A genetic disorder is a genetic problem caused by one or more abnormalities formed in the genome. Most genetic disorders are quite rare and affect one person in every several thousands or millions.
A genetic disorder is a genetic problem caused by one or more abnormalities formed in the genome. Most genetic disorders are quite rare and affect one person in every several thousands or millions.
Birth Defects was written for healthcare workers who look after individuals with birth defects, their families, and women who are at increased risk of giving birth to an infant with a birth defect. This book is being used in the Genetics Education Programme which trains healthcare workers in genetic counselling in South Africa. It covers: modes of inheritance, medical genetic counselling, birth defects due to chromosomal abnormalities, single gene defects, teratogens, multifactorial inheritance
Birth Defects was written for healthcare workers who look after individuals with birth defects, their families, and women who are at increased risk of giving birth to an infant with a birth defect. This book is being used in the Genetics Education Programme which trains healthcare workers in genetic counselling in South Africa. It covers: modes of inheritance, medical genetic counselling, birth defects due to chromosomal abnormalities, single gene defects, teratogens, multifactorial inheritance
A genetic disease is any disease caused by an abnormality in the genetic makeup of an individual. The genetic abnormality can range from minuscule to major - from a discrete mutation in a single base in the DNA of a single gene to a gross chromosomal abnormality involving the addition or subtraction of an entire chromosome or set of chromosomes. Some people inherit genetic disorders from the parents, while acquired changes or mutations in a preexisting gene or group of genes cause other genetic diseases. Genetic mutations can occur either randomly or due to some environmental exposure.
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2. Definition and prevalence
Chromosomal anomalies refers to missing, extra or irregular portion of
chromosomal DNA.
Most foetus with chromosomal anomalies do not survive.
Affects 1 out of 200 of new borns
3. Genetics and Disease
Most diseases have probable genetic and environmental basis.The
genetic conponent may be the major or the only factor leading to
manifestation of the disease,or it may merely predispose the individual to
get a disease in response to environmental stresses.Based on genetic
mechanism,disease may be of 5 types
1. Chromosomal disorders
2. Single gene disorders
3. Polygenic disorders
4. Mitochondrial disorders
5. Somatic cell disorders
4. Chromosomal disorders
MECHANISM OF CHROMOSOMAL ANOMALIES
Loss or gain of a whole chromosomes due to abnormalities in a cell division
may cause profound disturbances in the genetic constitution of the fetus
and affect its survival.
Depending upon the type of abnormality,the chromosome involved and
the type of imbalance ,there may be an early abortion ,still birth,neonatal
death,malformations or intellectual disability.
There are 2 types of chromosomal abnormalities.
1. Numerical
2. Structural
5. Numerical chromosome anomalies
Aneuploidies result from failure of chromosomes to separate normally
during cell division.This phenomenon is known as nondisjunction
Nondisjunction during meiosis 1 leads to formation of disomic or nullisomic
gametes, resulting in trisomic and monosomic zygotes respectively
Nondisjunction during meiosis 2 results in monosomic,disomic and
nullisomic gametes,leading to disomic ,trisomic and monosomic zygotes.
Common aneuploides in live born babies include Downs syndrome(trisomy
21),Edward syndrome(trisomy 18),Patau syndrome (trisomy 13) and Turners
syndrome(monosomy X)
6. STRUCTURAL CHROMOSOME ABNORMALITIES
Chief types of structural chromosome abnormalities are ;
1. Translocation
2. Inversion
3. Deletion
4. Duplication
5. Ring chromosome
6. Isochromosomes
13. Testing for Chromosomal Disorders
Laboratory testing for chromosomal disorders includes conventional
karyotyping,fluorescent in situ hybridization (FISH),quantitative PCR(qPCR),multiplex
ligation probe amplification(MLPA) and chromosomal microarray (CMA).
The process of making chromosome preparations by in vitro culture,staining,identification
and classification of chromosomes is called karyotyping.Karyotyping can be performed
on peripheral blood lymphocytes,bone marrow aspirate and tissue biopsy material.
The process is labor intensive and involves culture,followed by harvesting chromosomes
after the cells are arrested in mitosis
After preparation of slides,staining (usually Giemsa stain)is done to produce a banding
pattern unique for each chromosome.This enables detection of numerical and structural
abnormalities through the genome at a resolution of approximately 5 Mb
Molecular cytogenic techniques includes FISH,quantitative polymerase chain reaction
and MLPA.
14. DOWNS SYNDROME
It is the most common chromosomal disorder,occuring with a
frequency of 1:800 to 1:1000 newborns.
Chromosome number 21 is present in triplicate usually because of
meiotic non disjunction in either maternal or parental gamete.
Clinical features and Diagnosis
Poor intellectual ability
Physical retardation
Falt facial profile
Upward slant of eyes
Epicanthic folds
Nose is small with flat nasal bridge
Mouth shows a narrow short palate with small teeth and furrowed
proteuding tongue
Significant hypotonia
15. The skull appears small and brachycephalic with
flat occiput.
Ears are small and dysplastic.
Characteristic facial grimace on crying
Hands are short and broad
Clinodactyly(hypoplasia of middle phalanx of 5th
finger)and simian crease are usual
There is a wide gap between the first and the
second toe(sandle gap)
18. TURNER SYNDROME
Clinical features
Lymphedema of the dorsum of hands and feet.
