Mendelian Inheritance in Humans

Question 1

What is the difference between a Mendelian/monogenic and multifactorial disease?

Answer 1

Mendelian - one gene involved

Multifactorial - many genes involved

Question 2

Why are pedigrees used?

Answer 2

To infer mode of inheritance

Genetic counselling

Question 3

What are the five basic Mendelian patterns?

Answer 3

Autosomal dominant

Autosomal recessive

X-linked recessive

X-linked dominant

Y-linked

Question 4

What are the characteristics of an autosomal dominant disease?

Answer 4

Affected person usually has at least one affected parent

Affects either sex

Transmitted by either sex

Child of an unaffected x affected mating has 50% chance of the disease (assuming affected parents are heterozygous)

Question 5

What are some examples of austomal dominant phenotypes?

Answer 5

Achondroplasia - form of dwarfism

Polydactyl

Hairy mid-digit

Widow’s peak

Yellow dominant to green in Mendel’s peas

Purple flowers are dominant to white in Mendel’s peas

Smooth seeds are dominant to wrinkled in Mendel’s peas

Question 6

What are the characteristics of an autosomal recessive disorder?

Answer 6

Affected usually have unaffected parents

Parents are usually carriers

Affects either sex

Increased incidence of inbreeding

Carriers and non-carriers are indistinguishable

1 in 4 chance of being affected

Question 7

Give some examples of autosomal recessive conditions

Answer 7

Albinism

Sickle cell anaemia

Cystic fibrosis

Attached ear lobes

Question 8

What gene is affected in cystic fibrosis?

Answer 8

Cystic fibrosis transmembrane conductance regulator

Question 9

Name some characteristics of X-linked recessive disorders

Answer 9

Affects mainly males

Probability of male offspring of female carrier being affected is 0.5

Females only affected if father is affected AND the mother is a carrier

Affected males are usually born to parents with no symptoms, although the mother often has affected male relatives

Question 10

Name some examples of X-linked recessive disorders

Answer 10

Duchenne muscular dystrophy

Red-green colour blindness

Haemophilia

Question 11

What are the characteristics of X-linked dominant disorders?

Answer 11

Affects either sex

The child of an affected female has 50% chance of being affected

All female children of an affected male are affected

No male children of an affected male are affected