性连锁遗传ppt课件

1、Sutton and Boveri Hypothesis Sex-linked inheritanceSutton and Boveri Hypothesis内在逻辑关系性染色体与性别源于细胞学研讨性别也是“性状基因决议性状伴性遗传性状性别染色体多基因在染色体上将会有什么问题提出？寻觅基因的实体构造性别决议的染色体学说第一次将性别(性状)同染色体建立了联络1902年，Mcclung; 1905,Wilson遗传的染色体学说细胞学家对遗传学奉献1902-1903年，Sutton、Boveri伴性遗传明确了性状性别和基因的关系(1910,Morgan)染色体学说的直接证据(1916,Bridge)

2、内在逻辑关系性染色体与性别源于细胞学研讨性别也是“性状基因决议性状伴性遗传性状性别染色体性染色体和性别决议知识点： 性染色体 性别是由性染色体决议的 性别决议类型历史：性染色体和常染色体性染色体与性别决议有关，叫性染色体。常染色体与性别决议无亲密关系的染色体，叫常染色体。 性染色体的发现与X0型：Carl Rabl (May 2, 1853-Dec 24, 1917) was an Austrian anatomist. His most notable achievement was on the structural consistency of chromosomes during th

3、e cell cycle. In 1885 he published that chromosomes do not lose their identity, even though they are no longer visible through the microscope. H. v. HENKING (1891) He worked on spermatogenesis and its association with the egg development in the hemiptere Pyrrhocoris apterus and found out that half o

4、f the daughter cells contained one element more than the other during anaphase II. He was not sure whether it was an additional chromosome and thus called the structure X. Mendel遗传学定律,1865年宣读，1866年发表 1902年，Mcclung麦克朗在蚱蜢中发现，体细胞中染色体数体细胞中的染色体数，故他第一次把X体与性别决议联络起来，因他在卵发生过程中没有看到X体，而在有些精子发生过程中看到X体 ，故以为X体是雄性

5、所特有的。 1900年，Mendel规律重新发现，遗传学诞生1903年，Sutton、Boveri；遗传染色体学说1905, Wilson和他的同事对蛛蝽属的昆虫进展了广泛的研讨，发现：蛛蝽属的昆虫性别决议： ：1/26：1/27 ：7 1910年，Morgan，伴性遗传性别基因1916,Bridges遗传的染色体学说的直接证据 染色体与性别决议1、X0型性别决议: A(X) A(X) AA(XX) : A(X) A(0) AA(X0)蚱蜢，蝗虫，蛛蝽属的昆虫。 2、XY型性别决议XY型性别决议：这种雄体体细胞中含有2个异型性染色体，雌体细胞中含有2个同型性染色体。 : A(X) A(X) A

6、A(XX) : A(X) A(Y) AA(XY)XY型性决议的生物： 全部哺乳类、某些两栖类、鱼类、昆虫果蝇、异株的植物女娄菜。 人类的性别决议2n=46,：44XX,：44 + XYX染色体：中等大小，155Mb,已发现有基因1098个基因。Y染色体：很小，60Mb，包含基因数目78个；主要是睾丸决议基因，毛耳基因等剧烈致雄的基因男性特征。人类X和Y染色体图性染色体联会时部分配对3、ZW性别决议ZW型性别决议：雌体个体含有2个异型性染色体，雄体个体含有2个一样类型的性染色体。和XY性正好相反。 : A(Z) A(W) AA(ZW) : A(Z) A(Z) AA(ZZ)ZW型性决议的生物：鸟类

7、，蛾类，蚕和某些鱼类。 4. 性指数决议性别果蝇和线虫虽然也有X和Y染色体，但是其性别决议的机制是取决于性指数sex index即性染色体X和常染色体A组数的比。在人类和果蝇中性染色体和性别的关系XYXXXXXXXYXOXYYX:3A性指数1X:2A =0.52X:2A =13X:2A =1.52X:2A =11X:2A =0.51X:2A =0.51X:3A =0.33人的性别超雌超雄果蝇的性别超雌(不能成活)超雄5、染色体组的倍性决议性别蜜蜂Apis mellifera的性别决议由染色体组的倍性决议的。蜂皇是可育的雌蜂，染色体为2n=32条，经正常减数分裂产生的卵和精子为单倍体n=16，卵

