性连锁遗传课件

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Sutton and Boveri Hypothesis Sex-linked inheritance,Sutton and Boveri Hypothesis,内在逻辑关系,性染色体与性别源于细胞学研究,性别也是“性状”基因决定性状,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？,寻找基因的实体结构,性别决定的染色体学说第一次将性别(性状)同染色体建立了联系（1902年，Mcclung; 1905,Wilson）,遗传的染色体学说细胞学家对遗传学贡献（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),内在逻辑关系,性染色体与性别源于细胞学研究,性别也是“性状”基因决定性状,伴性遗传性状,性别,染色体,性染色体和性别决定,知识点：,性染色体,性别是由性染色体决定的,性别决定类型,历史：,性染色体和常染色体,性染色体与性别决定有关，叫性染色体。,常染色体与性别决定无密切关系的染色体，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 the 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 of 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体与性别决定联系起来，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 ，故认为X体是雄性所特有的。,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行了广泛的研究，发现：,蛛蝽属的昆虫性别决定：,：1/2（6）：1/2（7）,：（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,染色体与性别决定,1、X0型性别决定,: A(X) A(X) AA(XX),: A(X) A(,0,) AA(X,0,),蚱蜢，蝗虫，蛛蝽属的昆虫。,2、XY型性别决定,XY,型性别决定：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) AA(XX),: A(X),A(,Y,) AA(X,Y,),XY,型性决定的生物：,全部哺乳类、某些两栖类、鱼类、昆虫（果蝇）、,异株的植物（女娄菜）。,人类的性别决定,2n=46,：44XX,：44 + XY,X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；主要是睾丸决定基因，毛耳基因等强烈致雄的基因（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性别决定,ZW型性别决定：雌体个体含有2个异型性染色体，雄体个体含有2个相同类型的性染色体。和XY性正好相反。,: A(Z) A(W) AA(ZW),: A(Z) A(Z) AA(ZZ),ZW型性决定的生物：鸟类，蛾类，蚕和某些鱼类。,4.,性指数决定性别,果蝇和线虫虽然也有X和Y染色体，但是其性别,决定的机制是取决于,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,XY,XX,XXX,XXY,XO,XYY,X:3A,性指数,1X:2A,=0.5,2X:2A,=1,3X:2A,=1.5,2X:2A,=1,1X:2A,=0.5,1X:2A,=0.5,1X: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、染色体组的倍性决定性别,蜜蜂（,Apis mellifera,）的性别决定由染色体组的倍性决定的。,蜂皇是可育的雌蜂，染色体为2,n,=32条，经正常减数分裂产生的卵和精子为单倍体,n,=16，卵和精子结合又形成2,n,=32的合子，将发育成蜂皇和工蜂（2,n,=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染色体是否杂合,小茧蜂（,Habrobracon,）二倍体（2,n,=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（2,n,=20）,的雄蜂，其性别决定取决于性染色体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,。,基因与性别决定,由复等位基因性别,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定的。,喷瓜,（,Ecballium elaterium,）,的性别决定,基因和显隐性关系,决定性别,基因型,a,D,a,D,a,D,，a,D,a,+,，a,D,a,d,a,+,两性,a,+,a,+,，a,+,a,d,a,d,a,d,a,d,2. 由二对基因决定,玉米（,Zea mays,）一般都是雌雄同株的。雌花的花序在叶腋呈穗状，由显性基因,Ba,控制，其隐性等位基因为,ba,(barren)。雄花的花序在顶端，由显性基因,Ts,（tassels）控制，其表型性别和基因型密切相关。,玉米的性别决定,基因型,性别,表型,BaBa TsTs,顶端长雄花序，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,环境与性别的决定,1 后螠：,海底 雌虫,吻 雄虫,改变条件：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%,。,发病原因,染色体不分离嵌合体,46XX/47XXY,46XY/45XO,47XYY/45XO,双受精,46XY/46XX,2、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，男性新生儿中达到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,原因,The extra X chromosome is retained because 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, but is much more common among pregnancies that do not survive to term (miscarriages and stillbirths).,Signs and symptoms,monosomy 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 hairline, low-set ears, and webbed necks,47, XYY AND 47,XXX,47, 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 X syndrome are born in the United States each day,sex-related inheritance,一、果蝇作为遗传材料的优点和伴性遗传：,双翅目昆虫，体型小，容易饲养。,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,),只有,4,对染色体，容易发现连锁现象,当然包括性连锁现象,。,易杂交、好控制,存在多线染色体，便于观测染色体结构变化,如何解读试验结果？,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若存在白眼雌蝇分离比就不符合3:1,5.F2白眼雌蝇是纯合ww,6.F1白眼雄蝇不可能从P红眼雄蝇获得红眼基因,7.,F2白眼雌蝇是不可能存在的？,根据已有知识如何去解释试验现象？,1.F1白眼雄蝇w,0,2.F1红眼雌蝇w,W,3.P红眼雄蝇自然状态下,W0,4.P白眼雌蝇ww,5.F2白眼雌蝇不可能出现,6.为什么P红眼雄蝇自然状态下W0？,6.为什么P红眼雄蝇自然状态下W,0,？ Morgan的假设,Mendel:,基因,成对,存在，分别来自双亲,红眼雄蝇自然状态下红眼基因,(W,0,),不成对,遗传的染色体学说：基因也应在性染色体上,性别决定染色体学说：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,，,其传递遵循,Mendel,颗粒遗传规律,。