Heredity and Environment

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Heredity and Environment
Chapter Three

• Human Growth and Development
• Georgia Perimeter College Decatur Campus

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Heredity and Environment

• Individual development arises from the
  interaction of two prime forces genetic
  instructions inherited at conception and
  environmental factors that allow those
  instructions to be expressed.
• Human development begins when the sperm
  penetrates the membrane of the ovum. The
  reproductive cell (gamete) contains all the
  instructions about how life should be formed.
• The combined genetic material from both gametes
  duplicates itself, forming two complete sets of
  genetic code. The two sets move toward opposite
  sides of the zygote and the cells begin to
  specialize in different functions.
• The first stage of human development is called a
  zygote. The zygote goes through three stages of
  growth Duplication, Division, and
  Differentiation. During duplication, the
  combined genetic material from both gametes
  duplicates itself, forming two complete sets of
  genetic code. Then these two sets move toward
  opposite sides of the zygote, and the zygote
  divides down the middle. Differentiation occurs
  when cells begin to specialize, taking different
  forms and reproducing at various rates, according
  to their specific function.

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A Dividing Zygote
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The Gene

• The basic unit of genetic instruction is the
  gene, which is a discrete segment of a
  chromosome, which is a molecule of DNA.
• The instructions of each gene are written in a
  chemical code made up of pairs of only four
  chemical bases - Adenine, Guanine, Cytosine, and
  Thymine. These chemicals only combine in four
  pair combinations A-T, T-A, G-C, and C-G.
• The Human Genome Project is an international
  effort to map the complete human genetic code
• Chromosomes
• Of the 23 pairs of human chromosomes 22 are
  closely matched pairs. The 23 chromosome is the
  sex chromosome. The 23 chromosome determines the
  gender of the child.

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Human Chromosomes

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Continuity and Diversity

• The male is XY and the female is XX. The Y
  chromosome determines male gender development.
•  
• Crossing-over of genes. Just before a chromosome
  pair divides to form a sperm or ova,
  corresponding segments of the pair are sometimes
  broken off and exchanged, altering the genetic
  composition of both pair members.
• Monozygotic and Dizgotic Twins.

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Dizygotic Twins

• Dizygotic twins are referred to as fraternal.
  They are created when two eggs are fertilized by
  two separate sperm to create two embryos (see
  figure 1).Fraternal twins are by far the most
  common type of twins. About 72 of all US twins
  are dizygotic. It is even possible for fraternal
  twins to have different fathers. Sometimes
  fraternal twins can appear to be identical in an
  ultrasound. It all depends on where the ova are
  implanted. If they implant separately they will
  appear totally separate. It is possible, however,
  that the placentas can become fused due to being
  implanted together and appear to look like one
  placenta.

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Monozygotic Twins
Monozygotic twins occur when one egg is
fertilized by one sperm and then splits (see
figure 4). Identical twins have exactly the
same DNA. Identical twins can only be the SAME
sex. You can not have boy/girl identical twins.
The time of division after fertilization
determines what kind of identical twins occur.
Phenotype is the constellation of observable
traits Genotype is the genetic endowment of the
individual. Whether a gene trait becomes
expressed in the phenotype is determined by two
levels of interactions (1) interaction among the
proteins synthesized according to the specific
genes that affect the trait and (2) ongoing
interaction between the genotype and the
environment. Additive Genes. When genes interact
additively, the phenotype reflect the
contributions of all the genes that are
involved. Nonadditive patterns are shown in the
phenotype when one gene is more dominant that the
others. This is known as the dominant-recessive
pattern. Incomplete dominance is when the
phenotype is influenced primarily, but not
exclusively by the dominant gene.

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Genes

• X-Linked Genes. Some genes are X-linked because
  they are located only on the X chromosome. This
  suggests that males, who only have one X
  chromosome are more susceptible to certain
  inherited disorders than female.
• In regard to the dominant-recessive pattern, a
  dominant gene may not penetrate the phenotype,
  and result in a mosaic, a mixture of cells, some
  with and without dominance.
• Genetic imprinting. The tendency of certain genes
  to be expressed differently when they are
  inherited from the mothers than when they are
  inherited from the father.
• Evidence that imprinting has occurred comes from
  examining the pedigree. If a disorder is always
  expressed when inherited only from the male or
  female parent, genomic imprinting should be
  suspected. It is the sex of the transmitting
  parent, not the affected offspring that is
  important. Another clue is discordance among
  monozygous twins

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Dominance and Recessive Genes

