Patho ch 6 Genetics
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What are congenital disorders?
disorders that are present at birth whether the cause is genetic, environmental, or both.
congenital disorders associate with structural defects attributable to errors in fetal development
2 known causes for congenital malformations?
Human genes are organized into ____ different chromosomes.
What happens to chromosomes during cell division?
Chromatids separate at the centromere and form 2 new chromosomes
Human chromosomes are ____ meaning they occur in pairs.
Of the 23 pairs of chromosomes 22 are homologous and are called ____, and the remaining pair of chromosomes are called the _____ chromosomes.
What is the difference b/t sex chromosomes of males and females?
What is the advantage to the species of sexual reproduction?
allows genetic variability which allows for adaptation and evolution in a changing environment
egg and sperm
special form of cell division that results in germ cells that are haploid
chromosomes are single instead of inpairs
in sex cells
What occurs during the first phase of meiosis?
Pairs of homologous chromosomes with duplicated sister chromatids come in close contact and portions of them are exchanged
portions of chromosomes are exchanged during the first stage of meiosis
What is the purpose of crossing over?
results in mixing of the maternal and paternal genes of the cell to form a new combination of genes
The first stage of meiosis results in ____ cells, each with ______ chromosomes.
What occurs during the second stage of meiosis?
the sister chromatids are pulled apart and form four cells with 23 chromosomes each
What is the end result of meiosis?
4 genetically unique germ cells
Where are genes located on a chromosome?
alternate forms of a gene that will decide what characteristics are displayed in the person's phenotype
How many alleles does a person have for each gene?
2 - one given by each parent
If both alleles for a gene are identical, the person is said to be ____ for that gene.
If 2 different alleles are present, the individual is _______ for that gene.
What are monogenic/single-gene traits?
traits that involve only one gene locus
How can we predict monogenic transmission of traits?
If a disease is carried on the recessive allele and a person is heterozygous for the disease they are said to be ____, because they are able to pass the defective recessive gene to their offspring even though they do not exhibit the trait.
Codominant expression of traits?
alleles that are not clearly dominant or recessive and result in a blending expression of the trait
Important example of codominant expression of traits?
What are the alleles in blood typing?
A, B, and O
Most traits result form the interaction of several gene loci and are called _____.
Polygenic traits are often affected by environmental factors, in which case they are called ______.
height, weight, and BP
permanent change in DNA structure
4 potential mutagens?
- 1. viruses
- 2. radiation,
- 3. chemicals
- 4. products of normal cellular metabolism
Why do most mutations occur?
due to copying errors during DNA replication
What is required for most DNA repair mechanisms?
What usually occurs if it is not available?
a normal complementary DNA template
if both strands of DNA are damaged there will usually be irreparable damage at that break
2 types of genetic mutations?
- 1. point mutation
- 2. frameshift mutation
single base pair substitution
addition or deletion of 1 or more bases that changes the frame of the DNA sequence and causes it to be read wrong and produce a protein with severely altered amino acids
How can a point mutation affect the body?
may cause codons to signify an abnormal amino acid that will be included in the sequence of a protein and may or may not cause problems
Sickle cell anemia is an example of a _____ _____ disorder in which a single amino acid substitution causes significant dysfunction.
How are the majority of genetic disorders obtained?
inherited from parents but may be caused by mutations
4 groups of disorders that are genetic in origin?
- 1. chromosomal aberrations
- 2. mendelian single-gene disorders
- 3. polygenic or multifactorial disorders
- 4. variety of single-gene defects that do not follow classic mendelian patterns of inheritance
What are chromosomal defects generally caused by?
abnormal number of chromosomes or alterations in the structure of 1 or more chromosomes
an abnormal nuber of chromosomes
What is the most common cause of aneuploidy?
paired homologous chromosomes fail to separate normally during 1st or 2nd meiotic division resulting in germ cells that have an abnormal number of chromosomes
What are the germ cells like that result from nondisjunction?
1 has 22 chromosomes and the other has 24
1 chromosome lags behind and is left out of the newly formed cell nucleus
What is the result of anaphase lag?
one daughter cell has normal number of chromosomes and one is missing one chromosome
chromosome is missing one chromosome
condition of having too many chromosomes
3 factors that increase the risk of aneuploidy occuring?
- 1. advanced maternal age
- 2. abnormalities in parental chromosome structure
- 3. abnormalities in crossing over
______ involving the autosomes is not usually compatible with life, but ____ ____ may result in a viable fetus with severe disability as in down syndrome.
Disorders involving extra or missing ____ chromosomes are more common and less debilitating.
