Being Human Sexuality

Here are ultra-detailed, structured lecture notes combining your PowerPoint slides + lecture captions, organised clearly for revision. I’ve also added explanations, examples, and links between ideas so you actually understand, not just memorise.

🧠 SCIENCE OF BIAS – WEEK 5

Genetics of Sexual Orientation, Gender Identity & Eugenics

1. 🧬 THE HUMAN GENOME (FOUNDATIONS)

What is the human genome?

  • The human genome = complete set of DNA in humans

  • Stored in 23 chromosome pairs (46 total)

  • DNA = instructions for:

    • Development

    • Functioning

    • Growth

    • Reproduction

Structure of DNA

  • 4 bases:

    • Adenine (A) ↔ Thymine (T)

    • Guanine (G) ↔ Cytosine (C)

  • ~3 billion base pairs

  • ~99% identical across humans

  • ~1% variation → explains individual differences

Genes & Alleles

  • Gene = unit of DNA coding for a protein

  • Alleles = different versions of a gene

  • Effects of alleles:

    • Different protein shapes

    • Different protein amounts
      → leads to variation in traits

Types of Genetic Variation

  1. Large-scale

    • Chromosomal abnormalities

    • Rare but major effects

  2. Medium-scale

    • Copy Number Variations (CNVs)

    • Insertions/deletions

  3. Small-scale

    • SNPs (Single Nucleotide Polymorphisms)

    • Occur ~every 1000 base pairs

    • Main source of individual differences

Key Idea

👉 Most human variation comes from small genetic differences, not big mutations.

2. 🧪 HOW GENETICS STUDIES WORK

Types of studies

  • Twin studies

    • Compare:

      • Monozygotic (100% shared genes)

      • Dizygotic (~50% shared genes)

    • If identical twins are more similar → genetic influence

  • Molecular genetics

    • Directly analyse DNA

Heritability (CRUCIAL EXAM CONCEPT)

Definition:

Heritability = proportion of variation in a trait explained by genetic differences in a population

⚠ VERY IMPORTANT:

  • Applies to populations, NOT individuals

  • Example:

    • 40% heritable ≠ 40% chance you have it

    • It means 40% of variation between people is genetic

Key Findings

  • Almost all traits are somewhat heritable

  • Traits are polygenic:

    • Many genes

    • Each with tiny effects

    • Add together probabilistically

3. 🌈 GENETICS OF SEXUAL ORIENTATION

Heritability Estimates

  • Same-sex behaviour: ~20–40% heritable (Western samples)

👉 Meaning:

  • Genetics explains part of variation

  • Remaining variation = environmental factors

    • Includes:

      • Social environment

      • Biological factors (e.g. prenatal hormones)

Important Limitations

  • Twin studies:

    • Don’t identify specific genes

    • Don’t model gene–environment interaction well

Ganna et al. 2019 GWAS study

Key Study (≈500,000 participants)

Findings:

  1. ❌ No “gay gene”

  2. ✔ Many genes with small effects

  3. ✔ Spread across whole genome (not just sex chromosomes)

  4. ✔ Partly different between males & females

  5. ✔ Cannot predict individual orientation

Specific gene links (tentative):

  • Smell-related genes

  • Hormone sensitivity genes (in males)

Challenge to Traditional Models

Alfred Kinsey

  • Proposed sexuality = single continuum

Genetics suggests:

  • ❌ Not one spectrum

  • ✔ Multiple independent dimensions

    • Attraction to men ≠ inverse of attraction to women

Key Conclusion

👉 Sexual orientation is:

  • Biologically influenced

  • Highly complex

  • Not deterministic

  • Not reducible to a single scale

4. transgender_symbol emoji GENETICS OF GENDER IDENTITY

Current Evidence

  • Moderate–strong heritability (twin studies)

  • Environmental influences also important

  • Very limited molecular genetic evidence

Key Gaps

  • Small sample sizes

  • Poor understanding of environmental factors

  • Lack of diversity in research populations

Important Insight

👉 Environmental ≠ purely social

  • Includes prenatal biological environment

5. 🧠 CORE THEMES IN BEHAVIOURAL GENETICS

1. Probabilistic (NOT deterministic)

  • Genes increase likelihood

  • Do not fix outcomes

2. Polygenic

  • Many genes → tiny effects

3. Population-level explanations

  • NOT individual prediction

6. ⚠ ETHICAL CONSIDERATIONS

A. FEAR OF Eugenics

Definition:

