Answer
Probability : 27/64
Work Step by Step
Autosomal Recessive Inheritance
- only the Homozygous is affected.
- Heterozygous is carrier (usually asymptomatic ).
- more than one member in the family is usually
affected.
- male & female have the same risk.
Types of Marriage
1-
parents : pp X pp.
eggs : all p .
sperm : all p.
offspring : pp.
Marriage of 2 affected ( pp ) result in 100% affected
( i.e. all the children will have Phenylketonuria ).
2-
parents : pp X Pp.
eggs : all p.
sperm : 1/2 P, 1/2 p.
offspring : 1/2 Pp, 1/2 pp.
( 50% affected & 50% carrier ).
3-
parents: Pp X Pp.
eggs : 1/2 P, 1/2 p.
sperm : 1/2 P, 1/2 p.
offspring : 1/4 PP, 1/2 Pp, 1/4 pp.
( 25% affected PP, 50% carriers Pp, 25% normal
pp)
- these are average figures because the actual
number of affected children will depend on the size
of the family
if number affected is 0 ( non of the 3 children have
the disorder )
Probability : 27/64.