201. A couple has already had three daughters. For their next pregnancy, assume normal independent fertilisation events and equal frequencies of \(X\)-bearing and \(Y\)-bearing sperm. What is the probability that the next child will be chromosomally \(XY\)?
ⓐ. \(\frac{1}{2}\)
ⓑ. \(\frac{1}{4}\)
ⓒ. \(\frac{1}{8}\)
ⓓ. \(\frac{3}{4}\)
Correct Answer: \(\frac{1}{2}\)
Explanation: Each pregnancy represents a new fertilisation event. The ovum contributes \(X\), while the sperm contributes either \(X\) or \(Y\). Under the stated assumption that the two sperm classes are equally represented, the probability of receiving a \(Y\)-bearing sperm in the next fertilisation is \(\frac{1}{2}\), giving an \(XY\) zygote. The outcomes of earlier pregnancies do not alter the chromosome carried by the sperm involved in a later independent fertilisation. Having three daughters previously does not reduce or increase the next pregnancy's chromosomal probability. This is a direct application of independence in repeated biological probability events and corrects the misconception that previous outcomes create a compensating tendency in later pregnancies. In human \(XX/XY\) sex determination, ova carry \(X\), whereas sperm occur as \(X\)-bearing and \(Y\)-bearing classes; previous births do not alter the chromosomal probability for a later independent conception.
202. In a family with four independent pregnancies, what is the probability, under the simple equal-probability human model, that all four zygotes are \(XX\)?
ⓐ. \(\frac{1}{2}\)
ⓑ. \(\frac{1}{4}\)
ⓒ. \(\frac{1}{8}\)
ⓓ. \(\frac{1}{16}\)
Correct Answer: \(\frac{1}{16}\)
Explanation: Under the stated model, each pregnancy has a probability of \(\frac{1}{2}\) of producing an \(XX\) zygote. Given that the pregnancies are treated as independent events, the probability that all four satisfy the same \(XX\) condition is obtained by multiplying the four probabilities: \[ \frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}=\frac{1}{16} \] The calculation does not imply that families of four must contain a particular fixed number of daughters. It gives the probability of one specified four-pregnancy outcome under the simple chromosomal model. Each fertilisation remains an independent event determined by whether an \(X\)-bearing or \(Y\)-bearing sperm fertilises the \(X\)-bearing ovum. Each human fertilisation is an independent event under the simple equal-\(X\)/\(Y\) gamete model; previous births do not alter the chromosomal probability for a later independent conception.
203. Evaluate the statements that follow about human sex determination.
I. Human females are homogametic with respect to sex chromosomes.
II. Human males normally form \(X\)-bearing and \(Y\)-bearing sperm.
III. The ovum determines whether the zygote becomes \(XX\) or \(XY\).
IV. Each pregnancy has an independent chromosomal sex outcome under the simple model.
ⓐ. I and III only
ⓑ. I, II and IV only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Human females have the \(XX\) complement and normally produce only \(X\)-bearing ova, making them homogametic. Human males possess \(XY\) and form two sperm classes, carrying either \(X\) or \(Y\). The variable sex-chromosome contribution comes from the sperm rather than the ovum. Fertilisation by an \(X\)-bearing sperm produces an \(XX\) zygote, while a \(Y\)-bearing sperm produces \(XY\). Under the simple probability model, separate pregnancies are independent, so previous births do not change the chromosomal probability for a later pregnancy. The third statement incorrectly assigns the determining chromosomal variation to the maternal gamete rather than to the two sperm classes produced by the heterogametic father. In human \(XX/XY\) sex determination, ova carry \(X\), whereas sperm occur as \(X\)-bearing and \(Y\)-bearing classes; previous births do not alter the chromosomal probability for a later independent conception.
204. In a large set of human fertilisations, investigators record approximately equal numbers of \(XX\) and \(XY\) zygotes. Assuming no differential survival is being considered, which inference is most directly supported?
ⓐ. Ova occur in equal \(X\)-bearing and \(Y\)-bearing classes.
ⓑ. Every sperm carries both \(X\) and \(Y\).
ⓒ. \(X\)- and \(Y\)-bearing sperm occur in roughly equal proportions.
ⓓ. Each parent contributes either two sex chromosomes or none.
Correct Answer: \(X\)- and \(Y\)-bearing sperm occur in roughly equal proportions.
