Rucete ✏ AP Biology In a Nutshell
11. Mendelian Genetics and Probability — Practice Questions 2
This chapter explores how alleles segregate during gamete formation, how traits are inherited independently, and how probability can predict inheritance outcomes.
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(Multiple Choice — Click to Reveal Answer)
1. Which part of meiosis ensures the law of segregation is followed?
(A) Prophase I
(B) Metaphase I
(C) Anaphase I
(D) Telophase II
Answer
(C) — The homologous chromosomes, and thus alleles, separate during anaphase I.
2. What genotype results from a test cross between a heterozygous dominant and a homozygous recessive individual?
(A) All dominant
(B) All recessive
(C) 1:1 ratio of dominant to recessive
(D) 3:1 ratio of dominant to recessive
Answer
(C) — Half will inherit the dominant allele, and half will inherit the recessive.
3. What term refers to having two different alleles for a single trait?
(A) Homozygous
(B) Diploid
(C) Heterozygous
(D) Recessive
Answer
(C) — Heterozygous means having two different alleles, such as Aa.
4. What phenotype would result from genotype pp if purple flowers (P) are dominant to white (p)?
(A) Purple
(B) Pink
(C) White
(D) Blended
Answer
(C) — The recessive trait (white) is only expressed when both alleles are recessive (pp).
5. What fraction of offspring from a cross of Pp × pp will have purple flowers?
(A) 1/4
(B) 1/2
(C) 3/4
(D) 1
Answer
(B) — 50% will be heterozygous (Pp) and display the dominant purple trait.
6. What is the expected genotypic ratio of offspring from a TT × Tt cross?
(A) 1 TT : 1 Tt
(B) 1 TT : 2 Tt : 1 tt
(C) 2 Tt : 1 tt
(D) All Tt
Answer
(A) — Half will be TT and half will be Tt.
7. In a monohybrid cross with two heterozygous parents, what proportion of offspring are expected to be homozygous dominant?
(A) 1/4
(B) 1/2
(C) 3/4
(D) None
Answer
(A) — A cross of Aa × Aa produces one AA genotype among four possibilities.
8. Which of the following best represents a dihybrid genotype?
(A) Aa
(B) AaBb
(C) AB
(D) AAbb
Answer
(B) — AaBb represents heterozygosity at two loci, making it dihybrid.
9. How can you determine the unknown genotype of a dominant phenotype organism?
(A) Perform mitosis
(B) Conduct a test cross
(C) Cross it with another dominant
(D) Analyze phenotype alone
Answer
(B) — A test cross with a homozygous recessive partner reveals the unknown genotype.
10. What ratio results when two heterozygous individuals are crossed for a single trait?
(A) 9:3:3:1
(B) 1:1
(C) 1:2:1
(D) 3:1
Answer
(D) — A classic monohybrid Aa × Aa cross produces 3 dominant : 1 recessive phenotypic ratio.
11. Which genotype will always produce recessive phenotype in offspring?
(A) AA
(B) Aa
(C) aa
(D) aA
Answer
(C) — Only the homozygous recessive (aa) genotype expresses the recessive trait.
12. What ratio of phenotypes would you expect from a heterozygous × homozygous recessive cross?
(A) 3:1
(B) 2:1
(C) 1:1
(D) 4:0
Answer
(C) — Half the offspring show the dominant phenotype and half the recessive.
13. What is the function of a Punnett square?
(A) It shows physical traits
(B) It illustrates gene linkage
(C) It predicts offspring genotype and phenotype probabilities
(D) It records chromosomal mutations
Answer
(C) — Punnett squares predict possible allele combinations and outcomes from a genetic cross.
14. Which genotype combination represents a heterozygous × homozygous dominant cross?
(A) Aa × AA
(B) Aa × aa
(C) AA × aa
(D) Aa × Aa
Answer
(A) — Heterozygous (Aa) crossed with homozygous dominant (AA).
15. Which situation would likely result in a 100% dominant phenotype in offspring?
(A) Aa × Aa
(B) AA × aa
(C) Aa × aa
(D) AA × Aa
Answer
(D) — All offspring will inherit at least one dominant A allele.
