enger−ross: concepts in iii. cell division and 10

Enger−Ross: Concepts in Biology, Tenth Edition

III. Cell Division and Heredity

10. Mendelian Genetics © The McGraw−Hill Companies, 2002

CHAPTER 10

Chapter Outline

10Mendelian Genetics

10.1 Genetics, Meiosis, and Cells10.2 Single-Gene Inheritance Patterns

Dominant and Recessive Alleles •Codominance • X-Linked Genes

10.3 Mendel’s Laws of Heredity

10.4 Probability Versus Possibility10.5 Steps in Solving Heredity Problems:

Single-Factor Crosses10.6 The Double-Factor Cross

10.7 Alternative Inheritance SituationsMultiple Alleles and Genetic Heterogeneity •Polygenic Inheritance • Pleiotropy

OUTLOOKS 10.1: The Inheritance of Eye Color

10.8 Environmental Influences on Gene Expression

Understand the concepts of genotype and phenotype.

Understand the basics of Mendelian genetics.

Work single-gene and double-factor genetic problems.

Understand how a person’s sex can influence the expression of their genes.

Understand how genes and their alleles interact.

• Explain how a person can have the allele for a particular trait but not show it.

• Determine if the children of a father and a mother with a certaingene combination will automatically show that trait.

• Understand how people inherit varying degrees of traits such as skin color.

• Determine the likelihood that a particular trait will be passed on tothe next generation.

• Determine the chances that children will carry two particular genes.

• Explain why men and women inherit some traits differently.

• Use the concepts of dominant alleles and recessive alleles,incompletely dominant alleles, and X-linkage to explain inheritance patterns.

Key Concepts Applications




10.1 Genetics, Meiosis, and CellsWhy do you have a particular blood type or hair color? Whydo some people have the same skin color as their parents andothers have a skin color different from that of their parents?Why do flowers show such a wide variety of colors? Why isit that generation after generation of plants, animals, andmicrobes look so much like members of their own kind?These questions and many others can be better answered ifyou have an understanding of genetics.

A gene is a portion of DNA that determines a character-istic. Through meiosis and reproduction, genes can be trans-mitted from one generation to another. The study of genes,how genes produce characteristics, and how the characteristicsare inherited is the field of biology called genetics. The firstperson to systematically study inheritance and formulate lawsabout how characteristics are passed from one generation tothe next was an Augustinian monk named Gregor Mendel(1822–1884). Mendel’s work was not generally accepted until1900, when three men, working independently, rediscoveredsome of the ideas that Mendel had formulated more than 30 years earlier. Because of his early work, the study of thepattern of inheritance that follows the laws formulated byGregor Mendel is often called Mendelian genetics.

To understand this chapter, you need to know somebasic terminology. One term that you have already encoun-tered is gene. Mendel thought of a gene as a particle thatcould be passed from the parents to the offspring (children,descendants, or progeny). Today we know that genes areactually composed of specific sequences of DNA nucleotides.The particle concept is not entirely inaccurate, because a par-ticular gene is located at a specific place on a chromosomecalled its locus (locus = location; plural, loci).

Another important idea to remember is that all sexu-ally reproducing organisms have a diploid (2n) stage.Because gametes are haploid (n) and most organisms arediploid, the conversion of diploid to haploid cells duringmeiosis is an important process.

2(n) → meiosis → (n) gametes

The diploid cells have two sets of chromosomes—one setinherited from each parent.

n + n gametes → fertilization → 2n

Therefore, they have two chromosomes of each kind andhave two genes for each characteristic. When sex cells areproduced by meiosis, reduction division occurs, and thediploid number is reduced to haploid. Therefore, the sexcells produced by meiosis have only one chromosome ofeach of the homologous pairs that were in the diploid cellthat began meiosis. Diploid organisms usually result fromthe fertilization of a haploid egg by a haploid sperm. Thusthey inherit one gene of each type from each parent. Forexample, each of us has two genes for earlobe shape: onecame with our father’s sperm, the other with our mother’segg (figure 10.1).

10.2 Single-Gene Inheritance PatternsIn diploid organisms there may be two different forms of thegene. In fact, there may be several alternative forms or allelesof each gene within a population. In people, for example,there are two alleles for earlobe shape. One allele producesan earlobe that is fleshy and hangs free, whereas the otherallele produces a lobe that is attached to the side of the faceand does not hang free. The type of earlobe that is present isdetermined by the type of allele (gene) received from eachparent and the way in which these alleles interact with oneanother. Alleles are located on the same pair of homologouschromosomes—one allele on each chromosome. These allelesare also at the same specific location, or locus (figure 10.2).

The genome is a set of all the genes necessary to specifyan organism’s complete list of characteristics. The termgenome is used in two ways. It may refer to the diploid (2n)or haploid (n) number of chromosomes in a cell. Be sure toclarify how this term is used by your instructor. The genotypeof an organism is a listing of the genes present in that organ-ism. It consists of the cell’s DNA code; therefore, you cannotsee the genotype of an organism. It is not yet possible toknow the complete genotype of most organisms, but it isoften possible to figure out the genes present that determine aparticular characteristic. For example, there are three possible

172 Part 3 Cell Division and Heredity

Figure 10.1

Genes Control Structural FeaturesWhether your earlobe is free (a) or attached (b) depends on thegenes you have inherited. As genes express themselves, their actionsaffect the development of various tissues and organs. Some people’searlobes do not separate from the sides of their heads in the samemanner as do those of others. How genes control this complexgrowth pattern and why certain genes function differently thanothers is yet to be clarified.

(a) (b)




genotypic combinations of the two alleles for earlobe shape.Genotypes are typically represented by upper- and lowercaseletters. In the case of the earlobe trait, the allele for free ear-lobes is designated “E,” whereas that for attached earlobes is“e.” A person’s genotype could be (1) two alleles for attachedearlobes, (2) one allele for attached earlobes and one allelefor free earlobes, or (3) two alleles for free earlobes.