8、和精子结合又构成2n=32的合子，将发育成蜂皇和工蜂2n=32。蜂皇产生的部分单倍体的卵，少数未经过受精但也能发育，长成雄蜂n=16，雄蜂经假减数分裂pseudomeiosis产生两个单倍体n=16的精细胞，发育成精子。6、取决于X染色体能否杂合小茧蜂Habrobracon二倍体2n=20为雌蜂，单倍体n=10为雄蜂。在实验室中，人们获得了二倍体2n=20的雄蜂，其性别决议取决于性染色体X是纯合型还是杂合型。性染色体X有不同的三种类型: Xa、Xb、Xc。雌性的性染色体为一对杂合型的X染色体： XaXb、XaXc 或 XbXc；雄性为纯合型：XaXa、XbXb 或 XcXc。基因与性别决议由复

9、等位基因性别 植物中有异形性染色体的并不多，但有的植物却明显由基因控制，如葫芦科的一种喷瓜Ecballium elaterium，其性别是由复等位基因决议的。喷瓜（Ecballium elaterium）的性别决定基因和显隐性关系决定性别基因型aD aDaD，aDa+，aDada+两性a+a+，a+adadadad2. 由二对基因决议玉米Zea mays普通都是雌雄同株的。雌花的花序在叶腋呈穗状，由显性基因Ba 控制，其隐性等位基由于ba (barren)。雄花的花序在顶端，由显性基因Tstassels控制，其表型性别和基因型亲密相关。玉米的性别决定基因型性别表型BaBa TsTs顶端长雄花序

10、，叶腋长雌花序Ba_ tsts顶端和叶腋都长雌花序baba Ts_顶端长雄花序，叶腋不长花序baba tsts顶端长雌花序，叶腋不长花序环境与性别的决议1 后螠：海底 雌虫吻 雄虫改动条件：间性2、爬行类 性别和孵化温度3、植物雄花和雌花4、激素与性别人妖与太监雄性动物生殖器官切除人染色体异常与疾病1、真两性畸形有3种类型：一侧为卵巢，另一侧为睾丸，称为单侧性真两性畸形，这种类型占40%。两侧均为卵睾(即在一个性腺内既有卵巢组织又有睾丸组织)，卵巢组织与睾丸组织之间有纤维组织相隔，称为双侧性真两性畸形，这种类型占20%。一侧为卵睾，另一侧为卵巢或睾丸，这种类型40%。发病缘由染色体不分别嵌合体

11、46XX/47XXY,46XY/45XO,47XYY/45XO双受精46XY/46XX2、Klinefelter综合征(klinefelters syndrome)47XXY，非典型染色体核型为48, XXXY；49, XXXXY；及嵌合型如46, XY/47, XXY；46, XX/47, XXY 等等。 本病的发病率相当高，男性新生儿中到达1.2。根据白种人的资料，身高180cm的男性患病率为1260，在精神病患者或刑事收容机构中为1100，在因不育而就诊者中约为120。 患者男性第二性征发育差，有女性化表现 缘由The extra X chromosome is retained bec

12、ause of a nondisjunction event during meiosis I (gametogenesis). Nondisjunction occurs when homologous chromosomes, in the case the X and Y sex chromosomes, fail to separate, producing a sperm with an X and a Y chromosome.Turner综合征This condition occurs in about 1 in 2,500 female births worldwide, bu

13、t is much more common among pregnancies that do not survive to term (miscarriages and stillbirths). Signs and symptomsmonosomy X (absence of an entire sex chromosome, the Barr body) is most common. There are characteristic physical abnormalities, such as short stature, swelling, broad chest, low hai

14、rline, low-set ears, and webbed necks 47, XYY AND 47,XXX47, XYY : This condition occurs in about 1 in 1,000 newborn boys. Five to 10 boys with 47,XYY syndrome are born in the United States each day. 47,XXX:Triple X syndrome occurs in around 1 in 1,000girls. On average, five to ten girls with triple

15、X syndrome are born in the United States each day sex-related inheritance一、果蝇作为遗传资料的优点和伴性遗传：双翅目昆虫，体型小，容易豢养。 25时12天就可以完成一个世代，生活史短，后代多。(不要忘了，经典遗传学是统计遗传学，多的后代很有必要；群体大，便于选择突变体)只需4对染色体，容易发现连锁景象, 当然包括性连锁景象。易杂交、好控制存在多线染色体，便于观测染色体构造变化如何解读实验结果？1.群体正常2.红眼显性怎样作测交3.F1红眼雌蝇是杂合子4.F2假设存在白眼雌蝇分别比就不符合3:15.F2白眼雌蝇是纯合ww6

