Chromosome – a long stretch of DNA containing the genetic code for the individual
Embryo – an early stage of development, after fertilisation and before body structures have formed
Foetus – the unborn offspring that develops from the embryo
Gene – a segment of DNA that codes for a protein
Gamete – the sex / reproductive cells (sperm and egg / ova in humans)
Zygote – a single cell consisting of the fertilised egg
Zygotes and Chromosomes
Human life begins as a zygote
Zygotes contain 46 chromosomes, in 23 pairs (one chromosome in each pair inherited from mother and the other from the father)
Karyotype - an individual's collection of their chromosomes
Chromosomes are found in the nucleus of the cell, and they contain the deoxyribonucleic acid (DNA) which is the genetic code for an individual
The Genetic Code
- The DNA has a characteristic double-stranded helix structure
- The DNA in our chromosomes contain our genes - genes are segments of DNA that contain instructions to produce proteins
- Environmental factors interact with genes to modify their expression and the amount of protein produced, leading to variation in proteins between people and contributing to the variations in traits and abilities between individuals
Aneuploidy - occurs when an individual does not have the typical 46 chromosomes
- Risk of aneuploidy increases with increasing maternal age
Sex chromosome aneuploidy
- Triple X (XXX) syndrome - females possess an extra X chromosome, causes tall height and affects verbal abilities, 1 in 1000 female birth chance
- Keinfelter syndrome (XXY) - males possess an extra X chromosome. 1 in 1000 male births, males tend to be tall with weaker muscles, fertility and verbal abilities may be affected
-XYY syndrome (XYY) - males possess an extra Y chromosome, 1 in 1000 male births; males are typically tall and have learning difficulties
- Turner syndrome (X) - females missing an X chromosome, 1 in 2000 to 1 in 5000 female births, females are typically short with a webbed neck, fertility, and spatial abilities are affected
Autosomal aneuploidies
Down syndrome (trisomy 21) - individuals possess an extra copy of chromosome 21, 1 in 1000 births, intellectual disability and characteristic facial features; individuals tend to be short with low muscle tone
Patau syndrome (trisomy 13) – individuals possess an extra copy of chromosome 1; 1 in 10,000 to 1 in 21,700 births, associated with intellectual disability and polydactyly (extra toes or fingers)
Edwards syndrome (trisomy 18) – individuals possess an extra copy of chromosome 18; 1 in 5,000 births, associated with intellectual disability, individuals tend to be short
• Both Patau syndrome and Edwards syndrome are associated with high infant mortality, with only up to 10% of infants reaching their first birthday
Multiple Pregnancies
- May occur due to one or multiple zygotes
- Twins may be monozygotic (identical) or dizygotic (non-identical/fraternal twins)
Stages in Prenatal development
- There are approximately 38 weeks/9 months between conception and birth
- Development is predetermined and under genetic instructions
There are three stages of development:
• Zygote – the germinal stage, lasting the first two weeks
• Embryo – the embryonic stage, lasting between weeks 3-8
• Foetus – the foetal stage, lasting from week 9 to birth
Germinal stage
- occurs in the first 2 weeks after the zygote is formed
Embryonic stage
- occurs 3-8 weeks after the zygote is formed
- embryo is surrounded by a protective sad of amniotic fluid
Foetal stage
First trimester
- occurs between 9-38 weeks after the zygote is formed
- foetus starts to be able to move, lungs start to expand and contract, heartbeat is audible using a stethoscope or ultrasound, external genitalia are formed
- start of bone development, eyelids and toenails forming
Second trimester
- Foetus is large enough for the mother to feel kicks
- Almost all neurons have developed by the end of the trimester
- Organs, muscles, and nervous system become more organised
- Foetus shows thumb sucking, swallowing, yawning, hiccupping, as well as sensitivity to sound and the opening and closing of the eyes
Third trimester
- foetus is able to cry, swallow, digest, and excrete
- foetus spends more time awake
- increased coordination between systems
- early signs of personality may be seen
Prenatal risks and teratogens
Teratogens- agents that cause developmental deviations
Genetic variations: mutations, chromosomal atypicalities
Drugs
Tobacco
Alcohol
Radiation
Metals (Mercury, lead)
Maternal infectious diseases (Zika virus, rubella, herpes simplex)
Parental factors e.g. age, nutrition, stress
Sensitive periods in development
- embryonic stage is when there is greatest vulnerability to damage
- Central nervous system is vulnerable to damage throughout the whole of the embryonic and foetal stages
Alcohol as a teratogen
• Foetal alcohol syndrome
• Partial foetal alcohol syndrome
• Alcohol-related neurodevelopmental disorder
Symptoms of foetal alcohol spectrum disorders include:
• Characteristic facial appearance (small eyes with a small upper lip and flat groove between the nose and upper lip)
• Short height, with a low body weight and small head size
• Effects on brain development, specifically learning and behavioural difficulties, and poor coordination
Postnatal development
-Humans are relatively helpless just afer birth
- Human brains mature relatively late (age 25) compared to chimpanzee brains (age 6)
Physical growth during childhood
The fastest growth occurs in infancy, with brain weight doubling by age 5 months
- Growth slows during early and mid childhood
- There is then a growth spurt during puberty, and this occurs earlier in girls than boys
Changes in body proportions
- at birth the head grows fastest, but by early childhood the trunk and legs have started to grow fastest
Postnatal brain development
- At birth we have 100 billion neurons
- Our brains grow approximately fourfold after birth
- Increase in brain size is due to the growth of neural connections and the production of glial cells
- Glial cells make up around half of the cells in our brains, and their numbers rapidly increase from the second trimester to age 3
Lateralisation of the Cerebral cortex
The two hemispheres of the brain show lateralisation during development
Left hemisphere
- Sensory input from and motor control of the right side of the body
- Language processing
- Analytical processing
Right hemisphere
- Sensory input from and motor control of the left side of the body
- Emotional and social processing
- Holistic processing
Brain plasticity
- Brains plasticity allows regions of the brain to take over new functions
- Greater plasticity is seen in early development - this allows for cortical reorganisation and enables a better recovery from brain injury, especially for language skills, though more complex skills may not show a full functional recovery
- Brain plasticity allows us to learn skills due to our experiences - stimulating the brain is essential for both the development of universal skills and individual skills