WEEK 3 – Threats to Prenatal 1

Development
Chromosomes, DNA, Genes

Genotype and Phenotype

Genetic Inheritance

Threats to prenatal, perinatal and postnatal development

Autosomal and Sex-linked disorders

Environmental threats

Educational Implications

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 Chromosomes, DNA, Genes, Allele 2

Chromosomes: They are thread-like

structures located inside the nucleus
of animal and plant cells.

A chromosome is an organized
package of DNA found in the nucleus
of the cell.

Different organisms have different

numbers of chromosomes.

Each human has 46 chromosomes,

23 each
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 Chromosomes, DNA, Genes, Allele 2

Twenty-two (22) of these are

called autosomes and the 23rd is
the sex chromosome.

Autosomes look the same in both

males and females.

The 23rd pair, the sex

chromosomes, differ between
males and females.

Females have XX and males have

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 Chromosomes, DNA, Genes, Allele 2

Karyotype: This is where all

chromosomes appear in their exact
pairs in humans or other organisms
from a single cell.

A karyotype is made up of 46
chromosomes (44 autosomes/life
chromosomes and 2 sex
chromosomes)

It is used to determine the sex, or

possible genetic disorders of an
individual.
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 Normal Male and Female Karyotype 4

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 Chromosomes, DNA, Genes, Allele 2
Deoxyribonucleic Acid (DNA): It
is an extremely long chain of
molecules that contains all the
information necessary for the life
functions of a cell.

DNA contains the specific instructions

that make each type of living creature
unique in terms of development,
survival and reproduction.

To carry out these functions, DNA

sequences must be converted into
messages that can be used to
produce proteins, which are the
complex molecules that do most of
the work
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 Chromosomes, DNA, Genes, Allele 2

Gene: The fundamental unit of

heredity; a specific section of DNA
that tells a cell how to make a
specific protein.

Proteins are used by the cell to

perform certain functions, to grow,
and to survive.

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 Chromosomes, DNA, Genes, Allele 2
Allele: An allele is a variant form of a
gene.

Some genes have a variety of different

forms, which are located at the same
position, or genetic locus, on a
chromosome.

Humans are called diploid organisms

because they have two alleles at each
genetic locus, with one allele inherited
from each parent.

If the two alleles are the same, the

individual is homozygous for that gene.

If the alleles are different, the individual

is heterozygous.
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 Genotype and Phenotype 7

• The genotype is a person’s unique combination of genes or genetic

makeup.
• Thus, the genotype is a complete set of instructions on how that
person’s body synthesizes proteins and thus how that body
is supposed to be built and function.

• A phenotype is an individual's observable traits, such as height, eye

color, and blood type.
• Not all the instructions in the genotype may be carried out (or
expressed).

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Dominant and Recessive Genes/Alleles
• Genes/Alleles can be dominant or recessive.

• Dominant alleles show their effect even if the individual only has one
copy of the allele (also known as being heterozygous?). For example, the
allele for brown eyes is dominant, therefore you only need one copy of
the 'brown eye' allele to have brown eyes (although, with two copies you
will still have brown eyes).

• If both alleles are dominant, it is called codominance?. The resulting

characteristic is due to both alleles being expressed equally. An example
of this is the blood group AB which is the result of codominance of the A
and B dominant alleles.

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Dominant and Recessive Genes/Alleles
• Recessive alleles only show their effect if the individual has two copies of
the allele (also known as being homozygous?). For example, the allele for
blue eyes is recessive, therefore to have blue eyes you need to have two
copies of the 'blue eye' allele.

• Some of the traits or characteristics passed through to children through

genetic inheritance include eye colour and blood type. Some health
conditions and diseases can be passed on genetically too

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Eye Colour
• The allele for brown eyes (B) is dominant over the allele for blue eyes (b).
• So, if you have one allele for brown eyes and one allele for blue eyes
(Bb), your eyes will be brown.
• This is also the case if you have two alleles for brown eyes, BB.
• However, if both alleles are for the recessive trait (in this case, blue eyes,
bb) you will inherit blue eyes.

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Eye Colour

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 Blood Group 13
• For blood groups, the alleles are A, B and O.
• The A allele is dominant over the O allele. So, a person with one A allele
and one O allele (AO) has blood group A.
• Blood group A is said to have a dominant inheritance pattern over blood
group O.

