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CAMPBELL BIOLOGY TENTH EDITION Reece • Urry • Cain • Wasserman • Minorsky • Jackson 14 Mendel and the Gene Idea Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick © 2014 Pearson Education, Inc. Drawing from the Deck of Genes • What genetic principles account for the passing of traits from parents to offspring? • The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) • The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) • Mendel documented a particulate mechanism through his experiments with garden peas Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants? © 2014 Pearson Education, Inc. Figure 14.1a Mendel (third from right, holding a sprig of fuchsia) with his fellow monks. Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments. • Advantages of pea plants for genetic study: – There are many varieties with distinct heritable features, or characters (such as flower color); many character variants (such as purple or white flowers) called traits – Mating of plants can be controlled – Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) – Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another © 2014 Pearson Education, Inc. Mendel started his experiments with True-Breeding Peas • Mendel tracked only those characters that varied in an “either-or” manner, rather than a “more-orless” manner. – He worked with flowers that were either purple or white. – He avoided traits such as seed weight, which varied on a continuum. • Mendel started his experiments with varieties that were true-breeding – When true-breeding plants self-pollinate, all their offspring have the same traits as their parents. © 2014 Pearson Education, Inc. Terminology for Mendel’s Experiments • In a typical breeding experiment, Mendel would cross-pollinate two contrasting, true-breeding pea varieties, a process called hybridization. • The true-breeding parents are the P (parental) generation • Their hybrid offspring are the F1 (first filial) generation • Mendel would then allow the F1 hybrids to selfpollinate to produce an F2 (second filial) generation © 2014 Pearson Education, Inc. Mendel’s Experiment- The Law of Segregation Figure 14.2 1.Removed stamen’s from truebreeding purple flower. 2. Transferred pollen (sperm) from stamens of true breeding white flower to carpel of purple flowers. 3. The pollinated carpel matured into a pod. 4. Planted the pod seeds. Results: The F1 offspring were all purple. When the purple flower’s pollen was transferred to white flower’s carpel all of the F1 were also purple. © 2014 Pearson Education, Inc. F1 hybrid plants are allowed to self-pollinate what traits do have in F2 generation 1. Repeated experiment as before. All F1 are purple. 2. Allowed F1 to crosspollinate itself. Results: F2 plants both purple and white flowered were produced in a ratio of 3:1. © 2014 Pearson Education, Inc. Fig. 14.3 Dominant and Recessive Traits • Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids • Mendel called the purple flower color a dominant trait and the white flower color a recessive trait • Mendel observed the same pattern of inheritance in six other characters in pea plant, each represented by two trait • What Mendel called a “heritable factor” is what we now call a gene Copyright © 2014 Pearson Education Inc. Mendel’s Model • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring • Four related concepts make up this model • These concepts can be related to what we now know about genes and chromosomes © 2014 Pearson Education, Inc. • The first concept is that alternative versions of genes account for variations in inherited characters • For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers • These alternative versions of a gene are now called alleles • Each gene resides at a specific locus on a specific chromosome • The DNA at that locus can vary in its sequence of nucleotides. • The purple-flower and white-flower alleles are two DNA variations at the flower-color locus. © 2014 Pearson Education, Inc. Alleles, alternative versions of a gene Enzyme C T A A A T C G G T Allele for purple flowers Locus for flower-color gene G A T T T A G C C A CTAAATCGGT Enzyme that helps synthesize purple pigment Pair of homologous chromosomes Allele for white flowers A T A A A T C G G T T A T T T A G C C A ATAAATCGGT Figure 14.4 Absence of enzyme One allele results in sufficient pigment • The second concept is that for each character an organism inherits two alleles, one from each parent • Mendel made this deduction without knowing about the role of chromosomes • A diploid organism inherits one set of chromosomes from each parent. • Each diploid organism has a pair of homologous chromosomes and, therefore, two copies of each gene. • The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation • Alternatively, the two alleles at a locus may differ, as in the F1 hybrids © 2014 Pearson Education, Inc. • The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance • In the flower-color example, the F1 plants inherited a purple-flower