What usually causes alteration in chromosome structure?
breakage and loss or rearrangement of pieces of the chromosomes during meiosis or mitosis
What alteration in chromosome structure may occur during crossing over?
if crossin over goes wrong portions of chromosomes may be lost, attached upside down, or attached to the wrong chromosome
How are gene locations described?
may be on the long arm (q arm) or the short arm (p arm) of the chromatid
4 common types of chromosomal rearrangements?
- 1. translocation
- 2. inversions
- 3. deletions
- 4. duplications
exchange of pieces of DNA b/t nonhomologous chromosomes
Why may a person have no symptoms from a chromosomal translocation?
What complications may occur for this person?
if not genetic material is lost may have no symptoms
at increased risk of producing abnormal gametes
chromosomal translocation in which no genetic material is lost and no symptoms are evident
exchange of a long chromatid arm for a short one results in the formation of one very large chromosome and one very small one
occur when sister chromatids separate incorrectly at the centromere such that the 2 identical short arms remain together and the long arms do too
removal and upside-down reinsertion of a section of chromosome involving no net loss or gain of genetic material and may not have consequences to the individual
What problems may be caused by inversion if the individual has no symptoms from it?
during meiosis when homologous pairs attempt to pair up the chromosome with an inverted section may not pair up properly and duplications or loss of genes at the time of crossing over may occur and offspring may be affected
loss of chromosomal material
How does deletion occur?
break in the arm of a single chromosome results in a fragment of DNA with no centromere that will be lost at the next cell division
What may occur if deletions occur at both ends of a chromatid?
may cause free ends to attach to once another and from a ring chromosome
extra copies of a portion of DNA are made
Trisomy 21? AKA?
chromosomal disorder in which individuals ahve an extra copy of chromosome 21
_____ is the most common of the chromosomal disorders and a leading cause of mental disability that occurs in about 1 in 700 live births.
down syndrome/trisomy 21
Symptoms of trisomy 21?
- 1. intellectual disability
- 2. protruding tongue
- 3. low-set ears
- 4. epicanthal folds - eye folds
- 5. poor muscle tone
- 6. short stature
Children with Down syndrome are often afflicted with what 3 conditions?
- 1. congenital heart deformities
- 2. increased susceptibility to respiratory tract infections
- 3. leukemia
In 95% of cases the extra chromosome 21 is thought to come from the _____ and the incidence of trisomy21 is clearly associated with maternal _____.
Trisomy 18 AKA?
Trisomy 13 AKA?
What usually occurs with trisomy 18 and trisomy 13?
most pregnancies are lost before term and liveborn infants usually do not survive more than a few days or weeks
The few trisomy conditions compatible with life involve chromosomes containing what?
the smallest numbers of genes
What causes Cri du Chat syndrome?
deletion of part of the short arm of chromosome 5
4 manifestations of Cri du Chat syndrome?
- 1. sever mental retardation
- 2. round face
- 3. congental heart anomalies
- 4. characteristic cry that sounds like a cat crying
What causes the characteristic cry of Cri du Chat Syndrome?
What is usually the prognosis for those affected with it?
usually survive to adulthood and thrive better than those with the trisomies
4 examples of autosomal chromosomal disorders?
- 1. trisomy 21/Down Syndrome
- 2. Trisomy 18/ Edwards Syndrome
- 3. Trisomy 13/ Patau Syndrome
- 4. Cri du Chat syndrome
3 examples of sex chromosome disorders?
Multiple X females and double Y males
- Klinefelter syndrome
- Turner syndrome
The incidence of ______ syndrome is about in 600 live-born males making it themost common sex chromosome abnormality.
What causes Klinefelter Syndrome?
a male has an extra X chromosome or numerous extra X chromosomes
XXY XXXY XXXXY
What occurs in Klinefelter Syndrome?
They are males because they have a Y chromosome but the extra X causes abnormal sexual development and femininization
When is Klinefelter syndrome usually diagnosed?
at puberty when lack of secondary sexual characteristics may become apparent
7 S&S of Klinefelter Syndrome?
- 1. lack of secondary sex characteristics
- 2. testicular atrophy and infertility
- 3. tall stature with long arms and legs
- 4. feminine hair distribution
- 5. gynecomastia
- 6. high-pitched voice
- 7. marginally impaired intelligence
What is the cause of the S&S of Klinefelter Syndrome?
How is it treated?
lack of testosterone
What causes Turner Syndrome?
a female has only one X chromosome and no Y chromosome or the second X chromosome is structurally abnormal
Where does the missing or damaged X chromosome in Turner Syndrome come from in the majority of cases?
7 S/S of Turner syndrome?
- 1. ovaries fail to develop or fail prematurely
- 2. short stature
- 3. webbing of the neck
- 4. wide chest
- 5. lymphedema of the hands and feet at birth (localized fluid retention and tissue swelling)
- 6. congenital heart defects
- 7. failure to develop secondary sexual characteristics
What occurs in multiple X females and double Y males?