How would individuals with each of these three geno-types look? The way each combination of alleles expresses(shows) itself is known as the phenotype of the organism.The phrase gene expression refers to the degree to which agene goes through transcription and translation to showitself as an observable feature of the individual.

A person with two alleles for attached earlobes willhave earlobes that do not hang free. A person with one allelefor attached earlobes and one allele for free earlobes willhave a phenotype that exhibits free earlobes. An individualwith two alleles for free earlobes will also have free earlobes.Notice that there are three genotypes, but only two pheno-types. The individuals with the free-earlobe phenotype canhave different genotypes.

Alleles Genotypes PhenotypesE = free earlobes EE Free earlobese = attached earlobes Ee Free earlobes

ee Attached earlobes

The expression of some genes is directly influenced bythe presence of other alleles. For any particular pair of allelesin an individual, the two alleles from the two parents areeither identical or not identical. Persons are homozygous for

a trait when they have the combination of two identical alle-les for that particular characteristic, for example, EE and ee.A person with two alleles for freckles is said to be homozy-gous for that trait. A person with two alleles for no frecklesis also homozygous. If an organism is homozygous, the char-acteristic expresses itself in a specific manner. A personhomozygous for free earlobes has free earlobes, and a personhomozygous for attached earlobes has attached earlobes.

Individuals are designated as heterozygous when theyhave two different allelic forms of a particular gene, forexample, Ee. The heterozygous individual received oneform of the gene from one parent and a different allelefrom the other parent. For instance, a person with oneallele for freckles and one allele for no freckles is heterozy-gous. If an organism is heterozygous, these two differentalleles interact to determine a characteristic. A carrier isany person who is heterozygous for a trait. In this situa-tion, the recessive allele is hidden, that is, does not expressitself enough to be a phenotype.

Dominant and Recessive AllelesOften, one allele in the pair expresses itself more than theother. A dominant allele masks the effect of other alleles forthe trait. For example, if a person has one allele for free ear-lobes and one allele for attached earlobes, that person has aphenotype of free earlobes. We say the allele for free ear-lobes is dominant. A recessive allele is one that, when pres-ent with another allele, has its actions overshadowed by theother; it is masked by the effect of the other allele. Havingattached earlobes is the result of having a combination oftwo recessive characteristics. A person with one allele forfree earlobes and one allele for attached earlobes has a phe-notype of free earlobes. The expression of recessive alleles isonly noted when the organism is homozygous for the reces-sive alleles. If you have attached earlobes, you have two alle-les for that trait. Don’t think that recessive alleles arenecessarily bad. The term recessive has nothing to do withthe significance or value of the allele—it simply describeshow it can be expressed. Recessive alleles are not less likelyto be inherited but must be present in a homozygous condi-tion to express themselves. Also, recessive alleles are not nec-essarily less frequent in the population (see table 11.1).Sometimes the physical environment determines whether ornot dominant or recessive genes function. For example, inhumans genes for freckles do not show themselves fullyunless a person’s skin is exposed to sunlight (figure 10.3).

CodominanceIn cases of dominance and recessiveness, one allele of thepair clearly overpowers the other. Although this is common,it is not always the case. In some combinations of alleles,there is a codominance. This is a situation in which bothalleles in a heterozygous condition express themselves.

Chapter 10 Mendelian Genetics 173

Sickle-cellhemoglobin

Attachedearlobes

Blood type A Blood type O

Normalhemoglobin

Freeearlobes

Figure 10.2

A Pair of Homologous ChromosomesHomologous chromosomes contain genes for the samecharacteristics at the same place. Note that the attached-earlobeallele is located at the ear-shape locus on one chromosome, and thefree-earlobe allele is located at the ear-shape locus on the othermember of the homologous pair of chromosomes. The other twogenes are for hemoglobin structure (alleles for normal and sickledcells) and blood type (alleles for blood types A and O). The examplespresented here are for illustrative purposes only. We do not reallyknow if these particular genes are on these chromosomes. It ishoped that the Human Genome Project, described in chapter 9, willresolve this problem.




A classic example of codominance in plants involvesthe color of the petals of snapdragons. There are two allelesfor the color of these flowers. Because neither allele is reces-sive, we cannot use the traditional capital and small letters assymbols for these alleles. Instead, the allele for white petals isgiven the symbol FW, and the one for red petals is given thesymbol FR (figure 10.4). There are three possible combina-tions of these two alleles:

Genotype PhenotypeFWFW White flowerFRFR Red flowerFRFW Pink flower

Notice that there are only two different alleles, red andwhite, but there are three phenotypes, red, white, and pink.Both the red-flower allele and the white-flower allele par-tially express themselves when both are present, and thisresults in pink.

A human example involves the genetic abnormality,sickle-cell disease (see figure 7.11). Having the two recessivealleles for sickle-cell hemoglobin (HbS and HbS) can result inabnormally shaped red blood cells. This occurs because thehemoglobin molecules are synthesized with the wrong aminoacid sequence. These abnormal hemoglobin molecules tend toattach to one another in long, rodlike chains when oxygen isin short supply, that is, with exercise, pneumonia, emphy-sema. These rodlike chains distort the shape of the red bloodcells into a sickle shape. When these abnormal red blood cellschange shape, they clog small blood vessels. The sickled redcells are also destroyed more rapidly than normal cells. Thisresults in a shortage of red blood cells, a condition known asanemia, and an oxygen deficiency in the tissues that havebecome clogged. People with sickle-cell anemia may experi-ence pain, swelling, and damage to organs such as the heart,lungs, brain, and kidneys.

Sickle-cell anemia can be lethal in the homozygousrecessive condition. In the homozygous dominant condition(HbAHbA), the person has normal red blood cells. In the het-erozygous condition (HbAHbS), patients produce both kindsof red blood cells. When the amount of oxygen in the bloodfalls below a certain level, those able to sickle will distort.However, when this occurs, most people heterozygous forthe trait do not show severe symptoms. Therefore thesealleles are related to one another in a codominant fashion.However, under the right circumstances, being heterozygouscan be beneficial. A person with a single sickle-cell allele ismore resistant to malaria than a person without this allele.


