Genetics Flashcards
The genetic code for every feature of each human is contained where?
Within the nucleus of each cell of the body.
On what molecule is the genetic code is contained within a long string of nucleotides on?
The genetic code is contained within a long string of nucleotides on a molecule called deoxyribonucleic acid (DNA).
Each gene is the result of what?
Each gene is the result of a string of DNA that contains the instructions for how to form specific proteins. These proteins are responsible for how our body functions.
DNA codes for hundreds to thousands of genes are bundled into individual chromosomes - what are chromosomes?
Chromosomes are single molecules of DNA.
Chromosomes come in pairs.
Therefore, each human has two copies of each gene, one on each chromosome in a pair. One chromosome in each pair comes from each parent.
We have 23 chromosome pairs or 46 chromosomes in total.
Sex chromosomes vs-non sex chromosomes
One of the pairs of chromosomes are the sex chromosomes. The sex chromosomes are the X chromosome and the Y chromosome. Males have an X and a Y chromosome and females have two X chromosomes. The other 44 chromosomes are called autosomes. They are the non-sex chromosomes.
What is a genotype?
Genotype refers to the genes that we have. For example, having the gene that codes for brown eyes.
What is a phenotype?
Phenotype refers to the physical expression of the genes that we have. For example, actually having brown eyes.
A person’s DNA is the result of half the genes from their mother combined with half the genes from their father.
A mother’s egg contains half her DNA, and a father’s sperm contains half his DNA. The DNA that makes up the egg or sperm is selected at random and is impossible to predict. The process of creating a gamete (egg or sperm) that contains half the genetic material of a normal cell is called what?
Meiosis
What is the pincipal of Mendelian inheritance?
When there is a single gene that codes for a physical feature, the phenotype is the result of the combination of the two genes (one on each chromosome). Some genes are more dominant than others, and the phenotype will reflect the more dominant gene. This is called Mendelian inheritance.
Dominant vs recessive genes
Genes that are more dominant over other genes are known as dominant, and genes that aren’t expressed when paired with more dominant genes are called recessive.
Some genetic conditions exhibit a simple inheritance pattern called Mendelian Inheritance - what does this mean?
This type of inheritance only occurs where the disease is caused by a single abnormal gene on one of the non-sex chromosomes (i.e. not the X or Y chromosomes).
These chromosomes are called “autosomes”.
Potential outcomes of inheritance of a autosomal recessive condition when two parents are carriers?
The children of these parents have a 1 in 4 (or 25%) chance of having the disease, and 2 in 4 (or 50%) chance of being a carrier and a 1 in 4 (or 25%) chance of having normal genes.
When one parent has an autosomal recessive disease, and the other parent is a carrier, what are the chances of their children inheriting the condition?
The children of these parents have a 50% chance of having the disease and a 50% chance of being a carrier.
Autosomal dominant condition - possible outcomes when one parent has the condition and the other parent is disease free?
The children of these parents have a 2 in 4 (or 50%) chance of having the disease, and 2 in 4 (or 50%) chance of having normal genes.
What is a chromosome disorder?
Chromosome disorders are condition where there is either a structural abnormality, an extra abnormal portion or an abnormal number of chromosomes compared to normal.
Deletion disorders
Deletion disorders occur where a portion of a chromosome is missing.
These syndromes are very rare
One example is cri du chat, which is caused by a missing portion of chromosome 5. Patients have learning, developmental and speech and language difficulties and a characteristic “cat like cry” as infants.
Duplication disorder
Duplication disorders occur where a portion of a chromosome is duplicated.
The chromosome contains twice the number of copies of that gene.
One example is Charcot-Marie-Tooth, which can be caused by a duplication of the short arm of chromosome 17. Patients suffer with sensory and motor neuropathy and have characteristic pes cavus (high arching foot).
What is a translocation disorder?
Translocation disorders occur where a portion of one chromosome is directly swapped with a portion of another chromosome.
The swap can be balanced (reciprocal translocations), where a portion of one chromosome is swapped with a portion of another.
Alternatively they can be unbalanced (nonreciprocal translocations), where a portion of one chromosome leaves the first chromosome and attaches to the other without any exchange taking place.
What do translocation disorders lead to?
Translocation does not usually lead to a specific genetic syndrome, but often predisposes to other conditions such as cancer and infertility. One example is the “Philadelphia chromosome” translocation in acute myeloid leukaemia, which is a reciprocal translocation between chromosome 9 and chromosome 22.
