Chapter 5 Sex Determination and Sex Chromosomes 1 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 any chromosome not considered and sex chromosome and is not involved in sex determination Sex chromosomes a type of chromosome in the genome that is involved in the determination of the sex as well as the development of sexual characteristics in the organism X Y in humans Heterogametic sex a sex that produces gametes containing unlike sex chromosomes In mammals the male is the heterogametic sex XY Homogametic sex the sex that produces gametes that do not differ with respect the sex chromosome content in humans the female is the homogametic sex XX XY sex determination system used for mammals and some insects i Lygaeus sex determination ii Males are the heterogametic sex and females are the homogametic sex ZW sex determination system used for reptiles birds and some amphibians and insects i Protenor sex determination ii ZW is heterogamous female 2 Know the evidence which indicates that the Y chromosome determines sex in mammals existence of turner and klinefelter syndrome know what the genetic makeup is of people with turner and klinefelter syndrome and what the characteristics phenotypes of these syndromes are We know the Y chromosome determines sex in mammals because of the karyotype of chromosomes i The Y chromosome does indeed determine maleness We know this by looking at turner and klinefelter syndrome Klinefelter syndrome 47 XXY and turner syndrome 45 X i The number states the total number of chromosomes present and the info after the comma indicates the deviation from the normal diploid content ii They both result from nondisjunction or the failure of the sex chromosomes to segregate properly during meiosis Klinefelter phenotype tall long arms and legs large hands and feet Have genetalia and internal ducts that are male but their testes are rudimentary and fail to produce sperm Slight enlargement of breasts are common and rounded hips Intelligence is usually below the normal range as well i Turner syndrome phenotype has female external genitalia and internal ducts but the ovaries are rudimentary They have short stature usually under 5 ft tall skin flaps on the back of the neck and underdeveloped breasts A broad shield like chest is sometimes noted Intelligence is normal i Those with klinefelter have more than 1 X chromosome Those with turner syndrome only have a single X chromosome The phenotypes of those with kilnefelter and turner syndrome showed scientists that the Y chromosome determines maleness in humans In it s absence the person s sex is female even if only a single X chromosome is present The presence of a Y chromosome in an individual with klinefelter syndrome is sufficient to determine maleness even though male development is not complete Similarly in the absence of a Y chromosome like those with turner syndrome no masculation occurs 3 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 Pseudoautosomal regions PARs are present on both ends of the Y chromosome and share homology with regions on the X chromosome and synapse and recombine with it during meiosis The presence of such a pairing region is critical to segregation to X and Y chromosomes during male gametogenesis The remainder of the chromosome 95 does not synapse recombine with the remainder of the X chromosome 4 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 Know where the SRY gene is located on the Y chromosome Sex determining region Y SRY controls male sexual development It is absent from the X chromosome At 6 8 weeks of development this gene becomes active in XY embryos A protein called the testes determining factor TDF is encoded by SRY and causes the undifferentiated gonadal tissue of the embryo to form testes It is located adjacent to the PAR of the short arm of the Y chromosome This was proved through transgenic mice These animals are produced from fertilized eggs injected with foreign DNA that is subsequently incorporated into the genetic composition of the embryo In normal mice a chromosomal region designated Sry has been identified and is comparable to SRY in humans When mouse DNA contains Sry is injected into normal XX mouse eggs most of the offspring develop into males SRY expressed gonads develop into testes i Cells in testes secrete anti mullerian hormone which blocks mullerian ducts from developing into uterus and ovaries testosterone which stimulates the formation of internal and external male structures SRY not expressed gonads develop into ovaries XX males have a translocation from the Y to the X XY females have a deletion from part of the Y 5 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 Because females have 2 X chromosomes and males only have one 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 It is important on the X chromosome because without proper dosage diseases like turner and klinefelter or embryonic lethality could occur Dosage compensation in mammals i In mammalian females one X is from the mom and the other X is from the dad So how are the X s inactivated ii The Lyon hypothesis the inactivation of X chromosomes occurs randomly and in somatic cells at a point early in development Once inactivation has occurred all descendant cells have the same X chromosome inactivated as their initial progenitor cell Dosage compensation in fruit flies i Y chromosome is not involved in the sex determination of fruit iii flies ii Activity of genes is doubled in males compared with females iii Factors that cause a fly to develop into a male are not located on the sex chromosomes but are located on autosomes Some
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