Biology Practice Test: Mendelian Genetics and Punnett Squares
- Mendelian Genetics
- 10 questions
- 13 minutes
This test practices Mendel's laws with Punnett squares and probability. Tip: each child is a separate event, so earlier children never change the odds for the next one.
Tap an option to mark your answer. You can change your mind, or tap the same option again to leave the question blank. The correct answers and explanations appear when you finish the test.Tap an option to answer; tap it again to leave the question blank.
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Question 1
A pea plant has the genotype Pp and purple flowers. What does "purple flowers" describe?
Correct answer: C
The phenotype is the trait you can observe, such as flower color. The genotype is the pair of alleles, here Pp.
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Question 2
According to Mendel's law of segregation, what fraction of the gametes made by a plant with genotype Rr will carry the r allele?
Correct answer: A
The two alleles separate when gametes form, and each gamete receives just one. Half the gametes get R and half get r.
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Question 3
Mendel crossed true-breeding purple-flowered peas (PP) with true-breeding white-flowered peas (pp). The F1 plants were then crossed with each other. Of the purple-flowered plants in the F2 generation, what fraction are heterozygous?
Correct answer: A
The F2 genotypes are 1 PP : 2 Pp : 1 pp. Only the 3 purple plants count here, and 2 of those 3 are Pp. The answer 1/2 counts Pp out of all four boxes, including the white plant.
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Question 4
Two heterozygous tall pea plants (Tt × Tt) are crossed. What is the expected genotype ratio of the offspring?
Correct answer: B
The Punnett square gives TT, Tt, Tt and tt. The 3 : 1 ratio describes phenotypes (tall to short), not genotypes.
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Question 5
A plant with genotype AaBb is crossed with a plant with genotype aabb. The two genes are on different chromosomes. What fraction of the offspring are expected to be aabb?
Correct answer: B
For each gene, Aa × aa gives aa half the time, and Bb × bb gives bb half the time. So 1/2 × 1/2 = 1/4. The fraction 1/16 would be correct for AaBb × AaBb.
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Question 6
In some cattle, red coat (R) and white coat (W) are codominant. Cattle with genotype RW are roan: their coat has both red and white hairs. Two roan cattle are mated. What fraction of their calves are expected to be roan?
Correct answer: C
RW × RW gives 1 RR : 2 RW : 1 WW, so half the calves are RW and roan. The other half are red or white.
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Question 7
Cystic fibrosis is an autosomal recessive disorder. Two parents who are both carriers already have one child with cystic fibrosis. What is the probability that their next child will have it?
Correct answer: B
Each pregnancy is an independent event. For Cc × Cc, every child has a 1/4 chance of being cc, no matter what happened before.
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Question 8
Huntington's disease is caused by a dominant allele (H). A father who is heterozygous (Hh) has children with a mother who does not carry the allele (hh). What is the probability that a child inherits the disorder?
Correct answer: D
Hh × hh gives Hh and hh in equal numbers. Because one H allele is enough to cause the disorder, half the children are expected to inherit it.
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Question 9
Mendel's law of independent assortment is most reliable for two genes that are
Correct answer: A
Genes on different chromosomes are sorted into gametes independently. Genes close together on the same chromosome are linked and are usually inherited together.
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Question 10
A mother has blood type AB (IAIB) and a father has blood type O (ii). Which blood types can their children have?
Correct answer: D
The children get IA or IB from the mother and i from the father, so they are IAi (type A) or IBi (type B). No child can be AB or O.
Questions you leave blank are not counted as wrong.
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