16. Which of these traits is most likely controlled by a single gene with two alleles?
(A) Eye color
(B) Height
(C) Blood type
(D) Mendel’s pea flower color
Answer
(D) — Mendel studied traits like flower color controlled by one gene with two alleles (P/p).
17. If two parents are both carriers for a recessive trait, what is the chance their child will express the trait?
(A) 0%
(B) 25%
(C) 50%
(D) 100%
Answer
(B) — There is a 1 in 4 chance that the child will inherit two recessive alleles (aa).
18. What outcome results from a cross between a homozygous dominant and homozygous recessive individual?
(A) 100% recessive
(B) 50% dominant, 50% recessive
(C) All heterozygous
(D) All homozygous dominant
Answer
(C) — All offspring will be heterozygous (Aa) and show the dominant phenotype.
19. Which of these represents a possible allele pair for a single gene trait?
(A) XY
(B) AB
(C) A and a
(D) RRYY
Answer
(C) — A and a are two alleles of the same gene (dominant and recessive).
20. What is the chance of two heterozygous parents having an offspring with the same genotype as themselves?
(A) 0%
(B) 25%
(C) 50%
(D) 75%
Answer
(C) — There’s a 2 in 4 chance of offspring being heterozygous (Aa).
21. A plant with genotype Tt is crossed with another Tt plant. What are the expected genotypes of the offspring?
(A) TT, Tt, tt
(B) TT only
(C) Tt and tt only
(D) All Tt
Answer
(A) — The genotypic ratio is 1 TT : 2 Tt : 1 tt.
22. A person has the genotype Aa for a trait. What percentage of their gametes will carry the recessive allele a?
(A) 0%
(B) 25%
(C) 50%
(D) 100%
Answer
(C) — Gametes will carry each allele 50% of the time: 50% A, 50% a.
23. Which genetic principle explains why offspring resemble their parents but are not identical?
(A) Incomplete dominance
(B) Chromosome theory of inheritance
(C) Segregation and independent assortment
(D) Linked inheritance
Answer
(C) — Alleles segregate and assort independently, creating variation in offspring.
24. In which of these crosses will none of the offspring express the recessive phenotype?
(A) Aa × Aa
(B) Aa × aa
(C) AA × Aa
(D) Aa × AA
Answer
(C) — All offspring will have at least one dominant A allele.
25. What is the chance of producing a homozygous dominant offspring from Aa × Aa?
(A) 25%
(B) 50%
(C) 75%
(D) 100%
Answer
(A) — 1 out of 4 offspring will be AA.
26. In a dihybrid cross where both parents are heterozygous (AaBb × AaBb), what is the probability of getting offspring that are homozygous dominant for both traits (AABB)?
(A) 1/4
(B) 1/8
(C) 1/16
(D) 3/16
Answer
(C) — Each homozygous dominant genotype has a 1/4 chance (AA and BB), so 1/4 × 1/4 = 1/16.
27. Which scenario best illustrates the law of independent assortment?
(A) A tall pea plant always produces yellow seeds
(B) A gene for color influences seed shape
(C) Alleles for height segregate independently from alleles for color
(D) Both alleles for height remain linked in gametes
Answer
(C) — Independent assortment means genes for different traits are inherited independently.
28. What proportion of offspring from a cross between AaBb × AaBb will show both dominant phenotypes?
(A) 1/4
(B) 9/16
(C) 3/16
(D) 1/2
Answer
(B) — 9/16 show both dominant phenotypes in a typical dihybrid cross.
29. In a genetic disorder caused by a recessive allele, what is the likelihood that two carrier parents (Aa) will produce an affected child?
(A) 25%
(B) 50%
(C) 75%
(D) 0%
Answer
(A) — Each child has a 25% chance of inheriting both recessive alleles (aa).
30. If a male with genotype XʳY and a female carrier XʳX has children, what is the probability that a son will have the disorder?
(A) 0%
(B) 25%
(C) 50%
(D) 100%
Answer
(C) — Sons have a 50% chance of inheriting the affected X from their mother.
31. In a dihybrid cross, how many offspring out of 16 would you expect to be homozygous for one trait and heterozygous for the other (e.g., AAbb or aaBb)?
(A) 4
(B) 6
(C) 8
(D) 2
Answer
(A) — There are four combinations like AAbb, aaBB, AAbb, aaBb, each occurring at 1/16 or 2/16 frequency combined.
