Chapter 3: biological foundations, genetics and prenatal development Flashcards
Pre and Early Life Cells
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Gametes:
- reproductive cells: sperm and ova
- Only contain 1⁄2 normal complement of DNA
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Zygote: new cell formed by conception.
- Full complement of DNA (1⁄2 from sperm + 1⁄2 from egg)
- All other cells are derived from the zygote by Mitosis
Deoxyribonucleic acid (DNA):
Genetic information for each cell
Genes:
segments of DNA that code for individual traits
Chromosomes
- Storage structures for DNA
- Rod-like structures visible in cell nucleus
- 46 chromosomes in 23 pairs
• Sex Chromosomes
- Males = XY
- Females = XX
Mitosis
- Mitosis is the process that ensures that the duplicate cell is identical in genetic makeup to the original cell
- The replication of genetic material during cell multiplication and the transfer of genetic information during reproduction are central to understanding development and growth
Meiosis
Genotype vs Phenotype
• Genotype
– The genetic code of the individual; sum of all genes
– Together with the environment determines phenotype
• Phenotype
– Traits as displayed or expressed
– Examples: height, eye colour or IQ
Phenotype = Genotype + Environment
Alleles
- There are two alleles for each gene, one from each parent
- Different alleles can produce variation in inherited characteristics
- Some alleles are dominant, some are recessive, and some are co-dominant
Dominant and Recessive Inheritance
Dominant alleles
- Always expressed in phenotype
- e.g., brown eyes, curly hair, facial dimples, type A blood, Huntington’s disease
- Possible genotypes: DD, Dd or dD (capitalisation indicates dominant allele)
Recessive alleles
- Must be paired with another recessive allele (aka one from each parent) to be expressed
e. g., blue eyes, colour-blindness, baldness, type O blood, PKU
Possible genotype: dd
Co-dominance:
• Both alleles expressed • Example of blood group
Homozygous alleles
Heterozygous alleles
Polygenic traits
- Homozygous: alleles for a trait are identical
-
Heterozygous: alleles for a trait are different
- Can be a carrier for a recessive allele (e.g., haemophilia)
- Polygenic traits: many genes as well as environment influence character (most traits are polygenic)
Summary of concepts
Determination of sex
- Ova - X; Sperm - X or Y
- Sex of zygote determined by which type of sperm fertilises the ovum
- 30% more male than female zygotes
- 6% more live male births
- Some traits are inherited in a sex linked manner
Genetic abnormalities
- Chromosome abnormality
- e.g., Down syndrome
- Dominant gene disorders
- e.g., Huntington’s disease
- Recessive gene disorders
- e.g., Sickle-cell disease
- Multifactorial
- e.g., congenital heart disease
Down syndrome
- Trisomy 21 – an extra 21st chromosome
- Most common autosomal disorder
- Responsible for 40% of moderate to severe mental retardation
- Symptoms include distinct facial features, mental retardation, speech problems, and slow motor development.
- Incidence increases with maternal age
- 1 in 1900 births at 20yrs; 1 in 300 at 35yrs; 1 in 30 at 45yrs
Phenylketonuria (PKU)
- Frequently occurring recessive disorder
- By 1 year infants are permanently retarded
- However, if identified early, these children are given a diet low in phenylalanine and usually attain an average level of intelligence and have a normal lifespan
- Inheriting unfavourable genes does not always lead to an untreatable condition if the environment (diet) can be adjusted
Abnormalities of the Sex Chromosomes
- These disorders produce less severe problems and deficits usually result in fewer problems than defects of the autosomes
- Often not recognised until adolescence
- XO = Turner’s syndrome (Female, short, infertile)
- XXY = Kleinfelter’s syndrome (Male, sterile, learning disorders)
- XYY = Male, tall, low IQ (especially verbal IQ)
Genetic counseling
• Helps prospective parents assess risk of bearing a child with a genetic defect
- Especially helpful when:
- Parents already have biological children with a defect
- Family history
- Ethnicity
- Women 35 years or older
Behavioural genetics
• Behaviour genetics: study of the nature/ nurture interaction
- Remembering that:
- Nature = genetics
- Nurture = environment
Phenotype = Genotype + Environment
Examples of behavioural genetics
Examples of how behavioural genetics can be analysed in humans:
Adoption studies: dissimilar nature, similar nurture
Twin studies: identical or fraternal, raised separately or together
Prenatal development
Gestation
Gestational age
• Gestation: The approximately 38-week period of development between conception and birth
• Gestational Age: Age of unborn baby, usually dated from the first day of an expectant mother’s last menstrual cycle
Stages of prenatal development (4)
- Conception
- Germinal (0 - 2 weeks)
- Blastocyst implants in uterus wall
- Embryonic (3 - 8 weeks)
- Umbilical cord connects embryo to placenta
- Amniotic sac develops
- Foetal
- Week 9 to birth
Germinal stage of development
Foetal Stage Development: 8 Weeks to Birth
• Organs and body become more complex
- “Finishing touches” – toenails, eyelids
- Appearance of bone
Two types of multiple births
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Monozygotic (MZ)
- One egg – one sperm cell
- Identical twins
- Share 100% of genes
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Dizygotic (DZ)
- Two eggs – two sperm cells
- More common
- Fraternal twins
- Share 50% of genes – just like non-twin siblings
