TWO ADDITIONAL COMPONENTS OF HUMAN SEXUALITY

1. TWO ADDITIONAL COMPONENTS OF HUMAN SEXUALITY • Gender Identification & Sexual Orientation • What is known about their genetic regulation? • Gender Identification • Ability of children to identify with and absorb gender roles • Most male infants grow up to think of themselves as boys and then men • Most female infants think of themselves as girls and then women

2. TRANSEXUALS • Individuals who have made a gender identification that is discordant with their biological sex • Consider themselves as women trapped in a male body or men trapped in a female body

3. TRANSVESTITES • Men who dress in women’s clothing • Not considered to be a failure of gender identification but rather individuals who internalize both gender roles and express one or the other at various times in their lives • How do we assign gender to a newborn baby?

4. GENDER ASSIGNMENT • At birth a newborn’s gender is assigned on the basis of the appearance of it’s external genitals • Recorded on the birth certificate • 1/2,000 births are children born with ambiguous genitalia • Treatment may include sexual correction of the external genitalia • Sometimes to a sex different than the chromosomal sex

5. GENDER IDENTITY • 2000 William Reiner of John Hopkins University followed the fates of 14 children born with pelvic field defects • These individuals were XY with normal testicles but no penis • 12 were reassigned as female and all behaved as boys throughout childhood • 6 declared themselves male sometime between the ages of 5-12 years old

6. Level • Events • Timing • Fertilization • Chromosomal • /genetic sex • XY=male • XX=female • Gonadal sex • Undifferentiated structures becomes testis or ovary • 9-16 weeks post fertilization • Phenotypic sex • Internal and external reproductive structures • 8 weeks post fertilization to puberty • Gender identity • Strong feelings of being male or female develop • childhood Levels of sexual identity

7. GENDER IDENTITY • Gender is not just another train • It is the keystone trait, the rest of our identity develops from our gender • Decisions regarding gender must be based on • Gonadal sex • Chromosomal sex • Presence or absence of functional SRY locus

8. SEXUAL ORIENTATION • Look at sexual orientation as a variable phenotype in the population • Make a distinction between gender identification and sexual orientation • Most gay men and lesbian women have a gender identification consistent with their biological sex • But they are physically attracted to members of the same sex • There may be some evidence that sexual orientation may have a genetic basis • Must look at the heritability of a trait

9. HERITIBILITY • Estimate of how much of the variation in a trait is genetic and how much is environmental • One method of doing this is by twin studies • Compare how alike ( or concordant) identical twins are with how alike fraternal twins are for the same trait • Identical twins have the same genotype, if reared together have similar environment • Fraternal twins have different genotypes but similar environment

10. SEXUAL ORIENTATION • Studies done in 1991 on twins • Males: • In identical twins, when one twin is gay about 52% of the time the other twin is also gay • In fraternal twins when one is gay about 22% of the time the other twin is also gay • In male siblings when one is gay about 13% of the time the brother is also gay

11. SEXUAL ORIENTATION • Studies done in 1991 on twins • Females: • In identical twins, when one twin is a lesbian about 50% of the time the other twin is also a lesbian • In fraternal twins, when one is a lesbian about 16% of the time the sister is also a lesbian • In female siblings when one is a lesbian about 13% of the time the sister is also a lesbian

12. SEXUAL ORIENTATION • Determination of sexual orientation can not be wholly genetic • If it were concordant then identical twins would either both be homosexual or heterosexual, the way that traits like Down syndrome or cystic fibrosis or color blindness are. • When a trait is wholly genetic then the concordance in twins is equal to one. • With homosexuality only 1/2 of the time are both twins gay.

13. SEXUAL ORIENTATION • There may be a genetic component because genetically identical individuals are more likely to be the same for sexual orientation that genetically different individuals • Siblings of gay individuals are also gay about 13% of the time • This is much higher than the national average incidence of gays, which is about 5 - 10% • Is this due to genetic or environmental factors?

