Today: Nature,
Nurture, and the
Brain
Next Week:
Digestion and
Nutrition
Week of 4/23:
Plants are Cool
5/2 Exam 3
4/27 bonus 3
4/30 homework 4
emester
Grades: Ugh
World
Grades:
A = 86+
B = 75-86
C = 60-75
D = 50-60
World
Nature, Nurture, and your
Brain
Children of alcoholic parents are likely to
become alcoholics…
Nature or Nurture?
What determines
human sexuality?
Twin Studies:
Chance of homosexual male having
homosexual brother: Bailey and Pillard (1991)
52% (29/56) of identical twins
22% (12/54) of fraternal twins
11% (6/57) of adoptive brothers
http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=306995
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 twins
22% (12/54) of fraternal twins
11% (6/57) of adoptive brothers
http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=306995
X/Y chromosomes in humans
CB 15.9
Sex-linked traits: Genes on the X chromosome
50% of males
affected,
0 % females
affected
Homosexual men are more likely to have gay
maternal uncle
50% of males
affected,
0 % females
affected
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/dispo
mim.cgi?id=306995
There is no gay gene. There are thousands of
genes and billions of interconnected neurons.
There are likely many genes affecting sexuality.
Evolutionary success = reproductive success
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
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
•Overdominance
•Sexually Antagonistic Selection
•Kin Altruism
Sickle-Cell Anemia:
an example of Overdominance
S=sickle-cell
H=normal
possible offspring
Dad
H or S
H HH
Mom or
S HS
HS
SS
Hb
75% Normal
25% Sickle-cell
Malaria
75% Resistant
25% Susceptible
An example of Overdominance:
S=sickle-cell
homozygous individuals have
H=normal
decreased reproduction, but
heterozygous individuals have possible offspring
increased reproduction.
Dad
H or S
H HH
Mom or
S HS
HS
SS
Hb
75% Normal
25% Sickle-cell
Malaria
75% Resistant
25% Susceptible
Overdominance in homosexuality:
The gay genes are passed on by heterozygous
individuals who can have a reproduction
advantage over individuals without these
genes.
Sexually Antagonistic Selection:
Genes may have an advantage in one sex and a
disadvantage in the other.
Large baby size is disadvantageous for
women, but advantageous for men.
Sexually Antagonistic Selection:
Genes for male homosexuality may be passed
advantageous in women, and visa versa.
Kin Altruism:
Homosexual family members may increase
reproductive success of the other family
members.
Kin Altruism
Kin Altruism:
Homosexual family members may increase
reproductive success of the other family
members.
How can genes for homosexuality, that inhibit
reproduction, be successfully inherited?
•Overdominance
•Sexually Antagonistic Selection
•Kin Altruism
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.
Environmental effects may also play a
role in determining sexuality.
Birth order: Each elder brother increases
probability of homosexuality by 1/3.
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.
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, 451–467
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, 451–467
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.
Both nature and nurture
determine human
sexuality.
Is one more of a choice?
Today: Nature,
Nurture, and the
Brain
Next Week:
Digestion and
Nutrition
Week of 4/23:
Plants are Cool
5/2 Exam 3
4/27 bonus 3
4/30 homework 4
emester