Medical genetics Wk 2

Question 1

what is gene interaction

Answer 1

a situation in which a single phenotype is affected by more than one set of genes

it can occur when two or more alleles of the same phenotype influence the outcome of particular phenotypic characters

Question 2

what can the interaction within alleles of a gene controlling a single character be

Answer 2

dominant
incomplete dominance and
codominance
are all called intra allele interaction

Question 3

describe epistatic and hypostatic genes

Answer 3

genes have a masking effect called the epistatic gene and the gene whose effect is masked is the hypostatic gene

Question 4

what is the bombay phenotype

Answer 4

in 1952 a very unusual situation provided information concerning the genetic basis of the H substance

this woman was subsequently shown to be homozygous for a rare recessive mutation in a gene designated FUT1 located on a chromosome 19q

it is a recessive condition at one locus masking the expression of the second locus

Question 5

when can the ABO system phenotype not be expressed

Answer 5

in individuals homozygous for the mutant form of the FUT1 gene even though they have the IA and/ or the IB alleles neither antigen is added to the cell surface and they are functionally type O

Question 6

what is a genetic mutation and a mutation

Answer 6

an alteration in the sequence of nucleotides in DNA

is any sort of genetic alteration that causes a change in the phenotype or expression of that gene

Question 7

what is epistasis

Answer 7

where the expression of one gene masks or modifies the effect of the second gene

the genes involved in the influence the same general phenotypic characteristic in an antagonistic manner which leads to masking

Question 8

give an example of epistasis

Answer 8

the homozygous presence of a recessive allele may prevent or override the expression of other alleles at a second locus- first locus is said to be epistatic and the second locus are said to be hypostatic to those of the second locus

Question 9

what is pleiotropy

Answer 9

where one gene ends up controlling multiple phenotypic traits in the organism

Question 10

what is marfan syndrome

Answer 10

is an autosomal dominant mutation in the gene encoding the connective tissue protein fibrillin

Question 11

how is marfan syndrome caused

Answer 11

caused by a mutation in a gene FBN1 found on chromosome 15 that determines the structure of fibrillin

Question 12

what is the name of the gene for marfan syndrome

Answer 12

FBN1

Question 13

what are the clinical manifestations of marfan syndrome

Answer 13

unusually long limbs, great stature or long toes in proportion to the persons height

Predisposition to cardiovascular disease: heart and blood vessels

shortness of breath, fatigue and palpitations

the walls of the aorta maybe weakened or stretched by aortic dilation and this will cause the aorta to tear

Question 14

when can marfan syndrome be detected

Answer 14

during childhood but not through a simple blood test

Question 15

what is the treatment for marfans syndrome

Answer 15

there is no cure but there are some treatments to improve the quality of life

however, new research and medical treatments, help those who are diagnosed early and get good healthcare and they have the same life span as everyone else

Question 16

what is sickle cell anaemia ( pleitropy)

Answer 16

is group of inherited red blood cell disorder

Question 17

why anaemia

Answer 17

normal rbcs last about 120 days in the bloodstream and then die
their main role is to carry oxygen but also remove carbon dioxide

in sickle cell anaemia - sickle cells die faster than normal rbcs usually 10 to 20 days

the bone marrow cannot make new rbcs fast enough to replace the dying ones

Question 18

inheritance of sickle cell anaemia

Answer 18

mutations on the HBB gene cause sickle cell disease

HBB gene encodes for the production of Hb

located on chromosome 11

Question 19

what are the symptoms of sickle cell anaemia

Answer 19

• swollen hands and feet
  -visual problems
  -fever
  -delayed growth
• pale skin

Question 20

what is phenylketonuria (pku)

Answer 20

is inherited from parents due to mutated PAH (phenylalanine hydroxylase) gene on chromosome 12

the body cannot change phenylalanine to tyrosine

Question 21

what are the symptoms of pku

Answer 21

-light hair, eyes and skin
eczema-like rash
seizures
hyperactivity
mental retardation
phenylketones in urine

Question 22

what is the treatment for pku

Answer 22

people must limit their intake of protein in their diet for their whole lives
special diet starting few days after birth
babies must drink a special formula without phenylalanine