Explanation: Human ova normally contribute \(X\), so the difference between an \(XX\) and an \(XY\) zygote lies in the chromosome carried by the fertilising sperm. If the two zygotic classes are observed in approximately equal numbers and differential survival is excluded as a confounding factor, the most direct inference is that \(X\)-bearing and \(Y\)-bearing sperm contribute to successful fertilisations at roughly equal frequencies. The observation does not require ova to carry \(Y\), nor does one sperm normally carry both sex chromosomes in this model. The zygote frequencies therefore support an inference about the relative contributions of the two gamete classes produced by the heterogametic parent.
205. Match each organismal system in Column I with the corresponding chromosomal principle in Column II. Each Column II entry is used once.
| Column I | Column II |
|---|
| P. Humans | 1. Female heterogamety of \(ZW\) type |
| Q. Birds | 2. Male heterogamety of \(XY\) type |
| R. Honey bee female | 3. Develops from a fertilised diploid egg |
| S. Honey bee male | 4. Develops from an unfertilised haploid egg |
ⓐ. P-1, Q-2, R-4, S-3
ⓑ. P-2, Q-3, R-1, S-4
ⓒ. P-3, Q-1, R-4, S-2
ⓓ. P-2, Q-1, R-3, S-4
Correct Answer: P-2, Q-1, R-3, S-4
Explanation: Humans use the \(XX\)-\(XY\) system, in which males are heterogametic. Birds use the \(ZZ\)-\(ZW\) system, in which females are heterogametic. Honey bees employ a different principle, haplodiploidy. Females arise from fertilised eggs and are diploid, whereas males arise from unfertilised eggs and are haploid. This comparison shows that chromosomal sex determination is not restricted to one pattern of sex-chromosome pairs. In humans and birds, sex is associated with different combinations of sex chromosomes, whereas in honey bees the decisive distinction is whether the egg is fertilised and whether the developing individual is diploid or haploid. This reasoning is consistent with the principle that haplodiploidy must be analysed through fertilisation status and chromosome-set number rather than by importing an \(XX/XY\) mechanism.
206. In honey bees, which symbolic relation correctly represents the basic haplodiploid pattern?
ⓐ. Fertilised egg \(\rightarrow n\) male; unfertilised egg \(\rightarrow 2n\) female
ⓑ. Fertilised egg \(\rightarrow 2n\) female; unfertilised egg \(\rightarrow n\) male
ⓒ. Fertilised egg \(\rightarrow 2n\) male; unfertilised egg \(\rightarrow 2n\) female
ⓓ. Fertilised egg \(\rightarrow n\) female; unfertilised egg \(\rightarrow n\) male
Correct Answer: Fertilised egg \(\rightarrow 2n\) female; unfertilised egg \(\rightarrow n\) male
Explanation: Honey bee sex determination is based on haplodiploidy. A fertilised egg receives one chromosome set from the mother and one from the father, producing a diploid \(2n\) individual that develops as a female. An unfertilised egg develops parthenogenetically and contains only the maternal chromosome set, producing a haploid \(n\) male. The difference is directly tied to fertilisation status and ploidy. This mechanism is distinct from human \(XX\)-\(XY\) and avian \(ZZ\)-\(ZW\) sex determination. The symbolic relation captures the essential causal chain: fertilisation changes chromosome-set number, and that ploidy difference is associated with development as female or male in the honey bee system.
207. If an unfertilised honey bee egg were experimentally caused to receive a paternal chromosome set before development, which immediate biological consequence would follow from the haplodiploid model?
ⓐ. It would remain haploid because fertilisation never affects ploidy.
ⓑ. It would necessarily lose the maternal chromosome set.
ⓒ. It becomes diploid instead of a normal haploid male-forming egg.
ⓓ. It would form an \(XY\) complement identical to that of a human male.
Correct Answer: It becomes diploid instead of a normal haploid male-forming egg.
Explanation: The defining difference in honey bee haplodiploidy is the number of chromosome sets. An unfertilised egg normally retains only the maternal set and is haploid. If it were to acquire an additional paternal chromosome set, the immediate genetic consequence would be restoration of diploidy. That changed ploidy is the relevant biological prediction; it shifts the egg away from the normal route that produces a haploid male. The model does not invoke an \(XY\) complement, since honey bee sex determination is not the human sex-chromosome mechanism. The reasoning depends on chromosome-set number rather than on assigning \(X\) and \(Y\) symbols to the bee system.
208. A honey bee male develops from an unfertilised egg. Which ancestry statement follows directly from this origin?
ⓐ. He has a mother but no father.
ⓑ. He has a father but no mother.
ⓒ. He has neither a mother nor grandparents.
ⓓ. He has two fathers but no mother.
Correct Answer: He has a mother but no father.
