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11. Mendelian Genetics and Probability — Practice Questions
This chapter explores Mendel’s laws of inheritance, monohybrid and dihybrid crosses, and how probability can be used to predict genetic outcomes.
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(Multiple Choice — Click to Reveal Answer)
1. What is the term for an organism with two identical alleles for a trait?
(A) Heterozygous
(B) Homozygous
(C) Diploid
(D) Recessive
Answer
(B) — An organism with two identical alleles (e.g., AA or aa) is homozygous.
2. Which of the following is an example of a genotype?
(A) Tall plant
(B) Tt
(C) Green pod
(D) Wrinkled seed
Answer
(B) — A genotype refers to the genetic makeup (like Tt), not the physical trait.
3. In a monohybrid cross of two heterozygous plants (Aa × Aa), what is the expected phenotypic ratio?
(A) 1:1
(B) 3:1
(C) 9:3:3:1
(D) 1:2:1
Answer
(B) — The phenotypic ratio is 3 dominant : 1 recessive.
4. Which law states that allele pairs separate during gamete formation?
(A) Law of dominance
(B) Law of segregation
(C) Law of independent assortment
(D) Law of recombination
Answer
(B) — Mendel’s law of segregation explains how alleles separate during meiosis.
5. What is the probability that two heterozygous parents (Aa) will produce an offspring with a homozygous dominant genotype (AA)?
(A) 1/2
(B) 1/4
(C) 3/4
(D) 1
Answer
(B) — A Punnett square for Aa × Aa gives 1 out of 4 offspring as AA.
6. In a dihybrid cross of two individuals heterozygous for both traits (RrYy × RrYy), what fraction of the offspring will show both recessive traits?
(A) 1/16
(B) 3/16
(C) 9/16
(D) 1/4
Answer
(A) — Only 1 out of 16 offspring will be homozygous recessive for both traits.
7. What is the phenotype of a pea plant with genotype rr if round seeds (R) are dominant to wrinkled (r)?
(A) Round
(B) Wrinkled
(C) Heterozygous
(D) Cannot be determined
Answer
(B) — A plant with rr genotype expresses the recessive wrinkled phenotype.
8. A test cross is used to determine:
(A) The number of chromosomes in a cell
(B) The genotype of an individual with a dominant phenotype
(C) The phenotype of a heterozygous organism
(D) Whether a trait is recessive
Answer
(B) — Test crosses determine if an organism with a dominant trait is homozygous or heterozygous.
9. If two heterozygous yellow hamsters (Yy) are crossed, and YY is lethal, what fraction of their surviving offspring will be yellow?
(A) 1/2
(B) 2/3
(C) 3/4
(D) 1/3
Answer
(B) — YY dies, so only Yy and yy survive. Of the two surviving types, 2/3 are yellow (Yy).
10. Which genetic cross would produce a 1:1 genotypic ratio?
(A) AA × aa
(B) Aa × Aa
(C) Aa × aa
(D) AA × Aa
Answer
(C) — Aa × aa produces 50% Aa and 50% aa — a 1:1 genotypic ratio.
11. Which of the following is an example of a phenotype?
(A) Aa
(B) Tt
(C) Tall plant
(D) Homozygous
Answer
(C) — A phenotype is the physical expression of a trait, like being tall.
12. What does a Punnett square show?
(A) The number of chromosomes
(B) The result of fertilization
(C) All possible allele combinations from a cross
(D) Mutations in a gene
Answer
(C) — A Punnett square predicts all possible genetic combinations in offspring from parental genotypes.
13. If a trait is recessive, how can it be expressed in the phenotype?
(A) Only if the organism is heterozygous
(B) Only if the dominant allele is present
(C) Only if both alleles are recessive
(D) Always expressed regardless of genotype
Answer
(C) — A recessive trait is only expressed when both alleles are recessive (homozygous recessive).
14. What is the probability of getting a heterozygous offspring from an Aa × aa cross?
(A) 0%
(B) 25%
(C) 50%
(D) 100%
Answer
(C) — The cross produces 50% Aa and 50% aa offspring.
15. Which of the following genotypes is considered heterozygous?
(A) AA
(B) Aa
(C) aa
(D) BB
Answer
(B) — Aa contains two different alleles and is heterozygous.
16. What is the phenotypic ratio in the offspring of a cross between two homozygous individuals (AA × aa)?
(A) 3:1
(B) 1:1
(C) 1:2:1
(D) 100% dominant phenotype
Answer
(D) — All offspring will be heterozygous (Aa) and show the dominant trait.