allele from one parent and a whiteflower allele from the other. • In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant © 2014 Pearson Education, Inc. • The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes • Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism • This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis © 2014 Pearson Education, Inc. • Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses • The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup • A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele © 2014 Pearson Education, Inc. Figure 14.5-3 P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: p P F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 2 P 1 2 p Sperm from F1 (Pp) plant F2 Generation P p PP Pp Pp pp P Mendel’s law of segregation accounts for the 3:1 ratio that he observed in the F2 generation Eggs from F1 (Pp) plant p 3 :1 The F1 hybrids produce two classes of gametes, half with the purpleflower allele and half with the white-flower allele. Useful Genetic Vocabulary • An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character - For flower color in peas- white flowers are homozygous recessive (pp) and purple flower are homozygous dominant (PP) for the flower-color gene. • An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character - For flower color in peas- purple flower are heterozygous dominant (Pp) • Unlike homozygotes, heterozygotes are not by truebreeding © 2014 Pearson Education, Inc. • Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition • Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup • Two organisms can have the same phenotype but different genotypes if one is homozygous dominant and the other is heterozygous. • In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes (homozygous dominant and heterozygous). © 2014 Pearson Education, Inc. Figure 14.6 3 Phenotype Genotype Purple PP (homozygous) Purple Pp (heterozygous) 1 2 1 Purple Pp (heterozygous) White pp (homozygous) Ratio 3:1 Ratio 1:2:1 1 The Testcross • How can we tell the genotype of an individual with the dominant phenotype? • Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual – If any offspring display the recessive phenotype, the mystery parent must be heterozygous © 2014 Pearson Education, Inc. The Testcross is used to determine genotype Figure 14.7 • A Purple pea can be either homozygous or heterozygous. What is the genotype? • The purple pea must be crossed with a homozygous white (recessive trait) pea. © 2014 Pearson Education, Inc. • Mendel’s law of segregation states that the two alleles for a heritable character segregate (separate) during gamete production and end up in different gametes. • If an organism has two identical alleles for a particular character, then that allele is present as a single copy in all gametes. All offsprings are puple. • If different alleles are present, then 50% of the gametes will receive one allele and 50% will receive the other mean half puple and half white flower. © 2014 Pearson Education, Inc. The Law of Independent Assortment • Mendel’s first testcross experiments followed only a single character, such as flower color. • All the F1 progeny produced in these crosses were monohybrids, heterozygous for one character. • A cross between two heterozygotes is a monohybrid cross. • Mendel identified the second law of inheritance by following two characters at the same time. • Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters • In one such dihybrid cross, Mendel studied the inheritance of seed color and seed shape. • A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently © 2014 Pearson Education, Inc. A Dihybrid Cross: Seed Color x Seed Shape • From his earlier experiments Mendel knew that the allele for yellow seeds is dominant (Y) and the allele for green seeds is recessive (y). • For seed shape, the allele for round seed shape is dominant (R) and the allele for wrinkled is recessive (r). • Mendel crossed two true-breeding pea varieties that differ in both color and seed shape: – Yellow round seeds (YYRR) x Green wrinkled seeds (yyrr) – The F1 will be dihybrids, heterozygous for 2 characters (YyRr) Are these 2 characters transmitted from parents to offspring as a package (Y and R alleles always stay together and y and r always stay together) or are seed color and seed shape inherited independently? © 2014 Pearson Education, Inc. Dihybrid Cross between F1 • YyRr F1’s will have the yellow round phenyotype no matter which hypothesis is correct. • What happens when the F1’s self pollinate and produce F2 offspring? • Dependent Assortment hypothesis2 classes of gametes produced: YR and yr so F2 phenotype will be in 3:1 (3 yellow round:1 green wrinkled) Independent Assortment hypothesis4 classes of gametes produced: YR, Yr, yR and yr 16 equally probable ways Y and R can combine so F1 phenotype will be in 9:3:3:1 9 yellow round 3 green round 3 yellow wrinkled 1 green wrinkled © 2014 Pearson Education, Inc. Figure 14.8 • Using a dihybrid cross, Mendel developed the law of