16、.F1白眼雄蝇不能够从P红眼雄蝇获得红眼基因7.F2白眼雌蝇是不能够存在的？根据已有知识如何去解释实验景象？1.F1白眼雄蝇w02.F1红眼雌蝇wW3.P红眼雄蝇自然形状下W04.P白眼雌蝇ww5.F2白眼雌蝇不能够出现6.为什么P红眼雄蝇自然形状下W0？6.为什么P红眼雄蝇自然形状下W0？ Morgan的假设Mendel:基因成对存在，分别来自双亲红眼雄蝇自然形状下红眼基因(W0)不成对遗传的染色体学说：基因也应在性染色体上性别决议染色体学说：性染色体不匹配Morgan的假设：控制果蝇眼色的基因在X染色体上，雄性个体的Y染色体没有这个基因，其传送遵照Mendel颗粒遗传规律。Abraxas

17、lacticolor-moth (蛾子)Abraxas grossulariata-醋栗尺蛾 Sex limited Inheritance in Drosophila. Science. Vol. 32, No. 812. p. 120-122. 1910.T. H. Morgan. Morgan实验景象的解释：几种伴性遗传的引见：1、人类色盲的传送规律：XbY XBXBXBY XBXb XBXb XBYXBXb XBY XBYXbY人类色盲家系图谱这是显性遗传么？是哪一类遗传方式？2、人类血友病的传送规律：血友病传送规律谱系解释：X连锁的近亲婚配 X连锁隐性遗传缺陷 红绿色盲(congen

18、ital dyschromatopsia of the protan and deutan type) 血友病hemophilia 进展性肌营养不良progressive muscular dystorphy 又称为假型肥大pseudomu-pertrophic、或 杜兴氏症Duchennes muscular dystrophy，DMD。 睾丸女性化 (testicular feminization syndrome)、 自毁容貌综合征(Lesch-Nyhan syndrome) 进展性肌营养不良和自毁容貌综合征X染色体连锁隐性遗传系谱的根本特点： 患者男性多于女性 男性患者，儿子正常。经过

19、女儿把致病基因传给外孙 1/2表现出隔代及交叉遗传。 双亲正常时，儿子能够是患者，女儿一定正常。 交叉遗传外公患者、外婆基因型正常，故可推断先证者外公患者的弟兄能够是患者3、连锁的显性遗传病抗维生素型佝偻病:X连锁显性遗传的特点患者双亲中必有一方患有此病，女性患者多于男性，但女性患者病情较轻。男性患者的后代中，女儿都是患者，儿子正常母亲正常。女性患者的后代中，子女各有1/2能够患病杂合子。未患病的后代，可以真实遗传。4、Y连锁遗传控制性状/疾病的基因位于Y 染色体上，基因随Y染色体而传送，由父 子 孙，这种遗传方式称为Y连锁遗传/限雄遗传/全男遗传。在Y染色体上已发现的基因：毛耳基因、睾丸决议

20、基因Y连锁遗传Y-linked inheritance人类的耳道长毛症 印第安人群中较为常见的毛耳缘hairy ear rims)，仅限于男性，青春期过后外耳道长出许多2-3cm的黑色长毛。5、鸡的伴性遗传鸡的芦花羽毛的遗传：B-芦花、b-非芦花 P:芦花(ZBW) 非芦花(ZbZb) 杂交 F1:非芦花(ZbW) 芦花(ZBZb) 雌鸡全部非芦花 雄鸡全部芦花 F2: 芦花 : 非芦花 : 芦花 : 非芦花 (ZBW) (ZbW) (ZBZb) (ZbZb) 1 ： 1 ： 1 ： 16、高等植物的伴性遗传几种雄性异配的植物物种常染色体数大 麻 (Cannabis sativa)20XXXY

21、茜 草 (Humulus lupulus)20XXXY酸 模 (Rumex anglocarpus)14XXXY女娄菜 (Melandrium album)22XXXY植物的伴性遗传枣椰树和石竹科女娄菜属Melandrium album的各个种宽叶B；狭叶b，对花粉是致死7、 不完全性连锁性染色体配对区的基因遗传8、限性遗传与从性遗传P486限性遗传P485从性遗传染色体学说的直接证据1、Bridges反复Morgan实验时所发现的异常景象1、按照性别决议初级、次级例外白眼雌蝇必需有两条X染色体、并且只能来源于母亲，例外红眼雄蝇XY？,且来源父亲？2、检查能否XY?或XX?染色体3、结果出乎预

