Campbell Biology 10. Chapter 14 Mendelian Genetics. A Global Approach. Chul-Su Yang, Ph.D., Lecture on General Biology 2

Transcription

1 Lecture on General Biology 2 Campbell Biology 0 A Global Approach th edition Chapter 4 Mendelian Genetics Chul-Su Yang, Ph.D., chulsuyang@hanyang.ac.kr Infection Biology Lab., Dept. of Molecular & Life Science, Hanyang University

2 Overview 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)

3 The particulate hypothesis is the idea that parents pass on discrete heritable units (genes) - This hypothesis can explain the reappearance of traits after several generations Mendel documented a particulate mechanism through his experiments with garden peas

4 Figure 4.

5 Figure 4.a

6 Concept 4. 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

7 Mendel s Experimental, Quantitative Approach Mendel s approach allowed him to deduce principles that had remained elusive to others A heritable feature that varies among individuals (such as flower color) is called a character Each variant for a character, such as purple or white color for flowers, is called a trait Peas were available to Mendel in many different varieties

8 Other advantages of using peas Short generation time Large numbers of offspring Mating could be controlled; plants could be allowed to self-pollinate or could be cross pollinated - Each flower has sperm-producing organs (stamens) and egg-producing organ (carpel) - Cross-pollination 타가수분 (fertilization between different plants) involves dusting one plant with pollen from another

9 Figure 4.2a TECHNIQUE Parental generation (P) 3 2 Stamens Carpel 4

10 Figure 4.2b RESULTS First filial generation offspring (F ) 5

11 Mendel chose to track only those characters that occurred in two distinct alternative forms He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)

12 In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation The hybrid offspring of the P generation are called the F generation When F individuals self-pollinate or crosspollinate with other F hybrids, the F 2 generation is produced

13 The Law of Segregation When Mendel crossed contrasting, truebreeding white- and purple-flowered pea plants, all of the F hybrids were purple When Mendel crossed the F hybrids, many of the F 2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F 2 generation

14 Figure 4.3 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F Generation (hybrids) All plants had purple flowers Self- or cross-pollination F 2 Generation 705 purpleflowered plants 224 white flowered plants

15 Mendel reasoned that only the purple flower factor was affecting flower color in the F hybrids Mendel called the purple flower color a dominant trait and the white flower color a recessive trait The factor for white flowers was not diluted or destroyed because it reappeared in the F 2 generation

16 Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits [Table 4.] What Mendel called a heritable factor is what we now call a gene

17 Table 4.

18 Mendel s Model Mendel developed a hypothesis to explain the 3: inheritance pattern he observed in F 2 offspring Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes

19 First, 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

20 Figure 4.4 Enzyme C T A A A T C G G T Allele for purple flowers Locus for flower-color gene Allele for white flowers G A T T T A G C C A CTAAATCGGT Pair of homologous chromosomes A T A A A T C G G T Enzyme that helps synthesize purple pigment Absence of enzyme One allele results in sufficient pigment T A T T T A G C C A ATAAATCGGT

21 Second, for each character, an organism inherits two alleles, one from each parent Mendel made this deduction without knowing about the role of chromosomes The two alleles at a particular locus 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 F hybrids

22 Third, 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 F plants had purple flowers because the allele for that trait is dominant

23 Fourth, (now known as the law of segregation), 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 organism This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

24 Mendel s segregation model accounts for the 3: ratio he observed in the F 2 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

25 Figure 4.5 P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP pp P p F Generation Appearance: Genetic makeup: Gametes: F 2 Generation / 2 Purple flowers Pp P / 2 p Sperm from F (Pp) plant P p Eggs from F (Pp) plant P p PP Pp Pp pp 3 :

26 Useful Genetic Vocabulary An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character 동형접합체 An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character 이형접합체 Unlike homozygotes, heterozygotes are not true-breeding

27 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 In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes

28 Figure 4.6 Phenotype Genotype Purple PP (homozygous) 3 Purple Pp (heterozygous) 2 Purple Pp (heterozygous) White pp (homozygous) Ratio 3: Ratio :2:

29 The Testcross 검정교배 How can we tell the genotype of an individual with the dominant phenotype? Such an 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

30 Figure 4.7 TECHNIQUE Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If purple-flowered or If purple-flowered parent is PP parent is Pp Sperm Sperm p p p p Eggs P P Pp Pp Pp Pp Eggs P p Pp pp Pp pp RESULTS or All offspring purple / 2 offspring purple and / 2 offspring white

31 The Law of Independent Assortment 독립유전의법칙 Mendel derived the law of segregation by following a single character The F offspring produced in this cross were monohybrids, individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross 단성잡종교배

32 Mendel identified his 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 F generation, heterozygous for both characters A dihybrid cross, a cross between F dihybrids, can determine whether two characters are transmitted to offspring as a package or independently 양성잡종교배