Figure 10.3

The Environment and Gene ExpressionThe expression of many genes is influenced by the environment. The allele for dark hair in the cat is sensitive to temperature and expressesitself only in the parts of the body that stay cool. The allele for freckles expresses itself more fully when a person is exposed to sunlight.

Figure 10.4

A Case of CodominanceThe colors of these snapdragons are determined by two alleles forpetal color, FW and FR. There are three different phenotypes becauseof the way in which the alleles interact with one another. In theheterozygous condition, neither of the alleles dominates the other.




Genotype PhenotypeHbA HbA Normal hemoglobin and nonresistance to malariaHbA HbS Normal hemoglobin and resistance to malariaHbS HbS Resistance to malaria but death from sickle-cell

anemia

Originally, sickle-cell anemia was found at a high frequencyin parts of the world where malaria was common, such astropical regions of Africa and South America. Today, how-ever, this genetic disease can be found anywhere in theworld. In the United States, it is most common among blackpopulations whose ancestors came from equatorial Africa.

X-Linked GenesPairs of alleles located on nonhomologous chromosomes sep-arate independently of one another during meiosis when thechromosomes separate into sex cells. Because each chromo-some has many genes on it, these genes tend to be inheritedas a group. Genes located on the same chromosome that tendto be inherited together are called a linkage group. Theprocess of crossing-over, which occurs during prophase I ofmeiosis I, may split up these linkage groups. Crossing-over

happens between homologous chromosomes donated by the mother and the father and results in a mixing of genes.The closer two genes are to each other on a chromosome, themore probable it is that they will be inherited together.

People and many other organisms have two types ofchromosomes. Autosomes (22 pairs) are not involved in sexdetermination and have the same kinds of genes on bothmembers of the homologous pair of chromosomes. Sex chro-mosomes are a pair of chromosomes that control the sex ofan organism. In humans, and some other animals, there aretwo types of sex chromosomes—the X chromosome and theY chromosome. The Y chromosome is much shorter than the X chromosome and has fewer genes for traits than foundon the X chromosome (figure 10.5). One genetic trait that islocated on the Y chromosome contains the testis-determininggene—SRY. Females are normally produced when two Xchromosomes are present. Males are usually produced whenone X chromosome and one Y chromosome are present.

Genes found together on the X chromosome are said tobe X-linked. Because the Y chromosome is shorter than theX chromosome, it does not have many of the alleles that arefound on the comparable portion of the X chromosome.Therefore, in a man, the presence of a single allele on his


Ichthyosis (dry scaly skin)

Duchenne muscular dystrophy

Retinosis pigmentosa (deposit of pigmentin retina of eye leading to blindness)

Night blindness

Ocular albinism (no eye pigment)Absence of sweat glandsX-linked cleft palate

Testicular feminization (cells do not respondto testosterone—develops femalecharacteristics but has testes)

Split hand/foot deformity

Fragile X (leads to mental retardation)

Hemophilia (blood will not clot)Color deficiency (blindness)

Centromere

X chromosome

Stature- and height-promoting genesSRY—testes-determining factor

Skeletal abnormalities

Promotes spermatogenesis

Y chromosome

Figure 10.5

Sex ChromosomesWhy is the Y chromosome so small? Is there an advantage to a species in having one sex chromosome deficient in genes? One hypothesisanswers yes! Consider the idea that, with genes for supposedly “female” characteristics eliminated from the Y chromosome, crossing-over andrecombining with “female” genes on the X chromosome during meiosis could help keep sex traits separated. Males would be males andfemales would stay females. The chances of “male-determining” and “female-determining” genes getting mixed onto the same chromosomewould be next to impossible because they would not even exist on the Y chromosome.




only X chromosome will be expressed, regardless of whetherit is dominant or recessive. A Y-linked trait in humans is theSRY gene. This gene controls the differentiation of theembryonic gonad to a male testis. By contrast, more than100 genes are on the X chromosome. Some of these X-linkedgenes can result in abnormal traits such as color deficiency,hemophilia, brown teeth, and at least two forms of musculardystrophy (Becker’s and Duchenne’s).

10.3 Mendel’s Laws of HeredityHeredity problems are concerned with determining whichalleles are passed from the parents to the offspring and howlikely it is that various types of offspring will be produced.The first person to develop a method of predicting the out-come of inheritance patterns was Mendel, who performedexperiments concerning the inheritance of certain character-istics in garden pea (pisum satium) plants. From his work,Mendel concluded which traits were dominant and whichwere recessive. Some of his results are shown in table 10.1.

What made Mendel’s work unique was that he studiedonly one trait at a time. Previous investigators had tried tofollow numerous traits at the same time. When this wasattempted, the total set of characteristics was so cumbersometo work with that no clear idea could be formed of how theoffspring inherited traits. Mendel used traits with clear-cutalternatives, such as purple or white flower color, yellow orgreen seed pods, and tall or dwarf pea plants. He was verylucky to have chosen pea plants in his study because theynaturally self-pollinate. When self-pollination occurs in peaplants over many generations, it is possible to develop a pop-ulation of plants that is homozygous for a number of charac-teristics. Such a population is known as a pure line.

Mendel took a pure line of pea plants having purpleflower color, removed the male parts (anthers), and dis-carded them so that the plants could not self-pollinate. Hethen took anthers from a pure-breeding white-flowered plantand pollinated the antherless purple flower. When the polli-nated flowers produced seeds, Mendel collected, labeled, andplanted them. When these seeds germinated and grew, theyeventually produced flowers.