Explanation: A male honey bee develops from an unfertilised egg, so his genome is derived from the maternal egg without genetic contribution from a father. He has a mother but no father. This unusual ancestry follows directly from parthenogenetic development of haploid males and does not mean that male bees lack all earlier ancestors. The mother herself arose from a fertilised egg and had both a mother and a father. Haplodiploidy thus creates asymmetry in pedigree relationships that cannot occur under ordinary biparental diploid reproduction. Understanding the ancestry pattern requires tracing whether fertilisation occurred at each generation rather than assuming that every individual must have two immediate genetic parents.
209. Study the chromosome-number relations in honey bees.
| Individual | Chromosome condition |
|---|
| Female | Diploid, \(2n=32\) |
| Male | Haploid, \(n=16\) |
Which inference is consistent with the table?
ⓐ. A male contains twice as many chromosome sets as a female.
ⓑ. Male and female bees differ only in sex-chromosome identity, not ploidy.
ⓒ. Female bees must arise without fertilisation.
ⓓ. Fertilisation restores diploidy in female-producing eggs.
Correct Answer: Fertilisation restores diploidy in female-producing eggs.
Explanation: The table shows that female honey bees are diploid with \(2n=32\), whereas males are haploid with \(n=16\). This difference reflects their developmental origins. A female-producing egg is fertilised, adding a paternal chromosome set to the maternal set and restoring diploidy. A male develops from an unfertilised egg and retains only one chromosome set. The sex difference is thus fundamentally associated with ploidy rather than an \(XX\)-\(XY\)-type chromosome pair. The numerical relation also confirms that the haploid male chromosome number is half the diploid female number. Chromosome count, fertilisation status and developmental sex are connected in one causal relation in the haplodiploid system. Male honey bees arise by parthenogenesis from unfertilised eggs, so their chromosome number is the haploid value; fertilisation status must therefore be established before chromosome number or genotype is inferred.
210. Assertion: A honey bee male can have a grandfather even though he has no father.
Reason: The male develops from an unfertilised egg, but his diploid mother developed from a fertilised egg and therefore had a father.
ⓐ. Assertion is true, but the stated Reason is false
ⓑ. Both are true, but Reason does not explain Assertion
ⓒ. Assertion is false, but the stated Reason is true
ⓓ. Both are true, and Reason explains the Assertion
Correct Answer: Both are true, and Reason explains the Assertion
Explanation: A honey bee male develops parthenogenetically from an unfertilised egg, so he has no father. His mother, however, is a diploid female. In the haplodiploid system, females arise from fertilised eggs and receive genetic material from both a mother and a father. The male can consequently have a maternal grandfather even though there is no paternal individual in his own immediate parentage. The Reason directly explains the apparently unusual ancestry described in the Assertion. This pattern is a distinctive consequence of haplodiploidy and can be understood only by tracing fertilisation status across generations rather than applying the ordinary assumption that every offspring has two genetic parents. The same conclusion is supported by the fact that honey bee sex determination depends on ploidy: fertilised eggs are diploid females, while unfertilised eggs remain haploid and develop as males.
211. A honey bee queen has chromosome number \(2n=32\). Under normal haplodiploid reproduction, how many chromosomes are expected in an unfertilised egg that develops into a male?
ⓐ. \(16\)
ⓑ. \(8\)
ⓒ. \(32\)
ⓓ. \(64\)
Correct Answer: \(16\)
Explanation: A diploid honey bee female has \(2n=32\), so the haploid chromosome number is \(n=16\). Eggs formed by the female are haploid and contain \(16\) chromosomes. If an egg is fertilised, a paternal haploid set is added and the diploid condition \(2n=32\) is restored in the developing female. If the egg remains unfertilised, it can develop parthenogenetically as a male while retaining the haploid chromosome number \(16\). The numerical calculation is biologically meaningful since it links meiosis, fertilisation status and sex determination. The male does not lose half of an already diploid embryonic chromosome set; rather, he develops directly from the haploid egg. Male honey bees arise by parthenogenesis from unfertilised eggs, so their chromosome number is the haploid value; chromosome counting follows directly from whether the egg was fertilised. Male honey bees arise by parthenogenesis from unfertilised eggs, so their chromosome number is the haploid value; fertilisation status must therefore be established before chromosome number or genotype is inferred.