,Abraxas 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试验现象的解释：,几种伴性遗传的介绍：,1、人类色盲的传递规律：,X,b,Y X,B,X,B,X,B,Y X,B,X,b,X,B,X,b,X,B,Y,X,B,X,b,X,B,Y,X,B,Y,X,b,Y,人类色盲家系图谱,这是显性遗传么？是哪一类遗传方式？,2、人类血友病的传递规律：,血友病传递规律谱系解释：,X连锁的近亲婚配,X连锁隐性遗传缺陷,红绿色盲,(congenital dyschromatopsia of the protan,and deutan type),血友病,（hemophilia）,进行性肌营养不良,（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),进行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基本特点：,患者男性多于女性,男性患者，儿子正常。通过女儿把致病基因,传给,外孙（,1/2,）,表现出隔代及交叉遗传。,双亲正常时，儿子可能是患者，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）的弟兄可能是患者,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,可能患病（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染色体而传递，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、睾丸决定基因,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(Z,B,W) 非芦花(Z,b,Z,b,),杂交,F1:非芦花(Z,b,W) 芦花(Z,B,Z,b,),雌鸡全部非芦花,雄鸡全部芦花,F2: 芦花 : 非芦花 : 芦花 : 非芦花,(Z,B,W) (Z,b,W) (Z,B,Z,b,) (Z,b,Z,b,),1 ： 1 ： 1 ： 1,6、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,),20,XX,XY,茜 草 (,Humulus lupulus,),20,XX,XY,酸 模 (,Rumex anglocarpus,),14,XX,XY,女娄菜 (,Melandrium album,),22,XX,XY,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重复Morgan试验时所发现的异常现象,1、按照性别决定初级、次级例外白眼雌蝇必须有,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、检查是否,XY?,或,XX?,染色体,3、结果出乎预料：,X0,、,XXY,4、性别决定出问题了？,X0,、,XXY,(,焦点1,),5、,X,w,X,w,Y,的,X,w,X,w,只能来源于他的母亲（白眼雌果蝇） (,焦点2,),6、修订性别决定理论（,实验支持,）、染色体分离异常（,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：,X,w,X,w,Y,。,（镜检的确如此）,初级例外红眼雄果蝇基因型和性染色体组成：,X,+,O,（镜检的确如此）,X染色体不分开现象,X染色体不分开现象,Bridges的推测（2）：,次级例外白眼雌蝇基因型和性染色体组成：,X,w,X,w,Y,（,镜检的确如此,）,次级例外用红眼雄蝇基因型和性染色体组成：,X,+,Y,（,镜检的确如此,）,正常交叉组红眼雌蝇中有,X,w,X,+,Y,基因型，比例2% （,镜检的确如此,）,正常交叉组白眼雄蝇中有,X,w,YY,基因型，比例2% （,镜检的确如此,）,2、Bridges关于果蝇性别决定的研究,Bridges（1932年）提出。果蝇：发现有的卵细胞在减数分裂过程中，性染色体不分离，形成异常的配子（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,2AXXX,3/2=1.5,雌,3AXXX,3/3=1,超雄,3AXY,1/3=0.3,雌,2AXXY,2/2=1,正常,2AXX,2/2=1,间性,3AXX,2/3=0.7,正常,2AXY,1/2=0.5,间性,3AXXY,2/3=0.7,果蝇性别决定的研究,X/A,1.0,1.0,1.0-0.5,0.5,0.5,育性,超,（高度不育）,间性,（不育）,超,（可育）,性别的发生,1、染色体平衡理论与基因组是有机整体,2、果蝇的性别发生,3、人类性别发生,4、拟南芥花发育,1、染色体平衡理论与基因组是有机整体,1.1、 果蝇(染色体平衡理论)-解释性别决定的机制:,认为在果蝇的X染色体上有许多性基因。在常染色体上有许多性基因。,性别决定于基因的,平衡,，即基因占优势，基因占优势。,1.2、 人类为代表的性别决定和剂量补偿效应(,剂量上达到平衡,),1.2、 人类等哺乳动物性别决定和剂量补偿效应(,剂量上达到平衡,),(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：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,染色体只有一条是有活性个的，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结构相同。,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,体细胞培养系,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（如果蝇），雄性个体,X,染色体基因超活性转录,异染色质化。只有一条,X,染色体有活性，另一条失活。如高等哺乳动物，形成巴氏小体。,雌性个体两条,X,染色体基因活性降低，达到和雄性个体相当水平,异配比如雄性动物,X,染色体上的基因活力增强,保持这种差异,1.3人类x染色体失活,并不意味全部基因关闭，仍然有基因转录，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: RNA,The X-inactivation center and inactivation,Genes,are shown in,bold, with the direction of transcription indicated by the arrow.,Brx,(brain X-linked),Tsx,(testis X-linked) 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 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 implicated 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 Reviews Genetics,2,61,.,Xist gene,The,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 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 that chromosome.,Tsix: antisense transcript of Xist,The,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,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 promoter,Methylation of the Xist promotor by DNA methyl 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 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 independent mechanism,Pluripotent 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 depleted 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 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结构相同。