General concepts and single gene disorders

Question 1

What is a phenotype?

Answer 1

The physical description of a character of an individual organism

Question 2

What is a character?

Answer 2

A structure, function or attribute determined by a gene or group of genes.

Question 3

What are traits?

Answer 3

Alternate forms of character

Question 4

What is a gene?

Answer 4

Section of DNA that codes for a specific protein, located on a chromosome

Question 5

What is a genotype?

Answer 5

The genes an individual has at a particular locus, responsible for the observed phenotype.

Question 6

What is penetrance?

Answer 6

The chance a given genotype will cause a particular phenotype.

Question 7

What is an allele?

Answer 7

Different types of the same gene, leading to the same character but a different trait.

Question 8

Are genetically determined disease phenotypes common in the population?

Answer 8

No, offspring carrying genes are less likely to reach reproductive age and pass on genes

Question 9

Are (poly)genetically and envrionmentally determined disease phenotypes common in the population?

Answer 9

Yes, symptoms are often less deadly therefore offspring carrying genes contributing to a diseased phenotype are more likely to pass the genes down to the next generation.

Question 10

Are completely environmental disease phenotypes common in the population?

Answer 10

No, very rare

Question 11

Describe autosomal dominant inheritance of disease

Answer 11

Two copies of the relevant gene but only one needs to be mutated to express the disease.

Question 12

Are heterozygotes affected by autosomal dominant diseases?

Answer 12

Yes, depending on the penetrance of the mutation.

Question 13

Describe autosomal recessive inheritance of disease

Answer 13

Two copies of the relevant gene but both copies must be mutated to get the disease. Heterozygotes are therefore described as unaffected carriers.

Question 14

What are the chances of two carriers (of autosomal recessive condition) having an affected child, carrier or normal child?

Answer 14

25% - affected
50% - carrier
25% - normal child

Question 15

Of the children had from two carrier parents (AR), of they are not affected, what is the chance they are a carrier?

Answer 15

2/3

Question 16

Describe X-linked recessive inheritance of disease

Answer 16

Diseased gene on X chromosome, females can be carriers, can make daughters carriers (50%) but can make sons have disease (50%)

Question 17

Why is a female carrying a diseased X chromosome (recessive) not normally affected?

Answer 17

The other acts as a backup, X inactivation is random and normally occurs to the diseased chromosome

Question 18

Describe X-linked dominant inheritance of disease

Answer 18

Mother will be affected, one mutant X and one normal.

If passed onto daughter, will be affected but due to X inactivation will survive. (50%)

If passed onto a son, then the boy only has a mutant X chromosome and a Y chromosome from father. He is unlikely to survive. (50%).

Question 19

Why can only females usually live with X-linked dominant conditions?

Answer 19

X-inactivation

Question 20

Describe mitochondrial inheritance of disease?

Answer 20

Only maternally inherited.

The mutation load varies per egg cell and is hard to predict (cells have many mitochondria).

Offspring can therefore either be carriers or affected.

Question 21

What is uniparental disomy?

Answer 21

Inheritance of two chromosomes from one parent and none from the other parent.

Question 22

Why is it bad to inherit two autosomes from one parent and none from the other?

Answer 22

Some genes are imprinted meaning that only one copy from one specific parent is used, therefore if that copy is not inherited due to uniparental disomy, we get a condition.

Question 23

What is a mutation?

Answer 23

Heritable alteration or change in the genetic material. These are usually pathogenic but also drive evolution

Question 24

What is a polymorphism?

Answer 24

A non-pathogenic alteration in DNA which may alter protein function but is not usually deleterious

Question 25

Why does incestual breeding result in more harmful mutations being expressed?

Answer 25

Closely related more likely to share same recessive mutations.

Question 26

What type of inheritance can lead to skipping of generations?

Answer 26

AR

Question 27

What are the chances of an affected AD to pass onto child?

Answer 27

1/2

Question 28

True or false, all individuals inheriting a mutant gene (AD) will be fully affected?

Answer 28

False

Question 29

Does mitochondrial DNA code for all the proteins in the respiratory chain?

Answer 29

No

Question 30

What percentage of alleles are shared by 2nd degree relatives?

Answer 30

25%