212. Examine the statements given below about honey bee haplodiploidy.
I. Females arise from fertilised eggs.
II. Males arise from unfertilised eggs.
III. Males are haploid and females are diploid.
IV. Male bees produce sperm through the same ploidy-reduction requirement as ordinary diploid males.
ⓐ. I and IV only
ⓑ. I, II and III only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and III only
Explanation: The defining structure of honey bee haplodiploidy is that fertilised eggs are diploid and develop as females, whereas unfertilised eggs remain haploid and develop as males. Thus ploidy and fertilisation status are directly linked with sex. A haploid male already possesses one chromosome set, so production of sperm cannot require the same reduction from \(2n\) to \(n\) that characterises meiosis in a diploid male. The fourth statement incorrectly transfers the ordinary diploid gametogenesis model to a haploid individual. The first three statements together express the core haplodiploid relation: \(2n\) females arise through fertilisation, whereas \(n\) males arise parthenogenetically from unfertilised eggs. The interpretation also depends on the fact that ABO family evidence should first restrict the possible parental genotypes, after which the gametes and offspring genotypes can be evaluated.
213. Arrange the events leading to a female honey bee in the correct order.
P. A queen produces a haploid egg.
Q. A sperm contributes a paternal haploid chromosome set.
R. Fertilisation restores diploidy.
S. The diploid individual develops as a female.
ⓐ. Q → P → S → R
ⓑ. P → R → Q → S
ⓒ. R → P → Q → S
ⓓ. P → Q → R → S
Correct Answer: P → Q → R → S
Explanation: The queen is diploid and produces haploid eggs. For a female-producing pathway, an egg is fertilised by a sperm that contributes another haploid chromosome set. Fusion of the maternal and paternal sets restores the diploid condition. The resulting diploid individual develops as a female under the honey bee haplodiploid system. The correct sequence begins with formation of the haploid egg, proceeds through paternal contribution at fertilisation, restores diploidy, and ends with female development. Placing diploidy before sperm contribution would reverse the causal process. This sequence makes clear that fertilisation is not simply associated with femaleness; it is the event that restores the second chromosome set in the female-producing developmental route. This reasoning is consistent with the principle that honey bee sex determination depends on ploidy: fertilised eggs are diploid females, while unfertilised eggs remain haploid and develop as males.
214. A honey bee pedigree shows individual P as a male and Q as P's mother. Q's parents are R, a female, and S, a male. Which relationship is necessarily supported by haplodiploidy?
ⓐ. P inherited one chromosome set from S directly.
ⓑ. P must have both a father and a mother.
ⓒ. S can be P's maternal grandfather through Q.
ⓓ. R cannot contribute genetically to P.
Correct Answer: S can be P's maternal grandfather through Q.
Explanation: Male P develops from an unfertilised egg produced by female Q, so P has no father and receives his chromosome set from Q. Q, however, is a diploid female and arose from a fertilised egg. Her parents can include male S as her father and female R as her mother. S is consequently P's maternal grandfather even though P himself has no father. The genetic contribution from S reaches P indirectly through Q rather than through a paternal parent of P. This ancestry pattern is a distinctive consequence of haplodiploidy and illustrates why ordinary pedigree expectations based on two immediate parents cannot be applied unchanged to honey bees.
215. In a honey bee colony, let F represent a fertilised egg and U represent an unfertilised egg. Which developmental relation is correct?
ⓐ. \(F\rightarrow n\) male and \(U\rightarrow2n\) female
ⓑ. \(F\rightarrow2n\) female and \(U\rightarrow n\) male
ⓒ. \(F\rightarrow2n\) male and \(U\rightarrow n\) female
ⓓ. \(F\rightarrow n\) female and \(U\rightarrow2n\) male
Correct Answer: \(F\rightarrow2n\) female and \(U\rightarrow n\) male
Explanation: Fertilisation combines a maternal haploid egg with a paternal haploid sperm, producing a diploid \(2n\) zygote that develops as a female honey bee. An unfertilised egg receives no paternal chromosome set and remains haploid \(n\); it can develop parthenogenetically as a male. The relation links three variables in one compact model: fertilisation status, ploidy and sex. This differs fundamentally from the human and avian systems, where sex is associated with particular sex-chromosome combinations in diploid offspring. In honey bees, the central distinction is whether a second chromosome set is supplied through fertilisation.
216. Suppose a honey bee queen produces two eggs with identical maternal chromosome numbers. Egg P is fertilised, while egg Q develops without fertilisation. Under the normal haplodiploid system, which comparison is expected?
ⓐ. P becomes diploid and develops as a female, whereas Q remains haploid and develops as a male.
ⓑ. P remains haploid and develops as a male, whereas Q becomes diploid and develops as a female.
ⓒ. Both become diploid, but P is female and Q is male.
ⓓ. Both remain haploid, and sex is determined only by an \(X\)- or \(Y\)-bearing sperm.
Correct Answer: P becomes diploid and develops as a female, whereas Q remains haploid and develops as a male.