14. SEXUAL ORIENTATION • Sibs are more genetically alike than non-related individuals in the population • The environment they are reared in is also more similar than non-related individuals • Ideal study would be to look at sibs reared apart where one sib is homosexual • Such a study would be difficult to do for obvious reasons • Twins reared apart where one or both is homosexual may not be a very large sample size

15. SEXUAL ORIENTATION • If there is a genetic component to sexual orientation, it seems to be not fully penetrant • 1/2 of the time an individual with the same genetic make up will not express the phenotype of homosexuality • To do genetic studies you must first define the phenotype clearly and unambiguously • Very difficult to do when looking at homosexuality

16. SEXUAL ORIENTATION • 1993 Dr. Dean Hamer and his colleges at NIH wanted to examine the genetics of male homosexuality • Used 2 methods to assess phenotype • Self assessment, where you ask the individual of they are homosexual • Kinsey scale is a set of psychological tests to determine sexual orientation • The 2 methods seem to correlate in Hamer’s study

17. Dr. Hamer’s First Study • First group consisted of 76 self identified gay men and their relatives over the age of 18 • Results • Brother of gay man had 14% chance of being gay • Compared to 5-10% chance for males in general population • Maternal uncles and sons of maternal aunts had a 7-8% chance of being gay • Did not see similar result for paternal uncles or sons of paternal aunts Here the chance of being gay was in the 1-5% range, the same as the general population

18. Dr. Hamer’s First Study • Results suggested an X linked determinant for sexual orientation • Because mom’s male siblings and her nephews seemed to have a higher chance of being homosexual than the general population • And they have the possibility of sharing an X chromosome with mom

19. Dr. Hamer’s First Study • Paternal uncles or cousins on father’s side can not share X chromosome with the male in question but maternal uncles or cousins can • Dad gave his homosexual son his Y chromosome • He got his X chromosome from mom • Hamer saw a higher frequency of concordance among relatives who could share an X chromosome • Wanted to further study this maternal component • So he did a second study

20. Dr. Hamer’s Second Study • Looked at 38 families in which at least 2 sons were gay • 38 pairs of homosexual brothers and their relatives • This study group consisted of gay men willing to participate and involve their families • Assessing the phenotype of volunteers is fairly straight forward • They are all openly gay

21. Dr. Hamer’s Second Study • Found maternal uncles and sons of maternal aunts had 10-13% chance of being gay • Compared to 7 - 8% from the first study • If the trait followed Mendelian inheritance would expect 50% for maternal uncle and 25% for son of maternal aunt • Still the results were encouraging and Dr. Hamer went on to look for the genetic component

22. Dr. Hamer’s Third Study • In this study Dr. Hamer tried to find a possible gene on the X chromosome • The mother of a homosexual male could be heterozygous for the gene • Looked at 40 pairs of gay brothers • These brothers should share a specific region of one of the mother’s X chromosomes • The region that predisposes them to homosexuality

23. Dr. Hamer’s Third Study • Brothers were as likely to have 2 different alleles as they were to both share one of the two given maternal alleles • Except for one region near the tip of the long arm of the X chromosome • In this region Xq 28 the two homosexual brothers shared the same allele in 33 out of the 40 cases • Interpreted this to mean that this region contains genes that may predispose an individual to homosexuality

24. Dr. Hamer’s Third Study • Dr. Hamer looked at 5 DNA markers in region Xq 28 • These sequences are highly variable in the general population • But in his study group 33 out of the 40 pairs of gay brothers were identical in this region • Now he had a DNA sequence that was more common among homosexual brothers • He concluded that this region could predispose a male to homosexuality

26. Dr. Hamer’s Third Study • Findings indicate a correlation between the genotype at Xq 28 and sexual orientation but it also means that there could be other genes or environmental influences at work • Went on to a fourth study • Looked at heterosexual brothers of these 40 homosexual brothers and found that only 22% of them shared the allele at Xq28 • Most of the heterosexual brothers did not carry the allele but 22% of them did

27. CRITISISMS OF THE WORK • Small sample size • 40 pairs of gay brothers • Work only identified DNA sequences that are more common among homosexual brothers, it did not identify a causative gene

28. IMPORTANT QUESTIONS TO CONSIDER • How do the researchers define phenotype? • Is it self identified? • What population did they study? • Are they openly gay individuals only? • What was the sample size? • What is the control group? • Does the study look at genetic component or environmental component or genetic environmental interaction?

29. Genetically Induced Behavior • The normal eye gene allows cells to break down tryptophan, which is required to make the hormone serotonin • Expression of the white gene in all cells reduces the levels of serotonin in the brain • In other animals reduced levels of serotonin in the brain is also associated with homosexual behavior