independent assortment. • The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation. • Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes • Genes located near each other on the same chromosome tend to be inherited together. © 2014 Pearson Education, Inc. The laws of probability govern Mendelian inheritance • Mendel’s laws of segregation and independent assortment reflect the same laws of probability that apply to tossing coins or rolling dice. • Values of probability range from 0 (an event with no chance of occurring) to 1 (an event that is certain to occur). • The probability of tossing heads with a normal coin is 1/2. • The outcome of one coin toss has no impact on the outcome of the next toss. Each toss is an independent event, just like the distribution of alleles into gametes. • Like a coin toss, each ovum from a heterozygous parent has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele. • The same probabilities apply to the sperm. © 2014 Pearson Education, Inc. The Multiplication and Addition Rules Applied to Monohybrid Crosses • The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities – Probability in an F1 monohybrid cross can be determined using the multiplication rule • Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1 chance 2 of carrying the dominant allele and a 1 chance of carrying the 2 recessive allele Figure 14.9 © 2014 Pearson Education, Inc. The Multiplication and Addition Rules Applied to Monohybrid Crosses • Each gamete has a 12 chance of carrying the dominant allele and a 12 chance of carrying the recessive allele • The probability that a heterozygous pea plant (Pp) will self-fertilize to produce a white-flowered offspring (pp) is the probability that a sperm with a white allele will fertilize an ovum with a white allele. • So, the probability of a white F2 plant is 12 X 12 = 1/4 © 2014 Pearson Education, Inc. The Multiplication and Addition Rules Applied to Monohybrid Crosses • • • • We can use the addition rule to determine the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous. The probability of an event that can occur in two or more different ways is the sum of the individual probabilities of those ways. The probability of obtaining an F2 heterozygote by combining the dominant allele from the egg (P) and the recessive allele from the sperm (p) is 1⁄4. The probability of combining the recessive allele from the egg (p) and the dominant allele from the sperm (P) is also 1⁄4. • Using the rule of addition, we can calculate the probability of an F2 heterozygote as 1⁄4 + 1⁄4 = 1⁄2. © 2014 Pearson Education, Inc. The rule of multiplication applies to dihybrid crosses • For a heterozygous parent (YyRr), the probability of producing a YR gamete is 1/2 × 1/2 = 1/4. • We can now predict the probability of a particular F2 genotype without constructing a 16-part Punnett square. • The probability that an F2 plant from heterozygous parents will have a YYRR genotype is 1/16 (1/4 chance for a YR ovum × 1/4 chance for a YR sperm). © 2014 Pearson Education, Inc. Solving Complex Genetics Problems with the Rules of Probability- A Dihybrid Cross YyRr • • • Seed color: Remember, in a monohybrid cross of Yy plants the probability of the offspring (F2) genotypes are: – ¼ for YY, ½ for Yy, and ¼ for yy Seed Shape: The same probabilities apply to the offspring (F2) genotype for seed shape: – ¼ for RR, ½ for Rr, and ¼ for rr Use the multiplication rule to determine the probability of each of the F2 genotypes in a dihybrid YyRr cross: – For YYRR = ¼ (YY) x ¼(RR) = 1/16 – For YyRR = ½ (Yy) x ¼ (RR) = 1/8 – For yyRR = ¼ (yy) x ¼ (RR) = 1/16 – For yyrr = ¼ (yy) x ¼ (rr) = 1/16 – For YyRr = ½ (yy) x ½ (Rr) = ¼ – For YYRr = ¼ (YY) x ½ (Rr) = 1/8 – For YYrr = ¼ (YY) x ¼ (rr) = 1/16 – For Yyrr = ½ (Yy) x ¼ (rr) = 1/8 – For yyRr = ¼ (yy) x ½ (Rr) = 1/8 © 2014 Pearson Education, Inc. Inheritance patterns are often more complex than predicted by simple Mendelian genetics • The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied • Many heritable characters are not determined by only one gene with two alleles • However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance © 2014 Pearson Education, Inc. Extending Mendelian Genetics for a Single Gene • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: – When alleles are not completely dominant or recessive – When a gene has more than two alleles – When a gene produces multiple phenotypes © 2014 Pearson Education, Inc. Degrees of Dominance • Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical – Mendel’s pea experiments. • In incomplete dominance the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties in Snapdragon. • In codominance two dominant alleles affect the phenotype in separate, distinguishable ways in human blood cell. © 2014 Pearson Education, Inc. Incomplete Dominance • Some alleles show incomplete dominanceheterozygotes show a distinct intermediate phenotype not seen in homozygotes. • This is not blending inheritance because the traits are separable (particulate), as shown in further