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

7. Gender is not just another train
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 %
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. Went on to a fourth study
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