Mendelian Genetics
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All Questions for: Mendelian Genetics
Question 1: Gregor Mendel's experiments with pea plants established which of the following fundamental principles?
- A Laws of segregation and independent assortment
- B The chromosome theory of inheritance
- C The structure of DNA
- D The central dogma of molecular biology
Explanation: Mendel formulated the laws of segregation and independent assortment through systematic crosses of pea plants, founding classical genetics.
Question 2: Mendel's Law of Segregation states that:
- A Two alleles of a gene separate during gamete formation, each gamete receiving one
- B Alleles of different genes assort independently
- C All offspring resemble the dominant parent
- D Alleles blend to produce an intermediate phenotype
Explanation: During gamete formation, the two alleles of a gene separate so that each gamete receives only one allele. This explains the 3:1 phenotypic ratio in monohybrid crosses.
Question 3: Mendel's Law of Independent Assortment applies to:
- A Genes located on different chromosomes or far apart on the same chromosome
- B Only genes on the same chromosome
- C Only sex-linked genes
- D Only mitochondrial genes
Explanation: Independent assortment applies to genes on different chromosomes (or far apart on the same chromosome), where alleles of one gene assort independently of alleles of another gene.
Question 4: A monohybrid cross between two heterozygotes (Aa × Aa) produces a phenotypic ratio of:
- A 3:1
- B 1:1
- C 1:2:1
- D 9:3:3:1
Explanation: Aa × Aa yields 1 AA : 2 Aa : 1 aa genotypically, which corresponds to a 3:1 phenotypic ratio (dominant : recessive).
Question 5: A monohybrid cross between a heterozygote and a homozygous recessive (Aa × aa) produces a phenotypic ratio of:
- A 1:1
- B 3:1
- C 9:3:3:1
- D All dominant
Explanation: Aa × aa yields 1 Aa : 1 aa, which corresponds to a 1:1 phenotypic ratio — the classic testcross ratio.
Question 6: A dihybrid cross between two double heterozygotes (AaBb × AaBb) produces a phenotypic ratio of:
- A 9:3:3:1
- B 3:1
- C 1:2:1
- D 1:1:1:1
Explanation: AaBb × AaBb yields the classic 9:3:3:1 ratio (dominant-dominant : dominant-recessive : recessive-dominant : recessive-recessive) when genes assort independently.
Question 7: A dihybrid testcross (AaBb × aabb) produces a phenotypic ratio of:
- A 1:1:1:1
- B 9:3:3:1
- C 3:1
- D 1:2:1
Explanation: AaBb × aabb yields a 1:1:1:1 ratio (AaBb : Aabb : aaBb : aabb) when genes assort independently. Deviations from this ratio indicate linkage.
Question 8: The physical appearance of an organism is called its:
- A Phenotype
- B Genotype
- C Karyotype
- D Allele
Explanation: Phenotype refers to observable traits, while genotype refers to the genetic makeup (the specific alleles present).
Question 9: An organism with two identical alleles for a gene is called:
- A Homozygous
- B Heterozygous
- C Hemizygous
- D Haploid
Explanation: Homozygous organisms carry two copies of the same allele (e.g., AA or aa). Heterozygous organisms carry two different alleles (e.g., Aa).
Question 10: An organism with two different alleles for a gene is called:
- A Heterozygous
- B Homozygous
- C Hemizygous
- D Diploid
Explanation: Heterozygous organisms have two different alleles (e.g., Aa), often showing the dominant phenotype when dominance is complete.
Question 11: A male with an X-linked gene has only one copy of the allele. This condition is called:
- A Hemizygous
- B Homozygous
- C Heterozygous
- D Haploid
Explanation: Hemizygous refers to having only one copy of a gene — as males do for X-linked genes. Because males have just one X, they express any allele on it regardless of dominance.
Question 12: Which of the following describes complete dominance?
- A The heterozygote shows the same phenotype as the dominant homozygote
- B The heterozygote shows an intermediate phenotype
- C Both alleles are expressed equally in the heterozygote
- D The heterozygote shows the recessive phenotype
Explanation: In complete dominance, the heterozygote shows the same phenotype as the dominant homozygote. For example, Aa looks identical to AA.
Question 13: In incomplete dominance, the heterozygote:
- A Shows an intermediate phenotype between the two homozygotes
- B Shows the same phenotype as the dominant homozygote
- C Shows both parental phenotypes simultaneously
- D Shows the recessive phenotype only
Explanation: Incomplete dominance produces an intermediate phenotype in the heterozygote. A classic example is pink flowers from crossing red and white snapdragons.
