Role of Chromosomes & Hormones (AQA A-Level Psychology): Revision Notes
Atypical Sex Chromosome Patterns
Most people are born with typical sex chromosomes - XX for females or XY for males. However, some individuals have different combinations of sex chromosomes, known as atypical sex chromosome patterns. These variations occur naturally and include conditions such as Klinefelter's syndrome and Turner's syndrome.
These chromosome variations are naturally occurring genetic differences that affect approximately 1 in every few hundred to few thousand individuals, depending on the specific condition.
Klinefelter's syndrome
Klinefelter's syndrome (KS) is a chromosomal condition affecting males who have an additional X chromosome, resulting in an XXY pattern instead of the typical XY. The condition was first identified by Dr Harold Klinefelter in the United States in 1942 and occurs in approximately 1 in every 750 males.
Physical characteristics
Males with Klinefelter's syndrome typically experience several distinctive physical features. They often have smaller than average testes and reduced testosterone production before and during puberty, which prevents the full development of typical male sexual characteristics. Many develop breast tissue, have reduced facial and body hair, and exhibit low muscle tone and reduced fertility. Physical proportions may also be affected, with a tendency towards longer legs and arms relative to torso size.
The reduced testosterone production is the primary cause of most physical and developmental characteristics associated with KS, making hormone therapy a crucial treatment consideration.
Psychological and cognitive effects
The condition can impact various aspects of psychological development and cognitive functioning. Language difficulties are common, affecting reading ability and creating challenges in academic settings. Many individuals display a notably passive or quiet temperament. Attention problems frequently occur, along with increased vulnerability to anxiety disorders and depression.
Mosaic Klinefelter's syndrome
A variant called Mosaic Klinefelter's syndrome (MKS) affects approximately 1 in 10 individuals with KS (roughly 1 in 7,500 males). In this form, only some body cells contain the extra X chromosome, while others have the typical XY pattern. Symptoms tend to be less pronounced than in standard KS. Rarely, some males may have multiple extra X chromosomes (XXXY or XXXXY), resulting in more severe manifestations.
Causes and inheritance
Klinefelter's syndrome is not inherited from parents but occurs during meiosis - the process of cell division that creates egg and sperm cells. An error called nondisjunction causes an egg or sperm cell to contain an extra X chromosome. When this combines with a normal gamete during conception, the resulting embryo has XXY chromosomes throughout its body cells.
Nondisjunction is the key mechanism behind both Klinefelter's and Turner's syndromes. This random error during cell division means these conditions are not passed down from parents but occur spontaneously during reproduction.
MKS occurs when similar errors happen during early embryonic development, affecting only some cells.
Medical complications and treatment
Various medical issues can arise from KS, including cardiovascular problems, circulatory and respiratory difficulties, diabetes, and kidney disorders. Older parents face slightly higher risks of having a child with the condition. Treatment typically involves testosterone supplements, enabling individuals to live relatively normal lives despite having a somewhat reduced average lifespan.
Research evidence
Research Study: Treatment Effectiveness
Simpson et al. (2003) demonstrated that behavioural and language difficulties associated with KS can be effectively addressed through androgen therapy (testosterone supplements) combined with psychological counselling. Earlier intervention produces better outcomes, supporting the view that negative effects of KS can be successfully managed.
Mortality Study: Long-term Health Outcomes
Swerdlow et al. (2005) conducted a mortality study of men diagnosed with KS in Britain between 1959 and 2003. They found that 461 of 3,518 patients had died (13%), representing higher mortality than non-KS patients. Primary causes included cardiovascular, nervous system and respiratory diseases, diabetes, epilepsy, and kidney disorders, suggesting that KS patients face elevated mortality risks due to various hormonal and genetic factors.
Brain Structure Research: Cognitive Basis
DeLisi et al. (2005) compared 11 KS participants with 11 non-KS controls using psychiatric interviews, cognitive tests, and MRI scans. Ten KS males showed some form of psychiatric disturbance, and generally had smaller frontal lobes, temporal lobes, and superior temporal gyrus brain regions. This may explain the language deficits observed in KS participants, suggesting a biological basis for cognitive difficulties experienced by some KS males.
Criminal Behaviour Study: Socioeconomic Factors
Stockholm et al. (2012) examined criminal patterns in 1,005 Danish men with standard KS and XYY variants. Men with KS showed higher conviction rates for sexual abuse, burglary and arson, but lower rates for traffic and drug offences compared to non-KS controls. However, when socioeconomic factors (education level, employment status, partner relationships) were considered, conviction levels became similar to controls, suggesting increased criminality results from poor socioeconomic conditions rather than the condition itself.
Turner's syndrome
Turner's syndrome (TS) is a chromosomal condition affecting females who have only one complete X chromosome in each cell, rather than the typical XX pattern. The second X chromosome is either missing entirely or incomplete. Dr Henry Turner first reported the condition in 1938, and it affects approximately 1 in 2,200 females.
Physical characteristics and development
Turner's syndrome occurs at conception, though most pregnancies with a single X chromosome end in miscarriage (about 1 in 10 miscarriages during the first trimester result from TS). Mosaic Turner's syndrome (MTS) exists where one copy of the X chromosome is missing or incomplete in some but not all cells.