Explanation: Both eggs begin as haploid products of the diploid queen. Egg P receives an additional haploid chromosome set through fertilisation, so its chromosome condition becomes diploid. Under the honey bee system, this route produces a female. Egg Q receives no paternal chromosome set and remains haploid; development from that unfertilised egg produces a male. The comparison isolates fertilisation as the changed condition while holding the original maternal egg state constant. The different outcomes follow from whether a second chromosome set is added, not from an \(X\)- or \(Y\)-bearing sperm system. Haplodiploidy links the presence or absence of fertilisation directly with both ploidy and sex.
217. A haploid honey bee male has \(n=16\). Which process best explains production of sperm without reducing chromosome number below the haploid state?
ⓐ. Meiosis produces sperm containing \(8\) chromosomes.
ⓑ. Mitotic divisions produce sperm that remain haploid.
ⓒ. Fertilisation precedes sperm formation and restores diploidy.
ⓓ. Cytokinesis fails so that each sperm becomes diploid.
Correct Answer: Mitotic divisions produce sperm that remain haploid.
Explanation: A honey bee male develops from an unfertilised egg and is already haploid, with \(n=16\). A ploidy-reducing meiotic division like that used by a diploid individual would reduce the chromosome complement below the normal haploid gametic condition. Instead, sperm production in the haploid male occurs by mitosis, so the sperm retain the haploid chromosome number. This feature follows directly from haplodiploidy: females are diploid and generate haploid eggs through meiosis, whereas males begin life with only one chromosome set. The distinction connects the male's developmental origin with his unusual mode of gamete production rather than treating bee gametogenesis as identical to ordinary diploid male meiosis.
218. A male honey bee fathers a diploid daughter. That daughter later produces an unfertilised egg that develops into a male. Which relationship is represented correctly?
ⓐ. The first male has produced a son directly through fertilisation.
ⓑ. The second male must have inherited a Y chromosome from the first male.
ⓒ. The daughter is haploid because her father was haploid.
ⓓ. The first male has no son, but the later male can be his grandson.
Correct Answer: The first male has no son, but the later male can be his grandson.
Explanation: A honey bee male can contribute sperm to a fertilised egg, and that fertilised egg develops as a diploid female. He can have daughters. A male bee, however, develops from an unfertilised egg, so a father does not contribute genetically to the immediate production of a male offspring. For that reason, a drone has no sons. His daughter can later produce an unfertilised egg, and a haploid male arising from that egg is the original drone's grandson. No \(Y\) chromosome is involved given that honey bees do not use the human \(XY\) mechanism. The unusual lineage follows from alternating fertilised female development and unfertilised male development within the haplodiploid system.
219. Assess the statements listed below about mutation and genetic variation.
I. A mutation involves an alteration of genetic material.
II. Mutation can create a genetic state that was not present before the alteration.
III. Recombination and mutation are identical processes because both always change DNA sequence.
IV. Mutation can contribute to heritable variation when the altered genetic material enters the reproductive lineage.
ⓐ. I, II and IV only
ⓑ. I and III only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Mutation is an alteration of genetic material and can generate a new genetic state. When such a change is transmitted through the reproductive lineage, it can contribute to inherited variation. Recombination also contributes to genetic variation, but it operates differently: it generates new combinations of existing alleles rather than necessarily altering the DNA sequence of an allele itself. The third statement incorrectly merges two distinct sources of variation. The distinction is important throughout inheritance studies. Segregation and recombination redistribute hereditary material that already exists in the parents, whereas mutation can alter that material. Both processes may increase genetic diversity, but the biological operation producing that diversity is not the same. Linkage modifies the joint frequencies of allele combinations without preventing segregation at the individual loci; reciprocal recombinant classes together make up the total recombination percentage.
220. An offspring carries a new allele produced by an alteration in genetic material, while another offspring carries a new combination of two parental alleles without either allele being changed. The two events are best classified, respectively, as:
ⓐ. recombination and point mutation
ⓑ. polyploidy and aneuploidy
ⓒ. mutation and recombination
ⓓ. segregation and mutation
Correct Answer: mutation and recombination
Explanation: The first observation states that the genetic material itself has been altered and a new allele has resulted. That is the defining relation for mutation. In the second observation, the alleles remain the parental alleles, but they occur in a combination not previously present together. That is recombination. The comparison separates alteration from rearrangement. Mutation can introduce a changed genetic sequence or chromosome state, whereas recombination reshuffles existing inherited variants into new associations. Both can contribute to variation among offspring, yet they should not be treated as interchangeable mechanisms. This distinction becomes especially useful when interpreting recombinant progeny from linked genes: non-parental allele combinations can arise without requiring a new mutation at either locus.
