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11. Mendelian Genetics and Probability — Practice Questions 3
This chapter reviews Mendel’s laws, monohybrid and dihybrid crosses, genotype vs. phenotype, and how test crosses and probabilities predict inheritance.
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(Multiple Choice — Click to Reveal Answer)
1. What principle is demonstrated by the separation of alleles during gamete formation?
(A) Law of dominance
(B) Law of independent assortment
(C) Law of segregation
(D) Law of probability
Answer
(C) — The law of segregation explains that allele pairs separate during meiosis.
2. A heterozygous tall pea plant is crossed with a short plant. What proportion of the offspring are expected to be tall?
(A) 25%
(B) 50%
(C) 75%
(D) 100%
Answer
(B) — The cross Tt × tt produces 50% tall (Tt) and 50% short (tt) plants.
3. What does a 3:1 phenotypic ratio suggest about the parents in a monohybrid cross?
(A) One is homozygous dominant
(B) Both are heterozygous
(C) One is heterozygous, the other recessive
(D) Both are homozygous
Answer
(B) — A 3:1 ratio results from crossing two heterozygotes (Aa × Aa).
4. What type of organism is always used in a test cross?
(A) Homozygous dominant
(B) Homozygous recessive
(C) Heterozygous
(D) Any organism with the dominant trait
Answer
(B) — A homozygous recessive organism is used to reveal the unknown genotype of a dominant-phenotype individual.
5. Which trait is likely dominant if two heterozygotes show that trait in the phenotype?
(A) Recessive
(B) Codominant
(C) Incomplete dominant
(D) Dominant
Answer
(D) — Dominant traits are expressed in heterozygous individuals.
6. What genotype is considered hybrid in genetics?
(A) AA
(B) Aa
(C) aa
(D) AABB
Answer
(B) — A hybrid is heterozygous, meaning it carries two different alleles.
7. What is the phenotype ratio in a cross between two heterozygous round-seed pea plants, where round (R) is dominant to wrinkled (r)?
(A) 1:1
(B) 3:1
(C) 9:3:3:1
(D) 1:2:1
Answer
(B) — Aa × Aa gives 3 round : 1 wrinkled phenotypes.
8. In a Punnett square, what do the boxes represent?
(A) Gametes
(B) Offspring genotypes
(C) Parent genotypes
(D) Phenotypic traits
Answer
(B) — The boxes show the possible genotype combinations of the offspring.
9. What is the genotype of a plant with two recessive alleles for stem length?
(A) SS
(B) Ss
(C) ss
(D) sS
Answer
(C) — The genotype ss means the plant is homozygous recessive.
10. Which of the following represents the expected outcome of a monohybrid cross involving complete dominance?
(A) 1:1 phenotype
(B) 3:1 phenotype
(C) 9:3:3:1 phenotype
(D) 1:2:1 phenotype
Answer
(B) — A cross like Aa × Aa produces 3 dominant : 1 recessive phenotypes when complete dominance is involved.
11. A genotype consisting of identical alleles is called:
(A) Homozygous
(B) Heterozygous
(C) Diploid
(D) Hybrid
Answer
(A) — Homozygous individuals have two identical alleles, such as AA or aa.
12. What is the chance that two heterozygous individuals (Rr) produce a homozygous recessive offspring?
(A) 100%
(B) 75%
(C) 50%
(D) 25%
Answer
(D) — The chance of rr offspring is 1 out of 4 in a typical Rr × Rr cross.
13. Which term describes the physical appearance of an organism for a trait?
(A) Genotype
(B) Phenotype
(C) Allele
(D) Gamete
Answer
(B) — The phenotype is the observable expression of a trait, like tall or short.
14. If a trait is controlled by a dominant allele, which genotype will NOT express it?
(A) AA
(B) Aa
(C) aa
(D) None of the above
Answer
(C) — Only the homozygous recessive genotype (aa) does not express the dominant trait.
15. How many alleles does an individual inherit for a single autosomal gene?
(A) One
(B) Two
(C) Three
(D) Four
Answer
(B) — One allele is inherited from each parent, making two total per gene.
