Typical Mendelian Inheritance

Question 1

What are mandelian disorders?

Answer 1

Single gene disorders

Question 2

What can cause mandelian disorders?

Answer 2

• mutations of one or both members of a pair of autosomal genes
• mutations in genes on the X or Y chromosomes

Question 3

Define homozygous

Answer 3

both alleles are identical

Question 4

Define heterozygous

Answer 4

Alleles are different

Question 5

Define trait

Answer 5

Gene-determined characteristic

Question 6

What is the pattern if a trait is expressed in a heterozygote?

Answer 6

Dominant

Question 7

What is the pattern if a trait is expressed in a homozygote?

Answer 7

recessive

Question 8

What are the typical features of autosomal dominant inheritance?

Answer 8

• both male and females affected ~ equally
• people affected in each generation
• both males and females can pass on the trait
• unaffected individuals do not pass on the trait

Question 9

What are the chances for a child of a heterozygote parent will be affected?

Answer 9

1 in 2 (50%)

Question 10

Define non-penetrance

Answer 10

An individual may have the mutant gene and yet a normal phenotype. They are able to pass the condition on to descendants producing a skipped generation

Question 11

What can affect penetrance?

Answer 11

May be age dependent (e.g. Huntington disease)

Question 12

What are the chances for the offspring of two heterozygotes (carriers) for a mutant gene? (AR)

Answer 12

• 1/4 homozygous normal
• 1/2 heterozygous
• 1/4 homozygous normal

Question 13

If an affected person has a child with a heterzygote, what are the chances the offspring are affected?

Answer 13

1 in 2

Question 14

Describe the inheritance pattern of autosomal recessive

Answer 14

• usually horizontal
• both males and females affected
• may be consanguinity in the family

Question 15

What are examples of AR conditions?

Answer 15

• cystic fibrosis
• phenylketonuria (PKU)
• spinal muscular atrophy (SMA)
• congenital adrenal hyperplasia

Question 16

Define compound heterozygote

Answer 16

an individual with two different mutant alleles at a locus

Question 17

What are the typical features of X-linked recessive inheritance?

Answer 17

• Males > females
• females typically carriers
• ‘Knight’s move’ pattern

Question 18

What are the expected proportions of inheritance in X-linked recessive diseases

Answer 18

• 25% affected boy
• 25 % normal boy
• 25% carrier girl
• 25% normal girl

Question 19

Who can be defined as an obligate carrier?

Answer 19

A woman with an affected child and an affected brother OR more than one affected child

Question 20

How can females be affected by X-linked recessive diseases

Answer 20

• Skewed X-inactivation

- Turners disease

Question 21

What are the features of X-linked dominant inheritance?

Answer 21

Both males and females affected

N.b - males more uniformly sever, females more variable due to X inactivation

Question 22

What is the pedigree pattern of X-linked autosomal dominant inheritance?

Answer 22

resembles autosomal dominant but with a lack of male to male transmission

Question 23

What are examples if X-linked autosomal dominant conditions?

Answer 23

• Vitamin D resistant rickets
• Incontentia pigmenti (male lethality)
• Rett syndrome (usually has male lethality)