22、料：X0、XXY4、性别决议出问题了？ X0、XXY(焦点1)5、XwXwY的XwXw只能来源于他的母亲白眼雌果蝇 (焦点2)6、修订性别决议实际实验支持、染色体分别异常实验支持Bridges推测1：初级例外白眼雌果蝇的基因型和性染色体组成：XwXwY。镜检确实如此初级例外红眼雄果蝇基因型和性染色体组成： X+O 镜检确实如此X染色体不分开景象X染色体不分开景象Bridges的推测2： 次级例外白眼雌蝇基因型和性染色体组成：XwXwY镜检确实如此 次级例外用红眼雄蝇基因型和性染色体组成：X+Y镜检确实如此 正常交叉组红眼雌蝇中有XwX+Y基因型，比例2% 镜检确实如此 正常交叉组白眼雄蝇中有X

23、wYY基因型，比例2% 镜检确实如此2、Bridges关于果蝇性别决议的研讨Bridges1932年提出。果蝇：发现有的卵细胞在减数分裂过程中，性染色体不分别，构成异常的配子XX、O等，产生不同性别的果蝇染色体组异常。性别染色体组成性指数超雌2AXXX3/2=1.5雌 3AXXX3/3=1超雄3AXY1/3=0.3雌2AXXY2/2=1 正常2AXX2/2=1间性3AXX2/3=0.7正常2AXY1/2=0.5间性3AXXY2/3=0.7果蝇性别决议的研讨X/A 1.01.01.0-0.50.50.5育性超（高度不育）间性（不育）超（可育）性别的发生1、染色体平衡实际与基因组是有机整体2、果蝇

24、的性别发生3、人类性别发生4、拟南芥花发育1、染色体平衡实际与基因组是有机整体1.1、 果蝇(染色体平衡实际)-解释性别决议的机制:以为在果蝇的X染色体上有许多性基因。在常染色体上有许多性基因。性别决议于基因的平衡，即基因占优势，基因占优势。1.2、 人类为代表的性别决议和剂量补偿效应(剂量上到达平衡)1.2、 人类等哺乳动物性别决议和剂量补偿效应(剂量上到达平衡)(1)猫:巴氏小体的发现：1949年，美学者MIBarr在雌猫的神经细胞核内间期发现一染色很深的小体，在雄猫中却没有这种小体，由于这种小体与性别、X染色体数有关，故称为X染色质体。(2)人： 女性：间期细胞核内有一个巴氏小体。 男性

25、：间期细胞核内没有巴氏小体。 用途：鉴定性别，检查各种X染色体变异 推论：Lyon假说体细胞间期Barr是小体的检查Lyon假说雌性动物体细胞X染色体只需一条是有活性个的，那么雌雄个体X染色体上的基因效果相当X染色体的失活是随机的，即雌性动物的染色体角度看是嵌合体失活发生在胚胎发育的早期，已失活细胞分裂产生的细胞群遗传构造一样。雌性动物伴性基因的作用产生嵌合体Lyon假说的证据Barr氏小体：人类染色体变异玳瑁猫体细胞培育系X连锁有关基因活性检测剂量平衡实现的途径两条X染色体都有活性，但转录速率不同假设蝇，雄性个体X染色体基因超活性转录异染色质化。只需一条X染色体有活性，另一条失活。如高等哺乳

26、动物，构成巴氏小体。雌性个体两条X染色体基因活性降低，到达和雄性个体相当程度异配比如雄性动物X染色体上的基因活力加强坚持这种差别1.3人类x染色体失活并不意味全部基因封锁，依然有基因转录，并且这些基因是散步在X-染色体上XIC：x染色体失活中心XIST: RNAThe X-inactivation center and inactivationGenes are shown in bold, with the direction of transcription indicated by the arrow. Brx (brain X-linked), Tsx (testis X-linked

27、) and Cdx4 (Caudal-4) are genes that lie within the Xic that do not have, at least for the moment, any defined X-inactivation function. The 2.1(2)P region shows differential histone H4 hyperacetylation in undifferentiated female and male embryonic stem (ES) cells and has been suggested as a possible

28、 regulatory element in X inactivation. P1 and P2 are the somatic Xist promoters, and P0 is a postulated Xist promoter in undifferentiated ES cells and early embryos. S12 and S19 are ribosomal protein pseudogenes found 5 to Xist in the mouse Xic. Blue bars indicate the regions that have been implicat

