Download ggender identificaiton and sexual orientation

TWO ADDITIONAL COMPONENTS
OF HUMAN SEXUALITY
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Gender Identification & Sexual Orientation
What is known about their genetic regulation?
Gender Identification
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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
TRANSEXUALS
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Individuals who have made a gender
identification that is discordant with their
biological sex
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Consider themselves as women trapped in a
male body or men trapped in a female body
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?
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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
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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
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Levels of sexual identity
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Level
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Events
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Timing
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XY=male
XX=female
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Fertilization
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Chromosomal
/genetic sex
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Gonadal sex
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Undifferentiated
structures becomes
testis or ovary
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9-16 weeks
post
fertilization
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Phenotypic sex
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Internal and external
reproductive structures
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8 weeks post
fertilization to
puberty
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Gender identity
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Strong feelings of being
male or female develop
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childhood
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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
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 Gonadal
sex
 Chromosomal sex
 Presence or absence of functional SRY
locus
SEXUAL ORIENTATION
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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
HERITIBILITY
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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
SEXUAL ORIENTATION
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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
SEXUAL ORIENTATION
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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
SEXUAL ORIENTATION
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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.
SEXUAL ORIENTATION
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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?
SEXUAL ORIENTATION
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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
SEXUAL ORIENTATION
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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
SEXUAL ORIENTATION
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1993 Dr. Dean Hamer and his colleges at NIH
wanted to examine the genetics of male
homosexuality
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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
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The 2 methods seem to correlate in Hamer’s study
Dr. Hamer’s First Study
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First group consisted of 76 self identified gay men
and their relatives over the age of 18
Results
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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
Dr. Hamer’s First Study
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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
Dr. Hamer’s First Study
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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
Dr. Hamer’s Second Study
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Looked at 38 families in which at least 2
sons were gay
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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
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They are all openly gay
Dr. Hamer’s Second Study
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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
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
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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
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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
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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
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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
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CRITISISMS OF THE WORK
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Small sample size
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40 pairs of gay brothers
Work only identified DNA sequences that
are more common among homosexual
brothers, it did not identify a causative gene
IMPORTANT QUESTIONS TO CONSIDER
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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?
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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