201. Which statement is most accurate about the number of ABO alleles in one diploid individual?
ⓐ. A person can possess all three alleles $I^A$, $I^B$, and $i$ together.
ⓑ. A person can carry only two alleles of the ABO gene, even though three alleles exist in the population.
ⓒ. A person with blood group AB carries four alleles because both traits are expressed.
ⓓ. A person with blood group O has no ABO allele.
Correct Answer: A person can carry only two alleles of the ABO gene, even though three alleles exist in the population.
Explanation: Multiple allelism refers to the existence of more than two alleles in a population, not inside one individual. Because humans are diploid, each person inherits only two alleles of the ABO gene, one from each parent. The three allelic forms are $I^A$, $I^B$, and $i$, but no individual can carry all three together. This distinction is important for avoiding a common confusion in blood group genetics.
202. Which blood group phenotype has neither A antigen nor B antigen on red blood cells?
ⓐ. A
ⓑ. B
ⓒ. O
ⓓ. AB
Correct Answer: O
Explanation: Blood group O lacks both A and B antigens on the red blood cell surface. This phenotype appears when the genotype is $ii$. Since neither $I^A$ nor $I^B$ is present, no A or B antigen is produced. The absence of both antigens is therefore the defining feature of blood group O.
203. Which statement best describes pleiotropy?
ⓐ. One gene influences more than one phenotypic trait.
ⓑ. One trait is controlled by many different genes only.
ⓒ. Two alleles of a gene assort independently during meiosis.
ⓓ. A heterozygote shows an intermediate phenotype.
Correct Answer: One gene influences more than one phenotypic trait.
Explanation: Pleiotropy occurs when a single gene has effects on multiple characteristics of the organism. The gene does not control just one isolated visible feature. Instead, one altered gene product can influence several body functions or structures. This is why one inherited disorder may show more than one symptom.
204. Which human disorder is a standard example of pleiotropy in genetics?
ⓐ. Colour blindness
ⓑ. Haemophilia
ⓒ. Down’s syndrome
ⓓ. Phenylketonuria
Correct Answer: Phenylketonuria
Explanation: Phenylketonuria is commonly used to explain pleiotropy because one defective gene leads to multiple effects in the body. The disorder is linked with abnormal metabolism of phenylalanine and may affect intellectual development and other features when untreated. A single genetic cause thus produces several manifestations. This makes it a good example of one-gene multiple effects.
205. Which statement best explains why pleiotropy can occur?
ⓐ. A single gene always produces many unrelated chromosomes.
ⓑ. One gene product may participate in more than one function or pathway in the body.
ⓒ. Two traits always fuse into one new trait in the F1 generation.
ⓓ. A recessive allele can never remain hidden in a heterozygote.
Correct Answer: One gene product may participate in more than one function or pathway in the body.
Explanation: Genes act through the products they encode, and those products may affect more than one biological process. If one such gene is altered, several traits may be influenced together. This is the basic logic behind pleiotropy. The concept therefore links one genetic change with multiple phenotypic consequences.
206. Which of the following is the best example of pleiotropic effect?
ⓐ. One gene determining only seed colour
ⓑ. One gene pair giving a $3:1$ phenotypic ratio
ⓒ. One mutant gene causing more than one type of bodily effect
ⓓ. Two different genes controlling one trait independently
Correct Answer: One mutant gene causing more than one type of bodily effect
Explanation: Pleiotropy is recognized when a single gene influences several phenotypic outcomes. A mutation in one gene may therefore affect metabolism, development, and visible characteristics together. That is different from a gene affecting only one trait or two separate genes affecting one character. The emphasis is on one cause producing multiple effects.
207. Which statement correctly compares pleiotropy with codominance?
ⓐ. Pleiotropy involves one gene affecting several traits, whereas codominance involves two alleles expressing together in a heterozygote.
ⓑ. Pleiotropy and codominance are two names for the same inheritance pattern.
ⓒ. Pleiotropy involves independent assortment, whereas codominance prevents segregation.
ⓓ. Pleiotropy occurs only in plants, whereas codominance occurs only in humans.
Correct Answer: Pleiotropy involves one gene affecting several traits, whereas codominance involves two alleles expressing together in a heterozygote.
