Lecture 17: Modes Of Inheritance

Question 1

What is meant by homozygous?

Answer 1

Someone with 2 identical copies of an allele

Question 2

What is meant by heterozygous?

Answer 2

Someone with 2 different alleles for a particular gene is heterozygous

Question 3

What is a single position on a gene/chromosome called?

Answer 3

A locus

Question 4

How many chromosomes do humans have?

Answer 4

46 chromosomes (23 pairs)

• 22 autosomal pairs
• sex chromosomes XX and XY

Question 5

What is meant by dominant autosomal?

Answer 5

A characteristic is dominant if it manifests in a heterozygote (i.e. 2 diff alleles at a locus)

Question 6

Describe the characteristics of dominant autosomal disorders.

Answer 6

• single gene/allele disease or trait
• disease passed down to offspring with multiple generations affected
• each affected person normally has one affected parent
• each child of an affected person has a 1 in 2 chance of being affected
• males and females equally affected and equally likely to pass on condition
• vertical pedigree pattern

Question 7

What is meant by a vertical pedigree pattern?

Answer 7

Disease transferred from parent to child

Multiple generations affected

Question 8

Name some examples of a dominant autosomal disorder?

Answer 8

Huntington’s disease

Osteogenesis imperfecta - Brittle Bone Disease

Question 9

What is the mutation in Huntington’s disease?

Answer 9

DNA expansion of a CAG repeat on chromosome 4

Question 10

What is the result of the Huntington’s disease mutation?

Answer 10

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

Question 11

Dominant autosomal disorders tend to be either…?

Answer 11

• Gain of function: gene now makes a protein with a new function e.g. longer lifespan/new location thus increasing their effect
• Dominant negative effect: mutated form interferes w/activity of proteins it binds to
• Insufficient: results in 1/2 amount of protein that’s not enough for normal function

Question 12

What is meant by autosomal recessive disorders?

Answer 12

2 copies of abnormal gene must be present in order for the disease or trait to develop

Question 13

What do autosomal recessive disorders tend to be?

Answer 13

loss of function mutations

Question 14

What is the chance of each subsequent sibling of a child affected by an autosomal recessive disorder also being affected?

Answer 14

1 in 4

Question 15

What pedigree pattern is observed in autosomal recessive disorders?

Answer 15

horizontal pedigree pattern

Question 16

What is an example of an autosomal recessive disorder?

Answer 16

Cystic fibrosis

Question 17

What is the mutation that causes cystic fibrosis?

Answer 17

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

Question 18

What is the result of mutations in cystic fibrosis?

Answer 18

• defective chloride ion channel
• loss of function, work less well, degraded faster or present in inadequate amounts
• impair airway defense, digestive issues

Question 19

What are the sex chromosomes?

Answer 19

X and Y
XX = female
XY = male

Question 20

What determines the sex of the progeny?

Answer 20

Sperm

Question 21

What sex is more affected by an alteration on a gene on the X chromosome and why?

Answer 21

Males since they only have one copy of X chromosome

Question 22

Who do X-linked recessive disorders mainly affect?

Answer 22

Males

Affected boys may have affected uncles

Question 23

What is the risk of a brother of a son affected by X-linked recessive disorder also having the disorder?

Answer 23

1 in 2 risk

Question 24

What are some examples of X-linked recessive disorders?

Answer 24

Haemophilia

Fabry’s disease

Question 25

What are X-linked dominant disorders similar to?

Answer 25

Autosomal dominant pattern

Question 26

Who is affected by X-linked dominant disorders if a father is affected?

Answer 26

All daughters affected (because they inherit one mutated X chromosome from dad)
No sons affected (because they only inherit Y not X)

Question 27

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

Answer 27

X-linked hypophosphatemia

Question 28

Who is affected by Y-linked disorders?

Answer 28

Only males

All sons of an affected father

Question 29

What pedigree pattern do Y-linked disorders express?

Answer 29

Vertical pedigree pattern (father to son)

Question 30

What is an example of a Y-linked disorder?

Answer 30

Retinitis Pigmentosa (production of defective protein)

Question 31

How are mitochondrial disorders inherited?

Answer 31

Maternally inherited

Question 32

What happens in mitochondrial disorders?

Answer 32

Mutations in mitochondrial DNA

Question 33

What pedigree pattern is usually seen in mitochondrial disorders?

Answer 33

Vertical pedigree pattern

Question 34

Why can mitochondrial conditions be variable within a family?

Answer 34

• many mitochondria in each cell
• all mitochondria inherited from mother
• random segregation
• severity of symptoms vary with: amount of WT to mutated mtDNA and severity of mutation

Question 35

What are some examples of mitochondrial disease?

Answer 35

Leber’s hereditary optic neuropathy - disrupt function of mitochondria leading to visual loss in young adulthood
(Commonly affect motor and nerve function and present as unrelated multi-system symptoms)

Question 36

In mitochondrial disorders children of affected are…?

Answer 36

Never affected

Question 37

In mitochondrial disorders how many children of an affected woman may be affected?

Answer 37

All children may be affected