You might be surprised to learn that all the plantsresulting from this cross had purple flowers. One of the pre-vailing hypotheses of Mendel’s day would have predictedthat the purple and white colors would have blended, result-ing in flowers that were lighter than the parental purpleflowers. Another hypothesis would have predicted that theoffspring would have had a mixture of white and purpleflowers. The unexpected result—all the offspring producedflowers like those of one parent and no flowers like those ofthe other—caused Mendel to examine other traits as welland formed the basis for much of the rest of his work. Herepeated his experiments using pure strains for other traits.Pure-breeding tall plants were crossed with pure-breedingdwarf plants. Pure-breeding plants with yellow pods were

crossed with pure-breeding plants with green pods. Theresults were all the same: the offspring showed the character-istics of one parent and not the other.

Next, Mendel crossed the offspring of the white-purplecross (all of which had purple flowers) with each other to seewhat the third generation would be like. Had the characteristicof the original white-flowered parent been lost completely?This second-generation cross was made by pollinating thesepurple flowers that had one white parent among themselves.The seeds produced from this cross were collected andgrown. When these plants flowered, three-fourths of themproduced purple flowers and one-fourth produced whiteflowers.

After analyzing his data, Mendel formulated severalgenetic laws to describe how characteristics are passed fromone generation to the next and how they are expressed in anindividual.

Mendel’s law of dominance When an organism hastwo different alleles for a given trait, the allele thatis expressed, overshadowing the expression of theother allele, is said to be dominant. The gene whoseexpression is overshadowed is said to be recessive.

Mendel’s law of segregation When gametes are formedby a diploid organism, the alleles that control a traitseparate from one another into different gametes,retaining their individuality.

Mendel’s law of independent assortment Members ofone gene pair separate from each other independ-ently of the members of other gene pairs.

At the time of Mendel’s research, biologists knew noth-ing of chromosomes or DNA or of the processes of mitosisand meiosis. Mendel assumed that each gene was separatefrom other genes. It was fortunate for him that most of thecharacteristics he picked to study were found on separatechromosomes. If two or more of these genes had beenlocated on the same chromosome (linked genes), he probablywould not have been able to formulate his laws. The discov-ery of chromosomes and DNA have led to modifications inMendel’s laws, but it was Mendel’s work that formed thefoundation for the science of genetics.


Table 10.1

DOMINANT AND RECESSIVE TRAITS IN PEA PLANTS

Characteristic Dominant Allele Recessive Allele

Plant height Tall DwarfPod shape Full ConstrictedPod color Green YellowSeed surface Round WrinkledSeed color Yellow GreenFlower color Purple White




10.4 Probability Versus PossibilityIn order to solve heredity problems, you must have anunderstanding of probability. Probability is the chance thatan event will happen, and is often expressed as a percentageor a fraction. Probability is not the same as possibility. It ispossible to toss a coin and have it come up heads. But theprobability of getting a head is more precise than just sayingit is possible to get a head. The probability of getting a headis 1 out of 2 (1⁄2 or 0.5 or 50%) because there are two sides tothe coin, only one of which is a head. Probability can beexpressed as a fraction:

the number of events that canproduce a given outcome

the total number ofpossible outcomes

What is the probability of cutting a deck of cards and gettingthe ace of hearts? The number of times that the ace of heartscan occur is 1. The total number of possible outcomes (num-ber of cards in the deck) is 52. Therefore, the probability ofcutting an ace of hearts is 1⁄52.

What is the probability of cutting an ace? The totalnumber of aces in the deck is 4, and the total number of cardsis 52. Therefore, the probability of cutting an ace is 4⁄52 or 1⁄13.

It is also possible to determine the probability of twoindependent events occurring together. The probability oftwo or more events occurring simultaneously is the productof their individual probabilities. If you throw a pair of dice,it is possible that both will be 4s. What is the probabilitythat both will be 4s? The probability of one die being a 4 is1⁄6. The probability of the other die being a 4 is also 1⁄6. There-fore, the probability of throwing two 4s is

1/6 × 1/6 = 1/36

10.5 Steps in Solving Heredity Problems:Single-Factor Crosses

The first type of problem we will consider is the easiest type,a single-factor cross. A single-factor cross (sometimes calleda monohybrid cross: mono = one; hybrid = combination) is agenetic cross or mating in which a single characteristic is fol-lowed from one generation to the next. For example, inhumans, the allele for Tourette syndrome (TS) is inherited asan autosomal dominant allele.

For centuries, people displaying this genetic disorderwere thought to be possessed by the devil since they dis-played such unusual behaviors. These motor and verbalbehaviors or tics are involuntary and range from mild (e.g.,leg tapping, eye blinking, face twitching) to the more violentforms such as the shouting of profanities, head jerking, spit-ting, compulsive repetition of words, or even barking like adog. The symptoms result from an excess production of thebrain messenger, dopamine.

If both parents are heterozygous (have one allele forTourette and one allele for no Tourette syndrome) what isthe probability that they can have a child without Tourettesyndrome? With Tourette syndrome?

Steps in Solving Heredity Problems—Single-Factor CrossesFive basic steps are involved in solving a heredity problem.

Step 1: Assign a Symbol for Each Allele.Usually a capital letter is used for a dominant allele and asmall letter for a recessive allele. Use the symbol T forTourette and t for no Tourette.

Allele Genotype PhenotypeT = Tourette TT Tourette syndromet = normal Tt Tourette syndrome

tt Normal

Step 2: Determine the Genotype of Each Parent and Indicate a Mating.Because both parents are heterozygous, the male genotype isTt. The female genotype is also Tt. The × between them isused to indicate a mating.

Tt × Tt

Step 3: Determine All the Possible Kinds of Gametes EachParent Can Produce.Remember that gametes are haploid; therefore, they canhave only one allele instead of the two present in the diploidcell. Because the male has both the Tourette syndrome alleleand the normal allele, half his gametes will contain theTourette syndrome allele and the other half will contain thenormal allele. Because the female has the same genotype, hergametes will be the same as his.

For genetic problems, a Punnett square is used. A Pun-nett square is a box figure that allows you to determine theprobability of genotypes and phenotypes of the progeny of aparticular cross. Remember, because of the process of meio-sis, each gamete receives only one allele for each characteris-tic listed. Therefore, the male will produce sperm with eithera T or a t; the female will produce ova with either a T or a t.The possible gametes produced by the male parent are listedon the left side of the square and the female gametes arelisted on the top. In our example, the Punnett square wouldshow a single dominant allele and a single recessive allelefrom the male on the left side. The alleles from the femalewould appear on the top.