Before birth, fluid accumulation (lymphoedema) may occur, causing swelling particularly in the neck, hands and feet. After birth, several physical features typically emerge including short stature, down-slanting eyes, and a distinctive short, webbed neck. A broad chest develops along with heart and kidney abnormalities.
The characteristic physical features of Turner's syndrome often become more apparent as the individual develops, with short stature being one of the most noticeable early indicators.
Reproductive and hormonal effects
The primary deficiency involves non-functioning ovaries, preventing girls with TS from developing breasts during puberty, experiencing periods, or becoming fertile. However, approximately 1 in 200 individuals with the condition can become pregnant naturally, and most have normal vaginal and uterine development. Some may experience typical pubertal changes, though about 1 in 200 with the condition can achieve natural pregnancy.
While most females with Turner's syndrome face fertility challenges, modern reproductive technologies and hormone treatments can help address many of these issues when treatment begins early.
Medical complications and treatment
Medical complications can include heart conditions, high blood pressure, urinary problems, vision and hearing difficulties, and osteoporosis (bone thinning). Psychological problems are generally minimal with TS, apart from potential social adjustment difficulties due to physical appearance and possible bullying experiences. Some girls may have minor learning difficulties, though some may even demonstrate advanced reading abilities.
Turner's syndrome is more common in girls born to older mothers and rarely affects multiple children in the same family. Treatment typically involves growth hormone administration alongside oestrogen and progesterone supplements, allowing individuals to live relatively normal lives with only slightly reduced average lifespan.
Research evidence
Longitudinal Mortality Study
Price et al. (1986) conducted a longitudinal study following 156 females with TS for 17 years. They found 15 deaths during this period (9%), compared with an expected 3.5 deaths in matched non-TS females. Major causes of death included cardiovascular and circulatory conditions, confirming that TS individuals have shorter than average lifespans.
Brain Development Research
Brown et al. (2002) performed MRI scans on 26 girls with TS and 26 gender- and age-matched non-TS girls. Results showed that TS girls had smaller posterior cerebral and cerebellar brain areas. No differences emerged between TS girls with maternally inherited versus paternally inherited X chromosomes. These findings suggest that TS affects growth in certain brain regions, potentially explaining some developmental effects associated with the disorder.
Fertility Treatment Research
Hewitt et al. (2013) reported that 30 pregnancies resulted from cryopreservation (extraction and freezing) of eggs from females with TS, but only when extracted from pre-pubertal girls. The progressive ovarian shrinkage that occurs with TS means egg extraction is unsuccessful after puberty begins.
Hormone Treatment Study
Quigley et al. (2014) provided 123 girls with TS either oestrogen supplements or placebo between ages 5-12. No differences in physical development appeared between groups before age 8.5 years, but between 8.5 and 12 years, those receiving oestrogen showed earlier and greater breast development compared with non-TS girls, though no other pubertal differences were noted. This suggests oestrogen supplements benefit girls between ages 8 and 12 and provide psychosocial advantages by normalising breast development.
Evaluation
Research comparing people with typical and atypical chromosome patterns enables researchers to examine which aspects of biological development and behaviour are influenced by genetics and chromosomes.
Studies of patients with KS and TS have led to therapeutic developments that improve quality and duration of life. For example, treating stunted physical growth in TS through growth hormone application, and treating KS with testosterone, helps individuals develop less passive characteristics.
The research into these conditions demonstrates the importance of early intervention and comprehensive treatment approaches that address both physical and psychological aspects of development.
The extraction of eggs from pre-pubescent girls with TS provides opportunities for later pregnancy and enhanced psychosocial benefits. However, ethical considerations require careful briefing of individuals about the technique and obtaining informed consent from all involved parties.
Although effects of both KS and TS stem directly from biological influences of genes and chromosomes, some effects may be more environmental. For instance, higher criminality rates in males with KS may result from socio-economic factors rather than the condition itself. Similarly, females with TS may experience psychological difficulties through being unable to have children rather than direct biological effects.
Ethical Considerations in Diagnosis and Treatment
Both KS and TS can be diagnosed prenatally through cytogenetic analysis of foetal cells. However, this raises ethical concerns as such diagnosis may create pressure for prospective parents to consider abortion.
Since both conditions are biological disorders caused by atypical chromosome patterns, research into them is justifiable as it could lead to effective gene therapies to correct the conditions.
Key Points to Remember:
- Atypical sex chromosome patterns occur when individuals have chromosome combinations other than XX or XY, affecting physical and psychological development
- Klinefelter's syndrome affects males with an extra X chromosome (XXY), causing reduced testosterone, physical feminisation, and cognitive difficulties that can be treated with hormone therapy
- Turner's syndrome affects females missing one X chromosome, resulting in short stature, non-functioning ovaries, and various medical complications treatable with growth hormone and oestrogen supplements
- Both conditions result from nondisjunction during meiosis rather than being inherited, and can have mosaic forms affecting only some body cells
- Research demonstrates that early intervention with appropriate treatments can enable individuals with these conditions to live relatively normal lives