INTRODUCTION TO GENETICS &GENETIC COUNSELLING Flashcards
What’s Human genetics
Human genetics is the scientific study of variation in humans
What are Genes?
Genes:These are sequences of DNA and hereditary elements
that carry traits and genetic intructions in humans
They are usually found in chromosones
What are Alleles
Alleles: These are variants of genetic instructions being
Carried by the genes
What are Chromosones
Chromosones: These are custodians of the genes. In human,
There are 23pairs of chromosones .
These consist of
22pairs of autosomal chromosones + one XY pair in Male
22pairs of autosomal chromosones + one XX pair in Female
When is it said for a person to be heterozygote for a gene?
If the member of a gene pair carry different instruction the
person is heterozygote for that gene e.g AS, the resulting trait may be determined by one member of the pair known as the
DOMINANT gene the other pair that is not manifested is known
as the RECESSIVE gene
What is Referred to as a KARYOTYPE?
Each individual has a chromosome that is characteristic in number and form
What is a Karyogram?
Karyogram
–Is the photographic representation of stained chromosomes arranged in order of decreasing length.
Much of what we know about genes was first discovered by?
Gregor Mendel
He Conducted experiments on?
the garden pea (Pisum sativum)
What was key to Mendel’s success? And why?
Garden peas also produce a large number of offspring, a key to Mendel’s success.
What are the 3 Mendel’s Laws?
Inheritance is particulate: “particles” called genes carry the information that makes parents tend to resemble their offspring.
–These “particles” segregate, so that individuals with two particles produce gametes with only one particle, the law of segregation.
–The “particles” for each gene segregate independently of each other, the law of independent assortment.
Phenotype vs. Genotype
An individual’s PHENOTYPE is his/her observable characteristics.
An individual’s GENOTYPE is his/her genetic composition of alleles.
The effect of every gene depends both upon the environment, and upon other genes
true or false
TRUE
Traditionally - 3 types of diseases
Namely?
a)genetically determined
b)environmentally determined
c)1. + 2.
up to ?% of pediatric in-patients have genetic abnormality
about ?% of spontaneous abortuses have chromosomal aberration
20%
50%
CLASSIFICATION GENETIC DISEASES
a)Chromosomal disorders
b)Single gene diseases
c)Mitochondrial disorders
d)Polygenic disorders
Autosomal recessive inheritance
characteristics (4)
Affected persons are usually born to unaffected parents
Parents of affected persons are usually asymptomatic but carry a single gene defect
Risk is increase in consanguinity
Children of both parents who are both heterozgous for the mutated gene have 25% chance of inheriting the disease
Autosomal recessive inheritance
Examples ?
cystic fibrosis
sickle cell anaemia
Tay-Sachs disease
Phenylketonuria
Mucopolysaccharidoses
Glycogen storage diseases
Galactosemia
Autosomal dominant
characteristics?
Affected person usually has at least one affected parent
It affects either sex
Affected person has 50% chance of passing the defect on to their children
Autosomal dominant
Example?
Achondroplasia
Huntington disease
Marfan syndrome
Familial hypercholesterolemia
Ehlers-Danlos syndrome
Polycystic kidney disease
Hereditary spherocytosis
X-LINKED RECESSIVE INHERITANCE
characteristics (3)
Affects males almost exclusively
Affected males are usually born to unaffected parents as mother is usually asymptomatic carrier, although she may have affected male relative.
There is no male to male transmission of the defect
X-LINKED RECESSIVE INHERITANCE
examples? (dch)
Duchene muscular dystrophy and
Colour blindness,
haemophilia
X-LINKED DOMINANT INHERITANCE
characteristics?
Affects either sex
females are often more mildly and variably affected than males
The child of an affected female has a 50% chance of being affected regardless of sex
All the daughters but none of the sons of affected male will be affected
X-LINKED DOMINANT INHERITANCE
characteristics?
X-LINKED DOMINANT INHERITANCE
Example ?
vitamin D resistant ricket
hypophosphatemic rickets
Klinefelter Syndrome
Incontinentia Pigmenti
Rett syndrome
X-LINKED DOMINANT INHERITANCE
Example ?
vitamin D resistant ricket
Klinefelter Syndrome
Incontinentia Pigmenti
Rett syndrome
hypophosphatemic rickets
Klinefelter Syndrome
Y-LINKED INHERITANCE
xtics? (4)
Would only affect males
Affected males would always have affected fathers
All the sons of affected father would be affected
As females are perfectly normal without Y gene, this code for either non-essential characters or for male specific functions.
Y-LINKED INHERITANCE
examples?
HYPERTRICHOSIS PINNAE (hairy ears)
GENETIC COUNSELLING
Definition:
This is communication of information and
advice about inherited condition
The person seeking the genetic advice (usually the parent or a guardian) is called the ?
CONSULTAND
The affected individual who caused the “person seeking the genetic advice” to seek the advice is called the ?
PROBAND, who is usually the child but the child may also be the CONSULTAND If of age
PURPOSE OF GENETIC COUNSELLING
(5)
- To provide concrete and accurate information about inherited Dxs
- Reassure worried parents about the inherited condition
- Allows affected parties to make informed decision on inherited Dxs
- Educate parents/people about the mode of inheritance
- To offer supports to affected families by health professionals
PLEASE NOTE THAT COUNSELLING SHOULD NOT BE?
(2)
A one-way discussion, After the health professional gives information he should thereafter listen to the views of affected parties
It should not be directional i.e Telling them what to do. The affected parents decide steps to take the professional should guide them to make the best decision.
PROCESS OF GENETIC COUNSELLING
(5)
History taking & pedigree construction
Examination of the Proband
Diagnosis making & Treatment
Counselling
Follow-up
Rules of pedigree construction?
(5)
a) Standardised symbols are used
b) By convention the male lines are placed on the LEFT
c) Female lines are placed on the RIGHT
d) All members of same generation are place on same line
e) Roman numerals are used for each generation
physical examination, The physician should? 6things
Look for dysmorphic features
Describe them accurately
Important anatomical landmarks used in description
Recording of the dermatoglyphics is important
Consider the summing up of the features in a syndrome
Consider consulting computerised database on
Dysmorphology or Encyclopedia on human malformation
for accuracy of tagged syndrome in the proband
useful investigations used in genetic counselling?
Chromosonal analysis: Karyotyping
DNA analysis: Polymorphisms & sequencing studies
Biochemical assays
Histopathology studies
Imaging studies
Rules of counselling?
7 rules
- This should take place under conducive and friendly environment
- Adequate time should be allocated to it
- should involve all aspects of the condition
- The education level of consultands should tailor the discussion, use lay languages if poorly educated
- Recurrence risks/ frequencies should be told
- Guilt & perceived stigma should be dealt with
- Reproductive options should be offered to the consultands
examples of Reproductive options that should be offered to the consultands?
Further pregnancy ± PRENATAL DIAGNOSIS
Artificial insemination by donors
In vitro fertilisation ±PRE IMPLANTATION DIAGNOSIS
Adoption
Contraception
Follow up?
- This is important in order to re-inforce the counselling
- Follow up on the clinical state of patients
- Allows for the physician to give information about new opportunities that could have emerged after the diagnosis
Such as;
New diagnosis tools
Treatment regimen
Preventive strategies etc.
