Genetic Factors and Disease

Question 1

What are the different causes of disease?

Answer 1

genetic, environmental and multifactorial

Question 2

Examples of genetic disease.

Answer 2

Huntington’s disease
haemophilia
cystic fibrosis
downs syndrome

Question 3

Examples of multifactorial disease.

Answer 3

(main cause of disease in developed countries)

• Schizophrenia
• diabetes
• spina bifida

Question 4

Examples of environmental disease

Answer 4

(Main cause of disease in third world countries)

• poor diet
• infection
• drugs
• alcohol

Question 5

autosomal

Answer 5

chromosomes 1-22

Question 6

locus

Answer 6

the position of DNA/a gene on the genetic map

Question 7

genotype

Answer 7

the genetic constitution of an individual

Question 8

phenotype

Answer 8

the physical characteristics of an individual and its interaction with the environment

Question 9

allele

Answer 9

one of several alternative forms of a gene at a specific locus

Question 10

polymorphism

Answer 10

frequent hereditary variations on the locus

Question 11

consanguinity

Answer 11

reproductive union between two relatives

Question 12

autozygosity

Answer 12

homozygous by descent
-alleles or chromosome segments of DNA that are identical as a result of inheritance through consanguinous mating

e.g. inheritance of the same mutant allele through the same family.

Question 13

homozygous

Answer 13

both alleles are the same at the locus

Question 14

heterozygous

Answer 14

alleles at the same locus that are different

Question 15

hemizygous

Answer 15

describes genes that are carried on an UNPAIRED chromosome e.g. a gene found on the locus of the X chromosome of a male.

Question 16

penetrance

complete penetrance?

incomplete penetrance?

Answer 16

the proportion/percentage of people with a gene/genotype who show the expected phenotype.

gene or genes for the trait are expressed in all of the population

The genetic trait is only expressed in parts of the population

Question 17

variable expression

Answer 17

variation in clinical features (type and severity) of a genetic disorder between individuals with the same gene alteration

Question 18

sex limitation

Answer 18

expression of a particular characteristic that is limited to one of the sexes.

Question 19

Autosomal dominant

Why may sometimes both parents seem unaffected, even though they must have at least one of the pathogenic allele?

Answer 19

1) A disease that can manifest in the heterozygous state
- Affects males and females in equal proportions

2) -Gonadal mosaicism
- the mother may have reduced penetrance/ variable expression i.e the disease is there but not clearly expressed

Question 20

Autosomal recessive

What is the chance of healthy siblings being carriers if both parents are carriers.

Give an example of an Autosomal recessive disease.

Answer 20

1) A disease that manifests in the homozygous state where two of the defective genes are required.
- males and females are affected in equal proportions

2)
2/3 chance because you eliminate the chance of the child being homozygous recessive, leaving you with three options and only two of them are carriers (heterozygous)

3)Cystic Fibrosis

Question 21

x-linked/sex-linked inheritance

What are the typical features of X-lnked inheritance?

Who can be carriers of X-linked disease?

Answer 21

Caused by a mutation in the genes of the X-chromosome.

-Usually only males are affected because they only contain 1 X chromosome
-Transmitted usually through unaffected females
-No male-to-male transmission because the X chromosome from males are transmitted to the female offspring only.
Sons only get their X chromosome from their mother.

3)Only females because males are always affected as they only have 1 X chromosome.

Question 22

Give an example of a dominant X-linked disease.

Give an example of a recessive X-linked disease.

Can males get this disease?

Answer 22

1) Alport’s Syndrome (kidneys)
2) Duchenne’s muscular dystrophy

Note: even though Duchenne’s muscular dystrophy is recessive, males only need one of the DMD gene in order to have the disease because it is X-linked.

Question 23

Lyonization

Why does this happen?

What is a barrbody?

Answer 23

1)
The process of an X chromosome inactivation. It causes only one of the female X chromosomes, in every somatic cell, to be active (functions properly). This can be skewed randomly.

2) This is so that females do not have double the amount of genetic information found on the x chromosome, as males do.
3) The name of an inactive X chromosome caused by lyonization. It is packaged by heterochromatin (therefore, cannot be transcripted)

Question 24

What is genomic imprinting?

What type of inheritance is this?

Give examples of genetic conditions that can be caused by genomic imprinting.

Answer 24

It is when 1 out of the 2 alleles of one gene is active. (One maternal/paternal allele is active, in certain genes.)

non-mendelian/mitochondrial/non-traditional inheritance because it doesn’t follow the Mendel’s laws where both alleles are inherited from each parent and are active.

Prada Willi syndrome and Angelman syndrome and, male infertility.

Question 25

What is mendelian inheritance?

Give examples of mendelian inheritance.

Answer 25

Inheritance of alleles (one from each parent). This follows Mendel’s laws.

X-linked, Autosomal recessive, Autosomal dominant

Question 26

What is Knudson’s two hit hypothesis

provide some examples

Answer 26

The hypothesis is that one gene must have two mutations for the phenotypic change to occur. It also states that most genes can either be inherited or acquired later in a person’s life.

• Sporadic cancers require 2 acquired mutations
• Hereditary cancers require an inherited mutation and 1 acquired mutation.

Question 27

What is an ideogram?

Answer 27

Diagram of chromosomes and their bands, representing the different genes. These are individually numbered to show the distance it is from the centromere.

Question 28

What are all the classifications of genetic disease?

Answer 28

1)Chromosomal
2)Mendelian: x-linked, Autosomal recessive/dominant
3)Mitochondrial/non-traditional: imprinting, mosaicism
(This is caused by a mutation that occurs within the mitochondrial DNA that is passed down from the mother so men cannot pass on mitochondrial mutations)