crosses. • Offspring of a cross between heterozygotes show three phenotypes: each parental phenotype and the heterozygous phenotype. • The phenotypic and genotypic ratios are identical: 1:2:1. • A clear example of incomplete dominance is the flower color of snapdragons. • A cross between a white-flowered plant and a redflowered plant produces all pink F1 offspring. • Self-pollination of the F1 offspring produces 25% white, 25% red, and 50% pink F2 offspring. © 2014 Pearson Education, Inc. Figure 14.10-3 Incomplete dominance in snapdragon flower color How do we know that this is not an example of blending? P Generation Red CRCR White CWCW CR Gametes CW F1 Generation Pink CRCW Gametes 1 2 CR 1 2 CW Sperm F2 Generation 1 1 2 2 CR 1 2 CW CR Eggs 1 2 C RC R C RC W C RC W CW CW CW Codominance • For example, the M, N, and MN blood groups of humans are due to the presence of two specific molecules on the surface of red blood cells. • People of group M (genotype MM) have one type of molecule on their red blood cells, people of group N (genotype NN) have the other type, and people of group MN (genotype MN) have both molecules present. • The MN phenotype is not intermediate between M and N phenotypes but rather exhibits both the M and the N phenotype. © 2014 Pearson Education, Inc. The Relationship Between Dominance and Phenotype • It is important to recognize that a dominant allele does not somehow subdue a recessive allele. • When a dominant allele coexists with a recessive allele in a heterozygote, they do not interact at all. • Alleles are simply variations in a gene’s nucleotide sequence. • For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype © 2014 Pearson Education, Inc. • Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain – At the organismal level, the allele is recessive – At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant – At the molecular level, the alleles are codominant © 2014 Pearson Education, Inc. Frequency of Dominant Alleles • Dominant alleles are not necessarily more common in populations than recessive alleles • For example, one baby out of 400 in the United States is born with extra fingers or toes • The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage • In this example, the recessive allele is far more prevalent than the population’s dominant allele © 2014 Pearson Education, Inc. Multiple Alleles • Most genes exist in populations in more than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. • The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither © 2014 Pearson Education, Inc. Multiple Alleles • • • • • • • • The ABO blood groups in humans are determined by three alleles: IA, IB, and i. Both the IA and IB alleles are dominant to the i allele. The IA and IB alleles are codominant to each other. Because each individual carries two alleles, there are six possible genotypes and four possible blood types. Individuals who are IAIA or IAi are type A and have type A carbohydrates on the surface of their red blood cells. Can donate blood to A and AB. Individuals who are IBIB or IBi are type B and have type B carbohydrates on the surface of their red blood cells. Can donate blood to B and AB. Individuals who are IAIB are type AB and have both type A and type B carbohydrates on the surface of their red blood cells. Can donate to AB but receive from all others. Type AB is universal recipient. Individuals who are ii are type O and have neither carbohydrate on the surface of their red blood cells. Can donate to all others. Type O is universal donor. © 2014 Pearson Education, Inc. Figure 14.11 Pleiotropy • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease © 2014 Pearson Education, Inc. Extending Mendelian Genetics for Two or More Genes • Some traits may be determined by two or more genes • Epistasis and polygenic inheritance are situations in which two or more genes are involved in determining phenotype. © 2014 Pearson Education, Inc. Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus. • For example, in Labrador retrievers and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair © 2014 Pearson Education, Inc. BbEe Eggs 1/ 1/ 1/ 1/ Sperm 1/ BE 4 1/ BbEe 4 bE 1/ 4 Be 1/ 4 be Figure 14.12 4 BE 4 bE 4 Be 4 BBEE BbEE BBEe BbEe BbEE bbEE BbEe bbEe BBEe BbEe BBee Bbee BbEe bbEe Bbee bbee be 9 : 3 : 4 Polygenic Inheritance • Quantitative characters are those that vary in the population in a gradual fashion • Quantitative variation usually indicates polygenic inheritance an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance • A cross between two AaBbCc individuals (with intermediate skin shade) produces offspring with a wide range of shades. © 2014 Pearson Education, Inc. Figure 14-13 × AaBbCc AaBbCc Sperm 1/ Eggs 1/ 8 1/ 8 1/ 8 1/ 8 1/ 1/ 8 1/ 1/ 8 8 1/ 8 1/ 64 15/ 8 1/ 1/ 8 8 8 1/ 8 1/ 8 1/ 8 8 1/ Phenotypes: 64 Number of dark-skin alleles: 0 6/ 64 1 15/ 64 2 20/ 3 64 4 6/ 64 5 1/ 64 6 Nature and Nurture: The Environmental Impact on Phenotype • Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype. • The phenotypic range is