16. In a cross between RR and rr individuals, what will be the phenotype of all offspring?
(A) All dominant
(B) All recessive
(C) Half dominant, half recessive
(D) 3:1 dominant to recessive
Answer
(A) — All offspring will be Rr (heterozygous) and show the dominant phenotype.
17. A heterozygous individual crossed with a homozygous recessive individual will produce what genotype ratio?
(A) 1:1
(B) 2:1
(C) 3:1
(D) 1:2:1
Answer
(A) — A Tt × tt cross results in 50% Tt and 50% tt offspring.
18. What tool is commonly used to predict the outcome of genetic crosses?
(A) Pedigree chart
(B) Karyotype
(C) Punnett square
(D) Genetic map
Answer
(C) — Punnett squares help visualize the possible outcomes of genetic crosses.
19. If two individuals with the genotype Aa are crossed, what is the probability of producing an Aa offspring?
(A) 1/4
(B) 1/2
(C) 3/4
(D) 0
Answer
(B) — In an Aa × Aa cross, 2 of the 4 outcomes are Aa (50%).
20. Which genotype results in expression of a recessive trait in complete dominance?
(A) Aa
(B) AA
(C) aa
(D) AABB
Answer
(C) — Only the homozygous recessive (aa) expresses the recessive trait.
21. What does the notation "F2 generation" refer to in Mendelian genetics?
(A) First set of parents
(B) First generation of offspring
(C) Second generation of offspring
(D) Final generation of a pedigree
Answer
(C) — F2 is the offspring of the F1 generation.
22. Which of the following is not a possible genotype in a standard monohybrid cross?
(A) AA
(B) Aa
(C) aa
(D) AaBb
Answer
(D) — AaBb is a dihybrid genotype involving two genes, not one.
23. What is the purpose of using a homozygous recessive individual in a test cross?
(A) To produce hybrid offspring
(B) To mask recessive traits
(C) To determine unknown genotype
(D) To express all dominant phenotypes
Answer
(C) — A test cross helps reveal whether the dominant-phenotype parent is heterozygous or homozygous dominant.
24. If round seeds (R) are dominant to wrinkled seeds (r), which genotype would produce wrinkled seeds?
(A) RR
(B) Rr
(C) rr
(D) RrR
Answer
(C) — Only rr will result in the recessive wrinkled phenotype.
25. Which of the following statements is true about dominant alleles?
(A) They are stronger than recessive alleles
(B) They are more common in populations
(C) They are always expressed when present
(D) They eliminate recessive alleles
Answer
(C) — A dominant allele is expressed in both homozygous (AA) and heterozygous (Aa) genotypes.
26. In a cross between AaBb × AaBb, what fraction of the offspring are expected to be homozygous for both dominant alleles (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 (1/4) × BB (1/4) = 1/16.
27. A dihybrid cross between AaBb × AaBb produces how many unique genotypes?
(A) 4
(B) 8
(C) 9
(D) 16
Answer
(C) — A dihybrid cross can produce 9 different genotypes.
28. In a population of 1000 individuals, 360 express a recessive trait (aa). What is the estimated frequency of allele a under Hardy-Weinberg equilibrium?
(A) 0.2
(B) 0.3
(C) 0.4
(D) 0.6
Answer
(C) — q² = 360/1000 = 0.36 → q = √0.36 = 0.6. So, allele a = 0.6.
29. Which of the following inheritance patterns explains blood type AB?
(A) Incomplete dominance
(B) Codominance
(C) Complete dominance
(D) Sex-linked
Answer
(B) — AB blood type results from codominant alleles IA and IB both being expressed.
30. A cross between two heterozygous parents for a lethal recessive gene results in what percentage of viable offspring?
(A) 100%
(B) 75%
(C) 50%
(D) 25%
Answer
(B) — 25% die due to homozygous recessive condition, leaving 75% surviving (AA and Aa).
31. A man with genotype IAIB and a woman with genotype ii have children. What blood types are possible in the children?
(A) A and B only
(B) AB only
(C) A, B, and AB
(D) All types
Answer
(A) — The children can inherit IA or IB from the father and i from the mother, resulting in types A or B.
