CHAPTER 6
IT MIGHT COME AS A SURPRISE to people when they learn that sperm, the ultimate manly cell, can be male or female. The male sperm carry the Y chromosome, and the female sperm carry the X chromosome. No disputing that. As we read in Chapter 4, when the male sperm wins the swimming race to get to the egg, the resulting baby will be male. If the female sperm gets there first, the baby will be female. What this means ultimately is that information present on the Y chromosome will govern the development of male characteristics that we are well familiar with. The lack of this information will mean that female characteristics will emerge.
An unresolved question in science is whether there are major differences between males and females outside the obvious anatomical ones. A related question is the scientific basis for whether humans are heterosexual or homosexual or somewhere along the spectrum of these orientations. It can be a fraught topic, with scientists sometimes being accused of bias in their studies or their interpretation of data. All scientists carry baggage – they are human after all. And yet the scientific question as to what makes you a male or a female, or makes you straight or gay, is as valid as any other, and is one many people are interested in. And we still have no clear-cut answer, unusual as that may seem. I would plead with you to leave your own baggage in the closet, and go back and collect it after you’ve read this chapter. By the end of it, you may wish to leave your baggage where it is, or add some rocks to it if I really annoy you.
THE Y CHROMOSOME (LEFT) AND THE X CHROMOSOME (RIGHT). CARRYING THE Y CHROMOSOME MAKES A MALE, ALTHOUGH THINGS MAY NOT BE THAT SIMPLE.
First, let’s look at gender. Once upon a time it seemed simple. In our species, as in nearly all species (barring creatures that, for example, don’t have any gender and procreate by simply dividing, or where a single creature has both male and female parts and can have sex with itself), there were males and there were females. The males had more hair, deeper voices, more muscle mass, could produce sperm from testes and ejaculate them via a penis. Some of these features have obvious functions such as producing sperm and some have a secondary feature of helping to attract a mate. These include, for example, a deep voice and a six-pack (not of beer), and are called secondary sexual characteristics. But of course some males might have less muscle or body hair (as is the case with most Asian men), so the idea of a spectrum is therefore important here. But all men have testicles. Females tend on average to be smaller in stature, with less muscle mass, higher-pitched voices, mammary glands to produce milk for their young and help attract a mate, and the ability to produce eggs and become pregnant.
MISS TIFFANY UNIVERSE, A BEAUTY CONTEST IN THAILAND WHERE ALL THE COMPETITORS WERE ONCE MEN.
So far so good. But then it gets more complicated. It turns out there is huge diversity among humans when it comes to defining gender. Tolerating that diversity is what makes us truly human. At one point, Facebook users could choose from one of 71 gender options, including asexual, cisgender, transgender and polygender1. ‘Man’ and ‘Woman’ were only two options out of the 71. What Facebook recognised was that gender identities are complex, and that for many people, describing themselves as a man or a woman may be inadequate. It’s certainly a mixed-up, muddled-up, shook-up world.
Our grandmothers would be very confused by this. But some people feel so fundamentally different they undergo gender reassignment, which is a combination of hormone therapy and surgery. Thailand is a particularly interesting country in this regard. There is a beauty contest there called Miss Tiffany Universe2. The women in this annual contest compete, as in other beauty contests, in swimsuits and ballgowns, but with one difference: they were all once men. The contest began 18 years ago in the hope that it would help break down the stigma that many in the Thai transgender community face. There are currently as many as 100,000 transgender women in South East Asia.
Gender identity is a person’s private sense and subjective experience of their own gender rather than being purely based on their anatomy3. Psychologists tell us that gender identity is usually formed by the age of three and is extremely difficult to change after that. What governs it is complex. It can be driven by cues from the environment, such as a child observing and imitating gender-linked behaviours. For males, these behaviours include assertiveness, ambitiousness and competitiveness, while for females, more modest behaviour and a focus on relationships feature. The environment might also involve gender-specific toys and clothes. But there are also hormonal and genetic influences at play. Some people have a mix of sexual characteristics. For example, a person with female genitalia as well as a deep voice and facial hair may have difficulty in identifying with one particular gender. A survey of the literature covering the years 1955–2000 suggests that as many as one in every 100 have intersex characteristics.