Robertsonian translocations
Robertsonian translocations occur in acrocentric chromosomes. These chromosomes are 13, 14, 15, 21 and 22. They have a longer long arm, which contains most of the genetic material, and a very short short arm with very little genetic information on it. When a person has a Robertsonian translocation, they loose the short arm completely, and the two long arms connect to each other at the centromere, essentially loosing a chromosome when they get rid of the two short arms. In this scenario the person is usually phenotypically normal, but has 45 chromosomes when counted and has a risk of problems in their offspring.
What is Trisomy?
Trisomy is where the person has an extra chromosome.
They have a total of 47 chromosomes.
They have three copies of a particular chromosome.
What is Patau syndrome?
Trisomy 13.
The syndrome varies in severity.
Patients have dysmorphic features, structural abnormalities affecting almost all areas of their body and learning disability. They have characteristic “rocker bottom feet”, where the soles of the feet are convex (rounded outwards) in shape.
What is Edwards syndrome?
This is trisomy 18. The syndrome varies in severity and affects almost all areas of the body, resulting in dysmorphic features and learning disability. They also have “rocker bottom feet”.
What is Mosaicism
Mosaicism is an interesting scenario where the chromosomal abnormality actually happens after conception. The abnormality occurs in a portion of cells in the body and not in others. The person therefore has different genetic material in different cells in their body. Each case is unique and the effects are unpredictable.
“Rocker bottom feet”, where the soles of the feet are convex, is a characteristic feature of what genetic condition?
Patau syndrome, and also Edwards syndrome
Trisomy 13
Patua syndrome
Trisomy 18
Edwards syndrome
Trisomy 21
Down’s syndrome
What is a mitochondrial myopathy?
Abnormal mitochondria lead to poor production of ATP, the molecule that provides energy in the body.
Poor production of ATP leads to myopathy (abnormal muscle function).
They are also responsible for rare forms of deafness, blindness, diabetes mellitus and epilepsy.
What are mitochondria
Mitochondria are organelles that live inside the cell cytoplasm.
The number of mitochondria in a cell varies depending on the function of that cell.
Myocytes (muscle cells) have thousands of mitochondria, whereas adipocytes (fat cells) have very few.
They are responsible for producing ATP for the cell.
Mitochondria contain their own DNA, separate from DNA in the cell the nucleus.
This DNA is arranged in a large circle, unlike the chromosomes found in the nucleus.
Mitochondrial Inheritance
At the time of conception, the sperm carrying the fathers genetic material enters the egg and the DNA in the nucleus of both cells combine.
The vast majority of the mitochondria in that first cell (called the zygote) come from the mother. All of the mitochondria in the sperm are in the tail, which does not enter the egg.
Therefore, the father does not contribute any mitochondria to the zygote and subsequently the fetus and child. Therefore, mitochondrial DNA is primarily from the mother. This is called maternal inheritance.
If we are looking at a specific disease gene in the mitochondria DNA, we need to consider that not all mitochondria within the mothers cells will be affected. The proportion of affected mitochondria that are passed to the offspring will determine whether that individual is affected.
Diagnostic genetic testing
Diagnostic testing involves testing a fetus or a person for a suspected genetic condition. We can test a fetus for a genetic condition via amniocentesis.
An example of this is antenatal testing for Down’s syndrome. Antenatal testing can have implications on the decision to continue the pregnancy.
Where a specific condition is suspected, for example Turner syndrome, it is possible to test directly for that condition in a child or adult.
Predictive genetic testing
Predictive testing involves testing a person for a specific gene mutation that has implications for them in the future.
Examples are the BRCA1 breast cancer gene or the gene for Huntington’s chorea.
Carrier genetic testing
Carrier testing involves testing parents or potential parents for the gene for a specific autosomal recessive condition in order to calculate the risk of passing it to their children. An example of this is testing for the cystic fibrosis gene.
When might genetic testing be undertaken in scenarios outside of diagnostic, predictive and carrier testing?
Genealogical testing
Forensic testing
Paternity testing
What is Karyotyping?
Karyotyping involves looking at the number of chromosomes, their size and basic structure.
In what conditions might karyotyping be helpful in diagnosing?
Down’s syndrome (trisomy 21)
Turner syndrome (45 XO)
What is microarray testing?
Microarray testing involves cutting up the genetic material from an individual using enzymes.
Different genes will have different molecular weights. The chopped up genetic material is then applied to a plate that separates molecules of different weights into different locations.
This can be used to see what genes the person expresses.
Applications of microarray testing?