17. Which combination of alleles would express a dominant trait?
(A) aa
(B) Aa
(C) bb
(D) All of the above
Answer
(B) — The presence of one dominant allele (A) is enough to express the dominant trait.
18. A dihybrid cross between two individuals heterozygous for two traits (AaBb × AaBb) results in how many phenotypes?
(A) 2
(B) 4
(C) 8
(D) 16
Answer
(B) — The phenotypic ratio is 9:3:3:1, totaling four different phenotypes.
19. If an individual is heterozygous for seed color (Yy), what gametes can it produce?
(A) Y only
(B) y only
(C) YY and yy
(D) Y and y
Answer
(D) — A heterozygous organism produces two types of gametes, one with Y and one with y.
20. Which pair represents alleles of the same gene?
(A) A and a
(B) A and B
(C) A and AA
(D) AA and aa
Answer
(A) — A and a are different forms (alleles) of the same gene.
21. What is the genotypic ratio of offspring from a heterozygous × heterozygous monohybrid cross (Aa × Aa)?
(A) 1:1
(B) 3:1
(C) 1:2:1
(D) 9:3:3:1
Answer
(C) — The genotypic ratio is 1 AA : 2 Aa : 1 aa.
22. What kind of inheritance pattern does a 9:3:3:1 phenotypic ratio suggest?
(A) Codominance
(B) Dihybrid cross with independent assortment
(C) Incomplete dominance
(D) Sex-linked inheritance
Answer
(B) — A 9:3:3:1 ratio is characteristic of a dihybrid cross with two independently assorted traits.
23. What does a lowercase letter (e.g., a) usually represent in genetics?
(A) A dominant allele
(B) A gamete
(C) A recessive allele
(D) A codon
Answer
(C) — Recessive alleles are commonly denoted by lowercase letters.
24. Which of the following is an example of complete dominance?
(A) Red × White = Pink
(B) A blood type from A and B alleles
(C) Aa and AA look the same
(D) Traits blend in heterozygotes
Answer
(C) — In complete dominance, the heterozygote expresses the same phenotype as the homozygous dominant.
25. How can a person inherit a recessive disorder?
(A) Only one dominant allele is needed
(B) One copy of the gene is enough
(C) Both parents must pass on a recessive allele
(D) It’s always passed from mother to child
Answer
(C) — Recessive disorders only appear when both inherited alleles are recessive.
26. In a dihybrid cross, what is the probability of producing offspring that are homozygous recessive for both traits (aabb) from two heterozygous parents (AaBb × AaBb)?
(A) 1/16
(B) 1/8
(C) 1/4
(D) 3/16
Answer
(A) — Only one out of sixteen offspring (1/16) will have the genotype aabb.
27. What does Mendel’s law of independent assortment state?
(A) Alleles for one gene always affect the alleles of another gene
(B) Dominant traits are inherited more often
(C) Genes for different traits can segregate independently during gamete formation
(D) Two dominant alleles will never appear together
Answer
(C) — This law explains how genes on different chromosomes assort independently into gametes.
28. A cross between two heterozygous individuals results in a 1:2:1 genotypic ratio. What type of inheritance does this suggest?
(A) Complete dominance
(B) Codominance or incomplete dominance
(C) Epistasis
(D) Sex linkage
Answer
(B) — In both codominance and incomplete dominance, the heterozygote expresses a distinct phenotype, leading to a 1:2:1 genotypic and phenotypic ratio.
29. If a female carrier for a recessive X-linked disorder has children with an unaffected male, what is the probability their son will have the disorder?
(A) 0%
(B) 25%
(C) 50%
(D) 100%
Answer
(C) — Sons receive one X chromosome from their mother, so there’s a 50% chance they inherit the affected X.
30. Which cross would produce a 9:3:4 phenotypic ratio in the F2 generation?
(A) A dihybrid cross with codominant alleles
(B) A monohybrid cross with incomplete dominance
(C) A dihybrid cross with recessive epistasis
(D) A test cross with sex-linked traits
Answer
(C) — Recessive epistasis modifies the expected 9:3:3:1 ratio into 9:3:4.
31. Which genotypic combination is responsible for blood type AB in humans?
(A) IAIA
(B) IAIB
(C) IBIB
(D) ii
Answer
(B) — AB blood type results from codominance between IA and IB alleles.