broadest for polygenic characters • Traits that depend on multiple genes combined with environmental influences are called multifactorial • The norm of reaction is the phenotypic range of a genotype influenced by the environment • For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity © 2014 Pearson Education, Inc. The Environmental Impact on Phenotype • For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity • Norms of reaction are generally broadest for polygenic characters • Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype © 2014 Pearson Education, Inc. Integrating a Mendelian View of Heredity and Variation • An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior • An organism’s phenotype reflects its overall genotype and unique environmental history © 2014 Pearson Education, Inc. Many human traits follow Mendelian patterns of inheritance • Humans are not good subjects for genetic research – Generation time is too long – Parents produce relatively few offspring – Breeding experiments are unacceptable • However, basic Mendelian genetics endures as the foundation of human genetics © 2014 Pearson Education, Inc. Pedigree Analysis • A pedigree is a family tree that describes the interrelationships of parents and children across generations • Inheritance patterns of particular traits can be traced and described using pedigrees • Information about the presence or absence of a particular phenotypic trait is collected from as many individuals in a family as possible, across generations. • The distribution of these characters is then mapped on the family tree. – For example, the occurrence of a widow’s peak (W) is dominant to a straight hairline (w). • Phenotypes of family members and knowledge of dominance/recessiveness relationships between alleles allow researchers to predict the genotypes of members of this family. – For example, if an individual in the third generation lacks a widow’s peak but both her parents have widow’s peaks, then her parents must be heterozygous for that gene. – If some siblings in the second generation lack a widow’s peak and one of the grandparents (first generation) also lacks one, then we know the other grandparent must be heterozygous, and we can determine the genotype of many other individuals. © 2014 Pearson Education, Inc. Figure 14.15 Key Male Female 1st generation (grandparents) Ww Affected male ww 2nd generation (parents, aunts, Ww ww ww Ww and uncles) Affected female ww Ww Ww ww Offspring, in birth order (first-born on left) Mating Ff FF or Ff ff Ff ff ff Ff Ff Ff ff ff FF or Ff 3rd generation (two sisters) WW or Ww ww Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Pedigrees can also be used to make predictions about future offspring We can use the multiplication and addition rules to predict the probability of specific phenotypes Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait? Recessively Inherited Disorders • Many genetic disorders are inherited in a recessive manner. • These range from relatively mild to life-threatening • Recessively inherited disorders show up only in individuals homozygous for the recessive allele • Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal • If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low • Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele • Most societies and cultures have laws or taboos against marriages between close relatives © 2014 Pearson Education, Inc. Recessively Inherited Disorders Albinism • Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair • Most recessive homozygotes are born to parents who are carriers of the disorder, but themselves lack the disorder Figure 14.16 © 2014 Pearson Education, Inc. Recessively Inherited Disorders Cystic Fibrosis • Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent – 1 in 25 people (4%) of European descent are carriers • The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell • Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine © 2014 Pearson Education, Inc. Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications • Sickle-cell disease affects one out of 400 AfricanAmericans • The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells • In homozygous individuals, all hemoglobin is abnormal (sickle-cell) • Symptoms include physical weakness, pain, organ damage, and even paralysis • Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms • About one out of ten African Americans has sickle cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes • Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous © 2014 Pearson Education, Inc. Recessively Inherited Disorders Sickle-Cell Disease • The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells. Glutamine is replaced by Valine. • The non sickle allele is incompletely dominant to the sickle-cell allele. • Carriers are said to have sickle-cell trait, about 1/10. • These individuals are usually healthy, although some may suffer symptoms of sickle-cell disease under blood-oxygen stress. © 2014 Pearson Education, Inc. Figure 14.17 Sickle-cell