遗传学英文课件：10 复杂性状遗传

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1、2021-12-10Inheritance of Complex Traits1Chapter 10Inheritance of Complex Traits2021-12-10Inheritance of Complex Traits2Chapter OutlineComplex Patterns of InheritanceStatistics of Quantitative GeneticsAnalysis of Quantitative traitsQuantitative Genetics of Human Behavioral Traits2021-12-10Inheritance

2、 of Complex Traits3航空报国英模：罗阳航空报国英模：罗阳2012年年11月月24日舰载机成功起降日舰载机成功起降“辽宁辽宁”号号2012年年11月月25日总指挥罗阳因心肌梗死逝世日总指挥罗阳因心肌梗死逝世原因何在？原因何在？2021-12-10Inheritance of Complex Traits4Cardiovascular Disease: A Combination of Genetic and Environmental Factors2021-12-10Inheritance of Complex Traits5Section 1Complex Patterns

3、 of InheritanceBreeding experiments and comparisons between relatives reveal that complex phenotypes may be influenced by a combination of genetic and environmental factors.2021-12-10Inheritance of Complex Traits61. Quantitative TraitsNature is full of examples of traits that vary continuouslybody s

4、ize, height, weight, the activity of a particular enzyme, reproductive ability, and general vigor and fitness, to name just a few.These are among the most interesting traits in biology, especially from an evolutionary standpoint.2021-12-10Inheritance of Complex Traits7One phenotype seems to blend im

5、perceptibly into the next; however, if we examine many individuals in a population, we usually find significant differences among them.Often this type of variation can be quantified by measuring the trait in a sample of individuals from the population.2021-12-10Inheritance of Complex Traits8We might

6、, for example, capture mice in a barn and weight them or collect corncobs from a field and count the number of kernels on each.With such a quantitative approach, the phenotype of every individual in the sample is reduced to a number.These numbers can be analyzed with a variety of statistical techniq

7、ues, enabling us to study the trait and , in particular to investigate its genetic basis.2021-12-10Inheritance of Complex Traits9Traits that are amenable to this kind of treatment are called quantitative traits (数数量性状量性状).Their essential characteristic is that they can be measured.2021-12-10Inherita

8、nce of Complex Traits10If can we link the evolution with genetics? 2021-12-10Inheritance of Complex Traits11 A growing body of evidence suggested that the complex inheritance of quantitative traits could be explained by Mendelian theory. Two insights made this possible:1. Quantitative traits are inf

9、luenced by many genes.2. Quantitative traits are influenced by many factors in environment. It is the combination of genetic and environmental influences that makes quantitative traits difficult to study. These traits depend on a complex mix of factors; that is, they are multifactoral.2021-12-10Inhe

10、ritance of Complex Traits12Wilhelm Johannsen1857-1921One of the first people to show that variation in a quantitative trait is due to genetic and environment factors.He studied the weight of seeds from the broad bean, Phaseolus vulgaris (蚕豆蚕豆).Among the plants available to him, seed weight varied fr

11、om 150 mg to 900 mg.He established lines from individual seeds across this range and maintained each line by self-fertilization for several generation.丹麦生物学家丹麦生物学家基因概念的提出者基因概念的提出者2021-12-10Inheritance of Complex Traits13Johannsen observed that seed weight also varied within each of the pure lines.Th

12、is residual variation in this trait is due to genetic differences because each line had been systematically inbred to make it homozygous for its genes.Rather, it must have been due to variation in uncontrolled factors in the environment.His work led to the realization that phenotypic variation in a

13、quantitative trait has two componentsone genetic, the other environmental.2021-12-10Inheritance of Complex Traits14H. Nilsson-Ehle He provided evidence that the genetic component of this variation could involve the contributions of several different genes.Nilsson-Ehle studied color variation in whea

14、t grains.When he crossed a white-grained variety with a dark red-grained variety, he obtained an F1 with an intermediate red phenotype.Self-fertilization of the F1 produced an F2 with seven distinct classes, ranging from white to dark red.2021-12-10Inheritance of Complex Traits15Inheritance of grain

15、 color in wheat.Three independently assorting genes (A, B, and C) are assumed to control grain color.Each gene has two alleles.The alleles for pigmentation (upper case letter) are semidominant over the alleles for lack of pigmentation (lower case letters).这就能用多基因的分离和组合来解释这就能用多基因的分离和组合来解释2021-12-10In

16、heritance of Complex Traits16The American geneticist East extended Nilsson-Ehles studies to a trait that did not show simple Mendelian ratios in the F2.East studied the lengthen of the corolla in tobacco flowers.In one pure line, the corolla length averaged 41mm; in another, it averaged 93 mm.2021-1