• Not all pairs of alleles will have the same
  phenotype dominance when AA Aa in phenotype, A
  is dominant, a is recessive. An allele can be
  dominant over one allele but recessive to another
  allele.
• Model of dominance from enzyme activity no
  copies produce no phenotype, one copy produces x
  amount of product and two copies produces 2x then
  the alleles are additive and there is no
  dominance (intermediate inheritance). If one copy
  of the allele produces as much product (or has as
  high a rate of flux) as a homozygote then there
  is dominance. There are cases where the
  heterozygote is greater in phenotypic value than
  either homozygote called overdominance
• X-Linked Recessive Inheritance is marked by
  individuals who are affected with the trait or
  condition in question are males. Mothers of the
  affected males are carriers, and the sisters of
  affected males may be either heterozygous or not
  carry the gene at all. The basis for X-linked
  inheritance is that females have two X
  chromosomes and males have only one X chromosome.

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X-Linked Dominant Inheritance

• X-Linked Dominant Inheritance
• X-linked dominant inheritance is very rare. It is
  marked by pedigrees in which
• all the daughters of an affected male are
  themselves affected
• none of the sons of an affected male is affected
  
• a heterozygous (carrier) female is affected and
  transmits the trait to half of her children, with
  males and females being equally affected
• twice as many females as males will be affected,
  in general.
• Y-Linked Inheritance
• A small number of genes have been mapped to the Y
  chromosome, and others have recently been
  discovered. Genes on the Y chromosome pass from
  father to sons only no daughters of affected
  males can inherit the gene, since none of their
  cells has a Y chromosome.

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Prenatal Prediction

• Prenatal Ultrasound. Using sound waves, an
  ultrasound can give a physician and eager parents
  a "window into the womb." Ultrasound can be used
  to determine the child's gestational age, to scan
  for abnormalities, and even to determine the
  baby's gender.

Common reasons for having an ultrasound include
determining the age of the fetus if you're unsure
about your last period, to determine a cause for
bleeding during pregnancy, to evaluate the health
of the fetus, to diagnose twins, and to check for
a breech birth.
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AFP Screening.

• AFP Screening. The alpha-fetoprotein (AFP) test
  is a screening device used to check for the
  possibility of Down's syndrome or spina bifida in
  the unborn child. It's a simple blood test given
  to the mother which checks the levels of three
  hormones in the blood. Varying amounts of the
  hormones can be a sign that the child has a
  disorder. About five percent of those women
  tested have a positive result, meaning that the
  hormone levels fall outside of the norm. However,
  only about one in thirty of those with a positive
  result actually have a problem. Over 95 of
  babies who test positive have neither Down's
  syndrome nor spina bifida.
• Chorionic villi sampling (CVS) is used to detect
  genetic defects in the unborn child early in
  pregnancy, at about ten weeks as opposed to
  sixteen for amniocentesis. The doctor inserts a
  long, thin tube into the uterus and snips off a
  small piece of the chorionic villi, tissue
  between the uterine lining and the fetal membrane
  which will become the placenta. The tissue is
  then analysed for genetic abnormalities.

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• Amniocentesis
• Performed usually at sixteen weeks or later,
  amniocentesis is a test used to determine if
  there are any genetic defects in the baby. Guided
  by ultrasound, a long needle is inserted into the
  uterus and fluid is extracted. The fluid is then
  evaluated by a laboratory for certain defects,
  infections, and/or fetal maturity.
• Amniocentesis poses a significant risk to the
  unborn child. Approximately one in 200 are
  miscarried as a result of the test. Your doctor
  should explain his or her reasons for asking you
  to have this test performed. It is usually
  recommended if the mother is over age 35, the
  couple has had a previous child with chromosomal
  defects, or if the parents carry certain genetic
  disorders.

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Fetoscopy

• Use of a special, fiber optic, tubular telescope
  to look at the fetus while it is still in the
  mother's uterus. Use of this scope requires a
  tiny incision in the mother's abdominal wall so
  that the telescope can be passed into the uterus
  to directly view the fetus.
• Dealing with a Negative Test Result
• An amniocentesis or other test may indicate that
  your unborn child has a genetic defect or
  disease. However, these tests cannot determine
  the severity of the problem. Many persons with
  genetic disorders such as spina bifida, Down's
  syndrome, and cystic fibrosis still lead rich and
  rewarding lives and are only mildly affected,
  though some disorders may present themselves as
  more severe.

