emester

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Today Nature, Nurture, and the Brain Next Week
Digestion and Nutrition Week of 4/23Plants are
Cool 5/2 Exam 3 4/27 bonus 34/30 homework 4
emester
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Grades Ugh
World
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Grades A 86 B 75-86 C 60-75 D 50-60
World
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Nature, Nurture, and your Brain
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Children of alcoholic parents are likely to
become alcoholics Nature or Nurture?
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What determines human sexuality?
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Twin Studies Chance of homosexual male having
homosexual brother Bailey and Pillard
(1991)52 (29/56) of identical twins22 (12/54)
of fraternal twins11 (6/57) of adoptive brothers
http//www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id
306995
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Correlation of homosexual brothers but not
homosexual brother and sister Correlation of
homosexual sisters but not homosexual sister and
brother
Chance of homosexual male having homosexual
brother Bailey and Pillard (1991)52 (29/56)
of identical twins22 (12/54) of fraternal
twins11 (6/57) of adoptive brothers
http//www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id
306995
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CB 15.9
X/Y chromosomes in humans
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Sex-linked traits Genes on the X chromosome
50 of males affected, 0 females affected
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Homosexual men are more likely to have gay
maternal uncle
50 of males affected, 0 females affected
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Cause or Effect? Brain differences? LeVay (1991)
interstitial nucleus of the anterior
hypothalamus 3 (INAH-3)2X larger in the
heterosexual men compared to women 2X larger in
the heterosexual men compared to homosexual men
http//www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id
306995
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There is no gay gene. There are thousands of
genes and billions of interconnected neurons.
There are likely many genes affecting sexuality.
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Evolutionary success reproductive success
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Evolutionary success reproductive success How
can a gene exist that inhibits reproduction
Homosexuality.
Genetics and Bisexuality V. Savolainen and L.
Lehmann (11 Jan. 2007) Nature 445 158
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Evolutionary success reproductive success How
can a gene exist that inhibits reproduction
Homosexuality. Three hypotheses
Your 1 most voted for Genetics and Bisexuality
V. Savolainen and L. Lehmann (11 Jan. 2007)
Nature 445 158
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• Overdominance
• Sexually Antagonistic Selection
• Kin Altruism

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Ssickle-cell Hnormal
Sickle-Cell Anemia an example of Overdominance
possible offspring Hb 75 Normal 25
Sickle-cell Malaria 75 Resistant 25
Susceptible
Dad
H or S
HS
HH
H or S
Mom
HS
SS
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An example of Overdominancehomozygous
individuals have decreased reproduction, but
heterozygous individuals have increased
reproduction.
Ssickle-cell Hnormal
possible offspring Hb 75 Normal 25
Sickle-cell Malaria 75 Resistant 25
Susceptible
Dad
H or S
HS
HH
H or S
Mom
HS
SS
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Overdominance in homosexuality The gay genes are
passed on by heterozygous individuals who can
have a reproduction advantage over individuals
without these genes.
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Sexually Antagonistic Selection Genes may have
an advantage in one sex and a disadvantage in the
other.
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Large baby size is disadvantageous for women, but
advantageous for men.
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Sexually Antagonistic Selection Genes for male
homosexuality may be passed advantageous in
women, and visa versa.
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Kin Altruism Homosexual family members may
increase reproductive success of the other family
members.
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Kin Altruism
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Kin Altruism Homosexual family members may
increase reproductive success of the other family
members.
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• How can genes for homosexuality, that inhibit
  reproduction, be successfully inherited?
• Overdominance
• Sexually Antagonistic Selection
• Kin Altruism

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• How can genes for homosexuality, that inhibit
  reproduction, be successfully inherited?Three
  hypotheses
• Overdominance
• Sexually Antagonistic Selection
• Kin Altruism
• Bisexuality would minimize the all or nothing
  effects of homosexuality.

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Environmental effects may also play a role in
determining sexuality. Birth order Each elder
brother increases probability of homosexuality by
1/3.
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Environmental effects on homosexuality Birth
order Each elder brother increases probability
of homosexuality by 1/3. Birth weight Boys
born as third sons have lower birth weight.
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Birth Weight, Sexual Orientation and the Sex of
Preceding Siblings
Fig 2. BIRTH WEIGHT, SEXUAL ORIENTATION AND THE
SEX OF PRECEDING SIBLINGS (2001) RAY BLANCHARD
AND LEE ELLIS J. biosoc. Sci. 33, 451467
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Homosexual males with older brothers have a lower
birth weight.
Fig 2. BIRTH WEIGHT, SEXUAL ORIENTATION AND THE
SEX OF PRECEDING SIBLINGS (2001) RAY BLANCHARD
AND LEE ELLIS J. biosoc. Sci. 33, 451467
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Environmental effects on homosexuality Birth
order Each elder brother increases probability
of homosexuality by 1/3. Birth weight Boys
born as third sons have lower birth
weight. Mothers may have immune reaction against
male proteins.
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Both nature and nurture determine human
sexuality. Is one more of a choice?
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Today Nature, Nurture, and the Brain Next Week
Digestion and Nutrition Week of 4/23Plants are
Cool 5/2 Exam 3 4/27 bonus 34/30 homework 4
emester