Probability =

MalegenotypeTt

Female genotypeTt

Possible femalegametesT & t

Possiblemale gametes

T & t

TT

t

t




Step 4: Determine All the Gene Combinations That CanResult When These Gametes Unite.To determine the possible combinations of alleles that couldoccur as a result of this mating, simply fill in each of theempty squares with the alleles that can be donated from eachparent. Determine all the gene combinations that can resultwhen these gametes unite.

Step 5: Determine the Phenotype of Each Possible GeneCombination.In this problem, three of the offspring, TT, Tt, and Tt, haveTourette syndrome. One progeny, tt, is normal. Therefore,the answer to the problem is that the probability of havingoffspring with Tourette syndrome is 3⁄4; for no Tourette syn-drome, it is 1⁄4.

Take the time to learn these five steps. All single-factorproblems can be solved using this method; the only variationin the problems will be the types of alleles and the number ofpossible types of gametes the parents can produce. Now let’sconsider a problem in which one parent is heterozygous andthe other is homozygous for a trait.

Problem: Dominant/Recessive PKUSome people are unable to convert the amino acid phenyl-alanine into the amino acid tyrosine. The buildup of phenyl-alanine in the body prevents the normal development of thenervous system. Such individuals suffer from phenylketonuria(PKU) and may become mentally retarded (figure 10.6). Thenormal condition is to convert phenylalanine to tyrosine. It

TT TT Tt

t

t Tt tt

is dominant over the condition for PKU. If one parent is het-erozygous and the other parent is homozygous for PKU,what is the probability that they will have a child who isnormal? A child with PKU?

Step 1:Use the symbol N for normal and n for PKU.

Allele Genotype PhenotypeN = normal NN Normal metabolism of

phenylalaninen = PKU Nn Normal metabolism of

phenylalaninenn PKU disorder

Step 2:Nn × nn

Step 3:n

Nn

Step 4:n

N Nnn nn

Step 5:In this problem, 1⁄2 of the progeny will be normal and 1⁄2 willhave PKU.

Problem: CodominanceIf a pink snapdragon is crossed with a white snapdragon,what phenotypes can result, and what is the probability ofeach phenotype?


-

Figure 10.6

PhenylketonuriaPKU is an autosomal recessive disorder located onchromosome 12. This diagram shows how thenormal pathways work (these are shown in gray).If the enzyme phenylalanine hydroxylase is notproduced because of a mutated gene, the aminoacid phenylalanine cannot be broken down, and isconverted into phenylpyruvic acid whichaccumulates in body fluids. There are three majorresults: (1) mental retardation becausephenylpyruvic acid kills nerve cells, (2) abnormalbody growth because less of the growth hormonethyroxine is produced, and (3) pale skinpigmentation because less melanin is produced(abnormalities are shown in color). It should alsobe noted that if a woman who has PKU becomespregnant, her baby is likely to be born retarded.Although the embryo may not have the geneticdisorder, the phenylpyruvic acid produced by thepregnant mother will damage the developing braincells. This is called maternal PKU.




Step 1:FW = white flowers FR = red flowers

Genotype PhenotypeFWFW White flowerFWFR Pink flowerFRFR Red flower

Step 2:FRFW × FWFW

Step 3:FW

FR

FW

Step 4:FW

FR FWFR

Pink flowerFW FWFW

White flower

Step 5:This cross results in two different phenotypes—pink andwhite. No red flowers can result because this would requirethat both parents be able to contribute at least one red allele.The white flowers are homozygous for white, and the pinkflowers are heterozygous.

Problem: X-LinkedIn humans, the gene for normal color vision is dominant andthe gene for color deficiency is recessive. Both genes are X-linked. People who are color blind are not really blind,but should more appropriately be described as having “colordefective vision.” A male who has normal vision mates witha female who is heterozygous for normal color vision. Whattype of children can they have in terms of these traits, andwhat is the probability for each type?

Step 1:This condition is linked to the X chromosome, so it hasbecome traditional to symbolize the allele as a superscript onthe letter X. Because the Y chromosome does not contain ahomologous allele, only the letter Y is used.

XN = normal color visionXn = color-deficientY = male (no gene present)

Genotype PhenotypeXNY Male, normal color visionXnY Male, color-deficientXNXN Female, normal color visionXNXn Female, normal color visionXnXn Female, color-deficient

Step 2:Male’s genotype = XNY (normal color vision)Female’s genotype = XNXn (normal color vision)

XNY × XNXn

Step 3:The genotype of the gametes are listed in the Punnett square:

XN Xn

XN

Y

Step 4:The genotypes of the probable offspring are listed in thebody of the Punnett square:

XN Xn

XN XNXN XNXn

Y XNY XnY

Step 5:The phenotypes of the offspring are determined:

Normal female Carrier female

Normal male Color-deficient male

10.6 The Double-Factor CrossA double-factor cross is a genetic study in which two pairs ofalleles are followed from the parental generation to the off-spring. Sometimes this type of cross is referred to as a dihy-brid (di = two; hybrid = combination) cross. This problem issolved in basically the same way as a single-factor cross. Themain difference is that in a double-factor cross you areworking with two different characteristics from each parent.

It is necessary to use Mendel’s law of independent assort-ment when considering double-factor problems. Recall thataccording to this law, members of one allelic pair separatefrom each other independently of the members of other pairsof alleles. This happens during meiosis when the chromosomessegregate. (Mendel’s law of independent assortment appliesonly if the two pairs of alleles are located on separate chromo-somes. We will assume this is so in double-factor crosses.)

In humans, the allele for free earlobes is dominant overthe allele for attached earlobes. The allele for dark hair domi-nates the allele for light hair. If both parents are heterozygousfor earlobe shape and hair color, what types of offspring canthey produce, and what is the probability for each type?

