Problem 1 – Prenatal Development
The basics of genetics
- Nucleus of each cell in humans contains 46 chromosomes arranged in 23 pairs
- Only cells that do not have 46 are gametes
- Chromosomes made up of DNA
- String of DNA that makes up each chromo can be further divided into genes
- Each gene controls/influences a particular feature of an organism
o A gene controlling a specific characteristic always appears in the same place (locus) on the same
chromosome in every individual
- 2 types of chromosomes: autosomal & sex chromosomes
o 22 autosomal pairs of chromosomes – normal chromos
o 23rd pair = sex chromo – either X or Y
- Normal female has XX normal male has XY
o Sex is determined by which gene is received from sperm bcuz women only have X chromos
- Genotype = characteristics and developmental sequences mapped in the genes of a specific individual
o Can be modified by individual experience into the phenotype
- Phenotype = the expression of a set of genetic info in a specific environment
o The observable joint result of genetic and environmental influences
- If one gene of a pair always exerts its effects = dominant (AA, Aa, aA)
o Dominant genes override the potential influence of the other recessive gene (aa)
o If you inherit a recessive gene for a trait from each parent, you will show the trait
o If you inherit a recessive gene from only one parent, you may never know you carry the gene
- Sex linked genes = genes located on one of the sex chromos
o X-linked inheritance = when a gene is carried on the X chromo
o Males only have 1 X = if they have the gene for an X-linked disease = they have the disease
o Females have 2 X so they would have to have the gene on both chromos to have the disease
Cell division:
- Mitosis: the process by which a single parent cell divides to make 2 new daughter cells
o Each daughter cell receives a complete set of chromos from the parent = genetically identical
o Cell’s nucleus (including chromos) duplicates itself & cell divides
- Meiosis: a type of cell division in sexually reproducing organisms that reduces the number of chromosomes in
gametes
o Cell duplicates its chromos but the divides twice = forms 4 cells
o Each daughter has only half the genetic material of the parent cell = 23 unpaired chromos
Genetic disorders:
- Down syndrome: caused by the presence of an extra copy of chromo 21 because of failure to separate during
meiosis
o Causes physical abnormalities e.g., flattened skull, short limbs & a protruding tongue
o Also causes disabilities involving motor and intellectual development
- Sex-linked chromosomal abnormalities – most common involve and extra X or Y chromo, or absence of one of
the X chromos in females
o Klinefelter syndrome: males have an extra X – making them XXY
▪ Undeveloped testes, enlarged breasts
▪ Impairments in language, academic, attentional and motor abilities
▪ 1/700 male births
o Fragile X syndrome: results from abnormality in the X chromo, which becomes constricted and often
breaks
▪ Prominent ears, long face, high-arched palate & soft skin
▪ Intellectual disability, learning disability or short attention span
▪ More common in males
o Turner syndrome (only females): either an X chromo is missing or part of one X is deleted
, ▪ Short, webbed neck, infertile
▪ Difficulty in math but verbal ability is good
▪ 1/2000 female births
o XYY syndrome: male has an extra Y
o Poly X: triple X- pattern (XXX) – normal in appearance
▪ 1/1000 female births
o Poly Y: triple Y- pattern (YYY)
▪ 1/1000 male births
- Genetic abnormalities
o phenylketonuria (PKU): individual cannot properly metabolize an amino acid
▪ easily detected but, if left untreated, results in intellectual disability and hyperactivity
▪ results from a recessive gene
▪ 1/10,000 – 20,000 births
o sickle-cell anemia: genetic disorder that affects the red blood cells and occurs most often in people of
African descent
▪ recessive gene causes the red blood cell to become a hook-shaped “sickle” that cannot carry
oxygen properly and dies quickly = body’s cells do not receive enough oxygen = anemia and
early death
▪ 1/500 African American children
o Huntington’s disease: dominant disease that causes CNS to deteriorate – causes muscle coordination
problems and mental deterioration
▪ Symptoms typically only appear until 35 or older
▪ 1/18,000-25,000 births
Prenatal development
- Begins with fertilization and ends with birth
- Lasts between 38 to 40 weeks
- Can be divided into 3 periods: germinal, embryonic & fetal
- The germinal period
o Begins at conception and ends when zygote is implanted in the uterus wall (implantation) - first 2 weeks
o Includes creation of fertilized egg (zygote), cell division & attachment of zygote to wall
o Approx. 1 week cell differentiation begins
