Sex and Differences

Question 1

Sex and gender – working definitions

Answer 1

Sex: Specific attributes that characterise maleness or femaleness. They can include chromosomal and other genetic attributes, as well as physiological, physical and behavioural differences.

Gender: A person’s internal sense of their gender (experienced gender) or the way in which their gender is perceived by others (expressed gender).

Sometimes portrayed as a dichotomy between biological (sex) and social or cultural (gender) differences, setting one against the other.

This ignores the fundamentally interactionist nature of human development. The distinction between definitions of sex and gender are fluid and overlapped. Some researchers will use the terms sex
difference and gender difference interchangeably.

Gender can be defined in multiple ways

Question 2

Asking questions about behaviour:
Why does a chaffinch sing in the Spring?

Answer 2

Illustrates the multiple ways you can answer the question why is the bird singing eg. about biological fitness- enhances representation of genes, how the behaviour has evolved, through mechanisms, mechanisms

This question can be approached from multiple directions – and a similar approach can be taken in relation to sex differences.

Question 3

So, why do chaffinches sing in the Spring?

Answer 3

• How did the behaviour evolve?
• How does it enhance biological fitness?
• How does it develop in an individual or social group?
• What are the physiological and brain mechanisms involved?
  These are Niko Tinbergen’s ‘four questions’ (Tinbergen 1963)

Question 4

Why are chimpanzees polygamous and common marmosets monogamous?

Answer 4

• How and when did this difference evolve in the primate group?
• What survival value does it have? How do the different patterns enhance biological fitness in the two species?
• How do these different behavioural patterns develop over an individual lifespan?
• What differences in physiological and brain function does the difference depend on?
  Niko Tinbergen’s ‘four questions’ again! For any behaviour you can think about it from that very broad perspective.

Question 5

• Why has sexual reproduction evolved?
• Sexual reproduction involves…

Answer 5

Why has sexual reproduction evolved given that:
* Asexual cell division of haploid cells, as in bacteria and many other single cell organisms is simpler, less prone to deleterious mutations accumulating in the population, avoids ‘waste of males’ (Maynard Smith 1971).

Sexual reproduction involves:
- more complex type of cell division (meiosis: diploid -> haploid) followed by fertilisation (haploid -> diploid)
- Deleterious mutations may accumulate, masked by a normal second copy of a gene
- Risks of finding / competing for mates

Reminder: haploid – having a single copy of each chromosome in a cell; diploid – having two copies …

Question 6

Mitosis and Meiosis

Answer 6

Mitosis: in a diploid cell (left) the chromosomes replicate and then separate into the daughter cells

Meiosis: chromosome duplication is followed by recombination and generation of haploid gametes – Fusion of two gametes in sexual reproduction restores the diploid state. Meiosis Involves interchange of genetic material between identical pairs of chromosomes.

Question 7

Sex – an evolutionary perspective

Answer 7

• Sun and planets condensed out of a spinning
  gaseous cloud about 5 billion years ago (bya). As that cloud span, some of the gas spread out and this is why the planets are all in the same plane as they rotate around the sun.
• Earth and a large asteroid (Theia) collided about
  4.5 bya, resulting in the formation of the moon and
  possibly also the large water content of Earth.
• Life also evolved some 4.5 bya.
• Sexual reproduction evolved in eukaryotes (organisms )
  at least 2 billion years after.
• Sexual reproduction remains predominant, even in those
  groups where parthenogenesis is also present
  (e.g. ants, bees, wasps, water fleas).

Question 8

Sex – an evolutionary perspective
Why did it persist?

What are the several competing/ complementary hypotheses?

Answer 8

• Recombination of different genotypes during meiosis allows favourable mutations to come together in the same individual.
• Recombination allows more rapid adaptation to fluctuating environments (e.g. to new parasites - the Red Queen hypothesis).
• Recombination enhances variation
• repair mechanisms are also associated with meiosis
• May help to avoid accumulation of deleterious mutations, especially in stressful environments.
• Parthenogenesis (as in bees, ants & wasps and also in water fleas and many other invertebrates) permits rapid exploitation of a favourable environment. Switch back to sex as things become more stressful, favourable season ends etc. Very rare in vertebrates.

Question 9

Sex differences and effect sizes

Answer 9

The magnitude of a difference between two groups can be expressed as the effect size (Cohen’s d – the difference in the means divided by the relevant standard deviation)

2 populations differ from a small amount to larger amount.