  • Idea of improving humans via selective breeding

Historical Context

  • Linked to early behavioural genetics

  • Included:

    • Forced sterilisation (e.g. Sweden, Canada)

  • Associated with:

    • Racism

    • Ableism

Types:

  • Active eugenics

    • Direct control (e.g. sterilisation)

  • Passive eugenics

    • Indirect policies maintaining status quo

    • Example:

      • Restrictions on reproduction in trans individuals

Why Eugenics is flawed

  1. Ignores diversity of values

  2. Based on biased definitions of “better humans”

  3. Traits are:

    • Complex

    • Interconnected

  4. Selecting one trait → harms others

  5. Violates reproductive freedom

B. DETERMINISM CONCERNS

Fear:

  • “Genes determine everything”

Reality:

  • Genetics = probabilistic

Dual interpretations:

  • Negative:

    • “Biology excuses behaviour”

  • Positive:

    • “This is natural and valid”

C. MISUSE OF GENETIC INFORMATION

Potential risks:

  • Discrimination (jobs, insurance, education)

  • Legal misuse

  • Stigma

  • Medicalisation of normal variation

D. CONVERSION THERAPY RISK

  • Misinterpretation:

    • “If not fully genetic → can be changed”

  • Leads to harmful practices

E. PRENATAL SELECTION

Techniques:

  • Prenatal diagnosis

  • Embryo selection

Ethical consensus:

  • ✔ Acceptable for serious diseases

  • ❌ Not acceptable for traits like:

    • Sexual orientation

    • Gender identity

    • Intelligence

F. MEDIA & MISCOMMUNICATION

Problem:

  • Sensational headlines:

    • “Gene for X”

Reality:

  • Oversimplification → public misunderstanding

G. RESEARCH BIAS

Issues:

  • Western samples

  • Lack of diversity in researchers

  • Influences:

    • Research questions

    • Interpretations

7. 🧩 BIG PICTURE SUMMARY

Scientific Takeaways

  • All traits influenced by genetics

  • Sexual orientation:

    • Partly genetic

    • Highly complex

  • No single gene

  • Not deterministic

Ethical Takeaways

  • High risk of misuse

  • Historical harms (eugenics)

  • Requires:

    • Ethical oversight

    • Responsible communication

    • Diverse research

Core Exam Concepts

👉 Be able to explain:

  • Heritability vs determinism

  • Polygenic traits

  • Twin vs molecular studies

  • Ganna et al. findings

  • Critique of Kinsey scale

  • Ethical risks (eugenics, misuse, determinism)

8. 🧠 EXTRA: HOW TO WRITE ABOUT THIS IN ESSAYS

If asked:

“Discuss genetics of sexual orientation”

Structure:

  1. Explain genome & heritability

  2. Twin studies (20–40%)

  3. Ganna et al. → polygenic

  4. Reject “gay gene” idea

  5. Critique Kinsey model

  6. Limitations (sample, replication)

  7. Ethical concerns

If you want, I can:

  • Turn this into exam-ready essay plans

  • Make flashcards

  • Or simplify into a 1-page cheat sheet

Here’s a high-quality MCQ quiz based directly on your lecture (slides + captions). I’ve made it exam-style with tricky options, then added answers + explanations at the end so you can test yourself properly.

🧠 MCQ QUIZ – Genetics, Sexual Orientation & Eugenics

Q1. What does heritability refer to?

A. The probability that an individual will develop a trait
B. The proportion of a trait caused by genes in an individual
C. The proportion of variation in a trait explained by genetic differences in a population
D. The number of genes responsible for a trait

Q2. Which statement about the human genome is correct?

A. 10% of DNA differs between humans
B. Humans have 23 chromosomes total
C. Over 99% of DNA is identical between individuals
D. DNA contains only two nucleobases

Q3. What is a key feature of most human traits?

A. They are controlled by a single gene
B. They are entirely environmentally determined
C. They are polygenic
D. They are deterministic

Q4. Twin studies estimate that same-sex sexual behaviour is:

A. 0–10% heritable
B. 20–40% heritable
C. 50–70% heritable
D. 80–100% heritable

Q5. What is the main conclusion of the Ganna et al. 2019 GWAS study?

