Objectives for Exam 2 PCB 3063, Spring 2012 Sex Determination and Sex Chromosomes (Chapter 5) •Know the difference between autosomes and sex chromosomes, understand the terms heterogametic sex and homogametic sex, and know the difference between the XY and the ZW sex determination systems. •autosomes- chromosomes that are not sex chromosomes; 22 sets of autosomes•sex chromosomes- 23rd set of chromosomes are the sex chromosomes•heterogametic sex- the sex that produces gametes containing unlike sex chromosomes. In mammals, the male is the heterogametic sex•homogametic sex- the sex that produces gametes that do not differ with respect to sex chromosome content; in mammals, the female is homogametic. •XO system- one X is male; two Xʼs (XX) is female•XY system- mammalian system; XY is male, XX is female•ZW system- reptiles, birds, some amphibians and insects; ZW is female, ZZ is male (sort of opposite of mammalian system)•Know the evidence which indicates that the Y-chromosome determines sex in mammals (existence of Turner and Klinefelter individuals), know what the genetic makeup is of people with Turner and Klinefelter Syndromes and what the characteristics (phenotypes) of these syndromes are. •existence of klinfelter and turner individuals was evidence that the Y-chromosome determine sex in mammals•Klinfelter syndrome: tall, testicular atrophy, female pubic hair pattern, minor breast development, poor beard growth. 47 chromosomes, XXY•Turner Syndrome: rudimentary ovaries, underdeveloped breasts, shield chest, short stature, webbed neck. 45 chromosomes, X•both syndromes result in sterility!!! •Know the difference in the genes that are carried by the X and the Y chromosomes, know what the PAR region is on the Y chromosome and why it is important. •X chromosome is really a garden variety chromosome that codes for many different things•Y chromosome has very few genes and basically just codes for male development (only contains about 2 dozen genes)•the SRY gene on the Y chromosome codes for TDF (Testis-determining factor)•PAR- (Pseudoautosomal region) share homology with regions on the X chromosome and synapse and recombine with it during meiosis•Understand how the SRY gene controls sex determination in mammals (initiates male development through regulation of testosterone and antimullerian hormone production during early development) and what the evidence is for the importance of this gene (XY females and XX males). •In humans, the absence of a Y chromosome leads to female development•At 6-8 weeks of development, SRY gene becomes active in XY embryos •SRY encodes a protein that causes the undifferentiated gonadal tissue of the embryo to form testes. •This protein is TDF•If SRY is expressed, the gonads develop into testis; if its not expressed, the gonads develop into ovaries•The cells in the testis secrete anti-Mullerian hormone•this blocks the Mullerian ducts from developing into the uterus and ovaries•Testosterone stimulates the formation of internal and external male structures•XY females have a deletion of part of the Y (the part containing the SRY gene)•XX males have a translocation from the Y to the X (part containing the SRY gene)•could have occurred during crossing over--PAR region•The XY females and XX males provided evidence for the SRY gene being responsible for male sex determination•Know where the SRY gene is located on the Y chromosome. •the SRY gene is located adjacent to the PAR of the short arm of the Y chromosome•Understand the concept of dosage compensation and why it is important on the X chromosome and how dosage compensation is achieved in mammals (X-inactivation) and in fruit flies (increase of activity of genes on the X in males).•females have 2 X chromosomes and males only have 1, so expression of the X is unbalanced•How is gene dosage regulated?•X-inactivation in human females-- “Lyon Hypothesis”•one X chromosome is inactivated in each cell in females during early development•Barr Body- inactivated X•Fruit flies instead double the activity of genes on the X in males compared with females to compensate•Know what a Barr body is and how inactivation of the X chromosome occurs during early development in mammalian females, and be able to state how many Barr bodies would be present in a cell that has 1, 2, 3, or 4 X chromosomes. •Barr Body- inactivated X•To find # of Barr Bodies: # Xʼs - 1....N-1 (because you only want one active X chromosome)•donʼt know all the details on how this is achieved•occurs in late blastula stage•1 of 2 Xʼs is inactivated•once that X is shut off, every daughter cell made has that X turned off•Understand the term genetic chimera (genetic mosaic), and be able to explain the pattern of inheritance of calico and tortiseshell coat colors in cats. •Females are genetic mosaics •in some parts of the body 1 X is being used and in other parts, the other X is being used•all the cells donʼt shut off the same X chromosome, therefore, if the female has different alleles, there is often a patchwork, or mosaic expression of those genes•Ex: calico and tortoiseshell cats are always female•X-linked coat color genes; females display different coat color alleles in different areas of the body•Know what the Xist gene does and where it is located.•The XIST is transcribed but not translated, it it the gene responsible for X-inactivation; it is located at XIC•Be able to provide an explanation of why individuals with Turner and Klinefelter syndromes are not phenotypically normal. •about 10% of the genes on the X that is shut off stay on and are expressedChromosome Mutations (Chapter 6) •Know the following terms and their significance: •Chromosome mutations or chromosome aberrations- change in total # of chromosomes, the deletion or duplication of genes or segments of a chromosome, and rearrangements of the genetic material either within or among chromosomes•passed to offspring•Nondisjunction- leads to monosomy and trisomy; where paired homologs fail to separate during segregation•doesnʼt allow the normal distribution of chromosomes into gametes•Aneuploidy- (2n +/- x chromosomes); general condition where an organism loses or gains one or more chromosomes, but not a complete set•monosomy- (2n - 1); loss of one chromosome; monosomy for the X chromosome occurs in humans, monosomy for any of the autosomes is usually not
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