32. Which inheritance pattern is indicated by a 1:2:1 phenotypic ratio in the offspring of two heterozygotes?
(A) Codominance or incomplete dominance
(B) Dihybrid cross
(C) Recessive epistasis
(D) Linked gene inheritance
Answer
(A) — A 1:2:1 phenotypic ratio suggests either codominance or incomplete dominance.
33. If a father has blood type AB and the mother is type O, what are the possible blood types of their children?
(A) A, B, AB, or O
(B) AB only
(C) A or B only
(D) O or AB only
Answer
(C) — The children can only inherit either the A or B allele from the father and an O allele from the mother.
34. What is the expected phenotypic ratio of a cross between two heterozygous individuals where one allele exhibits complete dominance?
(A) 3:1
(B) 1:2:1
(C) 1:1
(D) 9:3:3:1
Answer
(A) — A standard Aa × Aa cross with complete dominance gives 3 dominant:1 recessive phenotype.
35. Which situation would produce a 2:1 phenotypic ratio in a monohybrid cross?
(A) Lethal homozygous dominant allele
(B) Incomplete dominance
(C) Codominance
(D) Independent assortment failure
Answer
(A) — If the homozygous dominant genotype is lethal, it won’t survive, leaving a 2:1 ratio in surviving offspring.
36. Explain why two parents with dominant phenotypes can have a child with a recessive phenotype.
Answer
If both parents are heterozygous (e.g., Aa), there's a 25% chance their child will inherit both recessive alleles (aa) and express the recessive trait.
37. What does a 9:3:3:1 phenotypic ratio indicate about the genetic cross?
Answer
This ratio comes from a dihybrid cross of two individuals heterozygous for two independently assorting genes, suggesting Mendel's law of independent assortment.
38. Describe the difference between a genotype and a phenotype.
Answer
A genotype refers to the genetic makeup (e.g., Aa), while a phenotype refers to the observable trait (e.g., tall plant).
39. What is the purpose of performing a test cross?
Answer
A test cross determines whether an individual with a dominant phenotype is homozygous (AA) or heterozygous (Aa) by crossing it with a homozygous recessive individual.
40. Why does a heterozygous individual not express a recessive trait?
Answer
In complete dominance, the dominant allele masks the effect of the recessive allele, so the recessive trait is not expressed in heterozygotes.
41. What gametes can a parent with genotype AaBb produce?
Answer
AB, Ab, aB, and ab — one of each combination of alleles from the two genes.
42. How does codominance differ from incomplete dominance?
Answer
In codominance, both alleles are fully expressed (e.g., AB blood type). In incomplete dominance, the phenotype is a blend (e.g., red × white = pink flowers).
43. What genotypic ratio is expected from a cross between Aa × Aa?
Answer
1 AA : 2 Aa : 1 aa — classic Mendelian monohybrid cross ratio.
44. Describe the inheritance pattern of sex-linked recessive traits.
Answer
These traits are located on the X chromosome. Males (XY) express the trait if they inherit one recessive allele; females (XX) must inherit two recessive alleles.
45. Why are males more likely to express X-linked recessive disorders?
Answer
Males only have one X chromosome, so a single recessive allele will result in expression of the disorder.
46. How does the environment influence phenotype?
Answer
Environmental factors like temperature, nutrition, or light can affect gene expression, modifying the phenotype without changing the genotype.
47. What is the expected phenotypic ratio in the offspring of a cross between two individuals with genotypes AaBb and aabb?
Answer
1:1:1:1 — one of each combination of dominant and recessive traits due to independent assortment.
48. In which generation did Mendel observe a 3:1 phenotype ratio, and why?
Answer
In the F2 generation of a monohybrid cross, because two heterozygous individuals produced offspring in a 3:1 dominant-to-recessive ratio.
49. What is epistasis, and how does it affect inheritance patterns?
Answer
Epistasis occurs when one gene masks or modifies the expression of another gene, altering expected Mendelian ratios (e.g., 9:3:4).
50. Why does probability not guarantee specific genetic outcomes in small sample sizes?
Answer
Probability gives expected ratios, but in small numbers, random chance can lead to deviations from these expectations.