So what does science tell us about gender? Whatever about private sense and subjective experience, we know that it is the Y chromosome that governs the appearance of the male physical characteristics. What is it on that chromosome that is responsible? There is in fact a single gene, called SRY, that acts as a signal to set the developmental pathway towards maleness4. The presence of this gene starts off the process of what is delightfully called virilisation. In females, the lack of the Y chromosome means this process can’t begin.
Furthermore, females also undergo what is called X-inactivation: one of the two X chromosomes is actually shut off, as a double dose of the genes on the two X chromosomes might be dangerous. But humans can have a chromosomal arrangement that is inconsistent with their gender. For example there are XY females. They have a condition termed ‘androgen insensitivity’. Androgens (a class of hormones that includes testosterone) drive the development of maleness, following the SRY cue from the Y chromosome. People with androgen insensitivity are not able to respond to androgens, and so the characteristics don’t develop; instead the X chromosome in those people dominates and they become female.
Then there is the case of females (as defined by XX) whose bodies produce a lot of testosterone naturally. This could be because there are genetic changes in the genes for the proteins that make testosterone, and these changes step up production. Or perhaps the cells in the woman’s body are much more sensitive to testosterone. She will then develop male secondary characteristics.
As we saw in Chapter 3, this has recently become an issue in athletics. A study of elite athletes in 2000 revealed that 4.7 per cent of females had a testosterone level in the male range5. Samples were taken from 2,127 elite male and female athletes from the 2011 and 2013 World Championships. High testosterone was shown to give a ‘significant competitive advantages’ to women running in the 400 m, 400 m hurdles and 800 m. Female hammer throwers and pole vaulters had a particularly high level. Exercise can actually influence the level of testosterone. Intense exercise boosts it, while endurance exercise lowers it. A question is whether women athletes with naturally high testosterone should be banned from competing because of what is called an ‘androgen advantage’. Higher testosterone means more muscle mass and endurance, and perhaps a more aggressive competitive attitude to drive them, although this latter aspect hasn’t been proven.
The International Olympic Committee is currently reviewing their opinion on this matter, with athletes such as Dutee Chand and Caster Semenya awaiting the verdict. Semenya was asked to take a gender test just before winning the 800 m in the 2009 World Championships. She went on to win gold in the 800 m in London in 2012 and Rio in 2016. One question is, If women who produce testosterone naturally are banned, where might that stop? Should an athlete be banned just because they have an optimal biology to allow them to win? (Take the case of swimmer Michael Phelps, who has a particular leg/upper body ratio.)
One thing that is clear is that the issue of gender identity is now important for many people, and biochemistry alone will not be able to necessarily answer it. It has become so important that within international human rights law, the Yogyakarta Principles have been established. They came about following a 2006 international meeting of human rights groups in Yogyakarta in Indonesia, and were established due to a trend of people’s human rights being violated because of their gender identity or sexual orientation. An Irish human rights expert, Michael O’Flaherty, led the drafting and development of the principles. They cover non-discrimination, personal security (including a specific statement on the death penalty, which is still applied in certain countries for sexual activity between persons of the same sex), economic, social and cultural rights, rights to expression and opinion, freedom of movement and rights of participation in family life. The Yogyakarta Principles are something of a milestone in the recognition of different gender identities and sexual orientation in humankind.
CASTER SEMENYA, A SOUTH AFRICAN MIDDLE-DISTANCE RUNNER AND 2016 OLYMPIC GOLD MEDALIST. FOLLOWING HER VICTORY AT THE 2009 WORLD CHAMPIONSHIPS, SHE WAS SUBJECTED TO GENDER TESTING. IN 2010 SHE WAS CLEARED TO RETURN TO COMPETITION.