33 Figure 4.8 EXPERIMENT P Generation YYRR yyrr Gametes YR yr F Generation YyRr Predictions Predicted offspring of F 2 generation Eggs Hypothesis of dependent assortment / 2 / 2 YR yr Sperm / 2 YR / 2 3 / 4 YYRR YyRr / 4 yr YyRr yyrr or Hypothesis of independent assortment Eggs / 4 / 4 / 4 YR Yr yr Sperm / 4 YR / 4 Yr / 4 yr / 4 yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyrr yyrr Phenotypic ratio 3: / 4 yr YyRr Yyrr yyrr yyrr 9 / 6 3 / 6 3 / 6 / 6 Phenotypic ratio 9:3:3: RESULTS Phenotypic ratio approximately 9:3:3:

34 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 or those far apart on the same chromosome Genes located near each other on the same chromosome tend to be inherited together

35 Concept 4.2 The laws of probability govern Mendelian inheritance Mendel s laws of segregation and independent assortment reflect the rules of probability When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes independently of another gene s alleles

36 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 곱셈정리 Probability in an F monohybrid cross can be determined using the multiplication rule Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 2 chance of carrying the dominant allele and a 2 chance of carrying the recessive allele

37 Figure 4.9 Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm Sperm / 2 R / 2 r / 2 R R R R r Eggs / 4 / 4 / 2 r r R r r / 4 / 4

38 The addition rule states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities The rule of addition can be used to figure out the probability that an F 2 plant from a monohybrid cross will be heterozygous rather than homozygous

39 Solving Complex Genetics Problems with the Rules of Probability We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied

40 Figure 4.UN0 Probability of YYRR = / 4 (probability of YY) / 4 (RR) = / 6 Probability of YyRR = / 2 (Yy) / 4 (RR) = / 8

41 Figure 4.UN02 ppyyrr ppyyrr Ppyyrr PPyyrr ppyyrr / 4 (probability of pp) / 2 (yy) / 2 (Rr) / 4 / 2 / 2 / 2 / 2 / 2 / 4 / 2 / 2 / 4 / 2 / 2 Chance of at least two recessive traits = / 6 = / 6 = 2 / 6 = / 6 = / 6 = 6 / 6 or 3 / 8

42 Concept 4.3 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

43 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

44 Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

45 Figure 4.0 P Generation Red C R C R White C W C W Gametes C R C W F Generation Pink C R C W Gametes C R / 2 / 2 C W F 2 Generation Sperm / 2 C R / 2 C W Eggs / 2 C R C R C R C R C W / 2 C W C R C W C W C W

46 The Relation Between Dominance and Phenotype A dominant allele does not subdue a recessive allele; alleles don t interact that way 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

47 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

48 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

49 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

50 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: I A, I B, and i. The enzyme encoded by the I A allele adds the A carbohydrate, whereas the enzyme encoded by the I B allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

51 Figure 4. (a) The three alleles for the ABO blood groups and their carbohydrates Allele I A I B i Carbohydrate A B none (b) Blood group genotypes and phenotypes Genotype I A I A or I A i I B I B or I B i I A I B ii Red blood cell appearance Phenotype (blood group) A B AB O

52 Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy 다면발현 For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

53 Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes Epistasis 상위성 Polygenic inhertance 다인자유전

54 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

55 Figure 4.2 BbEe BbEe Eggs Sperm / 4 BE / 4 be / 4 Be / 4 be / 4 BE BBEE BbEE BBEe BbEe / 4 be BbEE bbee BbEe bbee / 4 Be BBEe BbEe BBee Bbee / 4 be BbEe bbee Bbee bbee 9 : 3 : 4

56 Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum 양적형질 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

57 Figure 4.3 AaBbCc AaBbCc Sperm / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 Eggs / 8 / 8 / 8 / 8 / 8 Phenotypes: Number of / 64 6 / 64 5 / / 5 64 / 64 6 / 64 / 64 dark-skin alleles:

58 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 norm of reaction 반응규격 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

59 Figure 4.4

60 Norms of reaction are generally broadest for polygenic characters Such characters are called multifactorial 다인성 because genetic and environmental factors collectively influence phenotype

61 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

62 Concept 4.4 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

63 Pedigree Analysis 가계도분석 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

64 Figure 4.5 Key Male Female Affected male Affected female Mating Offspring st generation 2nd generation 3rd generation Ww ww ww Ww Ww ww ww Ww Ww WW or Ww ww ww st generation 2nd generation FF or Ff 3rd generation Ff Ff ff Ff ff ff Ff ff Ff FF or Ff ff Widow s peak No widow s peak Attached earlobe Free earlobe (a) Is a widow s peak a dominant or recessive trait? b) Is an attached earlobe a dominant or recessive trait?

