MCDB 1041 Class 17 Human Sex Determination

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1 MCDB 1041 Class 17 Human Sex Determination Learning goals: Distinguish how "phenotypic" sex is different from "gonadal" sex Describe how hormones are involved in the development of different ductal systems in the male and female reproductive system Explain how the SRY gene is involved in sex determination. Explain how the outward sex characteristics can be mismatched with genetic sex (the sex chromosomes) Quiz 2 is on Monday! Homework 3 due Sunday night! Review session Friday (today) 3 pm B121 Porter

2 X chromosome about 2000 genes abou t 80 gene s Sex chromosomes determine primary sex The X and Y chromosome act as homologs even though they contain different genes Y chromosome about 80 genes -During meiosis in males, the replicated X and Y chromosomes pair together on the metaphase plate -Both chromosomes contain similar regions, at the tips of each chromosome, called the pseudoautosomal region

3 It is not the number of X chromosomes, but rather the presence of a Y chromosome that determines the identity of the gonad=chromosomal sex

4 There is a specific gene on the Y chromosome called the Sex determining Region of the Y or SRY When this piece is missing, XY individuals become female. When present in XX, become male => Thus, the Y chromosome (specifically the SRY gene) determines the gonad as male.

5 TWO STAGES TO SEXUAL DEVELOPMENT kidney Primary =Gonad determination the Y chromosome Male duct Female duct At the beginning of human development, either male or female development is possible. The gonads are indifferent or unspecialized Unspecialized gonads and two sets of reproductive ducts exist until week 6.

6 SECONDARY= HORMONES Internal reproductive connections AND external sex characteristics are dependent on hormones released from the gonad 1. Testes release Testosterone: required to maintain the male ductal system Anti-mullerian hormone (AMH): required to degrade the female ductal system 2. Ovaries release estrogen Estrogen is required to maintain the female ductal system

7 testosterone estrogen Testosterone is actually produced in both men and women, primarily in the gonads. Men produce about 10x as much testosterone as women. Estrogen is also produced in both men and women. In fact, testosterone is converted into estrogen This occurs in the liver and adrenal glands in both men and women And in the ovaries in women

8 Why don t sex and gender always match? Mutations A person could have a mutation in the SRY gene Hormones A person may be chromosomally male or female, but have a deficit or excess of hormone production or reception (this is also due to a mutation): most common example Androgen Insensitivity Syndrome (AIS) Chromosomal abnormalities Sometimes people have too many or too few sex chromosomes; this results primarily in infertility, but can also result in ambiguous external sex characteristics because the number of X chromosomes influences phenotype XO XXX XXY XYY

9 The Olympic games and gender testing International Olympic Committee (IOC) introduced sex testing in : eight female athletes failed gender tests but were all cleared on appeal; seven were found to have an "intersex" condition. 2000: the IOC had abolished universal gender testing but some women were still asked to prove their gender 2012: From now on, gender testing will involve hormonal levels, not chromosomal makeup Santhi Soundarajan Silver medal 2006 Asian Games Stripped of medal Caster Semenya Gold Medal 2009 Olympics Cleared They may have Androgen Insensitivity Syndrome

10 Use the front side of your handout to help draw a concept map of sex determination in humans Work together as a group on the white boards, then transfer your diagram to your handout so you can refer to it later Next, fill out the table in Part II

11 Read the case studies (Part III) and when you Are done talking about them with your group, meet with the table next to you, and share your ideas.

12 Summing up with some clicker questions.

13 A person with Androgen Insensitivity Syndrome is XY with a mutation in the testosterone receptor, so they make testosterone, but cannot respond to it. As a result they look female. What kind of gonad do they have? a. Testes b. Ovaries What internal reproductive structures do they have? a. Male ductal system b. Female ductal system c. Neither d. Both

14 An individual with a single X chromosome and no Y (sometimes written XO, called Turner syndrome) has: a. no ovaries and no female ductal system b. testes and a female ductal system c. testes and a male ductal system d. normal ovaries and female ductal system e. ovaries and a female ductal system, but is infertile