PCB3063 General Genetics Exam 2 Study Guide Fall 2014 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 Sex chromosomes determine the sex of an organism while autosomes do not 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 the homogametic sex XY System mammals some insects XO System some insects ZW System reptiles birds some amphibians insects 1 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 The Klinefelter Turner karyotypes their corresponding sexual phenotypes led scientists to conclude that the Y chromosome determines maleness in humans In the absence of Y the person s sex is female even if only a single X chromosome is present PCB3063 General Genetics Exam 2 Study Guide Fall 2014 Klinefelter Syndrome 47 XXY 1 500 males Tall stature Testicular atrophy Poor beard growth Minor breast development Female pubic hair pattern Turner Syndrome 45 X 1 3 000 females Short stature Webbed neck Shield chest Underdeveloped breasts Widely spaced nipples Rudimentary ovaries Brown nevi 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 The Y chromosome carries far fewer genes than the X Y chromosome contains about two dozen active genes 2 PCB3063 General Genetics Exam 2 Study Guide Fall 2014 Human X chromosome map Pseudoautosomal Region PAR A region on the Y chromosome that is also represented on the X chromosome Shares homology with regions on the X chromosome and synapse recombine with it during meiosis The presence of this pairing region is critical to segregation of X Y chromosomes during male gametogenesis Understand how the SRY gene controls sex determination in mammals and what the evidence is for the importance of this gene XY females and XX males Know where the SRY gene is located on the Y chromosome Sex Determining Region Y SRY The sex determining region of the Y chromosome found near the chromosome s pseudoautosomal boundary Critical gene that controls male sexual development Encodes a protein that causes the undifferentiated gonadal tissue of the embryo to form testes TDF Testis Determining Factor SRY expressed gonads develop into testis o Cells in testis secrete Anti Mullerian Hormone prevents Mullerian ducts from developing into uterus ovaries Testosterone stimulates the formation of internal external male structures 3 PCB3063 General Genetics Exam 2 Study Guide Fall 2014 SRY NOT expressed gonads develop into ovaries XX males have a translocation from the Y to the X XY females have a deletion of part of the Y The part where SRY should be Includes SRY so they have the region even though they lack a 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 and in fruit flies Females have two X chromosomes males only have one so expression of genes on the X is unbalanced between the two sexes Dosage Compensation A genetic mechanism that equalizes the levels of expression of genes at loci on the X chromosome In mammals this is accomplished by random inactivation of one X chromosome leading to Barr body formation o The Lyon Hypothesis In insects activity of genes on the X is doubled in males when compared to females 4 PCB3063 General Genetics Exam 2 Study Guide Fall 2014 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 chromosome X inactivation occurs during the late blastula stage Random choice in each cell of which X is inactivated The number of Barr bodies follows an N 1 rule where N is the total number of X chromosomes present Ex In an XX female N 2 so N 1 1 Barr body Understand the term genetic chimera genetic mosaic and be able to explain the pattern of inheritance of calico and tortoiseshell coat colors in cats Genetic Mosaic Chimera The presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg All female mammals are mosaic for genes on the X chromosome Calico tortoiseshell cats are always female X linked color coat genes Females display different coat color alleles in different areas of the body Such X linked coat color patterns do not occur in male cats because all their cells contain the single maternal X chromosome and are therefore hemizygous for only one X linked coat color allele 5 PCB3063 General Genetics Exam 2 Study Guide Fall 2014 Know what the XIST gene does and where it is located XIST X Inactive Specific Transcript Gene responsible for X inactivation Located in the X Inactivation Center Xic Transcribed but not translated DNA of inactive X becomes methylated Be able to provide an explanation of why individuals with Turner and Klinefelter syndromes are not phenotypically normal Possible explanations Chromosome inactivation does not normally occur in the very early stages of development of those cells destined to form gonadal tissues Not all genes on each X chromosome forming a Barr body are inactivated Research has shown that as many as 15 of the human X chromosomal genes escape inactivation Chromosome Mutations Chapter 6 Know the following terms and their significance Chromosomal Aberration Mutation Any duplication deletion or rearrangement of the otherwise diploid chromosomal content of an organism 6 PCB3063 General Genetics Exam 2 Study Guide Fall 2014 Nondisjunction A cell division error in which homologous chromosomes or the sister chromatids fail to separate and migrate to opposite poles Responsible for defects such as monosomy trisomy Aneuploidy A condition in which the chromosome number is not an exact multiple of the haploid set Monosomy One copy of a chromosome Monosomy of
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