29、ed in specific functions. Effects on choice and counting have not so far been distinguished in the regions indicated by light bars. The terminal two Xist exons, lying within the 65-kb deletion, have no effect on counting or choice.X-chromosome inactivation: counting, choice and initiation. Nature Re

30、views Genetics 2, 61.Xist geneThe Xist RNA, a large (17 kb in humans) transcript, is expressed on the inactive chromosome and not on the active one. It is processed in a similar way to mRNAs, through splicing and polyadenylation. However, it remains untranslated, suggested that this RNA gene evolved

31、 at least partly from a protein coding gene that became a pseudogene.The inactive X chromosome is coated with this transcript, which is essential for the inactivation.X chromosomes lacking Xist will not be inactivated, while duplication of the Xist gene on another chromosome causes inactivation of t

32、hat chromosome.Tsix: antisense transcript of XistThe Tsix is a antisense transcript gene of the Xist at the XIC center. The Tsix antisense transcript acts in cis to repress the transcription of Xist, which negatively regulates its expression. The mechanism behind how Tsix modulates Xist activity in

33、cis is poorly understood; but one theory believes that Tsix is involved in chromatin modification at the Xist locus and another believes that transcription factors of pluripotent cells play a role in Xist repression.Regulation factors of the Xist promoterMethylation of the Xist promotor by DNA methy

34、l transferases The Tsix antisense is believed to activate DNA methyl transferases that methylate the Xist promoter, in return resulting in inhibition of the Xist promoter and thus the expression of the Xist gene. dsRNA and RNAi pathway Dicer is an RNAi enzyme and it is believed to cleave the duplex

35、of Xist and Tsix at the beginning of X inactivation, to small 30 nucleotide RNAs(xiRNAs), These xiRNAs are believed to be involved in repressing Xist on the probable active X chromosome. Dicer levels were decreased to 5%, which led to an increase in Xist expression in undifferentiated cells.Tsix ind

36、ependent mechanismPluripotent cells transcriptional factors Pluripotent cells consist of transcriptional factors Nanog, Oct4 and Sox2 that seem to play a role in repressing Xist. (1) In the absence of Tsix in pluripotent cells, Xist is repressed. (2) Nanog or Oct4 transcriptional factors were deplet

37、ed in pluripotent cells, but Xist was in the upregulation, Nanog and Oct4 are involved in the repression of Xist expression.Polycomb Repressor Complex 2 (PRC2) The PRC2 has been observed to repress Xist expression independent of the Tsix antisense transcript, although the definite mechanism is still

38、 not known.玳瑁猫X染色体随机失活，发生在胚胎发育的早期，已失活细胞分裂产生的细胞群遗传构造一样。2 果蝇的性别发生2.1 Ratios of X chromosomes to autosomes in different sexual phenotypes in Drosophila melanogaster ( After Strickberger 1968. From: Chromosomal Sex Determination in Drosophila )X chromosomesAutosome sets(A)X:A ratioSex321.5Metafemale431.

39、33Metafemale441Normal female331Normal female221Normal female230.66Intersex120.5Normal male130.33Metamale2.2 Regulation cascade of Drosophila somatic sex determinationArrows represent activation, while a block at the end of a line indicates suppression. The msl loci, under the control of the Sxl gene

40、, regulate the dosage compensatory transcription of the male X chromosome. (After Baker et al. 1987.) From: Chromosomal Sex Determination in Drosophila2.3 Interpreting the x:a ratioX:A ratio is measured by competition between X-encoded activators and autosomally encoded repressors of the promoter of

41、 the Sxl gene. This female-specific activation of Sxl is thought to be stimulated by “numerator proteins encoded by the X chromosome including Sisterless-a and Sisterless-b. These proteins bind to the “early promoter of the Sxl gene to promote its transcription shortly after fertilization. The “deno

42、minator proteins are autosomally encoded proteins such as Deadpan and Extramacrochaetae blocking the binding or activity of the numerator proteins. The denominator proteins may actually be able to form inactive heterodimers with the numerator proteins2.3 The differential activation of the sxl gene i

43、n females and males two X chromosomes and two sets of autosomes (2X:2A), the numerator proteins (sis-a, sis-b, etc.) are not all bound by inhibitory denominator proteins (such as deadpan) on the autosomes. The numerator proteins activate the early promoter of the Sxl gene. Eventually, in both males

