1. Sex Determination

2. Sexual Differentiation
Autosomes
Sex chromosomes
Heterogametic sex (2 types of gametes)
Homogametic sex (1 type of gamete)
Males are not always heterogametic sex - females are heterogametic in birds, moths, fish and chickens
XX/XY – male heterogametic sex
ZZ/ZW – female heterogametic sex

3. Sexual Differentiation
Primary Sexual Differentiation – gonads
Secondary Sexual Differentiation – overall appearance of the organism
Unisexual, Dioecious, Gonochoric – containing only male or female reproductive organs
Bisexual, Monoecious, Hermaphorditic – both male and female reproductive organs

4. Sexual Differentiation
Chlamydomonas (Green algae)
Most of life in haploid stage
Asexually reproduce daughter cells by mitotic division
Unfavorable conditions – (such as nitrogen depletion) function as gametes to produce zygotes
Diploid Zygotes – withstand unfavorable conditions

7. Sexual Differentiation
Meiosis occurs then returns to the haploid state
Isogametes – gametes are indistinguishable (isogamous is the species producing the isogametes)
When forming zygotes, only “+” and “-” isogametes mate
Chemical difference between cells but no morphological difference

8. Sexual Differentiation
Zea mays A. Stamen - Male 1. Produce diploid microspore mother cells 2. Each mother cell undergoes meiosis to produce 4 haploid microspores 3. Each haploid microspore develops into a mature male microgametophyte (pollen grain)

9. Sexual Differentiation
B. Pistil (Female)
1. Produce diploid megaspore
After meiosis, only 1 haploid megaspore survives
Megaspore divides 3 times producing a total of 8 nuclei in one embryo sac
Two nuclei in the middle – endosperm nuclei

10. Sexual Differentiation
Micropyle end (where two sperm nuclei enter) – 3 nuclei/1 oocyte, 2 synergids
Antipodal nuclei – the three nuclei opposite from micropyle end
One sperm fertilizes endosperm nucleus; the other sperm fertilizes oocyte nucleus
Double fertilization – diploid zygote/triploid endonucleus
Each ear of corn contains as many as 1000 of these structures

11. Sexual Differentiation
C. elegans
Only 1000 cells
2 sexual phenotypes
Males – testis (functional)
Hermaphrodite – XX
Males – X (No Y chromosome)

12. Sexual Differentiation
6. Hermaphrodite – testis and ovaries a. During larval stage, testis produce sperm (stored) b. Ovaries produced but no oogenesis until adult stage c. Able to self fertilize d. If hermaphrodite mates with male – ½ hermaphrodite, ½ males

13. Sexual Differentiation
Protenor (insect)
1906 Edmund Wilson found female somatic cells contained 14 chromosomes, including 2 X sex chromosomes
Gametes from female contains 7 chromosomes, including 1 X chromosome

14. Sexual Differentiation
Gametes from the male contains 6 chromosomes without 1 X chromosome
Gametes from the male contains 6 chromosomes with 1 X chromosome
Fertilization by male containing
X sperm – FEMALE
6. Fertilization by male containing
O sperm – MALE
Protenor Mode of Sex Determination –
XX / XO

16. Sexual Differentiation
Lygaeus turicus
Insect has 14 chromosomes
12 autosomes and 2 X chromosomes – Female
12 autosomes and 1 X Y - Male
Females produce only X chromosomes
Males produce X and Y chromosomes
Lygaeus Mode of Sex Determination (XX/XY)

18. Sexual Differentiation
Humans
Tjio and Levan (1956) – discovered 46 diploid for humans; 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes

19. Sexual Differentiation in Humans
All human embryos undergo a hermaphroditic period
5th week of gestation, gonadal primordia arise
Primordail germ cells become cortex/inner medulla
Cortex – develop into ovary
Medulla – develop into testis

20. Sexual Differentiation in Humans
7th week of gestation, if XY chromosomes are present, medulla develops into testis. If no Y present, cortex forms ovarian tissue
Initiation of testis stimulates production of two hormones that is needed for male sex differentiation
Males produce spermatocytes at puberty
Females arrest eggs in meiosis by 25th week

21. Sexual Differentiation in Humans
Y chromosome in Male Development
Y chromosome was once believed to contribute very little to the makeup of males (Wasteland theory)
Pseudoautosomal regions (PARS) – regions on Y chromosome that share homology with X chromosome
These regions synapse/located on ends of Y chromosome

22. Sexual Differentiation in Humans
All the rest of Y is called NRY (nonrecombining regions)
TDF – testis determining factor
SRY – sex determining region Y (encodes for TDF)
Females X Y – missing SRY region on the Y chromosome
Males X X – attached SRY region to the X chromosome

24. Sexual Differentiation in Humans
Transgenic Mice studies
1. X X fertilized eggs injected with SRY gene produced male mice
2. SRY is the gene to determine maleness
3. Conserved throughout a diverse group of animals

25. Sexual Differentiation in Humans
David Page 1. Discovered 12 genes on the NRY 2. Two groups a. 5 NRY genes, homolog on the X chromosomes (housekeeping genes) b. 7 NRY genes, No homolog on the X chromosome encode proteins only expressed in testicular tissue

26. Sexual Differentiation in Humans
Dosage Compensation
Females (XX) should have a “genetic dosage” problem for all X-linked genes
Potential for females to produce twice as much X-linked gene products

27. Sexual Differentiation in Humans
Dosage Compensation
Barr Bodies – Murray Barr/Ewart Bertram’s experiments with female cats
Noticed a darkly stained body near the nuclear envelope of interphase cells
Stained positive with Feulgen stain
Dark body composed of inactivated X chromosome (suggested by Susumo Ohno)

29. Sexual Differentiation in Humans
1. Hypothesis for dosage compensation – one X chromosomes functional between males and females 2. No matter how many X chromosomes are present in females, all but 1 is inactivated (multiple Bar Bodies)29

30. Sexual Differentiation in Humans
Problems with theory If one X chromosome is inactivated, why isn’t the Turner, 45X normal? Why aren’t people with multiple X karyotypes normal? Explanation Not inactivated during early development of gonadal tissues or not all of the X is inactivated

31. Sexual Differentiation in Humans
Lyons Hypothesis 1. Answered the question: Which X chromosome is inactivated? Paternal or maternal 2. Mary Lyon/Liane Russell proposed hypothesis of random inactivation

32. Sexual Differentiation in Humans
Experiment with Female Mice -heterozygous for X-linked coat color
1. Pigmentation on X chromosome (expressing in patches)
Females have different patterns based on which X is inactivated
Males have only one type of patch expressed