17、2-10Inheritance of Complex Traits172021-12-10Inheritance of Complex Traits18Within each pure line, East observed some phenotypic variationpreassembly the result of environmental influences.By crossing the two lines, East obtained an F1 that had intermediate corolla length and approximately the same

18、amount of variation that had seen within each of the parental strains.When East intercrossed the F1 plants, he obtained an F2 with about same corolla length, on average, that he saw in the F1; however, the F2 plants were much more variable than the F1.2021-12-10Inheritance of Complex Traits19 The va

19、riability was due to two sources: the segregation and independent assortment of different pairs of alleles controlling corolla length, and environmental factors. Each inbred some of the F2 plants to produce an F3 and observed less variation within the different F3 lines than in the F2. The reduced a

20、mount of variation within the F2 lines was presumably due to the segregation of fewer allelic differences.2021-12-10Inheritance of Complex Traits20Thus, the complex inheritance pattern that East observed with corolla length could be explained by a combination of genetic segregation and environmental

21、 influences.2021-12-10Inheritance of Complex Traits21How many genes were involved in determining corolla length in Easts strains of tobacco?We can make a crude guess by comparing the F2 plants with each of the inbred parental strains.Lets suppose that the strain with the shorter corollas was homozyg

22、ous for one set of alleles and that the strain with the longer corollas was homozygous for another set of alleles.2021-12-10Inheritance of Complex Traits22Furthermore, lets suppose that the long-corolla alleles are semidominant, that all length-controlling genes assort independently, and that each g

23、ene makes an equal contribution to the phenotype.If corolla length were determined by one gene, with alleles a (for short corolla) and A (for long corolla), we would expect of F2 to have short corollas (like the short parental strain) and to have long corollas (like long parental strain). 2021-12-10

24、Inheritance of Complex Traits23If two genes determined corolla length, we would expect 1/16 to resemble the long-corolla parent.If three genes were involved, the frequency of each parental type in F2 would be 1/64, and if four genes were involved, it would be 1/256.With five genes, the parental freq

25、uencies in the F2 would each be 1/1024.2021-12-10Inheritance of Complex Traits24East studied 444 F2 plants and failed to find even one with either of the parental phenotypes.This failure would seem to rule out the hypothesis of four or fewer genes controlling corolla length.Thus, we can conclude tha

26、t at least five genes are responsible for the difference in corolla length between Easts two inbred strains.至少有至少有5个基因参与了花冠长度的调控！个基因参与了花冠长度的调控！2021-12-10Inheritance of Complex Traits252. Threshold Traits (阈性状阈性状) Continuously varying traits such as corolla length, grain color, and bean size are cont

27、rolled by multiple factors, both genetic and environmental.Geneticists have found that some discontinuous traits also appear to be multifactorial.For example, some humans are born with cleft lip, a condition due to an error in embryological development.2021-12-10Inheritance of Complex Traits26Althou

28、gh family studies have suggested that cleft lip is caused by genetic and environmental factors, it is not a quantitative trait in the usual sense; individuals either express it or they do not.Nonetheless, the genetic and environmental factors that predispose an individual to show it appear to vary q

29、uantitatively.Thus, although cleft lip is manifested discontinuously, its underlying variation seems to be continuous.2021-12-10Inheritance of Complex Traits27 Geneticists theorize that when the underlying variable exceeds a certain level, the trait appears. This type of trait is therefore called a

30、threshold trait. 当潜在的变量超出一定水平的当潜在的变量超出一定水平的时候，性状就会表现出来，这时候，性状就会表现出来，这就是阈性状就是阈性状A model for expression of a threshold trait.When the underlying variable, the “liability(易患性易患性),” reaches a threshold value, the trait is expressed. This variable is assumed to be continuously distributed in the populati

31、on.2021-12-10Inheritance of Complex Traits28There is, in effect, a genetic “liability” to develop the condition.However, only those individuals whose liability crosses the threshold are actually at risk.The underlying variable is a function of factors in the genotype as well as factors in the enviro

32、nment.Some individuals carry alleles that predispose them to develop cleft lip.2021-12-10Inheritance of Complex Traits29In humans, the evidence that traits such as cleft lip are influenced by genetic factors comes from comparisons between relatives, especially twins.Occasionally a fertilized human e

33、gg splits and forms two genetically identical zygotes.The individuals who develop from these zygotes are referred to as one-egg, or monozygotic (MZ), twins; they share 100% of their genes.2021-12-10Inheritance of Complex Traits30单羊膜腔单羊膜腔2021-12-10Inheritance of Complex Traits31More frequently, two i