32. In a pedigree chart, which pattern suggests an autosomal dominant trait?
(A) Trait skips generations
(B) Trait affects mostly males
(C) Every affected person has at least one affected parent
(D) Only homozygous individuals are affected
Answer
(C) — Autosomal dominant traits typically appear in every generation with affected individuals having at least one affected parent.
33. In a cross between Aa × Aa, what is the expected ratio of genotypes among the offspring?
(A) 1:1
(B) 1:2:1
(C) 3:1
(D) 9:3:3:1
Answer
(B) — The expected genotypic ratio is 1 AA : 2 Aa : 1 aa.
34. Which of the following is true about linked genes?
(A) They follow independent assortment
(B) They are found on different chromosomes
(C) They are inherited together more often than not
(D) They always undergo recombination
Answer
(C) — Linked genes are close together on the same chromosome and tend to be inherited together unless crossing-over occurs.
35. Which explanation best fits a phenotypic ratio of 9:3:4 in a dihybrid cross?
(A) Codominance
(B) Recessive epistasis
(C) Dominant epistasis
(D) Incomplete dominance
Answer
(B) — Recessive epistasis modifies the classic 9:3:3:1 ratio to 9:3:4.
36. What is the difference between genotype and phenotype? Provide an example of each.
Answer
Genotype refers to the genetic makeup (e.g., Aa), while phenotype is the observable trait (e.g., purple flowers). A plant with genotype Aa might have the purple phenotype if purple is dominant.
37. Explain Mendel’s law of independent assortment and how it applies to a dihybrid cross.
Answer
The law states that alleles for different genes segregate independently during gamete formation. In a dihybrid cross, this leads to combinations like AaBb producing AB, Ab, aB, and ab gametes.
38. How can a recessive allele reappear in a population after being hidden for generations?
Answer
If individuals carry the recessive allele (as heterozygotes), it can be passed on and expressed when two carriers produce a homozygous recessive offspring.
39. Why do offspring from the same parents often show variation in traits?
Answer
Due to the random combination of alleles from each parent, along with independent assortment and potential crossing-over during meiosis, offspring can have different trait combinations.
40. What is a test cross and how is it used to determine an unknown genotype?
Answer
A test cross is performed by crossing an organism showing the dominant phenotype with a homozygous recessive individual. The offspring’s phenotypes reveal whether the unknown genotype is homozygous or heterozygous.
41. Describe how incomplete dominance differs from complete dominance, using flower color as an example.
Answer
In complete dominance, red (R) dominates white (r), so Rr appears red. In incomplete dominance, Rr would show a blended pink phenotype.
42. How does a pedigree help identify the inheritance pattern of a trait?
Answer
Pedigrees show family relationships and trait inheritance over generations, allowing scientists to determine if a trait is dominant, recessive, autosomal, or sex-linked.
43. What genotypes result from a cross between two heterozygous individuals (Bb × Bb)?
Answer
The offspring genotypes would be 1 BB, 2 Bb, and 1 bb — in a 1:2:1 ratio.
44. Why are sex-linked traits more commonly expressed in males?
Answer
Males have only one X chromosome, so a single recessive allele on it will be expressed, while females need two copies to show the trait.
45. How does codominance appear in the phenotype of a heterozygote?
Answer
Both alleles are fully expressed. For example, in AB blood type, both A and B antigens appear equally on the surface of red blood cells.
46. A plant has genotype Aa. What are the possible genotypes of gametes it can produce?
Answer
The plant can produce gametes carrying either A or a allele.
47. What does it mean if a genetic disorder is said to be autosomal recessive?
Answer
The disorder appears only when an individual inherits two copies of the recessive allele. Carriers (heterozygous) do not show symptoms.
48. If two parents are heterozygous for a trait, what is the probability their child will also be heterozygous?
Answer
There is a 50% chance the child will inherit one dominant and one recessive allele, making them heterozygous.
49. What is epistasis and how can it affect expected Mendelian ratios?
Answer
Epistasis occurs when one gene masks or modifies the expression of another gene, altering typical phenotypic ratios such as changing 9:3:3:1 to 9:3:4.
50. Why might actual results from a genetic cross differ from predicted probabilities?
Answer
Because inheritance involves chance, small sample sizes, mutations, or environmental factors may cause observed ratios to deviate from theoretical predictions.