Single gene diseases and genetic screening

Question 1

What is the definition for genetic screening?

Answer 1

Genetic screening is defined as “search in the population for persons possessing certain genotypes that: (1) are already associated with disease or predisposition to disease, or (2) may lead to disease in their descendants”.

Question 2

What is a genetic test?

Answer 2

A genetic test examines a DNA of a person for abnormal genes, or analyses the number, arrangement, and characteristics of the chromosomes.

Question 3

How does a faulty gene trigger disease?

Answer 3

When a gene contains a mutation, the protein encoded by that gene will be abnormal. Some protein changes are insignificant, others are disabling.

Question 4

What is a pedigree analysis?

Answer 4

In a pedigree analysis, information about the presence/absence of a particular phenotypic trait is collected from as many individuals in a family as possible and across generations.
From a pedigree, the pattern of inheritance for a genetic trait can be determined.
This information can allow the geneticist to estimate the probability that a child might inherit a genetic disorder.
The distribution of these characters is then mapped on the family tree

Question 5

Give some examples of an autosomal recessive disorder?

Answer 5

Includes many nasty and fatal childhood diseases: sickle cell anaemia, cystic fibrosis, Tay Sachs

Question 6

Is Tay-Sachs recessive or dominant?

Answer 6

Lethal recessive disorder

Question 7

What is Taq-Sachs caused by?

Answer 7

Disfunctional enzyme - it fails to break down specific lipids in the brain

Question 8

What are the symptoms of Tay-Sachs?

Answer 8

The symptoms begin with seizures, blindness, and degeneration of motor and mental performance a few months after birth

Question 9

What population is Tay-Sachs, most common?

Answer 9

Among Ashkenazic Jews this disease occurs in one of 3,600 births, about 100 times greater than the incidence among non-Jews or Sephardic Jews

Question 10

What is the incidence of Cystic Fibrosis?

Answer 10

Cystic fibrosis strikes one of every 2,500 whites of European descent.

Question 11

How many people are carriers of Cystic Fibrosis?

Answer 11

1 in 25 whites

Question 12

What causes Cystic Fibrosis?

Answer 12

The normal allele codes for a membrane protein that transports Cl- between cells and the environment.
If these channels are defective or absent, there are abnormally high extracellular levels of chloride that causes the mucus coats of certain cells to become thicker and stickier than normal.
This mucous build-up in the pancreas, lungs, digestive tract, and elsewhere favours bacterial infections.

Question 13

What is the outcome of Cystic Fibrosis and what are the treatments?

Answer 13

Without treatment, affected children die before five, but with treatment can live past their late 20’s
Chest pounding, antibiotics used

Question 14

What is the incidence of Sickle cell?

Answer 14

1 in 400

Question 15

Among what population is Sickle cell most prominent?

Answer 15

African Americans

Question 16

What causes Sickle Cell?

Answer 16

It is caused by the substitution of a single amino acid in hemoglobin.
When oxygen levels in the blood of an affected individual are low, sickle-cell hemoglobin crystallizes into long rods.
This deforms red blood cells into a sickle shape.
This sickling creates a cascade of symptoms

Question 17

Sickle cell is an example of what?

Answer 17

Plieotrophy

Question 18

Why is the frequency of people with Sickle cell high among those heterozygous carriers with 1 allele?

Answer 18

Individuals with one sickle-cell allele have increased resistance to malaria, a parasite that spends part of its life cycle in red blood cells.
Heterozygous amounts of HbS causes cell fragility
In tropical Africa, where malaria is common, the sickle-cell allele is both a good and bad thing!
Homozygous normal individuals can die of malaria, homozygous recessive individuals die of sickle-cell disease, and carriers are relatively free of both

Question 19

What is Huntington’s Disease?

Answer 19

Degenerative disease of the nervous system

Question 20

Is Huntington’s disease dominant or recessive?

Answer 20

Dominant but doesn’t affect a person phenotypically until around 30-45

Question 21

Where is the Huntington’s Disease allele found?

Answer 21

Tip of chromosome 4

Question 22

A normal gene for Huntington’s has repeats of CAG around 11-34 time, but how many times does an affected Huntington’s disease gene have?

Answer 22

CAG - 42-120

Question 23

How is the severity of Huntington’s increased?

Answer 23

Increases in severity based on the number of triplicate repeats

Question 24

Name two x-linked recessive disorders

Answer 24

Colour-blindness

Muscular dystrophy

Question 25

Name a sex influenced trait

Answer 25

Pattern Baldness