Questions & explanations
1. In snapdragon (Antirrhinum), a cross between a red-flowered plant (RR) and a white-flowered plant (rr) produces all pink-flowered offspring (Rr). When two pink-flowered plants are crossed, what is the expected phenotypic ratio in the offspring?
- A. 1 red : 2 pink : 1 white
- B. 3 red : 1 white
- C. 1 red : 1 pink : 1 white
- D. All pink
Answer: A. 1 red : 2 pink : 1 white
In incomplete dominance, the heterozygous genotype (Rr) produces an intermediate phenotype (pink). Crossing two heterozygotes (Rr × Rr) yields a genotypic ratio of 1 RR : 2 Rr : 1 rr, which corresponds to a phenotypic ratio of 1 red : 2 pink : 1 white.
2. In a dihybrid cross involving two interacting genes, the phenotypic ratio in F2 is 9:7 instead of 9:3:3:1. This modified ratio is most likely due to:
- A. Complementary gene interaction
- B. Incomplete dominance
- C. Co-dominance
- D. Multiple alleles
Answer: A. Complementary gene interaction
A 9:7 ratio in F2 indicates complementary gene interaction, where both dominant alleles from two genes are required for a trait. The 9/16 have both dominants, while the remaining 7/16 lack at least one dominant and show the alternative phenotype.
3. In Mendel's experiments, the 'factors' that determine traits are now known as genes. Which statement about alleles is correct?
- Alleles are different forms of the same gene.
- Alleles are always dominant.
- Alleles are located on different chromosomes.
- Alleles are only found in homozygous individuals.
Answer: Alleles are different forms of the same gene.
Alleles are alternative versions of a gene that arise by mutation and are found at the same locus on homologous chromosomes. They can be dominant or recessive, and are present in both homozygous and heterozygous individuals.
4. In a dihybrid cross between pea plants with yellow round (YYRR) and green wrinkled (yyrr) seeds, the F2 generation shows a phenotypic ratio of 9:3:3:1. This is due to:
- Law of Segregation
- Law of Dominance
- Law of Independent Assortment
- Law of Purity of Gametes
Answer: Law of Independent Assortment
The Law of Independent Assortment states that alleles of different genes assort independently during gamete formation. This leads to the 9:3:3:1 phenotypic ratio in a dihybrid cross when genes are on different chromosomes.
5. In a monohybrid cross between two heterozygous tall plants (Tt), the phenotypic ratio in F2 generation is 3 tall : 1 dwarf. This is explained by:
- Law of Dominance
- Law of Segregation
- Law of Independent Assortment
- Law of Unit Characters
Answer: Law of Segregation
The Law of Segregation states that during gamete formation, the two alleles for a trait separate, so each gamete carries only one allele. This results in a 3:1 phenotypic ratio in the F2 generation of a monohybrid cross.
6. Human skin colour is controlled by at least three different genes, each with multiple alleles, resulting in a continuous range of phenotypes. This type of inheritance is called:
- A. Polygenic inheritance
- B. Multiple alleles
- C. Pleiotropy
- D. Co-dominance
Answer: A. Polygenic inheritance
Polygenic inheritance involves multiple genes contributing to a single quantitative trait, producing a continuous distribution of phenotypes. Skin colour is a classic example, with additive effects of several genes.
7. Phenylketonuria (PKU) is caused by a mutation in a single gene that leads to mental retardation, reduced hair and skin pigmentation, and other symptoms. This is an example of:
- A. Pleiotropy
- B. Polygenic inheritance
- C. Incomplete dominance
- D. Co-dominance
Answer: A. Pleiotropy
Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits. In PKU, a defect in the gene for phenylalanine hydroxylase affects brain development, pigmentation, and other systems.
8. In sweet pea plants, two genes (C and P) are required for purple flower colour. A cross between two white-flowered plants (CCpp × ccPP) produces all purple F1 (CcPp). This is an example of:
- A. Complementary gene interaction
- B. Incomplete dominance
- C. Co-dominance
- D. Polygenic inheritance
Answer: A. Complementary gene interaction
Complementary gene interaction occurs when two dominant alleles from different genes are both needed to produce a phenotype. Here, both C and P must be present for purple colour; absence of either results in white.
9. Mendel selected garden pea (Pisum sativum) for his experiments. Which of the following is NOT a reason for his choice?
- Pea plants have contrasting traits that are easily distinguishable.
- Pea plants are self-pollinating but can be cross-pollinated artificially.
- Pea plants have a long generation time, allowing detailed study.
- Pea plants produce a large number of offspring in each generation.
Answer: Pea plants have a long generation time, allowing detailed study.
Mendel chose pea plants because they have a short generation time, not long. The other options are correct reasons: they have contrasting traits, can be self- or cross-pollinated, and produce many offspring.
10. In the ABO blood group system, a person with genotype I^A I^B has blood type AB. This is an example of:
- A. Incomplete dominance
- B. Co-dominance
- C. Multiple alleles
- D. Pleiotropy
Answer: B. Co-dominance
Co-dominance occurs when both alleles are fully expressed in the heterozygote. In AB blood type, both I^A and I^B alleles produce their respective antigens on red blood cells, so both are expressed equally.
11. In a monohybrid cross between a pure tall (TT) and a pure dwarf (tt) pea plant, the F1 generation consists of all tall plants. This illustrates:
- Law of Segregation
- Law of Independent Assortment
- Law of Dominance
- Incomplete dominance
Answer: Law of Dominance
The Law of Dominance states that in a heterozygote, one allele (dominant) masks the expression of the other (recessive). Here, tall (T) is dominant over dwarf (t), so all F1 plants (Tt) are tall.
12. The ABO blood group system is controlled by three alleles: I^A, I^B, and i. This is an example of:
- A. Multiple alleles
- B. Polygenic inheritance
- C. Incomplete dominance
- D. Co-dominance
Answer: A. Multiple alleles
Multiple alleles refer to the presence of more than two alleles for a gene in a population. The ABO blood group has three alleles (I^A, I^B, i), although an individual carries only two.