Sex Determination & Sex Chromosomes PDF

Sex Determination & Sex Chromosomes Chapter 5 Sex Chromosomes & the Heterogametic Sex 5.1 In many organisms... Males are the heterogametic sex - produce unlike gametes (could be XY or XO) Female are the homogametic sex- gametes all have only X chromosome Figure 5-1 © 2016 Pearson Education, Inc. Sex...

Sex Determination & Sex Chromosomes Chapter 5 Sex Chromosomes & the Heterogametic Sex 5.1 In many organisms... Males are the heterogametic sex - produce unlike gametes (could be XY or XO) Female are the homogametic sex- gametes all have only X chromosome Figure 5-1 © 2016 Pearson Education, Inc. Sex Chromosomes & the Heterogametic Sex 5.1 In some organisms, female is heterogametic – ZZ/ZW notation -Used when female is heterogametic sex – Example: chickens – ZZ is homogametic male – ZW is heterogametic female Mistakes During Meiosis Nondisjunction: when a chromosome pair fails to separate during meiosis Results in too many or too few chromosomes Most abnormal embryos spontaneously abort Mistakes During Meiosis f Klinefelter Syndrome Klinefelter syndrome – Tall with long arms and legs—large hands and feet – Testes are rudimentary and fail to produce sperm. – Slight breast enlargement and hips often rounded – Intelligence below normal range – 47,XXY karyotype – Figure 5-2a © 2016 Pearson Education, Inc. Turner Syndrome Turner syndrome – Female external genitalia and internal ducts but ovaries rudimentary – Short stature; skin flaps on back of neck; flat underdeveloped breasts; broad, shield like chest – Normal intelligence © 2016 Pearson Education, Inc. Figure 5-2 © 2016 Pearson Education, Inc. 47,XXX Syndrome (Triplo-X_) Triplo-XXX – Three X chromosomes along with normal set of autosomes (47,XXX) – Results in female differentiation (1/1000 live births) Often 47,XXX women perfectly normal—unaware of condition Underdeveloped secondary sex characteristics, sterility, and mental retardation do occur. Tetra-X and penta-X karyotypes have been reported. © 2016 Pearson Education, Inc. 47,XYY 47,XYY Condition – Consistently shared characteristic found so far: Males are over 6 feet tall with subnormal intelligence. Nondisjunction of Sex Chromosomes XXY = tall, big, somewhat feminized male XYY = male, tall, mild mental impairment XXX = normal female XO = shorter female Y Chromosome Determines Maleness in Humans 5.2 X chromosome has 1,336 genes Y chromosome has 307 genes Y Chromosome Determines Maleness in Humans 5.2 PARs – regions on the ends of Y that share homology with regions of the X – Can do crossing over Y Chromosome Determines Maleness in Humans 5.2 MSY – male-specific region – 95% of Y chromosome – No crossing over with X in this region Y Chromosome Determines Maleness in Humans 5.2 Euchromatic region – has genes Heterochromatic region – no genes Y Chromosome Determines Maleness in Humans 5.2 SRY gene – responsible for male phenotype – Humans with XY, but missing SRY, are female – Humans with XX, but with SRY from Y attached, are male Y Chromosome Determines Maleness in Humans 5.2 SRY gene makes TDF protein TDF may be a transcription factor that acts as a master switch for the expression of other genes What do you think the phenotypic sex of the following individuals would be: – XXX – XXY – XYY – X0 – XXY with nonsense mutation in SRY gene Yay Problems! 1. A woman with Turner syndrome also has hemophilia, as did her father. Which of her parents underwent nondisjunction during meiosis, giving rise to the gamete responsible for the syndrome? 2. A couple has a child with XYY. In which parent could the nondisjunction have occurred, and during which meiotic division? Ratio of Males: Females Isn’t 1.0 5.3 Equal numbers of males and females conceived Slightly more males than females born Why????? Dosage Compensation Prevents Excessive Expression of X-linked Genes 5.4 X-inactivation – one X is turned off – Females are mosaics – Example: calico & tortoise shell cats Dosage Compensation Prevents Excessive Expression of X-linked Genes 5.4 Barr body – an inactivated X chromosome Evidence: in sex chromosome disorders, all but 1 X are inactivated Dosage Compensation Prevents Excessive Expression of X-linked Genes 5.4 X-inactivation center (Xic) – Major control unit on X chromosome – Genetic expression occurs only on inactivated X chromosome – Consists of X-inactive specific transcript (XIST) gene— critical to inactivation Dosage Compensation Prevents Excessive Expression of X-linked Genes 5.4 Drosophila and C. elegans Sex Determination Hmm… 5.5 Drosophila and C. elegans Sex Determination Y chromosome does not determine sex in Drosophila and C. elegans! – Drosophila: contains Y chromosome but has no role – C. elegans: has no Y chromosome Ratio of X Chromosomes to Autosomes Can Determine Sex 5.5 X- Drosophila and C. elegans Sex Determination 5.5 C. elegans has two sexual phenotypes – Males = XO Can mate with hermaphrodites – Hermaphrodites = XX Have both testes and ovaries Eggs are fertilized by stored sperm, can self fertilize Drosophila and C. elegans Sex Determination 5.5 Like drosophila, ratio of X chromosomes to autosomes that determines sex Temperature Controls Sex Determination in Reptiles 5.6 Temperature Controls Sex Determination in Reptiles 5.6 3 scenarios 1) Low temp = female, high temp = male 2) Low temp = male, high temp = female 3) High and low = female, intermediate = male Temperature Controls Sex Determination in Reptiles 5.6 Temperature effects on enzymes – Aromatase converts androgens (male hormones such as testosterone) to estrogens (female hormones such as estradiol) – Thermosensitive factors mediate transcription of gene More Problems! 1. A couple has a child with Klinefelter syndrome. In which parent could the nondisjunction have occurred, and during which meiotic division?