Genetics: modes of inheritance

Question 1

What is human genetic material encoded on?

Answer 1

chromosomes

Question 2

How many chromosomes do humans have?

Answer 2

46
23 pairs
22 autosomal pairs + sex chromosomes (XX or XY)

Question 3

What other DNA do we inherit other than chromosomal?

Answer 3

mitochondrial DNA
16500 bp, encodes 15 proteins, rRNA and some tRNAs

most mitochondrial proteins are encoded by cell genome

Question 4

Where is the mitochondrial DNA derived from?

Answer 4

the mother

Question 5

Homozygous for a gene?

Answer 5

contains 2 identical copies of an allele

Question 6

Heterozygous for a gene?

Answer 6

contains 2 different alleles

Question 7

What is a locus?

Answer 7

the position of the gene on a chromosome?

Question 8

What are single gene disorders?

Answer 8

affect all body systems
(most common)
cause a large health burden

Question 9

What are chromosomal disorders?

Answer 9

occur when the chromosome are damaged during the formation of the sperm or egg or during early development of the embryo

Question 10

What are complex disorders?

Answer 10

result from a combination of genetic and environmental factors interacting

eg. obesity, type II diabetes

Question 11

What is autosomal dominant?

Answer 11

A characteristic is dominant if it manifests in a heterozygote (ie two different alleles at a locus)

Question 12

What are we referring to when talking about dominance?

Answer 12

phenotype

Question 13

What gene is responsible for eye colour?

Answer 13

OCA-2
active OCA-2 –> melanocytes full of melanin (brown)

inactive OCA-2 –> melanocytes no melanin (blue)

Question 14

Dominant autosomal disease?

Answer 14

single gene/allele

Question 15

How is an autosomal dominant disease passed down?

Answer 15

Each child of an affected person has a 1 in 2 chance of being affected

Males and females are equally affected and equally likely to pass on the condition

Question 16

What pedigree diagram represents the autosomal dominant disease?

Answer 16

Vertical pedigree pattern

Question 17

What is huntingtons disease?

Answer 17

Symptoms usually start 30-50 years of age

Difficulty concentrating, depression, stumbling, involuntary jerking, problems swallowing

Abnormal intracellular Huntington protein aggregate gains a pathological function and is toxic to neurons resulting in cell death

Question 18

What is brittle bone disease?

Osteogenesis imperfecta

Answer 18

Bones break easily

Weakening connective tissue particularly bone

Question 19

What is the “gain of function” dominant autosomal disorder?

Answer 19

gene now makes a protein with a new function

eg longer lifespan/new location thus increasing their effect

Question 20

What is the “dominant negative effect” dominant autosomal disorder?

Answer 20

the mutated form interferes with the activity of proteins it binds

eg dimers or multimers which reduces activity

Question 21

What is the “Insufficient” dominant autosomal disorder?

Answer 21

mutant in one gene results in ½ the amount of a protein that is not enough for normal function (rare)

Question 22

What are autosomal recessive disorders?

Answer 22

two copies of the abnormal (non-working) gene must be present in order for the disease or trait to develop

Tend to be ‘loss of function’ mutations (eg deletions)

Question 23

Who are “carriers” of autosomal recessive diseases?

Answer 23

They have lost a single copy of a gene but the normal one is sufficient to maintain normal function

Question 24

What pedigree chart is used for an autosomal recessive disease?

Answer 24

A horizontal pedigree pattern

Question 25

What pedigree chart is used for an autosomal recessive disease?

Answer 25

A horizontal pedigree pattern

Question 26

What is Cystic Fibrosis?

Answer 26

Defective chloride ion channel
Results in impaired airway defense
Prone to respiratory infections
Digestive issues eg meconium ileus

Question 27

What mutations give rise to cystic fibrosis?

Answer 27

Various mutation in gene encoding chloride ion channel (CFTR gene on chromosome 7)

Question 28

What are the sex chromosomes?

Answer 28

Consist of an X and a Y chromosome

Determine the sex

Question 29

How many genes do the X and Y chromosomes contain?

Answer 29

• X-chromomsome 1000- 1300 genes (~850 protein coding)

- Y-chromosome 150 genes (50-70 protein coding)

Question 30

What are X-linked recessive disorders?

Answer 30

• Affects mainly males – effectively dominant
• Females can be carriers
• Females who are homozygous for the mutation (two copies) have the disorder

Question 31

What are the chances of being affected by having an unaffected father and carrier mother?

Answer 31

boys- 1/2

girls- 1/2 of being a carrier

Question 32

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

Answer 32

Haemophilia

- In some cases female carriers exhibit subtle signs of the disease

Question 33

What are X-linked dominant disorders?

Answer 33

• Similar to autosomal dominant pattern
• all daughters and no sons of an affected father are affected
  (Condition often milder and more variable in females than in males)

Question 34

What is an example of an X-linked dominant disorder?

Answer 34

hypophosphatemia

Results in vitamin D-resistance rickets

Question 35

What are Y-linked disorders?

Answer 35

• Affects only males
• All sons of an affected father

very few- bc only a few genes are encoded on Y chromosome

Question 36

What is an example of a Y-linked disorder?

Answer 36

Retinitis Pigmentosa

Cells of retina produce a defective protein

Question 37

What pedigree chart is used for a Y-linked disease?

Answer 37

vertical pedigree

Question 38

What are mitochondrial inherited disorders?

Answer 38

caused by mutations in Mitochondrial DNA

Question 39

How are mitochondrial disorders inherited?

Answer 39

Are maternally inherited

• All children of an affected woman may be affected
• Children of affected men are never affected

Question 40

Mitochondrial disease variability

Answer 40

• Mitochondria have multiple copies genome some normal some mutant (heteroplasmy)
• Only express disease effects above a threashold

Question 41

How else are mitochondrial diseases caused?

Answer 41

by mutations in cell genome and have mendalian or sex chromosome linked inheritance

Question 42

How does a mitochondrial disease develop?

Answer 42

• Random segregation
• Number of affected mitochondria
• Symptoms develop once threshold reached
• Cells different number mutant mitochondria
  can change with time

Question 43

When do mitochondrial diseases develop?

Answer 43

with age due to accumulation of mutant mitochondria

Question 44

What is an example of mitochondrial disease?

Answer 44

Motor and nerve function commonly affected

Leber’s hereditary optic neuropathy (Visual loss in young adulthood)