Screening for chromosomal abnormalities and many common genetic conditions
Looking for mutations in cancer cells
For research aimed at matching genes with phenotypes
Specific gene testing
Specific gene testing can be done by splitting the two strands of DNA and adding a “gene probe”.
The gene probe is made of single stranded DNA that contains complementary genetic code for a specific gene you want to test for.
When the strands of DNA are mixed with the gene probe and the gene probe matches the genetic material on the DNA, they will stick together.
This suggests the specific gene that matches the gene probe is present.
This is used to confirm whether a patient has a particular gene.
DNA sequencing
DNA sequencing is only used for research purposes, and has no role in routine clinical practice.
This involves splitting the two strands of DNA and watching as individual nucleotides are added to a single strand of DNA, ultimately revealing the exact sequence of nucleotides in that section of DNA.
What is Down’s syndrome?
Down’s Syndrome is caused by three copies of chromosome 21.
It is also called trisomy 21.
It gives characteristic dysmorphic features and is associated with a number of associated conditions.
The extent to which the person is affected and the associated conditions they have vary between individuals.
Dysmorphic features of downs syndrome?
Hypotonia (reduced muscle tone)
Brachycephaly (small head with a flat back)
Short neck
Short stature
Flattened face and nose
Prominent epicanthic folds (folds of skin covering the medial portion of the eye and eyelid)
Upward sloping palpebral fissures (gaps between the lower and upper eyelid)
Single palmar crease
Complications of Down’s syndrome?
Learning disability
Recurrent otitis media
Deafness. Eustachian tube abnormalities lead to glue ear and conductive hearing loss.
Visual problems such myopia, strabismus and cataracts
Hypothyroidism occurs in 10 – 20%
Cardiac defects affect 1 in 3, particularly ASD, VSD, patent ductus arteriosus and tetralogy of Fallot
Atlantoaxial instability
Leukaemia is more common in children with Down’s
Dementia is more common in adults with Down’s
Cardiac defects which may occur alongside Downs Syndrome?
ASD, VSD, patent ductus arteriosus and tetralogy of Fallot
First line antenatal Downs syndrome screening?
The combined test is the first line, most accurate and test of choice where possible.
This test is performed between 11 and 14 weeks gestation.
It involves combining results from ultrasound and maternal blood tests.
Ultrasound measures nuchal translucency, which is the thickness of the back of the neck of the fetus.
Down’s syndrome is one cause of a nuchal thickness over 6mm.
Maternal blood tests:
- Beta‑human chorionic gonadotrophin (beta-HCG). A higher result indicates a greater risk.
- Pregnancy‑associated plasma protein‑A (PAPPA). A lower result indicates a greater risk.
Antenatal testing for DS
The screening tests provide a risk score for the fetus having Down’s syndrome. When the risk of Down’s is greater than 1 in 150 (this result occurs in around 5% of tested women) the woman is offered amniocentesis or chorionic villus sampling. These tests involve taking a sample of the fetal cells, which then undergo karyotyping to give a definitive answer to whether the fetus is affected by Down’s or not.
Chorionic villus sampling (CVS) involves an ultrasound guided biopsy of the placental tissue. This is used when testing is done earlier in pregnancy (before 15 weeks).
Amniocentesis involves ultrasound guided aspiration of some amniotic fluid using a needle and syringe. This is later in pregnancy once there is enough amniotic fluid to make it safer to take a sample.
NIPT - Downs syndrome
Non-invasive prenatal testing (NIPT) is a relatively new test for detecting abnormalities in the fetus during pregnancy. It involves a simple blood test from the mother. The blood will contain fragments of DNA, some of which will come from the placental tissue and represent the fetal DNA. These fragments can be analysed and detect conditions such as Down’s.
NIPT is not a definitive test, but it does give a very good indication of whether the fetus is affected. This is gradually being rolled out in the NHS as an alternative to invasive testing (CVS and amniocentesis) for women that have a higher than 1 in 150 risk of Down’s syndrome.
Management of Downs Syndrome
Management involves supportive care from the multidisciplinary team to help them meet their needs:
Occupational therapy
Speech and language therapy
Physiotherapy
Dietician
Paediatrician
GP
Health visitors
Cardiologist for congenital heart disease
ENT specialist for ear problems
Audiologist for hearing aids
Optician for glasses
Social services for social care and benefits
Additional support with educational needs
Charities such as the Down’s Syndrome Association
Routine follow up investigations of importance for children with Down’s syndrome?