Step 1:Use the symbol E for free earlobes and e for attached ear-lobes. Use the symbol D for dark hair and d for light hair.

E = free earlobes D = dark haire = attached earlobes d = light hair

Genotype PhenotypeEE Free earlobesEe Free earlobesee Attached earlobesDD Dark hairDd Dark hairdd Light hair





Step 2:Determine the genotype for each parent and show a mating.The male genotype is EeDd, the female genotype is EeDd,and the × between them indicates a mating.

EeDd × EeDd

Step 3:Determine all the possible gametes each parent can produceand write the symbols for the alleles in a Punnett square.Because there are two pairs of alleles in a double-factorcross, each gamete must contain one allele from each pair—one from the earlobe pair (either E or e) and one from thehair color pair (either D or d). In this example, each parentcan produce four different kinds of gametes. The foursquares on the left indicate the gametes produced by themale; the four on the top indicate the gametes produced bythe female.

To determine the possible gene combinations in thegametes, select one allele from one of the pairs of alleles andmatch it with one allele from the other pair of alleles. Thenmatch the second allele from the first pair of alleles witheach of the alleles from the second pair. This may be done asfollows:

Step 4:Determine all the gene combinations that can result whenthese gametes unite. Fill in the Punnett square.

ED Ed eD edED EEDD EEDd EeDD EeDdEd EEDd EEdd EeDd EeddeD EeDD EeDd eeDD eeDded EeDd Eedd eeDd eedd

Step 5:Determine the phenotype of each possible gene combination.In this double-factor problem there are 16 possible ways inwhich gametes can combine to produce offspring. There arefour possible phenotypes in this cross. They are representedin the following chart.

Genotype Phenotype SymbolEEDD or EEDd or Free earlobes/dark hair *EeDD or EeDd

EEdd or Eedd Free earlobes/light hair ∧eeDD or eeDd Attached earlobes/dark hair ``eedd Attached earlobes/light hair +

EDED

EeDd

Ed

Ed

eD

eD

ed

ed

ED Ed eD edED EEDD EEDd EeDD EeDd

* * * *Ed EEDd Eedd EeDd Eedd

∗ ∧ ∗ ∧eD EeDD EeDd eeDD eeDd

* * `` ``ed EeDd Eedd eeDd eedd

∗ ∧ `` +

The probability of having a given phenotype is9⁄16 free earlobes, dark hair3⁄16 free earlobes, light hair3⁄16 attached earlobes, dark hair1⁄16 attached earlobes, light hair

For our next problem, let’s say a man with attachedearlobes is heterozygous for hair color and his wife ishomozygous for free earlobes and light hair. What can theyexpect their offspring to be like?

This problem has the same characteristics as the previ-ous problem. Following the same steps, the symbols wouldbe the same, but the parental genotypes would be as follows:

eeDd × EEdd

The next step is to determine the possible gametes that eachparent could produce and place them in a Punnett square.The male parent can produce two different kinds of gametes,eD and ed. The female parent can produce only one kind ofgamete, Ed.

EdeDed

If you combine the gametes, only two kinds of offspring canbe produced:

EdeD EeDded Eedd

They should expect either a child with free earlobes and darkhair or a child with free earlobes and light hair.

10.7 Alternative Inheritance SituationsSo far we have considered a few straightforward cases inwhich a characteristic is determined by simple dominanceand recessiveness between two alleles. Other situations, how-ever, may not fit these patterns. Some genetic characteristicsare determined by more than two alleles; moreover, sometraits are influenced by gene interactions and some traits areinherited differently, depending on the sex of the offspring.

Multiple Alleles and Genetic HeterogeneitySo far we have discussed only traits that are determined bytwo alleles, for example, A, a. However, there can be more





than two different alleles for a single trait. All the variousforms of the same gene (alleles) that control a particular traitare referred to as multiple alleles. However, one person canhave only a maximum of two of the alleles for the character-istic. A good example of a characteristic that is determinedby multiple alleles is the ABO blood type. There are threealleles for blood type:

Allele*IA = blood has type A antigens on red blood cell surfaceIB = blood has type B antigens on red blood cell surfacei = blood type O has neither type A nor type B antigens on

surface of red blood cell

In the ABO system, A and B show codominance whenthey are together in the same individual, but both are dominantover the O allele. These three alleles can be combined as pairsin six different ways, resulting in four different phenotypes:

Genotype PhenotypeIAIA Blood type AIAi Blood type AIBIB Blood type BIBi Blood type BIAIB Blood type ABii Blood type O

Multiple-allele problems are worked as single-factorproblems.

Polygenic InheritanceThus far we have considered phenotypic characteristics thatare determined by alleles at a specific, single place on homol-ogous chromosomes. However, some characteristics aredetermined by the interaction of genes at several different loci(on different chromosomes or at different places on a singlechromosome). This is called polygenic inheritance. The factthat a phenotypic characteristic can be determined by manydifferent alleles for a particular characteristic is referred to asgenetic heterogeneity. A number of different pairs of allelesmay combine their efforts to determine a characteristic. Skincolor in humans is a good example of this inheritance pat-tern. According to some experts, genes for skin color arelocated at a minimum of three loci. At each of these loci, theallele for dark skin is dominant over the allele for light skin.Therefore a wide variety of skin colors is possible dependingon how many dark-skin alleles are present (figure 10.7).

Polygenic inheritance is very common in determiningcharacteristics that are quantitative in nature. In the skin-color example, and in many others as well, the characteris-tics cannot be categorized in terms of either/or, but thevariation in phenotypes can be classified as how much orwhat amount (Outlooks 10.1). For instance, people showgreat variations in height. There are not just tall and shortpeople—there is a wide range. Some people are as short as1 meter, and others are taller than 2 meters. This quantita-tive trait is probably determined by a number of differentgenes. Intelligence also varies significantly, from those whoare severely retarded to those who are geniuses. Many ofthese traits may be influenced by outside environmental fac-tors such as diet, disease, accidents, and social factors. Theseare just a few examples of polygenic inheritance patterns.