alleles Low O2 Sickle-cell hemoglobin proteins Part of a fiber of sickle-cell hemoglobin proteins Sicklecell disease Sickled red blood cells (a) Homozygote with sickle-cell disease: Weakness, anemia, pain and fever, organ damage Sickle-cell allele Normal allele Sicklecell trait Very low O2 Sickle-cell and normal hemoglobin proteins Part of a sickle-cell fiber and normal hemoglobin proteins Sickled and normal red blood cells (b) Heterozygote with sickle-cell trait: Some symptoms when blood oxygen is very low; reduction of malaria symptoms Dominantly Inherited Disorders • Some human disorders are caused by dominant alleles • Dominant alleles that cause a lethal disease are rare and arise by mutation • Achondroplasia is a form of dwarfism caused by a rare dominant allele – Heterozygous individuals have the dwarf phenotype. – The 99.99% of the population who are not achondroplastic dwarfs are homozygous recessive for this trait. – Achondroplasia is another example of a trait for which the recessive allele is far more prevalent than the dominant allele. © 2014 Pearson Education, Inc. Figure 14.18 Parents Dwarf Dd Normal dd Sperm D d d Dd Dwarf dd Normal d Dd Dwarf dd Normal Eggs Dominantly Inherited Disorders • A lethal dominant allele can escape elimination if it causes death at a relatively advanced age, after the individual has already passed on the lethal allele to his or her children. • One example is Huntington’s disease, a degenerative disease of the nervous system (1:10,000 people) – The dominant lethal allele has no obvious phenotypic effect until the individual is about 35 to 45 years old. – Then the deterioration of the nervous system is irreversible and inevitably fatal. • Any child born to a parent who has the allele for Huntington’s disease has a 50% chance of inheriting the disease and the disorder. • Recently, molecular geneticists have used pedigree analysis of affected families to track the Huntington’s allele to a locus near the tip of chromosome 4. • The gene has now been sequenced. • This has led to the development of a test that can detect the presence of the Huntington’s allele in an individual’s genome. © 2014 Pearson Education, Inc. Multifactorial Disorders • Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components • No matter what our genotype, our lifestyle has a tremendous effect on phenotype • Little is understood about the genetic contribution to most multifactorial diseases © 2014 Pearson Education, Inc. Counseling Based on Mendelian Genetics and Probability Rules • Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease • Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders • It is important to remember that each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings • For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately • The tests enable people to make more informed decisions about having children • However, they raise other issues, such as whether affected individuals fully understand their genetic test results • Probabilities are predicted on the most accurate information at the time; predicted probabilities may change as new information is available © 2014 Pearson Education, Inc. In Utero Fetal Testing • In amniocentesis, the liquid that bathes the fetus is removed and tested usually between 14-16 weeks – Amniocentesis can be used to assess whether the fetus has a specific disease. • Fetal cells extracted from amniotic fluid are cultured and karyotyped to identify some disorders. • Other disorders can be identified from chemicals in the amniotic fluids. • In chorionic villus sampling (CVS), a sample of the placenta is removed and tested • Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero © 2014 Pearson Education, Inc. (a) Amniocentesis Ultrasound monitor (b) Chorionic villus sampling (CVS) Figure 14.19 1 Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Fluid Fetal cells Ultrasound monitor Fetus Placenta Chorionic villi Uterus Centrifugation Several hours Biochemical 2 Several and genetic tests weeks 1 Cervix Several hours Suction tube inserted through cervix Fetal cells 2 Several weeks Several hours 3 Karyotyping Figure 14.19 Newborn Screening • Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States • One common test is for phenylketonuria (PKU), a recessively inherited disorder that occurs in one of every 10,000–15,000 births in the United States © 2014 Pearson Education, Inc. Making a histogram and analyzing a distribution pattern of GENE Relationship among alleles of a single gene Complete dominance of one allele Description Heterozygous phenotype same as that of homozygous dominant Incomplete dominance Heterozygous phenotype intermediate between of either allele the two homozygous phenotypes Codominance Both phenotypes expressed in heterozygotes Example PP Pp CRCR CRCW CWCW IAIB Multiple alleles In the whole population, ABO blood group alleles some genes have more IA, IB, i than two alleles Pleiotropy One gene is able to affect Sickle-cell disease multiple phenotypic characters © 2014 Pearson Education, Inc.