44、and females, constitutive transcription of sxl starts from the late promoter. If Sxl is already available (i.e., from early transcription), the Sxl pre-mRNA is spliced to form the functional female-specific message. one X chromosome and two sets of autosomes (1X:2A), the numerator proteins are bound

45、 by the denominator proteins and cannot activate the early promoter. When the Sxl gene is transcribed from the late promoter, RNA splicing does not exclude the male-specific exon in the mRNA. The resulting message encodes a truncated and nonfunctional peptide, since the male-specific exon contains a

46、 translation termination codon. (After Keyes et al. 1992.) From: Chromosomal Sex Determination in Drosophila,Developmental Biology. 6th edition.2.4 Maintenance of sxl functionThe pattern of sex-specific RNA splicing in three major Drosophila sex-determining genes. In each case, the female-specific t

47、ranscript is shown at the left, while the default transcript (whether male or nonspecific) is shown to the right. Exons are numbered, and the positions of the termination codons and poly(A) sites are marked. (After Baker 1989.) From: Chromosomal Sex Determination in Drosophila2.2 Regulation cascade

48、of Drosophila somatic sex determinationArrows represent activation, while a block at the end of a line indicates suppression. The msl loci, under the control of the Sxl gene, regulate the dosage compensatory transcription of the male X chromosome. (After Baker et al. 1987.) From: Chromosomal Sex Det

49、ermination in DrosophilaControl of Sexual Behavior3 sex determination of human and animalSRY and Y chromosome genesMore steps and cascadeMore proteins took part in Human chromosome At each end of the human X and Y chromosomes are the pseudoautosomal regions (PAR), which recombine during meiosis and

50、therefore contain the same genes. The non-pseudosomal portion of the X (NPX) chromosome and male-specific portion of the Y (MSY) chromosome do not recombine with each other, and therefore contain genes that are no longer alleles. Sex differences between XX and XY organs arise because of differences

51、in dosage of NPX and MSY genes, and because females but not males inherit the paternal X chromosome imprint (Xp). Sex differences are also created because XX organs are a mosaic of cells that express different alleles at polymorphic X loci, whereas XY organs are not mosaic for this reason Human sex

52、determinationSRY was a TFSRY protein binding to DNA Schematic model of SRY-directed assembly of a male-specific transcriptional pre-initiation complex.sex determination in mice Cellular mechanism of SRY functionIn the cytoplasm, SRY is bound by calmodulin (CaM) and importin (Imp), which recognize th

53、e N- and C-terminal nuclear localization signals (NLSs) on SRY, respectively, and recruit it to enter the nucleus. At 10.5 dpc(days post coitum (dpc), SRY and steroidogenic factor 1 (SF1) bind directly to specific sites (TESCO, testis-specific enhancer of Sox9 core) that lie within the gonadal speci

54、fic enhancer of Sox9 (indicated by the coloured regions on the DNA) and pregulate Sox9 expression cooperatively.At 11.5 dpc, after initiation of Sox9 expression, an auto-regulation system operates in which SOX9 also binds directly to TESCO with SF1 to prolong and amplify Sox9 expression. Abbreviatio

55、ns: SOX9, SRY box containing gene 9; SRY, sex-determining region on the chromosome Y.Flower developmentThe ABC model of flower development was first formulated by George Haughn and Chris Somerville in 1988.It was first used as a model to describe the collection of genetic mechanisms that establish f

56、loral organ identity in the Rosids, as exemplified by Arabidopsis thaliana, and the Asterids, as demonstrated by Antirrhinum majus. Both species have four verticils (sepals, petals, stamens and carpels), which are defined by the differential expression of a number of homeotic genes present in each v

57、erticil. This means that the sepals are solely characterized by the expression of A genes, while the petals are characterized by the co-expression of A and B genes. The B and C genes establish the identity of the stamens and the carpels only require C genes to be active. It should be noted that type

58、 A and C genes are reciprocally antagonistic.Steps of flower developmentApical meristemInflorescence meristemFlower meristem on which flowers developFloral organ identity of ABCs.Barry Causiera,Zsuzsanna Schwarz-Sommerb,Brendan Daviesa.Review Floral organ identity: 20 years of ABCs.(Seminars in Cell

59、 & Developmental Biology,2021)ABC model overviewSimple rule that underlie the whorl specifications using floral homeotic mutantsClass A mutants have carpels in the first whorl instead of sepals, and stamens in the second whorl in place of petalsClass B mutants have sepals rather than petals in the second whorl and carpels in the rather than stamens in the third whorlClass C