Question 10

What is the difference between men and women height?

Answer 10

Male- 178.4 cm
Female- 164.7 cm

Question 11

Sexual dimorphism and breeding systems in primates

Answer 11

Sexual dimorphism is very variable
It can be very substantial
It relates to social patterns/ mating systems that you see

Sexual dimorphism tends to be high in primates with significant male
-male competition but low in those that either breed cooperatively or are relatively solitary.

Data from Plavcan (2012)
* Humans show less sexual dimorphism than great apes

Question 12

Effect sizes for different human behaviours or traits

Answer 12

Height as comparator

Some characteristics where the effect size is very large eg. gender identity and sexual orientation has high but social dominance has a much smaller effect size

Question 13

Sexual development
Chromosomal sex in humans

Answer 13

• Most human cells are ‘diploid’ (2 copies of each
  of the 23 chromosomes).
• Sperm and eggs are ‘haploid’ (a single copy)- produced by meisosis, with the fertilised egg becoming diploid again.
• Meiosis leads to recombination of gene
  variants between homologous chromosomes.
• One ‘pair’ of chromosomes look markedly different (the XY pair) and, in most cases, a fertilized egg will either be XX or XY.
• An XY chromosomal pair normally leads to male-like development, and XX to female-like development.
• But note that things are very different in other groups of vertebrates.
• What is it about the Y chromosome that leads to male-like development in mammals?

Question 14

Sex Determination in Mammals:
some experimental evidence from mice

Answer 14

Take about 1% of the Y chromosome
|
Inject into a large number of already fertilised mouse ova
|
Examine the resulting embryos
|
Many XX females, 2 XX embryos with a male phenotype, many XY males

The XX mouse embryos with male phenotype can be shown to have incorporated a gene (Sry) into one of their X chromosomes.

Koopman et al. (1991) Nature 351:117-121

Most of them were XX and they looked like female mice. Another half were XY and they looked like male mice.

Question 15

Sex Determination in Humans:
some experimental evidence

Answer 15

• The human SRY gene can produce masculinisation when incorporated into a developing mouse foetus (i.e. acts in the same way as the mouse Sry gene).
• A number of rare abnormalities of sex chromosomes (showing theres something important with Y chromosome), including:
  X0 - Turner’s syndrome (female phenotype)
  XXY – Klinefelter’s syndrome (male phenotype)
• Very rare instances of individuals with an XX genotype and male phenotype. Some of these individuals have been shown to have the SRY gene from the Y chromosome incorporated into one of their X chromosomes.
• Other rare instances include individuals with an XY genotype and female phenotype. These may be accounted for by mutations in the sex steroid signalling pathways.
• ? How does the SRY gene drive sexual differentiation ?

Question 16

Sex determination in other vertebrates
Pretty much constant in mammals

Answer 16

• Mammals share a similar sex-determining chromosomal mechanism as humans, with SrY as the ‘master switch’ on the Y chromosome.
• In birds the heterogametic sex is female W/Z and the male is ZZ. The Z chromosome includes the gene Dmrt1, a double ‘dose’ of which promotes testicular (male like) development. Dmrt1 is an ancient sex-determining gene, found also in some insects (e.g. Drosophilla) as well as amphibia and reptiles.
• In many amphibians and reptiles sex may either be determined through a ‘pure’ genetic mechanism, or be strongly influenced by environmental factors. (environmental factors affect sex and determination factors).
  Example here is of temperature-dependent sex reversal in the Bearded Dragon (Pogona vitticeps).

Question 17

Hormones and receptors

Answer 17

• Hormones are chemical messengers produced in one part of the body, typically then released into the blood and act at receptors on cells of another organ or tissue.
• Although most hormones bind to receptors on a cell surface, steroid hormones are fat soluble and can pass directly through the external cell membrane and bind to receptors.
• The hormone-receptor complex may then pass into the cell nucleus and directly affect gene transcription. This is a slow mechanism because diffusion processes take time.
• However steroids may also bind to different receptors at the cell surface, perhaps modifying the effects of a brain neurotransmitter and producing rapid effects on behaviour.

Question 17

Hormones and receptors

Answer 17

• Hormones are chemical messengers produced in one part of the body, typically then released into the blood and act at receptors on cells of another organ or tissue.
• Although most hormones bind to receptors on a cell surface, steroid hormones are fat soluble and can pass directly through the external cell membrane and bind to receptors.
• The hormone-receptor complex may then pass into the cell nucleus and directly affect gene transcription. This is a slow mechanism because diffusion processes take time.
• However steroids may also bind to different receptors at the cell surface, perhaps modifying the effects of a brain neurotransmitter and producing rapid effects on behaviour.