Loose skin folds at the nape of neck
Short stature
Short neck with webbing and low posterior hair line.
Anomalous ears
Prominent narrow and high arched palate,small
mandible and epicanthic folds may be noted.
Chest is broad shield like with widely spaced
hypoplastic nipples
19. At puberty, sexual maturation fails to occur.It has been recommended
that the diagnosis of Turners syndrome should be considered in all girls with
short stature.
Ultrasound may show streak ovaries and hypoplastic uterus.
Levels of FSH and LH are increased
20. Management
Height monitoring should be done using growth charts for Turner syndrome
Cardiac evaluation and measurement of BP is recommended at baseline
and every year
Treatment with growth hormone is recommended. While therapy may
increase the final height by 8-10 cm,the cost is prohibitive
Thyroid testing should be done in infancy or early childhood.if child is lagging
in growth.Routine evaluation is required after 10 years of age.
Counselling regarding behavioral problems due to short stature,amenorrhea
and sterility is an integral part of management
Ovarian hormone replacement is advised around 14 years
21. Conjugated estrogen(0.3 mg/day) or ethinyl estradiol
are given for 3-6 months.After 6-12 months ,cyclical
therapy with estrogen and progesterone is stated
Regular audiometry is advised in adulthood or earlier, if
indicated.
Evaluation for renal malformation by USG should be
done at first contact
22.
23. Single Gene Disorders
Single gene disorders are inherited as autosomal dominant,autosomal recessive or X
linked disorders due to mutation in the disease Specific genes.
Drawing and interpreting a pedigree is an integral part of diagnosis of single gene
disorders
Mutations refers to the heritable change in the DNA resulting in the perturbed protein
structure and function.
There are 3 types of mutations;
Substitution or point mutation
Deletion mutation
Insertion
24. AUTOSOMAL DOMINANT INHERITANCE
Generally it impairs the synthesis of
structural or non enzyme proteins
e.g:Huntington chorea and
connective tissue disorders.
These disorders manifest even if only
one of the alleles of the abnormal
gene is affected.
Physical examination of other siblings
and parents should be done to
uncover milder forms of the disorder.
25. AUTOSOMAL RECESSIVE INHERITANCE
They manifest only in homozygous state,i.e.both the
alleles are mutant genes.
Generally they affect synthesis of enzyme ,leading
to inborn errors of metabolism.
The parents of the affected individuals are
apparently normal but carry the mutant genes
E.g: Beta thalassemia, sickle cell disease,spinal
muscular atrophy,phenylketonuria and
galactosemia
26. X LINKED RECESSIVE INHERITANCE
Since in males,there is no corresponding locus for
a mutant alllele of the X chromosome on the
shorter Y chromosome, the mutant X linked
recessive gene expresses as a clinical disorder in
the male child because it is not suppressed by
ajormal alllele
In the female, the disorder does not manifest
clinically since the mutant gene is compensted for
by the normal allele in the other X chromosome
E.g:color blindness
27. X LINKED DOMINANT INHERITANCE
These are rare.Both heterozygous female
and hemizygous males are affected.
All the sons of the affected males are
normal and all daughters are
affected.The affected females transmitt
the disease to half of the sons and half of
the daughters.
E.g:hypophosphatemic type of vitamin D
resistant rickets
28. MITOCHONDRIAL INHERITANCE
Mutation within a mitochondrial gene can lead to phenotypic
defects and show a pattern of maternal genetic transmission .
Since mitochondria are only present in ovum and not
sperms,the inheritance is maternal
All affected daughters transmitt the disease.Sons are affected
but do not transmit the disease
E.g:Leigh disease,mitochondrial encephalopathy,lactic
acidosis,stroke like syndrome.
29. SOMATIC CELL GENETIC DISORDERS
In somatic cell disorders,the defects are restricted to specific
somatic cells,in contrast to previous 4 types of gamete disease in
which abnormality is present in all the cells,including germ line cells.
These include cancers which can arise due to genetic changes in
somatic cells alone.
30. THERAPY OF GENETIC DISORDERS
While genetic disorders cannot be cured completly,symptoms may be
ameliorated and irreversible damage or handicap prevented or reduced
through several therapeutic approaches
Specific diets
Providing deficient proteins
Promoting excretion of toxic substances
Avoid precipitating factors and drugs
Supportive care
31. PREVENTION OF GENETIC DISORDERS
Carrier screening: HbA2 levels are highly useful in identifying carriers of thalassemia trait pre pregnancy
or early in pregnancy.
Newborn screening:This is an example of secondary prevention by early diagnosis and treatment.
Newborn infants are screened routinely for some endocrine disorders and inborn errors of metabolism.
Prevention of Neural Tube Defects:Folic acid supplementation is recommended at a dose of 0.4 mg
daily from 1 month before to 3months after conception to prevent neural tube defects.
Maternal serum screening:Estimation of pregnancy associated plasma protein A(PAPP-A) and free
hCG int 1st trimester and serum alpha fetoprotein unconjugated estriol and inhibin A in the 2nd trimester
are useful biochemical markers to detect aneuploidies