Sexual orientation and the issue of homosexuality has been a fraught topic for humans over the millennia. From antiquity, many cultures shunned homosexuality and punished those who were gay. Others, however, such as the ancient Greeks, were more tolerant. The first record of a homosexual couple was Khnumhotep and Niankhkhnum, ancient Egyptians who lived around 2400 BC. They are depicted in a nose-kissing position, which was the most intimate pose in Egyptian art. It was perfectly normal for Azande warriors in northern Congo to take young male lovers. Native Americans who were homosexual were revered as special shamans with particular magical powers. There are many accounts of same-sex relationships in ancient Chinese literature. But in some cultures and religions it remains a difficult topic, being labelled as deviant.
Homosexuality is a common trait that occurs throughout nature and may in fact have evolved to provide advantages to a social species such as ourselves. And there have been two main lines of investigation into the scientific basis for heterosexuality and homosexuality. As ever with something as complex as this, the issue of the environment is one aspect, while the biological basis (e.g. whether genetic variation explains its variation in human populations) forms the second, although the consensus once again is that it will be a combination of the two. The overall view is that there will be biological factors but that these will be modified based on environmental factors such as exposure to hormones in the uterus or social factors.
First, we should provide definitions. Heterosexuality is defined as romantic attraction, sexual attraction or sexual behaviour between persons of the opposite sex. It also refers to a sense of identity with others similarly inclined. Homosexuality is a preference in these things for one’s own sex. What again has become clear is a continuum between the two, with bisexuality being defined as attraction to both sexes.
A DEPICTION OF KHNUMHOTEP AND NIANKHKHNUM, A SAME-SEX COUPLE FROM ANCIENT EGYPT WHERE HOMOSEXUALITY WAS OPENLY PRACTISED.
Factors that influence this trait concern exposure to testosterone in the womb. If there is androgen insensitivity then the brain might develop differently. Heterosexuality in females has been linked to a lower amount of masculinisation than that found in lesbians, for example. In one remarkable study, deletion of a single gene changed the orientation of female mice from straight to gay6. The gene in question has the highly appropriate name FucM, which was shown to be responsible for masculinising the mouse’s brain. Females lacking this gene avoided the advances of male mice and tried to mate with females instead. Disappointingly, the gene for FucK does not appear to be involved.
A lot of effort has gone into finding ‘the gay gene’, but consensus has yet to be found. The idea that there would be a gene for such a complex trait is now seen as non sensical. There is however substantial evidence for there being a genetic basis, mainly based on identical and fraternal twin studies. These studies show that if one twin is gay, the partner twin is much more likely to also be gay if they are identical rather than fraternal. The comparison between identical and fraternal twins is important, as identical twins share the exact same genes while fraternal twins don’t. In both cases the reasonable assumption is that the twins will be raised in very similar environments, and so the fact that identical twins are more likely than fraternal twins to both be gay points to a genetic basis7.
TWO BROTHERS SHARE A MANLY DRINK. IS THE YOUNGER ONE MORE LIKELY TO BE GAY?
But what might that genetic basis be? As association has been found with regions on chromosome 8, and with the Xq28 gene in the X chromosome, and these are being explored further8. One interesting development is that there are marks on genes (chemical marks called methylations) which associate with sexual orientation, with five regions being marked in people who are gay, but not in straight people9. These markings are thought to act as volume controls on genes – how much a gene is active. This is the science of ‘epigenetics’, ‘epi’ meaning ‘outside’. The gene is the same (in terms of DNA sequence), but it carries chemical markings. This pattern was able to predict sexual orientation in 67 per cent of cases, which is impressive, although the study needs to be replicated. The reason for such marks being there is not known, but it could be some environmental factor that attaches them. And they may well pass to the next generation, if the same markings are present in the sperm or egg, contributing to the heritability of homosexuality as a trait.