32. In a pedigree, how can you identify an autosomal recessive trait?
(A) It appears only in males
(B) It skips generations
(C) It affects all children of affected parents
(D) It never appears in carriers
Answer
(B) — Autosomal recessive traits often skip generations and appear only when both parents are carriers.
33. A plant with genotype RrYy is self-crossed. What is the probability of producing an offspring with the genotype RrYy?
(A) 1/2
(B) 1/4
(C) 1/8
(D) 1/16
Answer
(B) — In a dihybrid cross, the probability of Rr is 1/2 and Yy is 1/2; together, 1/2 × 1/2 = 1/4.
34. In a certain trait, heterozygotes show a blended phenotype. What type of inheritance does this represent?
(A) Incomplete dominance
(B) Codominance
(C) Complete dominance
(D) Recessive lethality
Answer
(A) — In incomplete dominance, heterozygotes have a blended intermediate phenotype.
35. If 75% of offspring from a cross show the dominant phenotype, what are the most likely genotypes of the parents?
(A) AA × aa
(B) Aa × Aa
(C) Aa × aa
(D) AA × AA
Answer
(B) — A 3:1 phenotypic ratio suggests a heterozygous × heterozygous (Aa × Aa) cross.
36. Explain Mendel’s law of segregation and how it applies to meiosis.
Answer
Mendel’s law of segregation states that each individual has two alleles for a gene, and these alleles separate during gamete formation. In meiosis, this occurs when homologous chromosomes separate in anaphase I.
37. What is a test cross, and why is it used in genetics?
Answer
A test cross involves crossing an individual with a dominant phenotype but unknown genotype with a homozygous recessive individual. It helps determine whether the dominant individual is homozygous or heterozygous.
38. Define genotype and phenotype and provide an example of each.
Answer
Genotype is the genetic makeup (e.g., Aa), while phenotype is the physical expression of the trait (e.g., tall plant). For example, a plant with genotype Aa shows the tall phenotype if tall is dominant.
39. How can two parents with dominant phenotypes produce a child with a recessive phenotype?
Answer
Both parents must be heterozygous (e.g., Aa). There’s a 25% chance that their child inherits the recessive allele from both (aa), expressing the recessive trait.
40. What is the probability of producing a homozygous recessive offspring from an Aa × Aa cross?
Answer
There is a 1 in 4 (25%) chance of producing an offspring with the genotype aa.
41. Describe the difference between codominance and incomplete dominance.
Answer
In codominance, both alleles are fully expressed (e.g., AB blood type). In incomplete dominance, the heterozygote shows a blend of both traits (e.g., red × white = pink flowers).
42. Why does a 9:3:3:1 phenotypic ratio occur in a dihybrid cross?
Answer
It occurs when two heterozygous individuals (AaBb × AaBb) are crossed and the genes assort independently, producing four phenotypic combinations in specific ratios.
43. What is a carrier in genetic terms, and when is this term commonly used?
Answer
A carrier has one copy of a recessive allele but does not express the trait. It’s commonly used in the context of autosomal or sex-linked recessive disorders.
44. In what situation would a Punnett square show only one genotype and one phenotype?
Answer
When both parents are homozygous dominant (e.g., AA × AA) or homozygous recessive (aa × aa), all offspring will have the same genotype and phenotype.
45. How does the concept of probability help predict genetic outcomes?
Answer
Probability allows us to calculate the likelihood of specific genotypes or phenotypes appearing in offspring, especially using tools like Punnett squares.
46. Give an example of a monohybrid cross and its expected genotypic ratio.
Answer
Crossing Aa × Aa results in genotypes: AA, Aa, and aa, in a 1:2:1 ratio.
47. If two traits are linked on the same chromosome, how might this affect inheritance patterns?
Answer
Linked genes tend to be inherited together and may not follow independent assortment unless crossing-over occurs between them.
48. What causes a 3:1 phenotypic ratio in the F2 generation of a monohybrid cross?
Answer
It results from crossing two heterozygous individuals, where 3 offspring show the dominant phenotype and 1 shows the recessive.
49. What information is necessary to complete a Punnett square?
Answer
You need the genotypes of the two parents to predict all possible genetic combinations in their offspring.
50. Why might two heterozygous individuals not produce an offspring with a recessive trait?
Answer
Although there is a 25% probability of recessive offspring, probability does not guarantee outcome—random chance may result in only dominant phenotypes.