Terms. Primary vs. secondary sexual differentiation. Development of gonads vs. physical appearances

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1 SEX DETERMINATION How is the sex of an organism determined? Is this process the same for all organisms? What is the benefit of sexual reproduction anyway?

2 Terms Primary vs. secondary sexual differentiation Development of gonads vs. physical appearances Individuals containing only male or female reproductive organs Unisexual, dioecious, gonochoric Individuals containing both male and female reproductive organs Bisexual, monoecious, hermaphroditic

3 Examples of Sexual Differentiation Chlamydomonas Sexual differentiation induced by environmental conditions + & - strains chemically different Fig. 5-1

4 Examples of Sexual Differentiation Zea mays Monoecious Many alleles involved in sex determination Fig. 5-2

5 Examples of Sexual Differentiation Homo- and heterogametic sexes Homogametic sex Like sex chromosomes (uniform gametes) E.g., XX Heterogametic sex Unlike sex chromosomes (unlike gametes) Hemizygous Gene present in single dose in otherwise diploid cell E.g., XY

6 Examples of Sexual Differentiation

7 Homo- & Heterogametic Sexes Heterogametic not necessarily the same as male E.g., many insects, most birds, some fish, reptiles, amphibians Birds Female (ZW) Male (ZZ)

8 An example of an organism which has different mating strains induced environmentally is A) Chlamydymonas B) Zea mays C) Protenor D) Lygaeus E) Homo sapiens

9 In organisms with an X and a Y chromosome, XY always means the organism is male. A) True B) False

10 Homo- & Heterogametic Sexes Humans Male = heterogametic sex (XY) Female = homogametic sex (XX)

11 Human Karyotype Fig. 5-5

12 Sex-Determination in Humans What actually determines maleness or femaleness in humans? Male = presence of Y or lack of 2nd X? Female = lack of Y or presence of 2nd X? Examined cases of aneuploidy Aneuploidy = presence or absence of individual chromosomes (2n +/- 1) Results from nondisjunction

13 Klinefelter & Turner Syndrome Klinefelter = 47, XXY Typically male with testicular atrophy & some feminine development Turner = 45, X Female with diminished ovaries Suggests Y determines maleness So what gene/genes are responsible?

14 The Y Chromosome MSY region Male specific region PAR region Involved in synapsis & crossover with X SRY region Sex-determining region Encodes gene product that triggers embryonic gonadal tissue to form testes Testis-determining factor (TDF) Fig. 5-7

15 Human Embryo and Sex Organ Development XY Embryo 7 Weeks old Wolffian vs Müllerian ducts TDF on Y chromosome governs development of testes

16 The Y Chromosome Further evidence from cases of SRY translocation XX but male phenotype SRY region attached to one X chromosome XY but female phenotype SRY region missing from Y chromosome

17 In humans, the default sex is A) male B) female

18 In humans, the key locus on the Y chromosome involved in determining maleness is the... A) MSY B) PAR C) SRY

19 The X Chromosome If females have 2 X chromosomes, shouldn t they produce twice the genetic products of X-linked genes?

20 Dosage Compensation One X chromosome is inactivated Barr body (sex chromatin body) Fig. 5-9

21 Dosage Compensation If extra X chromosomes are inactivated, why isn t a 45, X (Turner) a 47, XXY (Klinefelter) or any other poly-x individual normal? Gene products may be important prior to inactivation Inactivation may not be complete

22 Dosage Compensation Which X chromosome is inactivated (paternal or maternal)? Lyon Hypothesis X inactivation is random in somatic stem cells Occurs early in embryonic development Once inactivated, all progeny cells have the same X chromosome inactivated E.g., calico & tortoiseshell cats All female Alleles for black or orange coat color on X chromosome Patches of skin/fur develop from a single progenitor; each patch represents a different progenitor with a different inactivated X

23 Dosage Compensation How is the X chromosome inactivated? Region of X chromosome (X-inactivation center; Xic) X-inactive specific transcript (XIST) RNA produced, but not translated May form molecular cage around chromosome

24 In humans, an individual whose chromosome complement was 48, XXXY would have how many Barr bodies? Just type the number you want to submit!

25 Sex-Determination in Drosophila Males are XY Same as humans? XXY are normal females XO are sterile males Females are XX Presence of Y does not cause maleness Absence of Y does not cause femaleness Genes on autosomes and X chromosome contribute to sex determination

26 Sex-Determination in Drosophila Sex is determined by a ratio # of X chromosomes : # of haploid sets of chromosomes (n) n = 4 (3 autosomes, 1 sex chromosome) Normal female is 2X:2A (1:1, or 1.0) Normal male is XY:2A (1:2, or 0.5)

27 Sex-Determination in Drosophila Triploid females 3X:3A (1.0) normal female Metafemales 3X:2A (1.5) fertile female Males w/o Y 1X:2A (0.5) fertile male Metamales XY:3A (0.33) infertile male Genic balance theory

28 Sex Determination & Temperature Variation Observed in certain reptiles Temperature may influence synthesis of steroids involved in differentiation of ovaries & testes Fig. 5-12

29 In Drosophila, a triploid (3n) mutant with 12 autosomes, 2 X and 1 Y chromosomes would be most similar to a A) Male B) Female C) Metafemale D) Metamale