They usually appear normal.
Females may experience menstrual abnormalities and mental retardation if more than 4 X chromosomes are present
Males will usually be taller than average
disorders that result from the mutation of a single gene
4 things a gene affected by a mendelian disorder may code for?
- 1. abnormal enzymes
- 2. structural proteins
- 3. regulatory proteins
- 4. regulatory RNA molecules
2 ways mendelian disorders are generally classified?
1. the location of the gene (autosomal or sex chromosome)
2. mode of transmission (dominant or recessive)
What is it used for?
shows family relationships and the members who have been affected by disease
can use patterns to determine if a disease is dominant, recessive, or sex-linked
2 examples of autosomal dominant disorders?
Marfan Syndrome and Huntington Disease
What causes autosomal dominant disorders?
mutation of a dominant gene located on one of the autosomes
5 predictable patterns of inheritance associated with autosomal dominant disorders?
- 1. M & F are equally affected
- 2. affected individuals usually have an affected parent
- 3. offspring of an affected individual with a normal mate have a 50/50 chance of inheriting the disease
- 4. Unaffected individuals do not transmit the disease
- 5. mating of 2 heterozygous ppl = 75% chance of it affecting the offspring
What is usually involved in autosomal dominant disorders?
key structural proteins or regulatory proteins
What causes Marfan syndrome?
mutation in the fibrillin 1 gene on chromosome 15
What is fibrillin 1?
glycoprotein secreted by fibroblasts in to the extracellular matrix that provides important scaffolding for deposition of other matrix proteins such as elastin
What is Marfan Syndrome?
What are the S/S?
disorder of the connective tissues of the body
tall and slender with long, thin arms and legs and fingers
What is Huntington disease?
autosomal dominant disease that primarily affects neurologic function
S&S of Huntington Disease?
mental deterioration and involuntary movements of arms and legs that appears around age 40
What results due to the late onset of Huntington Disease?
it may be passed on to offspring before the parent knows they have it
What causes Huntington disease?
triplet repeats on chromosome 4 - the more of them there are the more likely the disease will develop
What causes autosomal recessive disorders?
mutation of a recessive gene located on an autosome
4 predictable patterns of inheritance followed by autosomal recessive disorders?
- 1. males and females are equally affected
- 2. in most cases the parents/relatives of the affected person are heterozygous carriers
- 3. Unaffected individuals may transmit the disease to offspring
- 4. the mating of 2 carriers results in a 25% chance of affected offspring and 50% chance of carrier offspring
How does this affect recessive disorders?
mating of related individuals
they are more likely to occur because related individuals are more likely to carry the same mutated genes
What is often affected with recessive disorders?
enzymatic functions are abnormal or the enzyme is absent which usually causes an inability to metabolize nutrients or to synthesize cellular components
All types of albinism involve disruption of ____ synthesis.
What type of albinism is recessive?
At risk for?
sunburn & skin cancer and generally exhibit impaired vision, nystagmus, and photosensitivity
Phenylketonuria - inability to met the amino acid phenylalanine b/c of lack of the enzyme phenylalanine hydroxylase
What causes the symptoms of PKU?
What are the symptoms?
accumulation of dietary phenylalanine in the body
children tend to be overly irritable and tremorous and have slowly developing mental retardation
What happens to excess phenylkietonuria?
excreted in the urine
What is done to prevent PKU from causing damage before it is detected?
screening for PKU is performed at birth
Symptom of PKU in infants?
defect in a membrane transporter for Cl in epi cells ass with production of abnormally thick secretions in glandular tissues
What is primarily affected by cystic fibrosis?
lung bronchioles and pancreatic ducts
What chromosome causes cystic fibrosis?
3 examples of autosomal recessive disorders?
albinism, PKU, cystic fibrosis
Example of a sex-linked disorder?
Sex-linked (x-linked) disorders?
mutation occurs on the sex chromosomes almost always the X
5 predictable patterns of inheritance for X-linked disorders?
- 1. affected ind are almost always male
- 2. affected fatehers transmit the defective gene to none of their sons but all of their daughters
- 3. a carrier female has a 1 in 2 chance of producing an affected son and a 1 in 2 chance of producing a carrier daughter
- 4. females are affected only in homozygous state
a bleeding disorder ass with a deficiency of factor VIII, a protein necessary for clotting
3 categories of disorders that are single-gene and do not follow mendelian principles of randomand independent assortment?
