biology : class12 Phenotypic Classes : Dihybrid Cross, Independent Assortment : Law of Independent Assortment MCQ Questions and Answers
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Phenotypic Classes
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MCQ Questions
1. In a typical Mendelian dihybrid cross between two heterozygous individuals, how many distinct phenotypic classes are expected in the F2 generation?
A.
Two
B.
Three
C.
Four
D.
Nine
Correct Answer:
Four
Explanation:
A typical dihybrid cross with complete dominance produces four distinct phenotypic classes in the F2 generation.
2. What is the usual phenotypic ratio in the F2 generation of a dihybrid cross involving two independently assorting gene pairs?
A.
3:1
B.
1:1:1:1
C.
9:3:3:1
D.
1:2:1
Correct Answer:
9:3:3:1
Explanation:
The four phenotypic classes in a standard dihybrid F2 cross occur in the ratio 9:3:3:1.
3. In the 9:3:3:1 phenotypic ratio of a dihybrid cross, which class is usually the largest?
A.
Both traits recessive
B.
First trait dominant and second trait recessive
C.
First trait recessive and second trait dominant
D.
Both traits dominant
Correct Answer:
Both traits dominant
Explanation:
The class showing both dominant phenotypes has the largest proportion, represented by 9 out of 16 offspring.
4. In a dihybrid F2 cross with a 9:3:3:1 ratio, what fraction of offspring show both recessive phenotypes?
A.
9/16
B.
3/16
C.
1/16
D.
12/16
Correct Answer:
1/16
Explanation:
The phenotypic class showing both recessive traits is the 1 part of the 9:3:3:1 ratio, or 1/16 of the offspring.
5. A standard dihybrid cross produces 160 offspring. How many are expected to show both dominant phenotypes?
A.
10
B.
30
C.
60
D.
90
Correct Answer:
90
Explanation:
Both dominant phenotypes occur in 9/16 of the offspring, and 9/16 of 160 equals 90.
6. In the F2 generation of a dihybrid cross, which phenotypic class represents 3/16 of the offspring?
A.
Both traits dominant
B.
Both traits recessive
C.
One trait dominant and the other recessive
D.
All offspring with identical phenotypes
Correct Answer:
One trait dominant and the other recessive
Explanation:
Each of the two mixed phenotypic classes occurs in 3/16 of the offspring in the 9:3:3:1 ratio.
7. Which set correctly lists the four phenotypic classes in a typical dihybrid F2 generation?
A.
Both dominant, first dominant only, second dominant only, both recessive
B.
Two dominant, two recessive, one mixed, one intermediate
C.
Tall, short, round, wrinkled
D.
Pure dominant, hybrid dominant, pure recessive, hybrid recessive
Correct Answer:
Both dominant, first dominant only, second dominant only, both recessive
Explanation:
The four classes are both dominant, dominant for the first trait and recessive for the second, recessive for the first and dominant for the second, and both recessive.
8. If two traits assort independently, the phenotypic classes of their dihybrid F2 offspring are produced because of the combination of:
A.
Only dominant alleles
B.
Only recessive alleles
C.
Phenotypes of both traits
D.
Only parental genotypes
Correct Answer:
Phenotypes of both traits
Explanation:
Independent assortment allows the phenotypic expression of the two traits to combine in four different ways.
9. In a standard dihybrid cross, what is the expected proportion of offspring showing a dominant phenotype for the first trait and a recessive phenotype for the second trait?
A.
9/16
B.
3/16
C.
1/16
D.
4/16
Correct Answer:
3/16
Explanation:
One mixed phenotypic class, with the first trait dominant and the second trait recessive, represents 3/16 of the F2 offspring.
10. A phenotypic ratio of 9:3:3:1 in a dihybrid cross indicates the presence of how many phenotypic classes?
A.
One
B.
Two
C.
Three
D.
Four
Correct Answer:
Four
Explanation:
The four numbers in the 9:3:3:1 ratio correspond to four distinct phenotypic classes.
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