Sex and Development

1. Sex and Development Chapter 1

2. Central Points • Fetus’ chromosomal sex determined at fertilization • Fetal development has many stages • During development, a fetus becomes male or female

3. 1.1 How Is the Sex Determined? • Complex interactions between the genes and environment early in fetal development • Secondary sex characteristics arenot directly related to reproduction

4. Human Chromosomes • 22 pairs of autosomes and 1 pair of sex chromosomes (=46 total chromosomes) • The chromosomal make of an individual can be determined by a karyotype • Sex chromosomes determine sex in humans Females XXMales XY

5. The Human Chromosome Set Giemsa – binds to where the DNA is rich in adenine and thymine

6. Sex Ratios • ~Equal numbers of X- and Y-bearing sperm are produced • Sex ratio: proportion of males to females, expect 1:1 • At birth, about 1:1.05 • Between ages 20 and 25, ratio is close to 1:1 • Ratio decreases with age

7. X and Y Chromosomes in Sex Determination

8. Procedures to Select Sex of a Baby Scientific procedures for sex selection: • Sperm sorting • Preimplantation genetic diagnosis (PGD)

9. Y X Layer with sperm 1) Sperm are separated from surrounding fluids, using a centrifuge. 2) Sperm are then placed in a saline solution containing a fluorescent dye, which binds to the DNA in the sperm. 3) As the sperm pass one by one through a detector, a laser bounces light off the dyed DNA. 4) Because X-bearing sperm have more DNA and reflect more light than Y-bearing sperm, each can be separated into different test tubes. Sperm Sorting

10. Sperm Sorting • Sperm sorted by laser based on how brightly they fluoresce • Sperm placed in uterus or eggs fertilized in a dish: $4,000–$6,000 (more if eggs must be collected) • Success rate: ~90% for female births, ~73% for male births • Acceptable method for some situations

11. Eight-cell embryos Single cells Embryos Transfer of selected embryos to patient Removal of single cell for sex chromosome analysis 1) Eggs are removed from the ovary, fertilized, and grown to the eight-cell stage. 2) Single cells are identified as either male or female. 3) Embryos of the desired sex are selected. 4) The selected embryos are transferred to the uterus for development. Preimplantation Genetic Diagnosis (PGD)

12. Preparation for Preimplantation Genetic Diagnosis (PGD) • Woman given hormone treatments • Multiple eggs surgically removed • Eggs placed in a dish with sperm until the eggs are fertilized by in vitro fertilization (IVF) • Embryos develop to the eight-cell stage, then PGD is performed

13. Preimplantation Genetic Diagnosis (1) • Process of PGD • Single cell removed from each embryo • Chromosomes analyzed to determine the sex • Embryos of the desired sex placed into the mother’s uterus • Unused embryos discarded or frozen for later use or donation to others

14. Preimplantation Genetic Diagnosis (2) • More invasive procedure increased risk for mother • $12,000–$15,000 • Success rate: ~100%

15. Stages in Human Fertilization

16. 1.2 Development: Fertilization to Birth • Fusion of sperm and egg, generally occurs in fallopian tube (oviduct) • Sperm deposited in vagina move through the cervix, uterus, and oviduct • Usually one sperm enters the egg (chemical changes prevent > one sperm) • Nuclei fuse to form a zygote with 46 chromosomes

17. Development: Fertilization through Implantation

18. Development: Embryo to Blastocyst • Embryo moves to the uterus (3–4 days), cell division continues • Forms a large hollow ball of cells or blastocyst • It contains an inner cell mass, an internal cavity, and an outer layer of cells • Implantation in uterine wall is complete by ~12 days

19. Development: The Chorion Forms • Produces human chorionic gonadotropin (hCG), which prevents loss of uterine lining • Chorion forms villi • Maternal tissues and villi form the placenta

20. In weeks 5 and 6, the embryo grows dramatically to a length of about 11 inches (28 cm). Most of the major organ systems, including the heart, are formed. Limb buds develop into arms and legs, complete with fingers and toes. The head is very large relative to the rest of the body because of the rapid development of the brain. By about 8 weeks, the embryo is large enough to be called a fetus. Although chromosomal sex (XX in females and XY in males) is determined at the time of fertilization, the fetus appears to be neither male nor female at the beginning of the third month. The sex organs cannot be seen in ultrasound scans until the 12th to 15th week. All the major organs have formed and are functional. By 16 weeks, major changes include an increase in size and further development of organ systems. Bony parts of the skeleton begin to form, and the heartbeat can be heard with a stethoscope. Fetal movements begin in the third month, and by the fourth month the mother can feel movements of the fetus’s arms and legs. It has a well-formed face, its eyes can open, and it has fingernails and toenails. Stepped Art p. 13

21. Third Trimester • Rapid growth • Circulatory system and respiratory system mature • Fetus doubles in size during the last eight weeks • Chances for survival outside the uterus increase rapidly

22. 1.3 Stages of Sexual Development • Formation of reproductive organs controlled by interaction between: • How genes function • Hormones • The maternal environment • Sex can be defined at several levels: • Chromosomes • Internal organs • External appearance

23. Stages of Sexual Development

24. Stages of Sex Development (1) • Chromosomal sex begins at fertilization • Zygote has either an XX or XY • Sex organs do not develop until week 7 or 8 • Expression of a gene on the Y chromosome causes the gonads to become testes • Sex is only one of thousands of traits

25. Stages of Sex Development (2) • Testes secrete testosterone, controls development of the male reproductive organs • If Y chromosome is not present, the gonad develops as an ovary • Female duct systems develop to form reproductive structures • Male duct systems degenerate • Phenotypic sex develops

26. SRY Gene • The “sex-determining gene” • A gene on the Y chromosome that confers the male sex • Sex-determining Region Y

27. Complete Androgen Insensitivity (CAI) (1) • Mutation of androgen receptor (AR) gene on the X chromosome • Lack receptors for testosterone or hormones derived from testosterone • Cells in the gonad cannot respond to hormone

28. Complete Androgen Insensitivity (CAI) (2) • Female duct system and external genitals develop • Individuals are chromosomal males (XY) • Physically appear to be females with well-developed breasts but limited pubic hair • Do not menstruate, infertile, and have undescended testes in their abdomens not ovaries

29. Individual with AIS

30. Barr Bodies • In females, one X chromosome is inactivated, forms Barr Body • Some cells express genes from father’s X chromosome, some express mother’s X chromosome • Males and XO individuals do not form Barr Bodies

31. Barr Bodies

32. Intersexuality • Chromosomal sex of a person is not consistent with his or her phenotypic sex • Or the phenotype is not classifiable as either male or female • Other conditions in this category: CAI, Klinefelter syndrome (47,XXY), and Turner syndrome (45,X)