- 1. disorders caused by long triplet repeat mutations - fragile X syndrome
- 2. disorders attributable to mitochondrial DNA mutations
- 3. disorders ass with genomic imprinting
Triplet repeat mutations?
repeting sequences of 3 nucleotides
Fragile X syndrome?
mental retardation caused by repeating of CGG on the X chromosome
Ppl with small number of repeats of CGG on X chromosome?
risk for passing fragile X to offspring but they are unaffected by it otherwise
Why does fragile X cause retardation?
Why does it affect males more strongly?
fragile X gene is cruial to the dev and function of cerebral neurons
no second X to offset S/S
How are mitochondrial gene mutations passed on?
mothers transmit mitochondrial DNA to sons and daughters but only daughters transmit them to their offspring
What are the S/S of mitochondrial gene mutation? Why?
dysfunction in tissues with high utilization of ATP: nerve, muscle, kidney, and liver cells
mitochondrial DNA codes for enzymes involved in oxidation phosphorylation reactions
process whereby maternal and paternal chromosomes are marked differntially within th e cell
2 examples of disorders caused by genomic imprinting?
How do they occur?
Prader-Willi syndrome, and Angelman syndrome
both are cuased by a deletion at the same locatio non chromosome 15
Difference b/t Prader-Willi and Angelman syndromes?
Prader-Willi is always on the paternally derived chromosome and Angelman is always on the maternally derived one
S/S of prader-Willi?
mental retardation, short , obesity, poor muscle tone, and hypogonadism
S/S of Angelman syndrome?
mental retardation, ataxia (involuntary movements), and seizures, laughing inappropriately
traits develop in response to more than one gene
traits and disorders influenced by mult. genes as well as env factors
study of developmental anomalies
factors that cause congenital malformation
4 factors that influence effects of teratogens?
- 1. amnt of exposure
- 2. dev stage of the fetus when exposed
- 3. prior condition of the mother
- 4. genetic predisposition of the fetus
2 stages of the intrauterine developent?
- 1. embryonic period - extends for conception to 9 weeks of dev
- 2. the fetal period which continues until birth
What usually occurs if an embryo is exposed to a teratogen before the 3rd week of gestation?
so few cells are damaged that no damage is done or so many are damaged that spontaneous abortion occurs
When is the embryo very vulnerable to teratogenesis?
Why may this be a period when the embryo is likely to be exposed to substances?
between the 3rd and 9th weeks (wk 4 & 5 are most vulnerable time)
organs are developing
mother may not realize she is pregnant
Vulnerable period for major morphologic abnormalities of the CNS & heart?
- CNS- 3-5 weeks
- heart - 31/ -5 1/2 weeks
Why is susceptiility to errors of morphogenesis significantly less after 3 months?
mostly growth is occuring, not new developement
Teratogens cause errors in morphogenesis by interfering with cell ---, ____, or ____.
proliferation, migration, or differentiation
Proven tetragenic chemicals and drugs?
thalidomide, alcohol, anticonvulsants, warfarin, folate antagonists, androgenic hormones, angiotensin-converting enzyme inhibotors, and organic mercury
fetal alcohol syndrome - causes growth retardation, developmental delay, learning and behavioral probs, malformations of the head and face, and cardiac defects
When is the risk for defects caused by rubella present?
just before conception and to 20 weeks gestation when the virus is still able to cross the placenta
S/S of rubella induced defects?
cataracts, deafness, and heart defects
Infectious agents know to cause birth defects?
TORCH - toxoplasmosis, others, rubella, cytomegalovirus, and herpes
all are able to cross placenta and cause damage
S/S of defects caused by infectious agents/
microcephaly, small eyes, pneumonitis, hepatomegaly and jaundice, petechiae, and purpura, heart defects, splenomegaly
protozoal infection that can be contracted from ingestion of raw or undercooked meat and from contact with cat feces
Conditions associated with higher risk of congenital anomalies? (6)
- 1. maternal age of 35 years or greater
- 2. history of previously bearing a child with chromosomal disorder
- 3. a known family history of x-linked disorders
- 4. a famlily history of inborn errors of metabolism
- 5. the occurrence of neural tube anomalies in previious pregnancy
- 6. awareness that the mother is a known carrier of a recessive genetic disorder
determine gestational age, fetal position, and placental location
detect visible congenital anomalies: spina bifida, heart defects, and malformations of face head, body, and limbs
When can amniocentesis be performed?
at 16 weeks gestation
chorionic villus sampling - removal of a piece of tissue directly from the chorion (outer membrane of the fetal sac)
When can CVS be conducted?
What does CVS test for?
genetic issues, shows genome
allows direct visualization fo the embryo as early as the first trimester of pregnancy
How is embryoscopy performed?
scope is inserted through the cervix and into the uterus
severe combined immunodeficiency
Symptoms of SCID?
compromised immune system and usually die from overwhelming infections unless env strictly controlled
replacing a damaged gene with a good one
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