o Blastocyst develops = a cluster of dividing cells made by a fertilized egg - the early stage of an embryo
o Trophoblast also develops = the outer layer of cells that later provides nutrition and support for the
embryo
o Implantation occurs after about 11-15 days
- The embryonic period
o Begins when implantation is complete – 3 to 8 weeks
o Rate of cell differentiation intensifies, support systems for cells form & organs appear
o 3 layers of cells form:
▪ Endoderm – the inner layer – develops into digestive & respiratory systems
▪ Mesoderm – middle layer – develops into the circulatory system, bones, muscles, excretory
system & reproductive system
▪ Ectoderm – outermost layer – becomes the nervous system, brain, sensory receptors & skin parts
o Life-support systems for the embryo develop rapidly
▪ Amnion – an envelope that contains amniotic fluid in which the embryo floats & provides temp &
humidity controlled env
▪ Umbilical cord – contains 2 arteries & 1 vein – connects embryo to placenta
▪ Placenta – a disk-shaped group of tissues – small blood vessels from the mother & offspring
intertwine but do not join
o Organogenesis = the process of organ formation during first 2 months of prenatal development
▪ While being formed, organs are very vulnerable to env changes
o 3rd week:
▪ neural tube that becomes spinal cord forms
▪ eyes begin to appear
▪ cells for the heart start to differentiate
o 4 week:
th
, ▪ Urogenital system becomes apparent
▪ Arm & leg buds emerge
▪ 4 chambers of the heart take shape
▪ Blood vessels appear
o 5th – 8th week:
▪ Arms & legs differentiate further
▪ Face starts to form & facial structures fuse
▪ Intestinal tract develops
- The fetal period
o Lasts about 7 months – from 2 months after conception to birth
o A lot of growth and development
o 3 months:
▪ Fetus becomes more active – moves arms, legs & head - opens & closes mouth
▪ Most facial features and limbs are distinguishable
▪ Genitals can be identified
o 4 months:
▪ Growth spurt occurs in body’s lower parts
▪ Mother can feel arm and leg movements
o 5 months:
▪ Skin structures (e.g. nails) have formed
▪ Fetus is more active
o 6 months:
▪ Eyes and eyelids are completely formed
▪ Fine layer of hair covers head
▪ Grasping reflex is present
▪ Irregular breathing movements occur
o Last 2 months:
▪ Fatty tissues develop
▪ Function of various organ systems step up
The basics of genetics
- Nucleus of each cell in humans contains 46 chromosomes arranged in 23 pairs
- Only cells that do not have 46 are gametes
- Chromosomes made up of DNA
- String of DNA that makes up each chromo can be further divided into genes
- Each gene controls/influences a particular feature of an organism
o A gene controlling a specific characteristic always appears in the same place (locus) on the same
chromosome in every individual
- 2 types of chromosomes: autosomal & sex chromosomes
o 22 autosomal pairs of chromosomes – normal chromos
o 23rd pair = sex chromo – either X or Y
- Normal female has XX normal male has XY
o Sex is determined by which gene is received from sperm bcuz women only have X chromos
- Genotype = characteristics and developmental sequences mapped in the genes of a specific individual
o Can be modified by individual experience into the phenotype
- Phenotype = the expression of a set of genetic info in a specific environment
o The observable joint result of genetic and environmental influences
- If one gene of a pair always exerts its effects = dominant (AA, Aa, aA)
o Dominant genes override the potential influence of the other recessive gene (aa)
o If you inherit a recessive gene for a trait from each parent, you will show the trait
o If you inherit a recessive gene from only one parent, you may never know you carry the gene
- Sex linked genes = genes located on one of the sex chromos
o X-linked inheritance = when a gene is carried on the X chromo
o Males only have 1 X = if they have the gene for an X-linked disease = they have the disease
o Females have 2 X so they would have to have the gene on both chromos to have the disease
Cell division:
- Mitosis: the process by which a single parent cell divides to make 2 new daughter cells
o Each daughter cell receives a complete set of chromos from the parent = genetically identical
o Cell’s nucleus (including chromos) duplicates itself & cell divides
- Meiosis: a type of cell division in sexually reproducing organisms that reduces the number of chromosomes in
gametes
o Cell duplicates its chromos but the divides twice = forms 4 cells
o Each daughter has only half the genetic material of the parent cell = 23 unpaired chromos
Genetic disorders:
- Down syndrome: caused by the presence of an extra copy of chromo 21 because of failure to separate during
