Plants that, if left to self-pollinate, produce offspring identical to themselves are called
_______________________________.
18. The offspring of crosses between parents with different traits are called
_______________________.
19. The process during sexual reproduction in which male and female sex cells join is called
_______________________.
20. The process of reduction division in which the number of chromosomes per cell is cut
in half is called _______________________.
H h
H HH Hh
h Hh hh 6.
3.
4.
1.
2.
5.
1.
6.
5.
4.
2.
3.
a. likelihood that something will happenb. shows the relative locations of genes on a chromosomec. physical characteristics of an organismd. containing one set of chromosomese. sex cellf. chemical factor that determines traitsg. specific characteristich. scientific study of heredityi. gene with more than two alleles
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14–1 Human HeredityBiologists can analyze human chromosomesby looking at a karyotype. A karyotype is apicture of the chromosomes from a cellarranged in homologous pairs.
Humans have 46 chromosomes. Two ofthese chromosomes, X and Y, are the sexchromosomes. Females have two X chromo-somes (XX). Males have one X and one Ychromosome (XY). The other 44 chromo-somes are called autosomes.
Human genes are inherited according tothe same principles of genetics described byMendel. To study the inheritance of humantraits, biologists use a pedigree chart. Apedigree shows the relationships within afamily. The inheritance of a certain trait in afamily can be traced using a pedigree. Fromthis, biologists can infer the genotypes offamily members.
It is difficult to associate an observedhuman trait with a specific gene. Manyhuman traits are polygenic, meaning thatthey are controlled by many genes. Theenvironment also influences many traits.
Some of the first human genes to beidentified were those that control bloodtype. Red blood cells can carry two differentantigens, called A and B. Antigens aremolecules that can be recognized by theimmune system. The presence or absence ofthe A and B antigens produces four possibleblood types: A, B, AB, and O. The ABOblood types are determined by a single genewith three alleles.
In addition to the ABO antigens, there isanother antigen on red blood cells called theRh antigen. People who have the Rh anti-gen are Rh positive. People without it areRh negative. A single gene with two allelesdetermines the Rh blood group.
There are several human genetic disor-ders, including phenylketonuria (PKU),Huntington disease, and sickle cell disease.PKU is caused by a recessive allele. It is
expressed only in individuals who haveinherited a recessive allele from each parent. Huntington disease is caused by adominant allele. It is expressed in any per-son who has that allele. Sickle cell disease iscaused by a codominant allele.
Scientists are beginning to understandwhich changes in the DNA sequence causecertain genetic disorders. Cystic fibrosis iscaused by the deletion of three bases in themiddle of the sequence for a protein. Thisdeletion inactivates the protein, which causes the symptoms of this disorder. Onlyone DNA base is changed in the allele thatcauses sickle cell disease. This base changeproduces a blood protein that is less solublethan normal.
14–2 Human ChromosomesThe two smallest human chromosomes,chromosomes 21 and 22, were the first chro-mosomes to have their DNA sequencesidentified. Both have many genes importantfor health. Both have regions of DNA thatdo not code for proteins.
Genes located on the X and Y chromo-somes, the sex chromosomes, are said to besex-linked. They are inherited in a differentpattern than genes located on autosomes.For example, all alleles linked to the X chro-mosome, including those responsible forcolorblindness, hemophilia, and Duchennemuscular dystrophy, are expressed in maleseven if they are recessive alleles. However,in order for these recessive alleles to beexpressed in females, there must be twocopies of them.
Summary
Name______________________________ Class __________________ Date ______________
Females have two X chromosomes.Males have only one. To account for thisdifference, one X chromosome in females israndomly turned off. The turned-off chro-mosome forms a dense region in the nu-cleus known as a Barr body. Barr bodies arenot found in males because their single Xchromosome must be active.
The most common error during meiosisis nondisjunction. Nondisjunction is thefailure of chromosomes to separate properlyduring meiosis. It causes abnormal num-bers of chromosomes to find their way intogametes. This may result in a disorder ofchromosome number. An example of auto-somal nondisjunction is Down syndrome, inwhich there is an extra copy of chromosome21. Nondisjunction can also occur in sexchromosomes. In Turner’s syndrome, afemale has only one X chromosome. InKlinefelter’s syndrome, there are extra Xchromosomes.
14–3 Human MolecularGeneticsBiologists can use techniques in molecularbiology to read, analyze, and even changethe DNA code of human genes. Genetictests are available to test parents for thepresence of recessive alleles for genetic disorders.
In a process called DNA fingerprinting,individuals can be identified by analyzingsections of DNA that have little or noknown function. These sections of DNAvary widely from one person to the next.
In 1990, scientists around the worldbegan the Human Genome Project. The goalwas to identify the DNA sequence for theentire DNA in a human cell. In 2000, thehuman genome was sequenced. Now theproject goal is to analyze these sequences.One way scientists are analyzing the DNAis by looking for genes. To do this, they lookfor promoter sequences. These aresequences that bind RNA polymerase.
Information about the human genomecan be used to cure genetic disorders bygene therapy. In one method of gene ther-apy, a virus is used to deliver the normalgene into cells to correct the genetic defects.The virus is changed so that it cannot causedisease. The normal gene is attached to theDNA of the virus. The inserted gene canmake proteins that correct the geneticdefect.
There are risks and problems with genetherapy. Having the power to manipulatehuman DNA doesn’t necessarily make itright. People in a society are responsible formaking sure that the tools made availableby science are used wisely.
Name______________________________ Class __________________ Date ______________
14. Is the following sentence true or false? Down syndrome occurs when an individual has
two copies of chromosome 21.
15. Circle the letter of the characteristic of Down syndrome.
a. dwarfism c. colorblindness
b. mental retardation d. muscle loss
16. Why does an extra copy of one chromosome cause so much trouble?
17. Circle the letter of each sentence that is true about sex chromosome disorders.
a. A female with the karyotype 45,X has inherited only one X chromosome and is sterile.
b. Females with the karyotype 47,XXY have Klinefelter’s syndrome.
c. Babies have been born without an X chromosome.
d. The Y chromosome contains a sex-determining region that is necessary for malesexual development.
Reading Skill PracticeWriting an outline is a useful way to organize the important facts in a section. Writean outline of Section 14–2. Use the section headings as the headings in your outline. Include only the important facts and main ideas in your outline. Be sure to includethe vocabulary terms. Do your work on a separate sheet of paper.
Name______________________________ Class __________________ Date ______________