4: Mendelian inheritance in humans

Question 1

What are pedigree charts used for?

Answer 1

Used to infer mode of inheritance
Genetic counselling e.g risk of offspring inheriting condition

Question 2

What are the pedigree symbols for males (unaffected and affected)?

Answer 2

Unaffected = square
Affected = filled in square

Question 3

What are the pedigree symbols for females (unaffected and affected)?

Answer 3

Unaffected = circle
Affected = filled in circle

Question 4

What is the pedigree symbol for unknown sex?

Answer 4

Diamond

Question 5

What is the pedigree symbol for a carrier?

Answer 5

Square/circle with dot in the middle

Question 6

What is the pedigree symbol for dead?

Answer 6

Square/circle with dash through

Question 7

What is the pedigree symbol for marriage?

Answer 7

line connecting shapes

Question 8

Describe autosomal dominant

Answer 8

• Affected person usually has at least 1 affected parent
• Affects either sex & transmitted by either sex = autosomal
• Child of unaffected x (hetero) affected mating has 50% chance of disease
• 8/14 with affected parent have disease = ~50%

Question 9

Give some examples of autosomal dominant phenotypes in humans and mendel’s peas

Answer 9

Humans:
Achondroplasia → form of dwarfism
Polydactyly → extra fingers/ toes
Window’s peak

Mendel’s peas:
Yellow dom to green
Purple flowers dom to white
Smooth seeds dom to wrinkled

Question 10

Describe an autosomal recessive

Answer 10

Typically, an affected person will not have affected parents
Parents usually carriers
Affects either sex
Often seen in pedigree where inbreeding is common
Carriers/non-carriers are indistinguishable
If 2 carriers mate offspring have ¼ chance of being affected and ½ chance of being a carrier

Question 11

Give some examples of autosomal recessive phenotypes

Answer 11

Albinism
Sickle cell anaemia
Cystic fibrosis
Attached/ detached earlobes in humans

Question 12

describe cystic fibrosis

Answer 12

→ in european pop.s it is the most common autosomal recessive disease
Affects lungs, increased mucus secretion
Median survival ~50 years (20 yrs ago was 31 years on av)
Heterozygotes may have (or had) a selective advantage through resistance to cholera, typhoid of other diseases
Genetics:
Affecteds have 2 inactive copies of the CFTR gene → involved in transport of chloride ions
Very large gene, so many diff mutations can cause CF = severity of disease depends on where mutation occurs
Most common mutation: Δ508

Question 13

What disorder do X-linked recessive disorders often get mistaken for?

Answer 13

Autosomal recessive

Question 14

Why do X-linked recessive disorders only commonly effect males?

Answer 14

XX females have another allele to compensate for the other, whereas XY males only have one → so if they have the disordered allele they will be affected

Question 15

In X-linked recessive disorders, what % chance do male offspring of female carriers have of being affected?

Answer 15

50% chance

Question 16

In X-linked recessive disorders, what is the only situation where females can be affected?

Answer 16

If father is affected and mother is a carrier

Question 17

Give some examples of X-linked recessive disorders

Answer 17

Duchenne Muscular dystrophy
Red-green colorblindness
Haemophilia

Question 18

What makes X-linked recessive disorders much more common within a family?

Answer 18

In breeding
e.g European Royal Families

Question 19

describe X-linked dominant disorder

Answer 19

Affects either sex
Affected male will always have affected daughters
Affected female’s offspring will have 50% chance of being affected
No male children of affected males are affected

Question 20

Give an example of x-dominant disorder

Answer 20

Quite rare so v few examples:
Hypophosphatemia → form of vitamin D-resistant rickets

Question 21

Describe X-chromosome inactivation

Answer 21

In mammals female are XX and males XY
This inequality compensated for by X-inactivation (lyonization)
In each cell of females one X is randomly inactivated
Occurs early in development and daughter cells have the same X inactivated

Question 22

Give an example of X-chromosome inactivation

Answer 22

Tortoiseshell cats → always female
Have 2 diff versions of a gene in their X chromosome
Dominant = orange
Recessive = black
Because some parts of the chromosome are inactive/ active the coat develops diff colours

= Because its X linked males will either be orange OR black (they only have 1 X)

Question 23

describe Y-linked disorder

Answer 23

Affects only males
All sons of an affected man are affected

Question 24

Give an example of Y-linked disorder, explain why there are very few examples

Answer 24

Very few examples:
Hairy ear rims possibly
Y chromosome eroding → has v few functioning genes
Beyond determining sex (SRY gene) it does very little