Regular thyroid checks (2 yearly)
Echocardiogram to diagnose cardiac defects
Regular audiometry for hearing impairment
Regular eye checks
Prognosis of Downs Syndrome
Prognosis varies depending on the severity of the associate complications. The average life expectancy is 60 years.
What is the underlying abnormality in Klinefelter syndrome?
Klinefelter syndrome occurs when a male has an additional X chromosome, making them 47 XXY.
Rarely people with Klinefelter syndrome can have even more X chromosomes, such as 48 XXXY or 49 XXXXY. This is associated with more severe features.
When does Klinefelter syndrome usually present?
When boys reach puberty
Features of Kleinfelder syndrome?
Taller height
Wider hips
Gynaecomastia
Weaker muscles
Small testicles
Reduced libido
Shyness
Infertility
Subtle learning difficulties (particularly affecting speech and language)
Klinefelter syndrome management options?
There is no way to treat the underlying genetic cause of Klinefelter syndrome. Treatment aims to help with the features of the condition:
Testosterone injections improve many of the symptoms
Advanced IVF techniques have the potential to allow fertility
Breast reduction surgery for cosmetic purposes
Multidisciplinary team input:
Speech and language therapy to improve speech and language
Occupational therapy to assist in day to day tasks
Physiotherapy to strengthen muscles and joints
Educational support where required for dyslexia and other learning difficulties
Prognosis/complications of Klinefelter syndrome?
Life expectancy is close to normal. Infertility can occasionally be treated with advanced IVF techniques.
There is a slight increased risk of:
Breast cancer compared with other males (but still less than females)
Osteoporosis
Diabetes
Anxiety and depression
What is the underlying abnormality in Turner syndrome?
45 XO - female has a single z chromosome and an empty space where the second X chromosome should be
Features of Turner syndrome
Short stature
Webbed neck
High arching palate
Downward sloping eyes with ptosis
Broad chest with widely spaced nipples
Cubitus valgus
Underdeveloped ovaries with reduced function
Late or incomplete puberty
Most women are infertile
What is cubitus valgus?
Cubitus valgus refers to an abnormal feature of the elbow, associated with Turner syndrome
When the arm is extended downwards with the palms facing forward, the angle of the forearm at the elbow is exaggerated, angled away from the body.
Three classic features of Turner Syndrome?
Short stature
Webbed neck
Widely spaced nipples
Turner Syndrome - associated conditions
Recurrent otitis media
Recurrent urinary tract infections
Coarctation of the aorta
Hypothyroidism
Hypertension
Obesity
Diabetes
Osteoporosis
Various specific learning disabilities
Management of Turner Syndrome
There is no way to treat the underlying genetic cause of Turner syndrome. Treatment aims to help with the symptoms of the condition:
Growth hormone therapy can be used to prevent short stature
Oestrogen and progesterone replacement can help establish female secondary sex characteristics, regulate the menstrual cycle and prevent osteoporosis
Fertility treatment can increase the chances of becoming pregnant
Patients need monitoring for the associated conditions and complications. Treatable conditions such as hypertension and hypothyroidism should be managed appropriate.
How is Noonan syndrome inherited?
Noonan syndrome is a genetic condition. There are a number of different genes that cause Noonan syndrome.
The majority for cases are inherited in an autosomal dominant way.
Features of Noonan syndrome?
Short stature
Broad forehead
Downward sloping eyes with ptosis
Hypertelorism (wide space between the eyes)
Prominent nasolabial folds
Low set ears
Webbed neck
Widely spaced nipples
Noonan syndrome: associated conditions
Congenital heart disease, particularly pulmonary valve stenosis, hypertrophic cardiomyopathy and ASD
Cryptorchidism (undescended testes) can lead to infertility. Fertility is normal in women.
Learning disability
Bleeding disorders
Lymphoedema
Increased risk of leukaemia and neuroblastoma
Noonan syndrome: implications on fertility
Men:
Cryptorchidism (undescended testes) can lead to infertility.
Women:
Fertility is normal in women.
What is the abnormality in Marfan Syndrome?
Marfan syndrome affects the gene responsible for creating fibrillin.
Fibrillin is an important component of connective tissue.
This means people with Marfan syndrome have features resulting from abnormal connective tissue.
Inheritance of Marfan Syndrome
Autosomal dominant
Main complication of Noonan syndrome?
Congenital heart disease
Noonan syndrome management
There is no treatment for the underlying genetic defect.
Management is supportive with involvement of the multidisciplinary team.
The main complication is congenital heart disease and often patients will require corrective heart surgery.
Features of Marfan syndrome?