• main diagnostic is newborn screening

Question 23

what is penetrance

Answer 23

is the percentage of individuals that show at least some degree of expression of mutant genotype

when the frequency of the phenotype is less than 100% - that is, when some of those who have the appropriate genotype completely fail to express it - the gene is said to show reduced penetrance

Question 24

what is expression

Answer 24

is the severity of expression of the phenotype among individuals with the same disease causing genotype

when the severity of disease differs in people who have the same genotype, the phenotype is said to have variable expressivity

Question 25

what is neurofibromatosis type 1

Answer 25

NF1 is an example of variable expressivity
it was first fully described by the physician von reckinghausen in 1882

the cytidine in the arginine codon (CGA) is deaminated to a uracil creating an inframe translational stop codon

Neurofibromin protein Tumor suppressors normally prevent cells from growing and dividing too rapidly or in an uncontrolled way

Question 26

what are the common symptoms

Answer 26

• brownish red spots in the coloured part of the eye called Lisch nodules
  -benign skin tumours called neurofibromas and larger benign tumours of nerves called plexiform neurofibromas
• scoliosis (curvature of the spine)
  -learning disabilities
  -vision disorders
  -mental disabilities
  -multiple cafe au lait spots
  -epilepsy

more likely to have a higher rate of cancer and cardiovascular disease than the population

Question 27

what is split hand deformity

Answer 27

autosomal dominant malformation with reduced penetrance is split hand deformity

is a rare genetic disorder where fingers or parts of fingers are missing

it gives a claw like appearance

orginates from the sixth or seventh week of development

Question 28

typical cases of split hand deformity

Answer 28

the lobster claw variety- absence of the third digit

monodactyly- presence of only the fifth digit and no cleft

Question 29

explain more on the split hand deformity

Answer 29

split hand defromity is inherited as an autosomal dominant trait and sometimes as an x linked recessive trait but it may also occur as a result of a random mutation

located on 7,10,3,2 and X

Question 30

what are imprinted genes

Answer 30

we only inherit one working copy of genes.

depending on the gene, either the copy from mom or the copy from dad is epigenetically silenced. Silencing usually happens through the addition of methyl groups during egg or sperm formation.

Question 31

what is dna methylation

Answer 31

attachment of the methyl groups to the bases of DNA
occurs in cytosine that follows gunaine at CpG dinucleotides

Question 32

what is Prader-Willi

Answer 32

are two very different disorders, but they are both linked to the same imprinted region of chromosome 15.

Question 33

what are the symptoms

Answer 33

learning difficulties, short stature, and compulsive eating.
* Individuals are missing gene activity that normally comes from dad.
* Happens when dad’s copy is missing, or when there are two maternal copies. Beginning in childhood the person becomes constantly hungry which often leads to obesity and type 2 diabetes.

Question 34

where is the gene located

Answer 34

The OCA2 gene (formerly called the P gene) is located on the segment of chromosome 15 that is often deleted in people with this disorder; provides instructions for making a protein called the P protein. This protein is located in melanocytes.

Question 35

what are the symptoms of angelmann syndrome

Answer 35

learning difficulties, speech problems, seizures, jerky movements, and an unusually happy disposition.
* Individuals are missing gene activity that normally comes from mom.
* Happens when mom’s copy is defective or missing, or when there are two paternal copies.

Question 36

where is the gene

Answer 36

Many of the characteristic features of Angelman
syndrome result from the loss of function of a gene
called UBE3A.
If the maternal copy of the UBE3A gene is lost because of a chromosomal change or a gene mutation,
a person will have no active copies of the gene in some parts of the brain.

Question 37

why is marfan syndrome an example of pleiotroy

Answer 37

fibrillin is an important part of connective tissue which affects multiple parts of the body such as joints, bones and eyes

Question 38

what is autosomal recessive

Answer 38

Autosomal recessive traits pass from both parents onto their child.

autosomal refers to the 22 chromosome

Question 39

what does tyrosine do

Answer 39

tyrosine is involved in the production / regulation of melanin

lack of melanin causes pku

Question 40

What is sickle cell trait

Answer 40

When only one recessive allele for sickle cell is present

Question 41

what gene is mutated in NF1

Answer 41

NF1 and its on chromosome 17