A. There is a single gene for sexual orientation
B. Sexual orientation is entirely environmental
C. Many genes of small effect contribute to sexual behaviour
D. Only sex chromosomes influence sexual orientation

Q6. Why can genetic findings NOT predict sexual orientation in individuals?

A. Genes do not influence behaviour
B. Traits are polygenic and probabilistic
C. Environmental factors do not exist
D. Twin studies are invalid

Q7. Which best describes environmental influences in this context?

A. Only social upbringing
B. Only cultural influences
C. Includes biological factors like prenatal hormones
D. Only parental behaviour

Q8. What challenge does genetic evidence pose to Alfred Kinsey’s theory?

A. Sexual orientation is fixed at birth
B. Sexual orientation is not measurable
C. It is not a single continuum from opposite- to same-sex attraction
D. Sexual orientation is entirely genetic

Q9. Why do molecular genetic studies often explain less variance than twin studies?

A. Twin studies are incorrect
B. Genes have no effect
C. Not all relevant genes have been identified yet
D. Environmental effects are ignored

Q10. What is meant by “polygenic”?

A. A trait is caused by one dominant gene
B. A trait is influenced by many genes with small effects
C. A trait is only environmentally determined
D. A trait is inherited only from one parent

Q11. Which is a major ethical concern about genetic research?

A. It is too simple to understand
B. It could be misused for discrimination
C. It has no real-world applications
D. It only applies to animals

Q12. What is Eugenics?

A. The study of genes in populations
B. The idea of improving humans through selective breeding
C. The natural evolution of humans
D. The process of genetic mutation

Q13. Why is eugenics scientifically flawed?

A. Traits are simple and predictable
B. Genes determine everything
C. Complex traits are probabilistic and interconnected
D. It is based on too much data

Q14. What is “passive eugenics”?

A. Direct sterilisation policies
B. Genetic mutation in populations
C. Policies that indirectly favour certain groups reproducing
D. Random genetic variation

Q15. Why is genetic determinism problematic?

A. It ignores environmental influences
B. It assumes genes have no effect
C. It makes research easier
D. It removes ethical concerns

Q16. Why is prenatal genetic selection controversial for traits like sexual orientation?

A. Traits are fully predictable
B. Traits are probabilistic and part of normal variation
C. It is impossible to do
D. It only affects adults

Q17. What is a major limitation of current research on gender identity?

A. Too many large studies exist
B. No environmental influences
C. Lack of large-scale molecular genetic studies
D. Over-replication

Q18. Why is media reporting of genetic findings problematic?

A. It is too technical
B. It often oversimplifies results (e.g., “gene for X”)
C. It ignores genetics entirely
D. It only reports accurate findings

Q19. What is the key takeaway about genes and behaviour?

A. Genes fully determine behaviour
B. Genes have no influence
C. Genes influence probabilities, not outcomes
D. Genes only affect physical traits

Q20. Which statement BEST summarises the lecture?

A. Sexual orientation is caused by one gene
B. Genetics has no role in behaviour
C. Behaviour is shaped by complex interactions between genes and environment
D. Environment alone explains human traits

✅ ANSWERS + EXPLANATIONS

  1. C – Population-level variation, NOT individuals

  2. C – >99% identical

  3. C – Most traits are polygenic

  4. B – ~20–40%

  5. C – Many small-effect genes

  6. B – Polygenic + probabilistic

  7. C – Includes biological environment

  8. C – Not a single continuum

  9. C – Missing many small-effect genes

  10. B – Many genes, small effects

  11. B – Risk of misuse/discrimination

  12. B – Selective breeding ideology

  13. C – Traits are complex & probabilistic

  14. C – Indirect structural policies

  15. A – Ignores environment

  16. B – Normal variation + probabilistic

  17. C – Lack of strong molecular evidence

  18. B – Oversimplification

  19. C – Probabilistic influence

  20. C – Interactionist view

If you want next:

  • I can make a harder MCQ set (exam-level traps)

  • Or a timed mock test with scoring + grade boundaries

  • Or MCQs for the evolution lecture too