Explanation: These two ideas refer to different kinds of genetic relationships. Pleiotropy is about one gene having multiple phenotypic effects, while codominance is about the simultaneous expression of two alleles in one heterozygous individual. Neither concept replaces the other. Keeping them separate helps avoid confusion when comparing inheritance patterns.
208. A genetic disorder affects amino acid metabolism and also produces developmental and functional abnormalities in the same individual. This pattern most strongly suggests
ⓐ. complete dominance
ⓑ. linkage
ⓒ. test cross inheritance
ⓓ. pleiotropy
Correct Answer: pleiotropy
Explanation: When one genetic defect leads to several distinct effects in the body, pleiotropy is the most appropriate explanation. The same altered gene can influence metabolism first and then produce wider consequences in development or physiology. This is the logic used in explaining disorders such as phenylketonuria. The pattern is therefore broader than a simple visible trait difference.
209. Which statement about pleiotropy is incorrect?
ⓐ. A single gene may influence more than one trait.
ⓑ. One inherited disorder may produce multiple effects in the same individual.
ⓒ. Pleiotropy means one trait is always controlled by many genes.
ⓓ. Phenylketonuria is commonly used as an example of pleiotropy.
Correct Answer: Pleiotropy means one trait is always controlled by many genes.
Explanation: Pleiotropy refers to one gene affecting several traits, not many genes affecting one trait. When a single gene has multiple effects, several features of the organism may be altered together. This is why one disorder may show more than one symptom. The idea is about one cause producing many consequences.
210. Why is phenylketonuria considered a pleiotropic disorder?
ⓐ. It affects only blood group inheritance.
ⓑ. It occurs only in heterozygous individuals.
ⓒ. It shows codominance between two alleles.
ⓓ. One defective gene leads to multiple effects in the body.
Correct Answer: One defective gene leads to multiple effects in the body.
Explanation: In phenylketonuria, a single gene defect disrupts phenylalanine metabolism and can lead to several manifestations. The disorder is therefore not limited to one isolated visible character. Multiple consequences arise from one genetic cause. This makes it a standard example of pleiotropy.
211. Which option best matches a pleiotropic gene?
ⓐ. A gene whose mutation affects metabolism as well as development
ⓑ. A gene that produces a $3:1$ ratio in the F2 generation
ⓒ. A gene with two alleles showing complete dominance
ⓓ. A gene that is always located on a sex chromosome
Correct Answer: A gene whose mutation affects metabolism as well as development
Explanation: A pleiotropic gene has more than one phenotypic effect. If mutation in a single gene changes metabolism and also affects body development or function, the gene is showing pleiotropy. This idea is broader than a single visible trait difference. It highlights how one gene product can influence several processes.
212. Which statement best explains the starch synthesis example in pea seeds?
ⓐ. Seed shape depends only on chromosome number.
ⓑ. A gene can influence phenotype through the amount or function of its product.
ⓒ. Every visible trait is independent of gene products.
ⓓ. Recessive alleles always fail to produce any effect at all.
Correct Answer: A gene can influence phenotype through the amount or function of its product.
Explanation: The starch synthesis example shows that genes act through their products rather than by magic or direct labeling of traits. A functional gene product can alter the amount of starch formed in the seed, which then influences seed appearance. This connects molecular action with visible phenotype. It also shows why phenotype depends on underlying biochemical events.
213. In the pea seed example, round seeds are associated with
ⓐ. effective starch formation in the seed
ⓑ. complete absence of sugar in the seed
ⓒ. failure of the seed to absorb water
ⓓ. total lack of gene activity
Correct Answer: effective starch formation in the seed
Explanation: In the standard explanation, the allele associated with round seeds allows better starch synthesis. More starch and less free sugar favor a fuller seed form, so the seed appears round. The visible phenotype is therefore linked to a biochemical difference inside the seed. This example helps relate gene action to phenotype.
214. Which statement is most accurate about dominance in the starch synthesis example?
ⓐ. Dominance is always absolute at every biochemical level.
ⓑ. A dominant allele always produces exactly the same amount of product in all genotypes.
ⓒ. A visible phenotype can show dominance even when biochemical differences still exist.
ⓓ. A recessive allele is never transmitted to the next generation.
Correct Answer: A visible phenotype can show dominance even when biochemical differences still exist.