65 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

66 Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner These range from relatively mild to lifethreatening

67 The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers 보인자 are heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents Albinism 선천성색소결핍증, 백색증 is a recessive condition characterized by a lack of pigmentation in skin and hair

68 Figure 4.6 Normal Aa Sperm Parents Normal Aa Eggs A a A AA Normal Aa Normal (carrier) a Aa Normal (carrier) aa Albino

69 If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low Consanguineous matings 동족교배 (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

70 Cystic Fibrosis 낭포성섬유증 Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent 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

71 Cystic fibrosis (CF), also known as mucoviscidosis, is an autosomal recessive genetic disorder that affects mostly the lungs, and also the pancreas, liver, and intestine. Difficulty breathing is the most serious symptom and results from frequent lung infections. Other symptoms including sinus infections, poor growth, and infertility affect other parts of the body. CF is caused by one of many different mutations in the gene for the protein cystic fibrosis transmembrane conductance regulator (CFTR). CFTR, 70kDa transmembrane glycoprotein and camp-dependent Cl- ion channel The underlying mechanism is abnormal transport of chloride and sodium across the epithelium, which is the cell layer that covers membranes over organs. This leads to thick, viscous secretions. The name cystic fibrosis refers to the characteristic scarring (fibrosis) and cyst formation within the pancreas, first recognized in the 930s.

72 Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications Sickle-cell disease affects one out of 400 African-Americans 겸상적혈구빈혈증 (sickle-cell anemia) The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells [Fig. 5.9] In homozygous individuals, all hemoglobin is abnormal (sickle-cell) The National Heart, Lung, and Blood Institute (NHLBI)

73 Fig. 4-UN 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

74 Figure 4.7 Sickle-cell alleles Low O 2 Sickle-cell hemoglobin proteins Part of a fiber of sickle-cell hemoglobin proteins Sickled red blood cells (a) Homozygote with sickle-cell disease: Weakness, anemia, pain and fever, organ damage Sickle-cell allele Normal allele Very low O 2 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

75 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 연골무형성증 is a form of dwarfism 왜소증 caused by a rare dominant allele An autosomal dominant genetic disorder, is caused by a change in the gene for fibroblast growth factor receptor 3 (FGFR3), which causes an abnormality of cartilage.

76 Figure 4.8 Dwarf Dd Parents Normal dd Eggs d Sperm D Dd Dwarf d dd Normal d Dd Dwarf dd Normal

77 Huntington s Disease: A Late-Onset Lethal Disease The timing of onset of a disease significantly affects its inheritance Huntington s disease is a degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Once the deterioration of the nervous system begins the condition is irreversible and fatal

78 Huntington s Disease: A Late-Onset Lethal Disease Expansion of a CAG (cytosine-adenine-guanine) triplet repeat stretch within the Huntingtin gene results in a different (mutant) form of the protein, which gradually damages cells in the brain, through mechanisms that are not fully understood. The most common genetic cause of abnormal involuntary writhing movements called chorea, which is why the disease used to be called Huntington's chorea 헌팅턴무도병

79 Multifactorial Disorders 다인자성장애 Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components Little is understood about the genetic contribution to most multifactorial diseases

80 Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

81 Counseling Based on Mendelian Genetics and Probability Rules Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders Probabilities are predicted on the most accurate information at the time; predicted probabilities may change as new information is available

82 Tests for Identifying Carriers 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

83 Fetal Testing In amniocentesis, the liquid that bathes the fetus is removed and tested 양수검사 In chorionic villus sampling (CVS), a sample of the placenta is removed and tested 임신초기융모검사 Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

84 Figure 4.9 (a) Amniocentesis Fetus Placenta Uterus Cervix Ultrasound monitor Amniotic fluid withdrawn Centrifugation (b) Chorionic villus sampling (CVS) Ultrasound monitor Fetus Placenta Chorionic villi Uterus Cervix Suction tube inserted through cervix Fluid Fetal cells Several hours 2 Several weeks Biochemical and genetic tests Several hours Fetal cells 2 Several weeks Several hours 3 Karyotyping

85 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 0,000 5,000 births in the United States

86 Figure 4.UN03 Relationship among alleles of a single gene Description Example Complete dominance of one allele Heterozygous phenotype same as that of homozygous dominant PP Pp Incomplete dominance of either allele Codominance Multiple alleles Pleiotropy Heterozygous phenotype intermediate between the two homozygous phenotypes Both phenotypes expressed in heterozygotes In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters C R C R C R C W C W C W I A I B ABO blood group alleles I A, I B, i Sickle-cell disease

87 Figure 4.UN04 Relationship among two or more genes Description Example Epistasis The phenotypic expression of one gene affects that of another BbEe BE BE be Be BbEe be be Be be Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc 9 : 3 : 4 AaBbCc