ExpertEasy1 markMultiple Choice
AQA GCSE · Question 01.5 · Inheritance Variation and Evolution
What percentage of the offspring in the Punnett square will have MSUD?
What percentage of the offspring in the Punnett square will have MSUD?
Answer options:
A.
25%
B.
50%
C.
75%
D.
100%
How to approach this question
1. Look at the completed Punnett square from the previous question. The possible genotypes are NN, Nn, Nn, and nn.
2. Identify the genotype that results in the MSUD phenotype. This is the homozygous recessive genotype, nn.
3. Count how many of the four squares contain this genotype. There is 1 square with nn.
4. Calculate the percentage: (1 out of 4) * 100%.
Full Answer
A.25%✓ Correct
25%
The Punnett square for a cross between two heterozygous parents (Nn x Nn) produces four possible genotypes for the offspring: NN, Nn, Nn, and nn. MSUD is a recessive condition, so an individual must have the genotype nn to be affected. Only one of the four possible outcomes is nn. Therefore, the probability or percentage is 1/4, which is equal to 25%.
Common mistakes
✗ Confusing the percentage of carriers (50%) with the percentage of affected individuals.
✗ Calculating the percentage of unaffected individuals (75%).
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