- Which of the following correctly describes codominance?
a) Both alleles express a blended phenotype
b) One allele completely masks the other
c) Both alleles are equally expressed in the phenotype
d) Neither allele is expressed in the phenotype
Answer: (c) Both alleles are equally expressed in the phenotype
Explanation: In codominance, both alleles contribute equally and independently to the phenotype of the organism. A common example is the AB blood group in humans, where both IA and IB alleles are expressed, resulting in both A and B antigens being present on the surface of red blood cells.
- If a woman with blood group AB marries a man with blood group O, what are the possible blood groups of their offspring?
a) A, B, AB, O
b) A, B
c) AB, O
d) Only O
Answer: (b) A, B
Explanation: The woman with blood group AB has the genotype IAIB, and the man with blood group O has the genotype ii. When crossed, the possible combinations of alleles in their offspring are IAi and IBi, meaning the children could either have blood group A (IAi) or blood group B (IBi). Blood groups AB and O are not possible.
- In a cross between a homozygous tall pea plant (TT) and a homozygous dwarf pea plant (tt), all offspring are tall. This is an example of:
a) Codominance
b) Incomplete dominance
c) Complete dominance
d) Pleiotropy
Answer: (c) Complete dominance
Explanation: Complete dominance occurs when one allele (in this case, T for tallness) completely masks the expression of the other allele (t for dwarfness). The heterozygous offspring (Tt) will exhibit the dominant phenotype, which is tall in this case.
- In a certain plant, yellow flowers (Y) are dominant over white flowers (y), and round seeds (R) are dominant over wrinkled seeds (r). If two plants with the genotype YyRr are crossed, what fraction of the offspring will have yellow flowers and wrinkled seeds?
a) 1/16
b) 3/16
c) 9/16
d) 12/16
Answer: (b) 3/16
Explanation: This is a dihybrid cross where both parents are heterozygous for flower color and seed shape. The phenotypic ratio for the cross will follow a 9:3:3:1 pattern. Of these, 9 will have yellow flowers and round seeds, 3 will have yellow flowers and wrinkled seeds, 3 will have white flowers and round seeds, and 1 will have white flowers and wrinkled seeds. Therefore, 3/16 of the offspring will have yellow flowers and wrinkled seeds.
- In the human ABO blood group system, which blood group is known as the universal recipient?
a) A
b) B
c) AB
d) O
Answer: (c) AB
Explanation: Individuals with blood group AB are called universal recipients because they have both A and B antigens on their red blood cells and no antibodies against A or B in their plasma. This allows them to receive blood from any ABO group without agglutination reactions.
- Which genetic disorder is caused by a recessive allele on an autosome?
a) Huntington’s disease
b) Down syndrome
c) Cystic fibrosis
d) Hemophilia
Answer: (c) Cystic fibrosis
Explanation: Cystic fibrosis is caused by a recessive allele located on an autosome (chromosome 7). An individual must inherit two copies of the faulty allele (one from each parent) to develop the disease. Huntington’s disease is an autosomal dominant disorder, Down syndrome is caused by trisomy 21, and hemophilia is an X-linked recessive disorder.
- Which of the following describes the law of independent assortment?
a) Alleles of a gene segregate from each other during gamete formation
b) Genes located on the same chromosome always assort together
c) Genes for different traits assort independently of each other during gamete formation
d) Homologous chromosomes fail to separate during meiosis
Answer: (c) Genes for different traits assort independently of each other during gamete formation
Explanation: Mendel’s law of independent assortment states that alleles of different genes assort independently during gamete formation, meaning the inheritance of one trait does not influence the inheritance of another. This law applies only when genes are located on different chromosomes or are far apart on the same chromosome.
- A female carrier for color blindness (XcX) marries a normal male (XY). What is the probability that their son will be colorblind?
a) 0%
b) 25%
c) 50%
d) 100%
Answer: (c) 50%
Explanation: Color blindness is an X-linked recessive disorder. The mother is a carrier (XcX), and the father is normal (XY). Sons inherit their X chromosome from their mother and their Y chromosome from their father. There is a 50% chance that a son will inherit the Xc (mutated) chromosome from the mother, making him colorblind.
- Which of the following is an example of a genetic disorder caused by non-disjunction?
a) Sickle cell anemia
b) Hemophilia
c) Down syndrome
d) Cystic fibrosis
Answer: (c) Down syndrome
Explanation: Down syndrome is caused by trisomy 21, which occurs due to non-disjunction, an error during meiosis where homologous chromosomes fail to separate properly. This results in an individual having three copies of chromosome 21 instead of two.
- Which of the following statements is incorrect regarding mutations?
a) Mutations can be beneficial, harmful, or neutral
b) Mutations are the source of new alleles in a population
c) Somatic mutations are heritable and passed on to offspring
d) Mutations can occur spontaneously or due to environmental factors
Answer: (c) Somatic mutations are heritable and passed on to offspring
Explanation: Somatic mutations occur in non-reproductive cells and are not passed on to offspring. Only mutations in germline cells (eggs or sperm) can be inherited. Somatic mutations may affect the individual but do not influence future generations.
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