Tall stature
Long neck
Long limbs
Long fingers (arachnodactyly)
High arch palate
Hypermobility
Pectus carinatum or pectus excavatum
Downward sloping palpable fissures
A patient that appears tall, has hypermobility or a murmur suggestive of mitral or aortic regurgitation, high arch palate and arm span longer than height, should be considered for what disorder?
Marfan syndrome
Marfan syndrome - associated conditions?
Lens dislocation in the eye
Joint dislocations and pain due to hypermobility
Scoliosis of the spine
Pneumothorax
Gastro-oesophageal reflux
Mitral valve prolapse (with regurgitation)
Aortic valve prolapse (with regurgitation)
Aortic aneurysms
Management of Marfan Syndrome
The greatest risk is from the associated cardiac complications, particularly valve prolapse and aortic aneurysms. Where these complications occur they may require surgical correction.
The aim of management is to minimise the blood pressure and heart rate to minimise the stress on the heart and the risk of complications developing. This is achieved by lifestyle changes, such as avoiding intense exercise and avoiding caffeine and other stimulants.
Preventative medications such as beta blockers and angiotensin II receptor antagonists can also help reduce the risk of complications.
Pregnancy has to be carefully considered, as it carries a significant risk of developing aortic aneurysms and associated complications.
Physiotherapy can be helpful in strengthening joints and reducing symptoms arising from hypermobility.
Genetic counselling is important in considering the implications of having children that may be affected by the condition.
Patients are also regularly followed up and monitored for complications. This often involves yearly echocardiograms and review by an ophthalmologist.
What is the genetic abnormality in Fragile X syndrome?
Fragile X syndrome is caused by a mutation in the FMR1 (fragile X mental retardation 1) gene on the X chromosome.
What is the role of the abnormal gene in Fragile X syndrome?
The FMR1 gene codes for the fragile X mental retardation protein, which plays a role in cognitive development in the brain.
Inheritance pattern of fragile X syndrome
It is X-linked, but it is unclear whether it is dominant or recessive.
Males are always affected, but females can vary in how much they are affected. This is because females have a spare normal copy of the FMR1 gene on their other X chromosome.
When the mother is phenotypically normal, the affected child may have inherited the X chromosome from their mother, or it may result from a de novo (random) mutation.
Features of Fragile X syndrome?
Fragile X syndrome usually presents with a delay in speech and language development. Other features are:
Intellectual disability
Long, narrow face
Large ears
Large testicles after puberty
Hypermobile joints (particularly in the hands)
Attention deficit hyperactivity disorder (ADHD)
Autism
Seizures
Management of Fragile X syndrome?
There is no cure for the condition.
Management is supportive and involves treating the symptoms.
This involves the multidisciplinary team to support the learning disability, manage autism and ADHD and treat seizures if they occur.
Life expectancy is similar to the general population depending on associated disabilities and complications.
What is the underlying abnormality in Prader-Willi Syndrome?
Prader-Willi Syndrome is a genetic condition caused by the loss of functional genes on the proximal arm of the chromosome 15 inherited from the father.
This can be due to a deletion of this portion of the chromosome, or when both copies of chromosome 15 are inherited from the mother.
Features of Prader Willi syndrome?
Constant insatiable hunger that leads to obesity
Poor muscle tone as an infant (hypotonia)
Mild-moderate learning disability
Hypogonadism
Fairer, soft skin that is prone to bruising
Mental health problems, particularly anxiety
Dysmorphic features
Narrow forehead
Almond shaped eyes
Strabismus
Thin upper lip
Downturned mouth
Feeding in Prader-Willi syndrome
A key feature everyone remembers for Prader-Willi syndrome is the the insatiable hunger.
Feeding can often be a challenge initially due to hypotonia and it is only later that the food seeking and excessive eating occur.
Management of Prader-Willi Syndrome
There is no cure. Carefully limiting access to food under guidance of a dietician is required to control weight.
This usually requires locking food in cupboards, putting a lock on the fridge and even controlling access to rubbish bins.
Under dietician guidance they usually require a lower than normal calorie intake, particularly as they tend to have lower activity levels due to poor muscle strength and tone.
Everyone that is in contact with the child will need to be educated about limiting access to food, including teachers, carers and relatives.
Growth hormone is indicated by NICE as a treatment for Prader-Willi Syndrome, aimed at improving muscle development and body composition.
Supportive care from the multidisciplinary team to manage features:
Dieticians play a very important role
Education support
Social workers
Psychologists or psychiatrists
Physiotherapists
Occupational therapists
Nice recommended medical treatment of Prader Willi syndrome?