• If a mother has the alleles A and O (AO), her blood group will be A
because the A allele is dominant. If the father has two O alleles (OO), he
has the blood group O.
• For each child that couple has, each parent will pass on one or the other
of those two alleles. This means that each one of their children has a 50
per cent chance of having blood group A (AO) and a 50 per cent chance
of having blood group O (OO), depending on which alleles they inherit.
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Blood Group Inheritance

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 Dominant and Recessive Genes/Alleles 15
• Examples of inheritance patterns include:

• Autosomal dominant – where the gene for a trait or condition is

dominant, and is on a non-sex chromosome
• Autosomal recessive – where the gene for a trait or condition is recessive,
and is on a non-sex chromosome
• X-linked dominant – where the gene for a trait or condition is dominant,
and is on the X-chromosome
• X-linked recessive – where the gene for a trait or condition is recessive,
and is on the X-chromosome
• Y-linked – where the gene for a trait or condition is on the Y-chromosome
• mitochondrial – where the gene for a trait or condition is in your
mitochondrial DNA, which sits in the mitochondria (powerhouse) of your
cells.
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 Gene Mutation 16

• A gene mutation is a change in one or more genes. It is an alteration in

the genetic material (the genome) of a cell of a living organism that can
be transmitted to the cell’s or the offspring.

• Some mutations can lead to genetic disorders or illnesses.

• There are hundreds of diseases caused by mutations in a single gene.

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Gene Mutation 17

Inherited
Disorders

Sex linked
Autosomal
disorders
disorders

Genetic Chromosomal Genetic Chromosomal

disorders disorders disorders disorders

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 Sex-Linked and Autosomal Disorders 18
• Sex-Linked Disorders: They involve the sex chromosome # 23 and occurs via
dominant-recessive patterns
• A recessive gene on the X chromosome is more likely to be expressed as the
phenotype in males because the Y chromosome has no allele that might
contract the gene.

• Autosomal disorders: They are caused by mutations in genes on the

autosomes, or numbered chromosomes. Individuals have two copies (alleles)
of every autosomal gene, one inherited from each parent.

• Autosomal dominant disorders are those that result from a mutation in one
copy of the gene.

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 Genetic and Chromosomal Disorders 19

• Genetic Disorders: These are abnormalities in chromosomes inherited from

one or both parents produces diseases in offspring

• Chromosomal Disorders: These are abnormality of chromosome numbers or

structure resulting from an error in cell division

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 Genetic Sex-Linked Disorders 20

1. Colour Blindness
• A recessive sex-linked disorder characterised by the inability to distinguish
between red and green colours due to a defect of the retina
• Dominant gene which permits for color vision is situated on the X
chromosome
• Y chromosome does not have this gene
• Male’s will be color blind if the X chromosome of the male gene has the gene
for color blindness

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 Genetic Sex Linked Disorders 24

2. Hemophilia A and B or Bleeder’s Disease

• A recessive sex-linked disorder carried on the X-chromosome which results in

the inability of the blood to clot
• Prevalent in males (Occurs in 1 in 5000 males)
• Can lead to death from internal bleeding
• Inheritance follows the same pattern as color blindness

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 Genetic Sex Linked Disorders 27

3. Ocular Albinism
• Caused by a change in the GPR143 gene which plays a signaling role important
in the pigmentation of the eye
• Similar pattern of inheritance as Colour Blindness and Hemophilia A and B
• Rarely occurs in females

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 Chromosomal Sex Linked Disorders 30

• Affects sex chromosomes as a result of an abnormality of chromosome

number or structure. They are not passed from parents to children.

1. Fragile X Syndrome
• Results from the breakage of the tip of an X chromosome
• Results in severe mental retardation, speech defects and severe deficits in
interpersonal interaction
• Usually, males are more severely affected by this disorder than females.
• Occurs in 1 out of 1200 males and 1 out of 2500 females

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 Chromosomal Sex Linked Disorders 34

2. Klinefelter Syndrome (XXY)

• A condition in males characterized by an extra X chromosome in most of their
cells instead of the normal XY pattern.
• Characteristics of such males: Relatively high pitched voices, feminine
contours, breast enlargement, little facial or body hair, sterile, small testes, an
inch taller than the average male, likely to be overweight, learning difficulties
as children

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 Chromosomal Sex Linked Disorders 38

3. Super Male Syndrome (XYY)

• A defect which occurs in males where they inherit an extra Y chromosome
• Characteristics of such males include: Height of above 6ft., slender during
adolescence, severe facial acne, poor co-ordination
• Defect occurs in between 1 in 900 births or as rare as 1 in 1500 or 2000

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 Chromosomal Sex Linked Disorders 40

4. Super Female Syndrome (XXX) or Trisomy 47

• Females with the defect inherit 3 or X chromosomes.
• Characteristics of such females: An inch taller than the average female, with
long legs and slender torso, emotionally immature for size during childhood
• Usually fertile but may have slight learning difficulties and usually in the low
range of normal intelligence

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 Chromosomal Sex Linked Disorders 43

5. Turner’s Syndrome (XO)

• A genetic disease caused by abnormal sex chromosomes which occurs in
females who inherit 1 X chromosome instead of 2.