meiosis
o Causes physical abnormalities e.g., flattened skull, short limbs & a protruding tongue
o Also causes disabilities involving motor and intellectual development
- Sex-linked chromosomal abnormalities – most common involve and extra X or Y chromo, or absence of one of
the X chromos in females
o Klinefelter syndrome: males have an extra X – making them XXY
▪ Undeveloped testes, enlarged breasts
▪ Impairments in language, academic, attentional and motor abilities
▪ 1/700 male births
o Fragile X syndrome: results from abnormality in the X chromo, which becomes constricted and often
breaks
▪ Prominent ears, long face, high-arched palate & soft skin
▪ Intellectual disability, learning disability or short attention span
▪ More common in males
o Turner syndrome (only females): either an X chromo is missing or part of one X is deleted
, ▪ Short, webbed neck, infertile
▪ Difficulty in math but verbal ability is good
▪ 1/2000 female births
o XYY syndrome: male has an extra Y
o Poly X: triple X- pattern (XXX) – normal in appearance
▪ 1/1000 female births
o Poly Y: triple Y- pattern (YYY)
▪ 1/1000 male births
- Genetic abnormalities
o phenylketonuria (PKU): individual cannot properly metabolize an amino acid
▪ easily detected but, if left untreated, results in intellectual disability and hyperactivity
▪ results from a recessive gene
▪ 1/10,000 – 20,000 births
o sickle-cell anemia: genetic disorder that affects the red blood cells and occurs most often in people of
African descent
▪ recessive gene causes the red blood cell to become a hook-shaped “sickle” that cannot carry
oxygen properly and dies quickly = body’s cells do not receive enough oxygen = anemia and
early death
▪ 1/500 African American children
o Huntington’s disease: dominant disease that causes CNS to deteriorate – causes muscle coordination
problems and mental deterioration
▪ Symptoms typically only appear until 35 or older
▪ 1/18,000-25,000 births
Prenatal development
- Begins with fertilization and ends with birth
- Lasts between 38 to 40 weeks
- Can be divided into 3 periods: germinal, embryonic & fetal
- The germinal period
o Begins at conception and ends when zygote is implanted in the uterus wall (implantation) - first 2 weeks
o Includes creation of fertilized egg (zygote), cell division & attachment of zygote to wall
o Approx. 1 week cell differentiation begins
o Blastocyst develops = a cluster of dividing cells made by a fertilized egg - the early stage of an embryo
o Trophoblast also develops = the outer layer of cells that later provides nutrition and support for the
embryo
o Implantation occurs after about 11-15 days
- The embryonic period
o Begins when implantation is complete – 3 to 8 weeks
o Rate of cell differentiation intensifies, support systems for cells form & organs appear
o 3 layers of cells form:
▪ Endoderm – the inner layer – develops into digestive & respiratory systems
▪ Mesoderm – middle layer – develops into the circulatory system, bones, muscles, excretory
system & reproductive system
▪ Ectoderm – outermost layer – becomes the nervous system, brain, sensory receptors & skin parts
o Life-support systems for the embryo develop rapidly
▪ Amnion – an envelope that contains amniotic fluid in which the embryo floats & provides temp &
humidity controlled env
▪ Umbilical cord – contains 2 arteries & 1 vein – connects embryo to placenta
▪ Placenta – a disk-shaped group of tissues – small blood vessels from the mother & offspring
intertwine but do not join
o Organogenesis = the process of organ formation during first 2 months of prenatal development
▪ While being formed, organs are very vulnerable to env changes
o 3rd week:
▪ neural tube that becomes spinal cord forms
▪ eyes begin to appear
▪ cells for the heart start to differentiate
o 4 week:
th
, ▪ Urogenital system becomes apparent
▪ Arm & leg buds emerge
▪ 4 chambers of the heart take shape
▪ Blood vessels appear
o 5th – 8th week:
▪ Arms & legs differentiate further
▪ Face starts to form & facial structures fuse
▪ Intestinal tract develops
- The fetal period
o Lasts about 7 months – from 2 months after conception to birth
o A lot of growth and development
o 3 months:
▪ Fetus becomes more active – moves arms, legs & head - opens & closes mouth
▪ Most facial features and limbs are distinguishable
▪ Genitals can be identified
o 4 months:
▪ Growth spurt occurs in body’s lower parts
▪ Mother can feel arm and leg movements
o 5 months:
▪ Skin structures (e.g. nails) have formed
▪ Fetus is more active
o 6 months:
▪ Eyes and eyelids are completely formed
▪ Fine layer of hair covers head
▪ Grasping reflex is present
▪ Irregular breathing movements occur
o Last 2 months:
▪ Fatty tissues develop
▪ Function of various organ systems step up