Growth hormone is indicated by NICE as a treatment for Prader-Willi Syndrome, aimed at improving muscle development and body composition.
Angelman syndrome: features
Delayed development and learning disability
Severe delay or absence of speech development
Coordination and balance problems (ataxia)
Fascination with water
Happy demeanour
Inappropriate laughter
Hand flapping
Abnormal sleep patterns
Epilepsy
Attention-deficit hyperactivity
disorder
Dysmorphic features
Microcephaly
Fair skin, light hair and blue eyes
Wide mouth with widely spaced teeth
Management of angleman syndrome
Like many other genetic syndromes, there is no cure and management focuses on a multi-disciplinary team approach to managing individual problems and supporting the patient and carers holistically.
Parental education
Social services and support
Educational support
Physiotherapy
Occupational therapy
Psychology
CAMHS
Anti-epileptic medication where required
What is the abnormality behind Angelman syndrome?
Angelman syndrome is a genetic condition caused by loss of function of the UBE3A gene, specifically the copy of the gene that is inherited from the mother.
This can be caused by a deletion on chromosome 15, a specific mutation in this gene or where two copies of chromosome 15 are contributed by the father, with no copy from the mother.
What is the underlying abnormality of William syndrome?
William syndrome is caused by a deletion of genetic material on one copy of chromosome 7, resulting in the person only having a single copy of the genes on this deleted region (on the other chromosome 7).
It usually the result of a random deletion around conception, rather than being inherited from an affected parent.
Features of William Syndrome?
Broad forehead
Starburst eyes (a star-like pattern on the iris)
Flattened nasal bridge
Long philtrum
Wide mouth with widely spaced teeth
Small chin
Very sociable trusting personality
Mild learning disability
Distinctive features to remember with William Syndrome
The distinctive features to remember with William syndrome are the very sociable personality, the starburst eyes and the wide mouth with a big smile.
It is worth remembering the association with supravalvular aortic stenosis and hypercalcaemia, as these are unique features
William Syndrome - aortic stenosis
Supravalvular aortic stenosis (narrowing just above the aortic valve)
Attention-deficit hyperactivity disorder
Hypertension
Hypercalcaemia
Management of William syndrome
Like many other genetic syndromes, there is no cure and management focuses on a multi-disciplinary team approach to managing individual problems and supporting the patient and family.
Echocardiograms and blood pressure monitoring are important to assess for aortic stenosis and hypertension.
A low calcium diet may be required to control hypercalcaemia, and they should avoid calcium and vitamin D supplements.
Achondroplasia
Achondroplasia is an autosomal dominant disorder associated with short stature. It is caused by a mutation in the fibroblast growth factor receptor 3 (FGFR-3) gene. This results in abnormal cartilage giving rise to:
short limbs (rhizomelia) with shortened fingers (brachydactyly)
large head with frontal bossing and narrow foramen magnum
midface hypoplasia with a flattened nasal bridge
‘trident’ hands
lumbar lordosis
In most cases (approximately 70%) it occurs as a sporadic mutation. The main risk factor is advancing parental age at the time of conception. Once present it is typically inherited in an autosomal dominant fashion.
What might trisomy 21 result from?
In DS the individual has an extra chromosome 21. This is the most common autosomal
trisomy. Trisomy 21 may result from:
* Meiotic non-disjunction (95%).
* Robertsonian translocation (5%).
* Mosaicism (1%).
What is Patau syndrome?
Trisomy 13 is also called Patau syndrome.
Clinical features are structural defects of the brain,
small eyes and other eye defects, cleft lip and palate, polydactyly and cardiac and renal
abnormalities.
What is Edwards’ syndrome?
Trisomy 18 is also called Edward’s syndrome.
Clinical features are low birth weight, prominent
occiput, small mouth and chin, flexed overlapping fingers, ‘rocker-bottom’ feet and cardiac
and renal abnormalities.
What translocation mutation in a parent increases a childs risk of down syndrome?
A parent with a translocation mutation of chromosome 14 and 21
When an extra chromosome 21 is joined onto another chromosome – usually 14 but
occasionally 13, 15, 21, 22 and Y – it is known as a Robertsonian translocation. A
chromosomal translocation can give rise to three copies of chromosome 21. Known as an
unbalanced translocation. It often arises from a parent who has a balanced Robertsonian
translocation. Translocation DS has a 3% change of recurring again if the father carries the
translocated chromosome and 10-15% if the mother does