*The symbols, I and i, stand for the technical term for the antigenic carbohydratesattached to red blood cells, the immunogens. These alleles are located on human chro-mosome 9. The ABO system is not the only one used to type blood. Others include theRh, MNS, and Xg systems.

Verylight

Medium Verydark

0 1 2 3 4 5 6Totalnumber ofdark-skingenes

Locus 1

Locus 2

Locus 3

d1d1

d2d2

d3d3

d1D1

d2d2

d3d3

d1D1

d2D2

d3d3

D1D1

D2d2

d3d3

D1d1

D2d2

D3D3

D1d1

D2D2

D3D3

D1D1

D2D2

D3D3

Figure 10.7

Polygenic InheritanceSkin color in humans is an example of polygenic inheritance. The darkness of the skin is determined by the number of dark-skin genes aperson inherits from his or her parents.





OUTLOOKS 10.1

The Inheritance of Eye Color

It is commonly thought that eye color is inherited in a simpledominant/recessive manner. Brown eyes are considered

dominant over blue eyes. The real pattern of inheritance, however, is considerably more complicated than this. Eye color is determined by the amount of a brown pigment, known asmelanin, present in the iris of the eye. If there is a large quantity of melanin present on the anterior surface of the iris,the eyes are dark. Black eyes have a greater quantity of melanin than brown eyes.

If a large amount of melanin is not present on the anteriorsurface of the iris, the eyes will appear blue, not because of a bluepigment but because blue light is returned from the iris (see illus-tration). The iris appears blue for the same reason that deep bod-ies of water tend to appear blue. There is no blue pigment in thewater, but blue wavelengths of light are returned to the eye fromthe water. People appear to have blue eyes because the bluewavelengths of light are reflected from the iris.

Just as black and brown eyes are determined by theamount of pigment present, colors such as green, gray, and hazel

are produced by the various amounts ofmelanin in the iris. If a very small amountof brown melanin is present in the iris, theeye tends to appear green, whereas rela-tively large amounts of melanin producehazel eyes.

Several different genes are probablyinvolved in determining the quantity andplacement of the melanin and, therefore, indetermining eye color. These genes interactin such a way that a wide range of eyecolor is possible. Eye color is probablydetermined by polygenic inheritance, justas skin color and height are. Some new-born babies have blue eyes that laterbecome brown. This is because they havenot yet begun to produce melanin in theiririses at the time of birth.

No melanin

Blue light

White light containsred, orange, yellow,green, and blue light

Melanin on theanterior surfaceof iris

Iris of the eyeis dark colored

Iris of the eyeappears blue

Iris of the eyeappears green or hazel

Some blue light

White light

Some melanin

PleiotropyEven though a single gene produces only one type of mRNAduring transcription, it often has a variety of effects on thephenotype of the person. This is called pleiotropy. Pleiotropy(pleio = changeable) is a term used to describe the multipleeffects that a gene may have on the phenotype. A goodexample of pleiotropy has already been discussed, that is,PKU. In PKU a single gene affects many different chemicalreactions that depend on the way a cell metabolizes theamino acid phenylalanine commonly found in many foods(refer to figure 10.6). Another example is Marfan syndrome(figure 10.8), a disease suspected to have occurred in formerU.S. president, Abraham Lincoln. Marfan syndrome is a dis-order of the body’s connective tissue but can also haveeffects in many other organs including the eyes, heart, blood,skeleton, and lungs. Symptoms generally appear as a tall,lanky body with long arms and spider fingers, scoliosis,osteoporosis, and depression or protrusion of the chest wall(funnel chest/pectus excavatum or pigeon chest/pectus cari-natum). In many cases these nearsighted people also showdislocation of the lens of the eye. The white of the eye(sclera) may appear bluish. Heart problems include dilation

of the aorta and prolapse of the heart’s mitral valve. Deathmay be caused by a dissection (tear) in the aorta from therupture in a weakened and dilated area of the aorta, calledan aortic aneurysm.

10.8 Environmental Influences on Gene Expression

Maybe you assumed that the dominant allele would alwaysbe expressed in a heterozygous individual. It is not so simple!Here, as in other areas of biology, there are exceptions. Forexample, the allele for six fingers (polydactylism) is domi-nant over the allele for five fingers in humans. Some peoplewho have received the allele for six fingers have a fairly com-plete sixth finger; in others, it may appear as a little stub. Inanother case, a dominant allele causes the formation of a lit-tle finger that cannot be bent like a normal little finger.However, not all people who are believed to have inheritedthat allele will have a stiff little finger. In some cases, thisdominant characteristic is not expressed or perhaps onlyshows on one hand. Thus, there may be variation in the




degree to which an allele expresses itself in an individual.Geneticists refer to this as variable expressivity. A goodexample of this occurs in the genetic abnormality neurofi-bromatosis type 1 (NF1) (figure 10.9). In some cases it maynot be expressed in the population at all. This is referred toas a lack of penetrance. Other genes may be interacting withthese dominant alleles, causing the variation in expression.

Both internal and external environmental factors caninfluence the expression of genes. For example, at conception,a male receives genes that will eventually determine the pitchof his voice. However, these genes are expressed differentlyafter puberty. At puberty, male sex hormones are released.

This internal environmental change results in the deeper malevoice. A male who does not produce these hormones retains ahigher-pitched voice in later life. Another characteristic whoseexpression is influenced by internal gene-regulating mecha-nisms is that of male-pattern baldness (figure 10.10).

A comparable situation in females occurs when anabnormally functioning adrenal gland causes the release oflarge amounts of male hormones. This results in a femalewith a deeper voice. Also recall the genetic disease PKU. If


(a)

(b) (c)

Figure 10.8

Marfan SyndromeIt is estimated that about 40,000 (the incidence is 1 out of 10,000) people in the UnitedStates have this autosomal dominant abnormality. Notice the lanky appearance to the bodyand face of this person with Marfan syndrome (a). Photos (b) and (c) illustrate their unusuallylong fingers.

