SECSI Talk

1. S.E.C.S.I. Talk-3
By- Debarshi Dasgupta
PALB-4137
Sr. M.Sc (Agri.)
Deptt. Of Agricultural Microbiology

2. Can EPIGENETICS explain HOMOSEXUALITY???

3. Homosexuality (from Ancient Greek homo, meaning "same", and Latin
sexus, meaning "sex") is romantic attraction, sexual attraction or sexual
behavior between members of the same sex or gender.
As a sexual orientation, homosexuality is "an enduring pattern of
emotional, romantic, and/or sexual attractions" to people of the same sex.
It "also refers to a person's sense of identity based on those attractions,
related behaviors, and membership in a community of others who share
those attractions."
WHAT IS HOMOSEXUALITY??

4. “Here on one occasion I saw what I took to be a
cock copulating with a hen. When he had finished,
however, and got off, the apparent hen turned out
to be a cock, and the act was again performed with
their positions reversed, the original “hen”
climbing on to the back of the original cock,
whereupon the nature of their proceeding was
disclosed.” -20th century naturalist George
Murray Levick’s observation in his fieldnotes
while observing Adelie’s penguins in Antarctica.

5. • HS is not a dichotomous alternative to heterosexuality in that there is
an empirically verified continuum between exclusive attraction to
same-sex and opposite-sex sexual partners.
• This continuum is usually measured by a Kinsey score that varies
from 0 (no attraction to same-sex partners) to 6 (exclusive attraction
to same-sex partners).
• Homosexuality has been widely prevalent in human beings as well as
animals.
Eg:- homosexual behavior has been recorded in 93 species of birds.
Even species as barnyard sheep have about 8% strictly
homosexual males.

6. CLASSICAL VIEW OF SEXUALORIENTATION/ PRENATAL ANDROGEN PARADIGM/ JOST
PARADIGM
• There is a long-term “organizational” influence of high vs. low androgen
exposure during fetal and perinatal development that leads to sexual
dimorphism of the brain, genitalia, and behavior at birth and early childhood.
• These organizational effects of fetal androgen exposure have a cellular-level
memory that controls the subsequent “activational” influence of androgens and
estrogens at puberty on the development of secondary sexual traits, including
sexual behavior.
• The classical view, however, cannot account for all sexual dimorphisms.

8. WHAT IS EPIGENETICS ???
• The study of cellular and physiological phenotypic trait variations
that are caused by external or environmental factors that switch
genes on and off and affect how cells read genes instead of being
caused by changes in the DNA sequence.
“EPI-MARKS”: Changes in chromatin structure that influences
transcription.
Nucleosome
repositioning DNA
methylation
Histone
modifications

9. REASONS FOR THE HYPOTHESIS
• HS has substantial realized heritability yet it has low concordance between
monozygotic twins in both sexes.
• HS prevalence is substantially higher than predicted by feasible forms of
mutation-selection balance.
• Mutations in humans that reverse sexually dimorphic fetal androgen profiles
only partially reverse sexual dimorphism in these individuals.
• Linkage studies looking for a genetic underpinning to sexual orientation have
not turned up any “major” homosexual genes.
Epigenetics fits the bill.

12. THE HYPOTHESIS IN A NUTSHELL
• Epi-marks originating in embryonic stem cells regulate the
androgen sensitivity of foetus. Also act as a buffer.
• They affect some sexual traits, vary in degree of expression,
produced at birth, erased between generations.
• When this erasure is escaped, sexually-antagonistic (SA) epi-
marks are produced.
• They contribute to gonad-trait discordances in opposite-sex
offspring.

13. WHEN WOULD HOMOSEXUALITY OCCUR??
• It occurs when a stronger-than-average
SA-epi-mark (influencing preference) is
transmitted to opposite gender offspring
and paired with a weaker-than-average
sex-specific epi-mark.

15. FUTUREDIRECTIONS
• Historical, social and religious norms have always undermined the
phenomena of homosexuality.
• Epigenetics is a feasible explanation for HS rather than genetic
polymorphism.
• It can explain gonad-trait discordances as well.
• Further developments in stem-cell technology and epigenetic
analysis can yield more insights.

16. THANK
YOU