Modes of inheritance

Question 1

How many pairs of chromosomes are in the human body?

Answer 1

23 pairs

Question 2

What does mitochondrial DNA encode?

Answer 2

15 proteins
rRNA
tRNA

Question 3

Where is mitochondrial DNA inherited from?

Answer 3

All mitochondrial DNA is inherited from mother

Question 4

What are complex genetic disorders?

Answer 4

Acquired both genetically and environmentally

Question 5

Why are brown eyes dominant?

Answer 5

Gene which regulates eye colour is OCA-2
It controls the amount of melanin in melanocytes
Active OCA-2 allows melanins to build up in melanocyte so you get brown eyes
Gene encoding blue eyes is inactive OCA-2 so no melanin produced
HERC2 controls the activity of OCA-2. You ned to have active HERC2 as well as active OCA-2 to have brown eyes
Active OCA-2 and inactive HERC2 = blue eyes

Question 6

What characteristics do autosomal dominant disorders have?

Answer 6

Gain in function- gene makes a protein with a new function
Dominant negative effect- mutated form interferes with activity of protein it binds
Insufficient- mutant gene results in 1/2 amount of a protein- not enough for normal function

Question 7

What is a carrier?

Answer 7

Carriers of recessive diseases have at least 1 copy of a gene but the normal one is sufficient to maintain normal function

Question 8

What characteristics do autosomal recessive disorders have?

Answer 8

Loss of function

Question 9

What causes cystic fibrosis?

Answer 9

Defective chloride ion channel
Caused by a mutation in gene encoding chloride ion channel- CFTR (cystic fibrosis transmembrane conductance regulator) gene on chromosome 7
Loss of function

Question 10

What are symptoms of cystic fibrosis?

Answer 10

Impaired airway defence
Prone to respiratory infection
Digestive issues e.g. Meconium ileus (bowel obstruction due to thick, sticky intestine)

Question 11

Who does x-linked recessive disorders affect?

Answer 11

Affects mainly males

Females can be carriers or affected (homozygous)

Question 12

What is an example of an x-linked recessive disorder?

Answer 12

Haemophilia- affects mostly males

In some cases, female carriers exhibit subtle signs of the disease e.g. Fabry’s disease

Question 13

How does an x-linked dominant pattern appear on a pedigree?

Answer 13

Similar to autosomal dominant

All daughters and no son’s of affected father are affected

Question 14

What is an example of an x-linked dominant disease?

Answer 14

X- linked Hypophosphatemia
Caused by PHEX gene mutation
Leads to overproduction of FGF21- FGF21 inhibits kidney phosphate resorption so kidneys can’t retain phosphate. Results in vit. D- resistant rickets

Question 15

How does a Y linked disorder present on a pedigree?

Answer 15

Vertical transmission
All sons of affected father affected
Affects only males

Question 16

What are examples of Y linked disorders?

Answer 16

Retinitis pigmentosa
Y- linked mutation in RPY gene
Cells of retina produce a defective protein

Question 17

What is special about mitochondria?

Answer 17

Have their own DNA

Shared an evolutionary past with bacteria- endosymbiosis

Question 18

What are mitochondrial proteins encoded by?

Answer 18

Majority of mitochondrial proteins encoded by nuclear genes

Question 19

How does mitochondria replicate?

Answer 19

By binary fission
Can lose or gain mutated genes
Mitochondria undergo random segregation

Question 20

Why are mitochondrial diseases variable?

Answer 20

Mitochondria have multiple copies of their genome- some are normal, some mutant (this is known as heteroplasmy)
Disease symptoms are only expressed above a threshold

Question 21

What determines the severity of symptoms in a mitochondrial disease?

Answer 21

The number of affected mitochondria
The number of mutant mitochondria can change with time
Symptoms develop with age due to an accumulation of mutant mitochondria

Question 22

What is an example of a mitochondria disease?

Answer 22

Leber’s hereditary optic neuropathy (LHON)
Visual loss in young adulthood, degeneration of optic nerve and retina
Typically in males, occasionally females