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Chapter 7: Outline • Sexual Differentiation Heterogametic • Human Male/Female – Y C-some – Dosage compensation • Drosophila Male/Female – X to autosome Ratio – Dosage compensation • Reptile Male/Female determination – Environmental factors (Temperature) Sexual Differentiation • Sexually reproducing organisms depend upon fertilization of gametes formed during meiosis • Fertilization relies on some form of sexual differentiation between individuals • Genetic basis is often related to different C-some composition between the sexes – The Heterogametic sex either lacks one C-some or contains a unique C-some Heterogametic Sex Determination • Protenor (insect): XX/X0 – Random distribution of the X c-some into only half of the male gametes – Females=XX (Homogametic) – Males=X0 (Heterogametic) • Lygaeus (insect): XX/XY – Random distribution of Y c-some into male gametes – Females=XX (Homogametic) – Males=XY (Heterogametic) • Males not always Heterogametic – ZZ/ZW Mode, where female=ZW Human Sex C-somes • Humans have a diploid (2n) number of 46 – 22 homologous pairs of Autosomes – 1 homologous pair of sex chromosomes – Typical Males=XY and Females=XX • X and Y C-somes – Very different in Appearance and DNA composition Heteromorphic – Pair during meiosis/mitosis Possible Interpretations • Y C-some determines maleness in humans – Have the Y then you will express male phenotype • Y C-some does not play a role in sexual development – Presence of two XX determines femaleness – Lack of the second X C-some determines maleness Human Sexual C-some Abnormalities • Klinefelter Syndrome (47,XXY) – Male genitalia and internal ducts – Testes are underdeveloped and fail to produce sperm • Turner Syndrome (45, X) – Female genitalia and internal ducts – Underdeveloped ovaries • XXX Syndrome – Female genitalia and internal ducts – Normal to sterility • XYY Condition – Male genitalia and internal ducts – Above average height Correlation of Y with Maleness • Appears to be a high degree of correlation between the Y C-some and the development of maleness • Molecular biologists also observed that in the absence of Y C-some, no male development occurs • What is so special about the Y C-some? Y C-some Structure • Shares very little homology with the X C-some – Less than 10% is shared between X and Y C-somes – Regions termed PARs (Pseudoautosomal regions) – A few allelic X genes • Rest is termed NRY (Nonrecombining region of Y) – Much of this is genetically blank (no genes) termed Heterochromatin – Some Y C-some specific genes have been discovered Observations • Some individuals who appeared to express Turner Syndrome (X0) were shown to have mutated Y C-some – Lost (missing) p arm of Y • Studies examining XX Males and XY Females established that translocations or mutations of Y p arm were present • By correlating the extent of Y alterations to phenotypes the interval bearing the male-determining factor (called TDF, testis-determining factor), was narrowed down to 35 kb on the p arm of Y SRY Gene • Sex-determining region of Y (SRY) was identified as the TDF – Testis-specific expression – XX (Males) and XY (Females) are often missing this SRY region – Highly conserved in mammals – Inserting this gene in XX mice (transgenic) and observed male development • Not the only gene on the Y C-some involved in Male development Genetic Dosage Problem • X C-somes are very similar to autosomes both in appearance and genetic content • Females contain 2x X genes – Do females producing 2 times X-linked genes than males? – How are males viable with only half the genetic X material? – Do males X c-some produce 2x the amount as female X? • How do humans compensate for this genetic dosage problem? Dosage Compensation • Females shut down one of their X C-some in somatic cells in a process called Lyonization – X-inactivation occurs in the first few days after fertilization (a few hundred cells) – In each cell, one of the X C-some becomes highly condensed forming a Barr body – Inactivation is stably transmitted to all subsequent progeny cells • Random average half maternal and half paternal • Can produce mosaics: patches of tissue expressing the allele in one X and alternating patch expressing other allele Drosophila Sex Determination • Males and females of drosophila have the same sex C-some composition as humans – Male=XY and Female=XX • Question then is does Maleness depend on the presence of the Y C-some? – Answer: No, Bridge showed this not to be the case Bridges’ Work • Drosophila have 3 pairs autosomes and 1 pair of sex C-somes • Observation: Offspring resulting from non-disjunction of X C-some (meiosis) in females (Result in 2n+1 or 2n-1) – XXY normal Female – X0 sterile males • Conclusion: Y C-some lacks male determining factor, but contains information essential for fertility Bridges’ Work • Observation: Offspring of females containing 3 copies of each C-some – Wide range of C-some complements occur during meiosis leading to C-some abnormalities in offspring • Conclusions: Ratio of X c-somes to the number of haploid sets of autosomes (A) present determines the sex of drosophila – > 1.0= Normal Females – < 0.5= Normal Males – Between 0.5 and 1.0= Intersex Dosage Compensation • Again males contain only half of the genetic material on the X c-some • Do they compensate for this like humans? – No, X C-some inactivation is not observed • Instead male X-genes are transcribed at twice the level as female X-genes – Master regulator Sxl is inactivated in males (other genes activated) causing enhanced X gene activity – Sxl is active in females to inactivate male-specific autosome gene(s) Drosophila Mosaics • Appearance of a unique fruit fly – Half of body expressed mutant phenotype (white eyes and miniature wing), while other side expressed wild phenotype • Other females of this cross were heterozygous for these X-linked genes • Bilateral Gynandromorph: Half of body developed as male and half as female Process to Form Mosaics • Female zygote (heterozygous for both genes) were to loss one of its X Csomes during the 1st mitotic division – Two cells would be produced one that was XX (Female) and one X0 (Male) • Progenitor cells then are responsible for producing one half of the flies body – Half would be Male – Half would be Female Temperature-Sensitive Sex Determination • Some species sex is determined not by C-some composition – Retiles, including all crocodiles, most turtles, and some lizards • Rather sex is determined according to the incubation temperature of the eggs during a critical period of embryonic development – Three distinct patterns of sex development have been observed 3 Patterns of TSD • Case I: 100% females at higher and 100% males lower temps • Case II: 100% males at higher and 100% females lower temps • Case III: 100% females at high and low temps, while males at intermediate temps • Important observations – In all cases, both M/F generated at certain temps – Pivotal temp (P) is fairly narrow • <5°C sometimes 1°C Mechanism of TSD • Thought to involve steroids (mainly estrogen) and the enzymes involved in their synthesis • Temp changes have been shown to alter estrogens, androgens, and inhibitors of the enzymes controlling their synthesis – Aromatase converts androgen (male hormone) to estrogen (female hormone)