34、ndependently fertilized eggs develop at the same time in the mother wombs.These two-egg, or dizygotic (DZ), twins are closely related as ordinary siblings; thus, they share 50% of their genes.Because of their genetic identity, we would expect MZ twins to be phenotypically similar than DZ twins.2021-

35、12-10Inheritance of Complex Traits32Similarity with respect to a trait like cleft lip is assessed by determining the concordance rate (一致率一致率)the fraction of twin pairs in which both twins show the trait among pairs in which at least one of them does.For cleft lip, the concordance rate has been esti

36、mated to be about 40% for MZ twins and about 4% for DZ twins.The much greater concordance rate for MZ twins strongly suggests that genetic factors influence an individuals likelihood of being born with cleft lip. 2021-12-10Inheritance of Complex Traits33Mental illnesses such as schizophrenia and man

37、ic-depressive disorder can also be regarded as threshold traits.For schizophrenia, the concordance rate ranges from 30 to 60% for MZ twins and from 6 to 18% for DZ twins; for manic-depressive disorder, the concordance rate is 70-80% for MZ twins and about 20% for DZ twins.Thus, twin studies suggest

38、that both of these mental illness are influenced by genetic factors.2021-12-10Inheritance of Complex Traits34Key PointsResemblances between relatives and responses to selective breeding indicate that complex traits have a genetic basis.Some complex traits can be quantified to permit genetic analysis

39、 based on Mendelian principles.Many genetic and environmental factors influence the variation observed in quantitative traits.2021-12-10Inheritance of Complex Traits35Key PointsPhenotypic segregations may provide a way to estimate the number of genes that influence a quantitative trait.Traits that a

40、re manifested when an underlying continuous variable (the liability) reaches a threshold value may be influenced by genetic factors.In humans, evidence that a threshold trait has a genetic basis comes from studies with twins.The concordance rate is the fraction of twin pairs in which both twins show

41、 a trait among pairs in which at least one of them does.2021-12-10Inheritance of Complex Traits36Section 2Statistics of Quantitative GeneticsThe frequency distributions of quantitative traits can be characterized by summary statistics.2021-12-10Inheritance of Complex Traits37The hallmark of quantita

42、tive traits is that they vary continuously in a population of individuals.This type of variation poses a formidable problem for the geneticist.Segregation ratios are difficult, if not impossible, to discern because the number of phenotypes is large and one phenotype blends imperceptibly into the nex

43、t.2021-12-10Inheritance of Complex Traits38For quantitatively varying traits, routine genetic analyses of the sort that we have done with seed color in peas and with human disorders such as albinism are out of the question.For these types of traits we must resort to a different kind of analysis, one

44、 that is based on statistical descriptions of phenotype in a population.2021-12-10Inheritance of Complex Traits391. Frequency DistributionThe first step in the study of any quantitative trait is to collect measurements of the trait from individuals in a population.Usually, only a small fraction of a

45、ll the individuals in the population can be measured.We call this group the sample (样本样本).The data from the sample can be presented graphically as a frequency distribution (频数频数分布分布).2021-12-10Inheritance of Complex Traits40 Frequency distributions and descriptive statistics of time to maturity in f

46、our populations of wheat. A and B are inbred strains that were crossed to produce F1 hybrids. The F1 plants were intercrossed to produce an F2. Seed from all four populations was planted in the same season to determine the time to maturity. In each case, data were obtained from 40 plants.2021-12-10I

47、nheritance of Complex Traits412. The Mean and the Modal ClassThe essential characteristics of a frequency distribution can be summarized by simple statistics calculated from the data.One of these summary statistics is called the mean or average =( Xk)/nX2021-12-10Inheritance of Complex Traits42The m

48、odal class (众数组众数组) in a sample that contains the most observations.In each of the distributions the mean is within or very close to the model class. Like the mean, it also captures the “center” of the distribution.This coincidence reflects the symmetry of the distributions; in each case, roughly eq

49、ual numbers are above and below the mean and the modal class.Not all distributions have this feature.2021-12-10Inheritance of Complex Traits43Some are skewed(偏离的偏离的), with most of the observations clustered at one end and only a few at the other end forming a long tail.Statisticians have developed a

50、n extensive theory about a particular type of symmetrical distribution called a normal distribution (正态分布正态分布).In this bell-shaped distribution, the mean and the modal class are located exactly in the center.2021-12-10Inheritance of Complex Traits443. The variance and the Standard DeviationThe data