• Characteristic of such females: Short in stature (4ft. 7inches), webbed necks,

small jaws and high arched palates, widely spaced breasts, broad shield
shaped chests, turned elbows, ovaries do not develop normally and do not
ovulate, slight mental retardation in some individuals.

• If diagnosed early injections of Growth Hormone can increase stature by a

few inches and estrogen replacement therapy can result in some breast
development and menstruation helping them to appear relatively normal
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 Genetic Autosomal Disorders 46

• Genetic Autosomal Disorders result from a defect on any of the first 22 pairs
of chromosomes
• They can be passed on from parents to offspring

1. Huntington Disease
• A dominant gene on chromosome 4 is responsible for the disorder
• Defect manifests at the age of 35
• Defect causes the degeneration of neurons producing dementia and random
jerking movements
• Death comes on an average of 12 years after the onset

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 Genetic Autosomal Disorders 49

2. Phenylketonuria (PKA)

• A recessive disorder that occurs in about 1 of 10,000 people. A defective gene

on chromosome 12 is responsible.
• Children with PKU are unable to metabolize phenylalanine, an amino acid.
• As a result, phenylalanine and its derivative phenylpyruvic acid, build up in the
body and cause permanent brain damage
• PKU can lead to intellectual disability, seizures, behavioral problems, and
mental disorders. It may also result in a musty smell and lighter skin.
• Treatment is available and children can develop average intelligence

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 Genetic Autosomal Disorders 52

3. Rhesus factor (Rh) incompatibility

• Rh factor is either positive (present) or negative (absent)
• Rh factor is only significant with respect to pregnancies when an Rh+ child is
born to an Rh- mother predisposing her to Rh disease

• If the blood of the Rh+ child’s blood crosses into mother’s blood during
pregnancy, labor or delivery the mothers body produces antibodies to attack
the baby’s blood which is foreign to her body

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 Genetic Autosomal Disorders 53

3. Rhesus factor (Rh) incompatibility

• The immune response of the mother to the baby’s blood results in the break
down of the baby’s red blood cells

• The result of the incompatibility doesn’t affect the health of the mother, but
the child may suffer complications such as jaundice, anemia, brain or heart
damage or death in severe cases

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 Genetic Autosomal Disorders 56

4. Sickle Cell Disease

• It is a genetic condition in which red blood cells are not shaped as they
should be.
• Sickle Cell Disease occurs as a result of a mutation in a gene on chromosome
11 that codes for the beta subunit of the hemoglobin protein
• Red blood cells look like round discs but in sickle cell disease, they're shaped
like sickles, or crescent moons, instead.
• These sickle shaped cells get stuck together and block small blood vessels.

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 Sickle Cell Cont’d 58
Symptoms include:
• Anemia: Sickle cells break apart easily and die, leaving you with too few
red blood cells.
• Episodes of pain: Periodic episodes of pain, called pain crises, are a major
symptom of sickle cell anemia.
• Swelling of hands and feet

• A child can only have sickle cell disease if they inherit sickling genes from
either parents in an autosomal recessive manner

• The main sickling genotypes are AA, AS, SS, CC, AC and SC. There are
those who are carriers of sickling genes but are do not necessarily have
sickle cell disease
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 Genetic Autosomal Disorders 61

2. Albinism
• Albinism is a group of inherited disorders that results in little or no
production of the pigment melanin, which determines the color of the
skin, hair and eyes.

• It is caused by mutations in one of several genes, and most types are

inherited in an autosomal recessive manner. Several genes provide
instructions for making one of several proteins involved in the
production of melanin.

• Different types of albinism can occur, based mainly on which gene

mutation caused the disorder
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 Types of Albinism 63

• Oculocutaneous albinism (OCA), the most common type, means

a person inherited two copies of a mutated gene

• It's the result of a mutation in one of seven genes, labeled from

OCA1 to OCA7.

• OCA causes decreased pigment in the skin, hair and eyes, as well
as vision problems. The amount of pigment varies by type, and
the resulting color of skin, hair and eyes also varies by and
within types

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Types of Albinism
• Albinism related to rare hereditary syndromes.
•
• For example, Hermansky-Pudlak syndrome includes a form of
OCA as well as bleeding and bruising problems and lung and
bowel diseases.

• Chediak-Higashi syndrome includes a form of OCA as well as

immune problems with recurrent infections, neurologic
abnormalities and other serious issues
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 Chromosomal Autosomal Disorders 65

1. Chromosomal Diseases (Down Syndrome)

• A chromosomal disorder characterized by features such as flat facial
features, small head and ears, short neck, bulging tongue, eyes that
slant upward, atypically shaped ears, poor muscle tone.