,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 chromosomes,Autosome sets,(A)X:A ratio,Sex,3,2,1.5,Metafemale,4,3,1.33,Metafemale,4,4,1,Normal female,3,3,1,Normal female,2,2,1,Normal female,2,3,0.66,Intersex,1,2,0.5,Normal male,1,3,0.33,Metamale,2.2 Regulation cascade of Drosophila somatic sex determination,Arrows 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 Determination in Drosophila,2.3 Interpreting the x:a ratio,X:A ratio is measured by competition between,X-encoded activators,and,autosomally,encoded repressors,of the promoter of 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 “denominator 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 proteins,2.3 The differential activation of the,sxl,gene in 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 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 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 translation termination,codon,.,(After Keyes et al. 1992.) From: Chromosomal Sex Determination in,Drosophila,Developmental,Biology. 6th edition.,）,2.4 Maintenance of sxl function,The pattern of sex-specific RNA splicing in three major Drosophila sex-determining genes. In each case, the female-specific transcript 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 Drosophila,2.2 Regulation cascade of Drosophila somatic sex determination,Arrows 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 Determination in Drosophila,Control of Sexual Behavior,3 sex determination of human and animal,SRY and Y chromosome genes,More steps and cascade,More 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 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 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 determination,SRY was a TF,SRY 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 function,In the cytoplasm, SRY is bound by,calmodulin,(,CaM,) and,importin,(Imp,), which recognize the 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,specific 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. Abbreviations: SOX9, SRY box containing gene 9; SRY, sex-determining region on the chromosome Y.,Flower development,The 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 floral 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 verticil.,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 A and C genes,are reciprocally antagonistic.,Steps of flower development,Apical meristem,Inflorescence meristem,Flower meristem on which flowers develop,Floral organ identity of ABCs.,Barry Causiera,Zsuzsanna Schwarz-Sommerb,Brendan Daviesa.Review Floral organ identity: 20 years of ABCs.(Seminars in Cell & Developmental Biology,2009),ABC model overview,Simple rule that underlie the whorl specifications using floral homeotic mutants,Class A mutants have carpels in the first whorl instead of sepals, and stamens in the second whorl in place of petals,Class B mutants have sepals rather than petals in the second whorl and carpels in the rather than stamens in the third whorl,Class C mutants have petals instead of stamens in the third whorl and sepals instead of carpels in the fourth,Thei,en, 2001. Nature vol. 414 p.491,What are MADS box genes?,The MADS box is a highly conserved sequence motif found in a family of transcription factors. The conserved domain was recognized after the first four members of the family, which were MCM1, AGAMOUS, DEFICIENS and SRF (serum response factor). The name MADS was constructed form the initials of these four founders.,M,CM1 from the budding yeast, Saccharomyces cerevisiae,A,GAMOUS from the thale cress Arabidopsis thaliana,D,EFICIENS from the snapdrag