Figure 10.9

Neurofibromatosis 1This abnormality is seen in many forms including benignfibromatous skin tumors, “café au lait” spots, nodules in the iris, andpossible malignant tumors. It is extremely variable in its expressivity,i.e., the traits may be almost unnoticeable or extensive. An autosomaldominant trait, it is the result of a mutation and the production of aprotein (neurofibromin) that normally would suppress the activity ofa gene that causes tumor formation.

Figure 10.10

Baldness and the Expression of GenesIt is a common misconception that males have genes for baldness andfemales do not. Male-pattern baldness is a sex-influenced trait in whichboth males and females may possess alleles coding for baldness.These genes are turned on by high levels of the hormone testosterone.This is another example of an internal gene-regulating factor.




children with phenylketonuria (PKU) are allowed to eatfoods containing the amino acid phenylalanine, they willbecome mentally retarded. However, if the amino acidphenylalanine is excluded from the diet, and certain otherdietary adjustments are made, the person will develop nor-mally. NutraSweet is a phenylalanine-based sweetener, sopeople with this genetic disorder must use caution when buy-ing products that contain it.

Diet is an external environmental factor that can influ-ence the phenotype of an individual. Diabetes mellitus, ametabolic disorder in which glucose is not properly metabo-lized and is passed out of the body in the urine, has a geneticbasis. Some people who have a family history of diabetes arethought to have inherited the trait for this disease. Evidenceindicates that they can delay the onset of the disease byreducing the amount of sugar in their diet. This change inthe external environment influences gene expression in muchthe same way that sunlight affects the expression of frecklesin humans (see figure 10.3).

SUMMARY

Genes are units of heredity composed of specific lengths of DNAthat determine the characteristics an organism displays. Specificgenes are at specific loci on specific chromosomes. The phenotypedisplayed by an organism is the result of the effect of the environ-ment on the ability of the genes to express themselves.

Diploid organisms have two genes for each characteristic.The alternative forms of genes for a characteristic are called alleles.There may be many different alleles for a particular characteristic.Organisms with two identical alleles are homozygous for a charac-teristic; those with different alleles are heterozygous. Some allelesare dominant over other alleles that are said to be recessive.

Sometimes two alleles express themselves, and often a genehas more than one recognizable effect on the phenotype of theorganism. Some characteristics may be determined by several differ-ent pairs of alleles. In humans and some other animals, males have

an X chromosome with a normal number of genes and a Y chromo-some with fewer genes. Although they are not identical, theybehave as a pair of homologous chromosomes. Because the Y chro-mosome is shorter than the X chromosome and has fewer genes,many of the recessive characteristics present on the X chromosomeappear more frequently in males than in females, who have two X chromosomes.

CONCEPT MAP TERMINOLOGY

Construct a concept map to show relationships among the follow-ing concepts.

law of independent assortment recessive allelelocus single-factor inheritanceoffspring X-linked traitprobability

KEY TERMS


allelesautosomescarriercodominancedominant alleledouble-factor crossgenegene expressiongenetic heterogeneitygeneticsgenomegenotypeheterozygoushomozygouslaw of dominancelaw of independent assortment

law of segregationlinkage grouplocus (loci)Mendelian geneticsmultiple allelesoffspringphenotypepleiotropypolygenic inheritanceprobabilityPunnett squarerecessive allelesex chromosomessingle-factor crossX-linked gene

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Topics Questions Media Resources

10.1 Genetics, Meiosis, and Cells

1. How many kinds of gametes are possible with each of the following genotypes?a. Aab. AaBBc. AaBbd. AaBbCc

Quick Overview• Mathematical description of meiosis

Key Points• Genetics, meiosis, and cells

10.2 Single-Gene InheritancePatterns

Quick Overview• Simple types of allele interactions

Key Points• Single-gene inheritance patterns





10.4 Probability Versus Possibility

Quick Overview• Brushing up your math skills

Key Points• Probability versus possibility

10.5 Steps in Solving HeredityProblems: Single-FactorCrosses

Quick Overview• Learning the strategy to story

problems

Key Points• Steps in solving heredity problems:

Single-factor crosses

10.6 The Double-Factor Cross 2. What is the probability of each of the following sets ofparents producing the given genotypes in their off-spring?

Parents Offspringa. AA × aa Aab. Aa × Aa Aac. Aa × Aa aad. AaBb × AaBB AABBe. AaBb × AaBB AaBbf. AaBb × AaBb AABB

3. If an offspring has the genotype Aa, what possible com-binations of parental genotypes exist?

Quick Overview• Expanding your strategy

Key Points• The double-factor cross

Animations and Review• Dihybrid cross

Interactive Concept Maps• Text concept map

10.7 Alternative InheritanceSituations

4. In humans, the allele for albinism is recessive to theallele for normal skin pigmentation.a. What is the probability that a child of a mother and

a father who are heterozygous will be albino?b. If a child is normal, what is the probability that it is a

carrier of the albino allele?5. In certain pea plants, the allele T for tallness is dominant

over t for shortness.a. If a homozygous tall and homozygous short plant

are crossed, what will be the phenotype and geno-type of the offspring?

b. If both individuals are heterozygous, what will be thephenotypic and genotypic ratios of the offspring?

6. What is the probability of a child having type AB bloodif one of the parents is heterozygous for A blood andthe other is heterozygous for B? What other genotypesare possible in this child?

Quick Overview• New ways to understand allele

interactions

Key Points• Alternative inheritance situations

Animations and Review• Beyond Mendel

Experience This!• Chart your own pedigree

Case Study• Should you need a license to be a

parent?

10.3 Mendel’s Laws of Heredity Quick Overview• Rules of thumb for genetics problems

Key Points• Mendel’s laws of heredity

10.8 Environmental Influenceson Gene Expression

Quick Overview• Nature versus nurture?

Key Points• Environmental influences on gene

expression

Topics Questions Media Resources

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