SME
Genetics
Pedigrees and Mendelian genetics
Use this for “Her brother has cystic fibrosis and her partner is a carrier 1 time in 25. What is the chance their child is affected?”
Diagram
P = p₁ × p₂ × 1/4
P
=
2
3
×
1
25
×
1
4
P
=
1
150
≈
0.00667
Both parents must be carriers, and then a child is aa one time in 4.
This is an example. Type your own values to start a new problem.
1
150
P
=
p₁
×
p₂
×
1
4
C
Chance both are carriers
worked out (not typed)
Formulas
C
=
p₁
×
p₂
2
75
=
2
3
×
1
25
P
=
C
×
1
4
1
150
=
2
75
×
1
4
Assumptions
•
The trait is autosomal recessive: only aa shows it, and both sexes show it alike.
•
An unaffected sibling of an affected child is a carrier 2 times in 3: AA, Aa or aA, with aa ruled out.
•
A partner from outside the family is a carrier as often as the population’s carrier frequency.
•
Two carriers have an aa child 1 time in 4, and each child is a fresh draw.
Step-by-step
WE KNOW
p₂ = 1/25, p₁ = 2/3
FIND
chance both are carriers (C), chance of an affected child (P)
Step 1 · Find chance both are carriers (C)
Use C = p₁ × p₂
Both parents must carry the allele; their chances are independent, so multiply.
C
=
2
3
×
1
25
C
=
2
75
Step 2 · Find chance of an affected child (P)
Use P = C × 1/4
Two carriers each pass the allele half the time, so a child is aa 1 time in 4.
P
=
2
75
×
1
4
P
=
1
150
Check
Put the numbers back in. Both sides should match.
2
75
=
2
3
×
1
25
✓
1
150
=
2
75
×
1
4
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
More problem types
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Expected counts for a modified dihybrid ratio
Use this for “A 9:3:4 cross gives 160 offspring. How many are expected in each class?”
Mode of inheritance from a pedigree
Use this for “Two unaffected parents have an affected daughter. Is the trait dominant, recessive or X-linked?”