The Role of Chromosomes & Hormones

• Chromosomes are made of nucleic acids (mainly DNA in humans) and protein and are found in the nucleus of cells
• Humans have 23 pairs of chromosomes in each cell (the 23rd pair determines the baby’s sex) with the chromosome pattern for a male being XY (one of the pair looks like and X and one like a Y) and for a female it is XX
• Chromosomes determine a baby’s sex at fertilisation: all eggs carry an X chromosome, and if the egg is fertlised by a Y-chromosome carrying sperm, the the child will be a male and if the sperm carries an X chromosome, then the baby will be female
• During prenatal development, individuals have gonads which are identical and have the ability to turn into either testes or ovaries, so if a Y chromosome is present it produces a protein which causes the gonads to become testes

The 23rd chromosome pair of a male 

• Hormones are chemical messengers that are released into the bloodstream from glands
• The main sex hormones are
• Testosterone - which stimulates the development of male characteristics and is produced in the testes
• Oestrogen - which stimulates the development of female characteristics and is produced in the ovaries
• Oxytocin - is produced in the hypothalamus and controls key aspects of the reproductive system (though testosterone inhibits its action in males) especially labour and breast-feeding in mothers, as well as nurturing behaviour in both sexes
• When it comes to determining the sex of a foetus, the chromosomes control the hormone action that decides if the gonads become testes or ovaries
• When the Y chromosome of the foetus in the womb is detected by the baby’s brain, approximately two months after conception, this slows down the oestrogen production that until then has been present in both the XX and the XY foetus, and increases testosterone production and the baby continues to grow as a boy
• If no Y chromosome is detected, then the oestrogen production continues and the baby develops ovaries and the characteristics of a girl
• The hormonal differences in XX and XY foetuses result in physical differences in bone and muscle growth and body shape 
• Testosterone affects the size of the suprachiasmatic nucleus (SDN): a brain area associated with our psychological gender identity, making it larger in males than in females 
• This may contribute to some of our psychological gender differences, though many psychologists debate whether functionally there is any difference between male and female brains

Though it is determined much earlier, a baby’s sex can usually be identified in the 18-20 week scan.

Research which investigates chromosomes and hormones

• Van Goozen et al. (1995) found transgender women (male-to-female) who were given oestrogen treatment showed decreases in aggression and visuospatial skills, while transgender men (female-to-male) treated with testosterone showed the opposite effects 
• Deady et al. (2006) conducted a correlational study in which they measured 27 young women’s testosterone levels, gave them the Bem Sex-role Inventory with extra questions about their mothering intentions and ambition to have children and found that high testosterone levels correlated with a low desire for children

Link to Issues & Debates: 

To identify all behaviour differences between males and females as being a result of their different chromosomes and hormones is biological determinism. This refers to the idea that all human behaviour is innate and it ignores the role of the environment, culture and socialisation in gendered behaviour. The idea that all human behaviour is innate is a form of hard determinism that views sex and gender as directly linked and something over which the individual has no control. Male and female sex determines gender which just becomes visible masculinity and femininity with associated personality traits, bodily strength and feminine and masculine choices of ways of life, including careers, childcare and preferred activities.