Question 14: In codominance, the heterozygote:
- A Expresses both alleles fully and simultaneously
- B Shows an intermediate phenotype
- C Shows only the dominant allele
- D Shows no phenotype
Explanation: In codominance, both alleles are fully expressed. The AB blood type is a classic example — both A and B antigens appear on red blood cells.
Question 15: A cross between two pink-flowered snapdragons (incomplete dominance) produces a phenotypic ratio of:
- A 1:2:1
- B 3:1
- C 1:1
- D 9:3:3:1
Explanation: Pink (Rr) × Pink (Rr) yields 1 Red (RR) : 2 Pink (Rr) : 1 White (rr), i.e., a 1:2:1 ratio — diagnostic of incomplete dominance.
Question 16: Human ABO blood groups are an example of:
- A Multiple alleles and codominance
- B Incomplete dominance only
- C Polygenic inheritance only
- D Sex-linked inheritance
Explanation: The ABO blood group system is governed by a single gene with three alleles (IA, IB, i). IA and IB are codominant, while both are dominant over i.
Question 17: A blood type AB individual has children with a blood type O partner. What is the possible blood type of their children?
- A Type A or Type B only
- B Type AB only
- C Type O only
- D All four types possible
Explanation: AB (IAIB) × O (ii) yields IAi (type A) and IBi (type B) offspring. Children can be type A or type B — not AB or O.
Question 18: A testcross is used to determine:
- A The genotype of an individual showing the dominant phenotype
- B The gender of the offspring
- C Whether the gene is X-linked
- D Whether mutation has occurred
Explanation: A testcross (crossing to a homozygous recessive individual) reveals the genotype of an individual showing the dominant phenotype — because the recessive parent cannot mask alleles passed from the unknown parent.
Question 19: In a testcross where the unknown parent is homozygous dominant, all offspring will:
- A Show the dominant phenotype
- B Show the recessive phenotype
- C Show a 1:1 ratio of dominant to recessive
- D Show an intermediate phenotype
Explanation: AA × aa yields all Aa (heterozygous) offspring, all expressing the dominant phenotype. A 1:1 ratio would indicate the unknown was heterozygous.
Question 20: The genotype of a person with type O blood is:
- A ii
- B IAIA
- C IBIB
- D IAIB
Explanation: Type O requires two i alleles (ii) since i is recessive to both IA and IB. The absence of A or B antigens on red blood cells defines type O.
Question 21: In a monohybrid cross Aa × Aa, what fraction of offspring will show the recessive phenotype?
- A 1/4
- B 1/2
- C 3/4
- D All offspring
Explanation: The cross produces 1/4 AA, 1/2 Aa, and 1/4 aa. Only the 1/4 aa genotype shows the recessive phenotype.
Question 22: What is the probability that a child of two heterozygous parents (Aa × Aa) will be homozygous dominant?
- A 1/4
- B 1/2
- C 3/4
Explanation: Aa × Aa produces 1/4 AA, 1/2 Aa, and 1/4 aa. The probability of AA (homozygous dominant) is 1/4.
Question 23: A plant with genotype AaBb is self-crossed. What fraction of offspring will be double homozygous recessive (aabb)?
- A 1/16
- B 1/4
- C 9/16
- D 1/2
Explanation: For each gene, Aa × Aa gives 1/4 aa. Since genes assort independently, P(aabb) = 1/4 × 1/4 = 1/16.
Question 24: Why did Mendel choose pea plants for his genetic experiments?
- A They had clear contrasting traits, short generation time, and easy controlled breeding
- B They had very long generation times
- C They had no genetic variation
- D They had complex traits influenced by many genes
Explanation: Pea plants offered clear-cut contrasting traits, short generation time, self-fertilization ability, and easy cross-breeding — ideal properties for quantitative analysis of inheritance.
Question 25: A pure-breeding red-flowered plant is crossed with a pure-breeding white-flowered plant (complete dominance). F1 offspring are:
- A All red (dominant phenotype)
- B All white (recessive phenotype)
- C Half red, half white
- D All pink (intermediate phenotype)
Explanation: In complete dominance, crossing homozygous dominant (RR) with homozygous recessive (rr) yields all heterozygous (Rr) F1 offspring — all showing the dominant phenotype.