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Chromosomal (miscount) abnormalities

• Chromosomal abnormalities refers to abnormalities
  in the number or organization of chromosomes. A
  common example is trisomy 21 or Down syndrome, in
  which there is an extra chromosome number 21.
• Down's syndrome is a condition that has a
  specific set of characteristics found in children
  who have an extra number 21 chromosome. Another
  name for this condition is trisomy 21.
  Characteristic findings are widely spaced first
  and second toes, a single hand (simian) crease,
  short fingers, a fold of tissue (epicanthal fold)
  across the inner aspect of the eye (near the
  nose), eyes that slant upward, decreased muscle
  tone, flattened back portion of the head, and a
  protruding tongue. The range of outcomes for
  these children is quite broad and many function
  quite well, although very few have normal
  intelligence.

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Klinefelter Syndrome

• There are 23 pairs of chromosomes found in each
  cell of the body, and each contain genes that
  determine our coloring, our features, and our
  sex. Women inherit two X chromosomes -- one from
  each parent, written 46, XX. Men inherit an X
  chromosome from their mothers and a Y chromosome
  from their fathers, written 46, XY. Some
  individuals, however, have an additional X
  chromosome in their chromosomal arrangement,
  referred to as 47,XXY, or more commonly, just
  XXY. The cause is unknown, yet XXY occurs in
  approximately 1 in every 500 to 1000 live male
  births, making it one of the most common
  chromosome variations.
• Turners syndrome is a a rare chromosomal disorder
  of females marked by the absence of one X
  chromosome, resulting in short stature and
  retarded sexual characteristics, low rear hair
  line, deformities of the spine, chest, and
  extremities, and cardiac defects.

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Fragile X syndrome?

• Fragile X syndrome is the most common inherited
  cause of mental impairment, affecting
  approximately 1 in 2,000 males and 1 in 4,000
  females worldwide. It is estimated that 1 in 259
  females and 1 in 755 males are carriers of the
  premutation.
• It is second only to Down syndrome as a cause of
  mental retardation. Both males and females may be
  affected by a wide variety of symptoms.
• Fragile X syndrome appears in children of all
  ethnic, racial and socio-economic backgrounds.
• Huntington Disease is an autosomally-inherited,
  dominant disorder in which the patient begins to
  exhibit symptoms in the third to fourth decades.
  Patients with Huntington Disease (HD) initially
  have a tendency to fidget which over months or
  years develops into jerky, choreiform movements.
  HD usually progresses over a 10 to 25 year
  period. As the disease progresses it leads to
  dementia and usually death from incorrect
  infection. There is a high incidence of suicide
  among patients with HD.

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Tourette Syndrome

• Tourette Syndrome is an inherited, neurological
  disorder characterized by repeated and
  involuntary body movements (tics) and
  uncontrollable vocal sounds. In a minority of
  cases, the vocalizations can include socially
  inappropriate words and phrases -- called
  coprolalia. These outbursts are neither
  intentional nor purposeful. Involuntary symptoms
  can include eye blinking, repeated throat
  clearing or sniffing, arm thrusting, kicking
  movements, shoulder shrugging or jumping.These
  and other symptoms typically appear before the
  age of 18 and the condition occurs in all ethnic
  groups with males affected 3 to 4 times more
  often than females. Although the symptoms of TS
  vary from person to person and range from very
  mild to severe, the majority of cases fall into
  the mild category. Associated conditions can
  include attention problems, impulsiveness and
  learning disabilities.Most people with TS lead
  productive lives and participate in all
  professions. Increased public understanding and
  tolerance of TS symptoms are of paramount
  importance to people with Tourette Syndrome.

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Genetic Counseling

• Genetic Counseling is a communication process,
  which deals with the human problems associated
  with the occurrence, or the risk of occurrence,
  of a genetic disorder in a family. This process
  involves an attempt by one or more appropriately
  trained persons to help the individual or family
  (1) comprehend the medical facts, including the
  diagnosis, the probable course of the disorder,
  and the available management (2) appreciate the
  way heredity contributes to the disorder, and the
  risk of recurrence in specified relatives (3)
  understand the options for dealing with risk of
  recurrence (4) choose the course of action which
  seems appropriate to them in view of their risk
  and their family goals and act in accordance with
  that decision and (5) make the best possible
  adjustment to the disorder in an affected family
  member and/or to the risk of recurrence of that
  disorder
• Most counseling involves the occurrence of a
  particular disease in a child and the concern of
  the parents as to whether their future children
  might be similarly affected. Parents may also
  want to know about the risk for the affected
  child's children or the children of unaffected
  sibs.