One interesting aspect of this concerns reproduction in the population as a whole. Why would genes persist in the population if their goal is to result in homosexuality, which could decrease the birth rate? Homosexuality seems to contradict the basic need for humans to reproduce. One reason is that the same gene variants that link to homosexuality may confer a reproductive advantage on heterosexual people. To support this, there is a study showing increased fertility in women related to gay people on their mother’s side but, for some reason, not their father’s10. The idea here is that the genes may confer being gay on the man, but if the woman has them she may not be gay but might be more fertile.
The second idea is that gay men make more diligent uncles, and so will help ensure the survival of their relatives, who are also carrying some of their DNA from their sister11. This would indeed make evolutionary sense, the mission here being to preserve the selfish genes of you and your family by being an attentive uncle – a ‘guncle’, as gay uncles are sometimes called. Your specific genes may not survive (you will have some genes that your siblings won’t have), but some of your genes will survive, in your nieces and nephews. The selfish genes would like that.
Another interesting possibility concerns creativity. For this, we must remember that most genes have multiple effects. The same protein (remember, genes code for proteins) can do different things. If one gene increased the chance of creativity and also being gay, it might spread, as being creative might present an evolutionary advantage. There is evidence that gay people tend to be more creative, so this presents another possibility12. Some, however, view the increased creativity in gay people as a coping strategy rather than something directly linked to being gay. And the link is correlative rather than being cause and effect.
Finally, one of the strongest predictors of whether a man is likely to be gay or not is the order a man was born relative to his brothers13. This is called the ‘fraternal birth order effect’. Several studies have shown that the more older brothers a man has, the more likely it is that he will be gay. It was noticed as far back as 1958 that gay men tend to have more older brothers. It didn’t matter how many older sisters they had. It was also shown that each older brother increased the chances of a later-born brother of being gay by 33 per cent. Overall, one in seven gay men owe their sexual orientation to this birth order effect. There is no such effect on the orientation of women.
What might be going on here? Well, all the action is in the uterus. This was shown because it didn’t matter if a gay brother was removed from his family at an early age. If a stepbrother came into a family with older brothers he didn’t have an increased chance of being gay. It has also been shown to occur across different countries and cultures. All of this points to something happening during development. The best hypothesis is that the mother’s immune system is somehow becoming sensitised to males as she has more pregnancies. Male foetuses produce what are called H-Y antigens, and these might be targeted by the female immune system as foreign. This somehow affects development in the foetus, and with each pregnancy this gets more pronounced, as the mother’s immune system gets better able to recognised these H-Y antigens. It’s almost as if the mother’s immune system is getting vaccinated each time against male antigens, such that a point is reached where a later brother is now being fully targeted in some way. Exactly what is being targeted and how the fact that it’s being targeted might alter sexual orientation is not known. But it remains a fascinating biological observation. And I wonder what it means for that most famous and magical of people in Irish folklore – the seventh son of a seventh son? He must always be fabulous …
There is a school of thought that asks, Why do we need to know the precise basis of gender or why someone is straight or gay? This is a reasonable point, as there is no obvious benefit from the research being done in terms of, say, a new treatment for a disease. If a gene variant that predicts being straight or gay is found, why will that matter? Might people change that gene and alter their fate? This is unlikely, as being straight or gay is probably much more complex than being determined by one or even a few gene variants.
The main reason people research this area is not to be patronising, but out of pure curiosity – to discover why things are the way they are. Can we explain complex biological phenomena in molecular terms? The uncovering of the scientific basis for being straight or gay, or male or female or transgender, has to be seen as science providing ammunition against those who try to malign or suppress or discriminate against people who aren’t like them.
One goal then is to promote a generally more tolerant society. All of these traits can be explained from our biology and our environments – part of what it is to be a member of the human race.. And most important, one thing that Mother Nature teaches us: without diversity, species die out. In spite of its best efforts, Facebook will most likely one day die, but we humans, in all our rich diversity, will hopefully prevail (see Chapter 19!).