Lecture 7: Human Pedigrees

Question 1

How many diseases following Mendelian inheritance patterns are there?

Answer 1

≈ 7300, numerous but rare.

Question 2

What are some examples of Mendelian genetic diseases?

Answer 2

Sickle cell anaemia

Cystic fibrosis

Achondroplasia

Marfan syndrome

Haemophilia

Huntington’s disease

Albinism

Question 3

What is a human pedigree chart?

Answer 3

A chart that diagrams the inheritance of a trait or health condition through generations of a family.

They help to determine how the disease is inherited and the probability of its prevalence in offspring.

Question 4

Describe the characteristics of an autosomal recessive disorder.

Answer 4

• Appears in the progeny of unaffected parents.

• Affected progeny includes both males and females.

Question 5

What are some examples of autosomal recessive disorders?

Answer 5

Albinism

Cystic fibrosis

Sickle cell anaemia

Question 6

State the characteristics of pedigrees of autosomal dominant disorders.

Answer 6

• Show affected males and females in each generation (every affected individual must have one affected parent).

• Show affected men and women transmitting the condition to equal proportions of their sons and daughters.

Question 7

What are some examples of Mendelian autosomal dominant disorders?

Answer 7

Pseudoachondroplasia

Huntington’s disease

Marfan syndrome

Question 8

What are some diseases caused by X-linked disorders?

Answer 8

Colour blindness

Haemophilia A

Duchenne muscular dystrophy

Question 9

What are the characteristics of an X-linked recessive disorder?

Answer 9

• Strong skew towards males being affected.

• Also compatible with an autosomal recessive pattern of inheritance.

Question 10

Define the characteristics of X-linked dominant disorder.

Answer 10

• Only females are affected in generation II.

• Chart is also compatible with autosomal dominant inheritance.

Question 11

State why are there differences in dominant & recessive X-linked and autosomal disorders? What are they?

Answer 11

Due to males having only one X chromosome, the inheritance patterns are different between X-linked and autosomal disorders.

• In X-linked recessive disorders males are much more likely to be affected as they only need to inherit one faulty copy, while females can be carriers.

• In X-linked dominant disorders, affected males will only transmit the conditions to daughters, but affected females will transmit the condition equally to their sons and daughters.

Question 12

What is the probability of a non-affected sibling in a dominant disorder being a carrier?

Answer 12

0, if they are non-affected and is a dominant disorder it cannot carry the mutant allele.

Question 13

What is the probability of a non-affected sibling in an autosomal recessive disorder being a carrier?

Answer 13

2/3, as they do not have the disorder they must be either AA or Aa.

Question 14

What is the probability of a non-affected female with an affected brother being a carrier in an X-linked recessive disorder? (Assume her brother is the only affected individual of this pedigree).

Answer 14

1/2, mother is an obligate carrier. Daughter has 50% chance of inheriting the mutant X.

Question 15

What is the probability of inheriting the mutant gene from an obligate carrier in an X-linked recessive disorder?

Answer 15

1/2, carrier must be female as X-linked. Chance of passing each chromosome is 50/50.

Question 16

What is the probability of inheriting the mutant gene from an unaffected parent in a dominant disorder?

Answer 16

0, by definition, a non-affected parent cannot carry the mutant allele in a dominant disorder.