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TWO ADDITIONAL COMPONENTS OF HUMAN SEXUALITY

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

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TWO ADDITIONAL COMPONENTS OF HUMAN SEXUALITY

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  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

  25. 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

  26. 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

  27. 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?

  28. 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

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