A01

• Chromosomes, composed of DNA, house genes that determine an organism's traits. Humans possess 46 chromosomes, organised into 23 pairs, with the 23rd pair dictating biological sex: XX for females and XY for males. While egg cells carry an X chromosome, sperm can carry either X or Y, thus determining the baby's sex upon fertilisation. The Y chromosome contains the 'sex-determining region Y' (SRY) gene, prompting male sex organ development in XY embryos.
• Gender development primarily unfolds through hormonal influences. Prenatally, hormones shape brain and reproductive organ development, while adolescence sees a surge in hormonal activity, fostering secondary sexual characteristics like pubic hair growth. Testosterone, crucial for male development, orchestrates male sex organ formation during foetal development, and high levels are associated with aggression, deemed adaptive for reproductive competition.
• Oestrogen, a female hormone,determines female sexual traits and menstruation, with fluctuations possibly triggering premenstrual symptoms. Oxytocin, abundant in women, particularly postpartum, stimulates lactation, reduces stress, and fosters bonding. Though traditionally linked to female intimacy, both sexes produce oxytocin during romantic activities.