Question 18

Hormones in the reproductive system

Answer 18

• Testosterone, estrogen and progesterone are steroid hormones secreted, respectively, by the testes and ovaries.
• Luteinising hormones (LH) and Follicle Stimulating Hormone (FSH) are large protein hormones secreted by the pituitary gland (both sexes) and known as ‘gonadotrophins’ because they stimulate hormone release from the testes and ovaries (gonads). These have a feedback approach on LH, FSH and GnRH
• Gonadotrophin Releasing Hormone (GnRH) is a smaller peptide secreted by the hypothalamus that stimulates the release of gonadotrophins.
• The steroid hormones inhibit the production of GnRH, LH and FSH – negative feedback control.

Question 19

Anatomy of hypothalamus and pituitary

Anterior pituitary vs posterior pituitary

Answer 19

Anterior pituitary:
- releasing hormones secreted into small blood vessels
- gonadotrophons released into the main blood circulation

Posterior pituitary:
- nerve cells secrete and release oxytocin and vasopressin from terminals

Question 20

What will a drug that blocks GnRH receptors stop?

What will blocking GnRH shut down?

Answer 20

A drug that blocks GnRH receptors (a GnRH receptor antagonist) will stop the secretion FSH and LH from the pituitary, and also the production of hormones by the gonads.

If you block GnRH you will shut down the production of LH and FSH and oestrogen and androgens. So GnRH dampens down production of those hormones.

Question 21

What are the clinical uses these drugs have?

Answer 21

• In cancers where the tumour growth is enhanced by steroids (e.g. ovarian, prostate).
• In procedures for in vitro fertilisation.
• In some gender-affirming treatments.

Question 22

Biosynthesis and action of of sex steroids

Answer 22

• Steroids, including testosterone and estrogen, as well as adrenal steroids such as cortisol, are synthesised from cholesterol in a number of steps
• The transformation of testosterone to either estradiol (E) or 5⍺dihydrotestosterone (DHT) requires the relevant enzyme.
• Mutations in the genes for the enzymes or receptors may
  reduce or abolish the action of sex steroids.
• Estrogen and testosterone are functionally important
  hormones in both sexes.
• Mutations of the genes for the pathway to cortisol can lead to excess androgen production - congenital adrenal hyperplasia – and masculinisation.

Question 23

The SRY gene and sex determination

Answer 23

SRY gene is pushing development in that particular direction

Question 23

Fetal testosterone and sexual differentiation

Answer 23

• Testes begin to produce testosterone
• (Reminder) - Testosterone can be metabolised to either 5a-dihydrotestosterone (another androgen) or estradiol (an oestrogen).
• This requires the enzymes 5a-reductase or aromatase respectively.
• Functional androgen and estrogen receptors are also required.
• Male-like differentiation of penis and testes requires 5a-dihydrotestosterone and occurs in weeks 6-12 of human pregnancy.
• Male-like brain differentiation of the brain requires the metabolism of testosterone to estradiol and occurs in the second half of pregnancy, though the evidence for this in humans is limited.
• So, in some cases, these processes may become dissociated – the biology does not necessarily result in chromosomal / gonadal / physical and ‘brain’ sex being the same.

Question 24

Human toddler toy preferences

Answer 24

A long running study of toy preferences in human toddlers has suggested that both biological factors (e.g. fetal testosterone exposure) and social factors contribute to a marked sex difference in toy preferences:
* Observational studies show a clear sex difference for some types of play (‘dolls vs cars’, rough & tumble), (- boys tend to play with boy-like toys and girls are more likely to play with girl like toys) and equal preference for others (e.g. puzzles, board games).
* Testosterone levels are higher in a male fetus from week 8 to week 24 of gestation, then again for about 5 months after birth.
* Two clinical syndromes (CAH – raised T levels, CAIS – insensitivity to androgens) are associated with increased preference for either raised ‘male’ toy preferences (CAH) or ‘female’ toy preferences (CAIS).
* For CAH, parental influences are likely to oppose the reported preference.
* Some evidence that toddler toy preferences may be associated with effects on adult sexual orientation (Li, Kung & Hines 2017).