• Caused by a duplication or an extra copy of their 21st chromosome

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 Types of Down Syndrome 66

• Trisomy 21: Here, there’s an extra copy of chromosome 21 in every

cell. This is the most common form of Down syndrome.

• Mosaicism: Occurs when a child is born with an extra chromosome

in some but not all of their cells. People with mosaic Down syndrome
tend to have fewer symptoms than those with trisomy 21.

• Translocation: In this type of Down syndrome, children have only an

extra part of chromosome 21. There are 46 total chromosomes.
However, one of them has an extra piece of chromosome 21
attached.
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 Environmental Threats 69

• Harmful substances such as drugs or radiation that invade the womb and
result in birth defects are called teratogens.

• Teratogens are especially damaging in the embryonic stage because it is a

critical period in prenatal development.

• Environmental pollutants and toxic wastes are also sources of danger to

unborn children. Some of these chemicals and radiations are:

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 Environmental Threats 70

• The term teratogen refers to any environmental agent that causes damage
during the prenatal period. Scientists chose this label (from the Greek
word teras , meaning “malformation”) because they first learned about
harmful prenatal influences from cases in which babies had been
profoundly damaged.

• But the harm done by teratogens is not always simple and

straightforward.

• It depends on the following factors:

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 Environmental Threats 71
• Dose . As we discuss particular teratogens, you will see that larger doses
over longer time periods usually have more negative effects.

• Heredity . The genetic makeup of the mother and the developing organism
plays an important role. Some individuals are better able than others to
withstand harmful environments.

• Other negative influences. The presence of several negative factors at once,

such as additional teratogens, poor nutrition, and lack of medical care, can
worsen the impact of a single harmful agent.
• Age. The effects of teratogens vary with the age of the organism at time of
exposure. To understand this last idea, think of the sensitive or critical period
concept.
• If the environment is harmful, then damage occurs, and recovery is difficult
and sometimes impossible.
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 Environmental Threats 72

• Prescription and Non-prescription Drugs

• Illegal Drugs
• Tobacco
• Alcohol
• Radiation
• Environmental Pollution from chemicals
(Mercury, Lead, Mercury, Pesticides, Carbon monoxide )
• Maternal Disease (Syphilis, Rubella, STD’s, HIV/AIDs)

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 Environmental Threats 73

• Other Maternal Factors

• Exercise
• Nutrition
• Drugs
• Poverty
• Emotional state
• Mother’s age

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 Alcohol 74

• Exposure to alcohol can result in varied abnormalities in childhood and later

years.
• A child exposed to alcohol in the womb can either have:

• Fetal alcohol spectrum disorder (FASD),

• Partial fetal alcohol syndrome (p-FAS), and
• Alcohol-related neurodevelopmental disorder (ARND)

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 Fetal Alcohol Syndrome 75

• It encompasses a range of physical, mental, and behavioral outcomes caused

by prenatal alcohol exposure
• Fetal alcohol syndrome (FAS), distinguished by

(a)slow physical growth

(b) a pattern of three facial abnormalities (short eyelid openings; a thin upper
lip; a smooth or flattened philtrum, or indentation running from the bottom of
the nose to the center of the upper lip), and
(c) brain injury, evident in a small head and impairment in at least three areas
of functioning—for example, memory, language and communication, attention
span and activity level (overactivity), planning and reasoning, motor
coordination, or social skills.
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 Partial Fetal Alcohol Syndrome 78

• Partial fetal alcohol syndrome (p-FAS), characterized by (a) two of the three
facial abnormalities just mentioned and (b) brain injury, again evident in at
least three areas of impaired functioning.

• Mothers of children with p-FAS generally drank alcohol in smaller quantities,

and children’s defects vary with the timing and length of alcohol exposure.

• Furthermore, recent evidence suggests that paternal alcohol use around the
time of conception can induce genetic alterations, thereby contributing to
symptoms (Ouko et al., 2009).

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 Alcohol-related neurodevelopmental disorder 79

• Alcohol-related neurodevelopmental disorder (ARND), in which at least three

areas of mental functioning are impaired, despite typical physical growth and
absence of facial abnormalities.

• Again, prenatal alcohol exposure, though confirmed, is less pervasive than in

FAS (Chudley et al., 2005; Loock et al., 2005).

• The more alcohol a woman consumes during pregnancy, the poorer the
child’s motor coordination, speed of information processing, attention,
memory, reasoning, and intelligence and achievement test scores during the
preschool and school years (Burden, Jacobson, & Jacobson, 2005; Korkman,
Kettunen, & Autti-Raemoe, 2003; Mattson, Calarco, & Lang, 2006).
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Educational Implications 80

DISCUSS
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