Explanation: The starch synthesis example shows that visible appearance and biochemical output need not behave in exactly the same way. A heterozygote may look similar to the dominant homozygote at the organism level, yet the amount of gene product may not be identical. This means dominance can seem complete for one trait but not for a biochemical measure. The example adds nuance to simple Mendelian ideas.
215. Which statement about the heterozygote in the starch synthesis example is correct?
ⓐ. It must always look wrinkled because one recessive allele is present.
ⓑ. It may appear round even though its biochemical state is not fully identical to the dominant homozygote.
ⓒ. It cannot form any starch at all.
ⓓ. It always shows codominance for seed shape.
Correct Answer: It may appear round even though its biochemical state is not fully identical to the dominant homozygote.
Explanation: A heterozygote can produce enough functional product to give the round seed phenotype. However, that does not necessarily mean its biochemical output is exactly the same as that of the homozygous dominant plant. This is why the example is useful in showing limits of oversimplified views of dominance. Visible phenotype alone may hide quantitative biochemical differences.
216. Why do wrinkled pea seeds develop in the starch synthesis example?
ⓐ. They contain too many chromosomes.
ⓑ. They are always produced by heterozygous plants.
ⓒ. They never absorb water during seed development.
ⓓ. Reduced starch synthesis leaves more sugar, affecting water balance and seed form.
Correct Answer: Reduced starch synthesis leaves more sugar, affecting water balance and seed form.
Explanation: When starch synthesis is reduced, more sugar remains in the developing seed. This changes water relations in the seed and affects how the seed dries later. As a result, the mature seed becomes wrinkled rather than round. The phenotype is therefore a downstream effect of altered gene product activity.
217. Which option best expresses the lesson from the starch synthesis example?
ⓐ. Genotype affects phenotype through biochemical pathways.
ⓑ. Dominant alleles are always more common in populations.
ⓒ. All recessive traits are caused by chromosome loss.
ⓓ. Seed shape in pea is unrelated to metabolism.
Correct Answer: Genotype affects phenotype through biochemical pathways.
Explanation: The example links a gene to its product and then to a visible trait. This shows that phenotype is not an isolated label but the outcome of biochemical processes under genetic control. Understanding that link makes inheritance more meaningful. It helps explain how genes produce observable characters.
218. Which statement is incorrect about the gene product and phenotype relationship?
ⓐ. Phenotype may depend on the function of a gene product.
ⓑ. Visible dominance can sometimes hide underlying biochemical differences.
ⓒ. A gene product may influence a trait indirectly through metabolism.
ⓓ. If two plants look alike, their biochemical state must always be exactly the same.
Correct Answer: If two plants look alike, their biochemical state must always be exactly the same.
Explanation: Similar visible phenotypes do not always guarantee identical biochemical conditions. Two genotypes may produce the same outward appearance while differing in the amount or efficiency of a gene product. The starch synthesis example highlights this possibility clearly. That is why phenotype and biochemical effect should not always be treated as perfectly identical levels of expression.
219. Which statement is most appropriate about the relationship between visible phenotype and biochemical effect in the starch synthesis example?
ⓐ. A visible phenotype always reveals the full biochemical state exactly.
ⓑ. Two plants may look similar in phenotype yet differ in the amount or efficiency of gene product.
ⓒ. Biochemical differences can exist only when two plants show different visible phenotypes.
ⓓ. A recessive allele has no effect at any level if the phenotype appears dominant.
Correct Answer: Two plants may look similar in phenotype yet differ in the amount or efficiency of gene product.
Explanation: The starch synthesis example shows that outward appearance does not always tell the whole story of gene action. A heterozygote may look like the dominant homozygote in visible phenotype, yet the underlying amount of functional product may differ. This means phenotype and biochemical state should not always be treated as identical. The example adds depth to the usual idea of simple dominance.
220. Which condition is most directly associated with the wrinkled seed phenotype in pea?
ⓐ. Excess starch deposition in the seed
ⓑ. Presence of both dominant alleles in the genotype
ⓒ. Simultaneous expression of two codominant alleles
ⓓ. Reduced starch formation with relatively higher sugar content
Correct Answer: Reduced starch formation with relatively higher sugar content
Explanation: Wrinkled seeds are linked with reduced starch synthesis in the seed. Because starch is formed less effectively, more sugar remains, which alters water balance during seed development and drying. This ultimately affects seed shape. The visible wrinkling is therefore a consequence of altered metabolism, not a direct surface feature alone.