51、in a frequency distribution could be dispersed, or they could be clustered.To measure the spread of data in a frequency distribution, we use a statistic called variance (方差方差).Data that are widely dispersed produce a large value for the variance, whereas data that are tightly clustered produce a sma

52、ll value.s2 = (X -X)2/(n-1)s2 = (X - 2021-12-10Inheritance of Complex Traits45 We should note two features of the variance.1. Firstly, it measures the dispersion of the data around the mean.When we calculate the variance, we take the mean to be central value of the distributions in the sample.1. Sec

53、ond, the variance is always positive.When we calculate the variance, we square the difference between each observation and the mean, and then sum the squared differences. 2021-12-10Inheritance of Complex Traits46Although the variance has desirable mathematical properties, it is difficult to interpre

54、t because the units of measurement are squared.Consequently, another statistic, called the standard deviation, is often used to be describe the variability of a sample.The standard deviation (s) is the square root of the sample variance.2SS=2021-12-10Inheritance of Complex Traits47Key PointsThe mean

55、 ( = (X )/n) and modal class point to be center of a frequency distribution.The variance (s2= (X - )2/(n-1) and standard deviation s= are statistics that indicate the extent to which data are scattered around the mean in a frequency distribution.X2SX2021-12-10Inheritance of Complex Traits48Section 3

56、Analysis of Quantitative traitsQuantitative geneticists focus their analyses on phenotypic variability as measured by the variance.2021-12-10Inheritance of Complex Traits49The key idea in quantitative genetics is that traits are controlled by many different factors in the environment and in the geno

57、type.Fisher hypothesized that a particular value of a quantitative trait, T, is the result of the combined influence of genetic and environmental factors.1 The Multiple Factor Hypothesis2021-12-10Inheritance of Complex Traits50He represented the effects of these factors as deviations from the over a

58、ll population mean:T= +g+eIn this equation, the represents the population mean, g represents the deviation from the genetic factors, and e represents the deviation from the mean that is due to environmental factors.2021-12-10Inheritance of Complex Traits51Quantitative phenotypes and the deviations o

59、f individual measurements from the population mean.2021-12-10Inheritance of Complex Traits52Some factors produce large values of T, and some produce small value of T.For each individual, these factors are different.Furthermore, Fisher emphasized that a multitude of factors are involved.He hypothesiz

60、ed that many genes contribute to a quantitative trait, and he assumed that many genes contribute to a quantitative trait, and he assumed that many aspect of the environment also make contributions.A trait that is controlled by many genes is polygenic (多基因的多基因的).2021-12-10Inheritance of Complex Trait

61、s532 Partitioning the Phenotypic Variance (表型方差的分割表型方差的分割)VT= Vg + VeVe=(VA+ VB + VC)/3=(1.92 days2 + 2.05 days2 + 2.88 days2)/3=2.28 days2Vg= VT Ve=14.26 days2 2.28 days2=11.98 days22021-12-10Inheritance of Complex Traits54VT= VG + Ve14.26 days2 = 11.98 days2 + 2.28 days2Form this partition, we see

62、 that most of the variance in maturation time in the F2 wheat population is due to genetic differences among the individuals.This genetic variability arose from the segregation and assortment of genes when the F1 plants reproduced.2021-12-10Inheritance of Complex Traits553. Broad-Sense Heritability

63、(广义遗传力广义遗传力)Often it is informative to calculate the proportion of the total phenotypic variance that is due to genetic differences among individuals in a population.This proportion is called the broad-sense heritability, symbolized H2.In terms of Fishers variance components,H2 +Vg/VT = Vg/(Tg+Ve)20

64、21-12-10Inheritance of Complex Traits56Broad-sense heritability:In quantitative genetics, the proportion of the total phenotypic variance that is due to genetic factors (因遗传因素导致的全部表型因遗传因素导致的全部表型方差的部分方差的部分).Because of the way it is calculated, the broad-sense heritability must lie between 0 and 1.202

65、1-12-10Inheritance of Complex Traits57If it is close to 0, little of the observed variability in the population is attributable to genetic differences among individuals.If it is close to 1, most of the observed variability is attributable to genetic differences.The broad-sense heritability therefore

66、 summarizes the relative contributions of genetic and environmental factors to the observed variability in a population.2021-12-10Inheritance of Complex Traits58However, it is important to note that this statistic is population-specific.For a given trait, different populations may have different values of the broad-sense heritability.Thus, the broad-sense heritability of one population cannot automatically be assumed to represent the broad-sense heritability of another population.2021-12-10Inher