Principles of Inheritance and Variation MCQs with Answers – Part 1 (Class 12 Biology)

Explanation: Genetics includes both heredity and the differences that appear among related individuals. Inheritance describes the transmission of characters from one generation to the next, while variation describes the extent to which offspring differ from their parents or from one another. These ideas are linked rather than independent: inheritance provides continuity across generations, whereas variation introduces diversity within that continuity. The fourth statement breaks this relationship by treating the two concepts as unrelated. A useful way to keep the terms distinct is to ask whether an observation concerns transmission of a parental character or a difference in its expression among progeny. That distinction supports later interpretation of genetic crosses and prevents heredity from being confused with simple resemblance caused by non-genetic circumstances.

Explanation: Resemblance of progeny to a parental form reflects transmission of a character across generations, which is inheritance. Differences among offspring represent variation. The same family can display both phenomena at once, so they should not be treated as mutually exclusive alternatives. Heredity is closely related to inheritance, but artificial selection requires deliberate human choice of particular variants for breeding and is not described in the observations. The decisive distinction is the direction of comparison: parent-to-progeny continuity points to inheritance, whereas differences among progeny or between progeny and parents point to variation. This paired interpretation becomes important when genetic crosses are used to study both stable transmission and differing phenotypic outcomes.

Explanation: Sexual reproduction brings genetic material from two parents into the same progeny and can create new combinations of inherited factors. This provides an important source of variation among offspring even when no new mutation has been deliberately produced. Artificial selection can later favour particular existing variants, but it does not require that the desired variation be created by directed mutation. Inheritance also does not mean exact phenotypic copying of one parent; characters are transmitted through genetic factors that may combine in different ways. The observation is specifically about novel combinations appearing after a cross, so the biologically relevant conclusion is the contribution of sexual reproduction to inherited variation.

Explanation: The Assertion is valid: when naturally occurring variation is present, humans can preferentially breed individuals showing a desired phenotype, so that the selected form becomes more common over generations. The Reason is not valid, since selection does not require a breeder to manufacture a directed mutation for the desired character. Selection operates on heritable differences that already exist or arise naturally. The biological point is the connection between variation and selective breeding, not a mechanism for purposefully designing mutations. Since the Reason introduces an unnecessary and incorrect requirement, it cannot explain the Assertion. This keeps artificial selection distinct from the origin of variation on which selection acts. Selection changes which variants are preferentially reproduced; it is not a purposeful mechanism for generating the required allele.

Explanation: Mendel's use of clearly contrasting traits created discrete categories that could be followed from parents into hybrid progeny. For flower colour, stable parental lines with distinct alternatives such as violet and white provide an unambiguous starting comparison. A continuous range of shades would make class boundaries less clear, while allowing many unrelated differences to vary simultaneously would complicate interpretation of the character being studied. Using only one parent would not test transmission between contrasting parental forms. The experimental value of contrast lies in traceability: when parental states are clearly defined, the appearance, disappearance, or reappearance of a trait in later generations can be recorded and compared without uncertainty about what phenotype was inherited.

Explanation: Genetic outcomes are probabilistic, so a small family can deviate from an expected proportion even when the underlying inheritance relation is valid. Large samples make chance fluctuations less dominant and allow numerical regularities to become clearer. Mendel's quantitative treatment then provided a way to compare the frequencies of different progeny classes rather than relying on impression alone. Repetition adds further support when similar patterns recur. The third statement is incorrect since an expected ratio is not a command that every small sample must match exactly. This distinction between expected probability and exact observed count is fundamental to genetic analysis: evidence becomes stronger through adequate numbers and reproducible patterns, not through demanding perfect ratios in every limited set of offspring.

Explanation: A single hybrid generation can reveal which phenotype appears immediately after crossing, but it cannot show the full behaviour of hereditary factors over time. Following successive generations allows an investigator to ask whether a trait that is absent from one generation reappears later and whether numerical relations recur. That temporal evidence was important in Mendel's work, since inheritance rules were supported by patterns extending beyond one set of hybrids. Repeated generations do not force offspring to copy parental genotypes, remove segregation, or make clear parental contrasts unnecessary. Their value is confirmation and extension: a proposed inheritance relation can be tested against new progeny produced from the earlier generation rather than being inferred from one observation alone. Inheritance preserves continuity across generations, while variation reflects differences among related individuals; parent-to-offspring resemblance and differences among offspring answer different biological questions.

Explanation: A character is a heritable feature that can occur in alternative forms, while a trait is a specific form of that character. Stem height is the character being followed, and tall and dwarf are its contrasting traits. This distinction matters when describing Mendel's experimental design: he selected pairs of contrasting traits for defined characters rather than treating each visible form as an unrelated character. Confusing the levels would later make allele notation and cross interpretation inconsistent. The relationship can be represented conceptually as one biological feature with two alternative expressions. Mendel's pea work used several such character-trait pairs so that transmission could be followed clearly from parental lines into later generations.

Explanation: A true-breeding line shows stable inheritance of a selected trait after repeated self-pollination. Consistent expression over generations gives the investigator confidence that the parental line is not regularly producing alternative forms of the character under study. Such stability was important for Mendel's parental crosses, since an unambiguous parental condition makes the appearance of the hybrid generation interpretable. The description does not imply heterozygosity for every gene, nor does it mean that the organism lacks all possible variation in every feature. True-breeding is applied to the relevant character or trait being followed. Repeated selfing and stable expression are the decisive observations that justify the classification in this experimental context.

Explanation: Mendel's \(14\) true-breeding varieties are understood as two contrasting forms for each of \(7\) characters. For example, stem height could be represented by stable tall and stable dwarf lines. The numerical relation is not a free-standing arithmetic fact; it reflects the experimental structure needed to compare two alternative traits of one character. It also does not imply that each variety differed from every other variety in all seven characters. The important biological unit is the contrasting pair: two dependable parental forms allow the inheritance of that character to be followed through a cross. This organisation made comparisons systematic and helped separate one character pair from another during analysis. The numbers are meaningful only when interpreted through that paired experimental design.

Explanation: The task requires checking whether both entries in each row are alternative traits of the character named in that row. Stem height is represented by tall and dwarf, pod shape by inflated and constricted, and seed shape by round and wrinkled. In row Q, violet is a flower-colour trait, but terminal describes flower position rather than flower colour. The row mixes traits belonging to two different characters. Mendel's design depended on keeping each character paired with its proper contrasting forms, since a cross can be interpreted cleanly only when the biological feature under comparison is defined. This distinction also prevents later confusion between a trait value and a separate character. True-breeding parents provide fixed starting allele combinations for interpreting later generations; a test cross becomes informative when a dominant phenotype could correspond to more than one genotype.

Explanation: True-breeding status is supported by stable transmission of the same trait through repeated self-pollination. If two contrasting flower-colour phenotypes continue to appear, the line does not meet that operational criterion for the character being tested. Self-pollination by itself is not sufficient; the outcome across generations is what establishes stability. Equal occurrence of two phenotypes would still contradict stable expression of one trait, and the observation does not show that flower colour is non-heritable. The reasoning is experimental rather than merely terminological: a parental line used to establish a clean Mendelian cross should provide a dependable starting phenotype. Persistent segregation within the supposed parental line would make later hybrid results difficult to interpret.

Explanation: Flower position is represented by axial and terminal, pod shape by inflated and constricted, seed shape by round and wrinkled, and pod colour by green and yellow. These mappings depend on keeping the biological character separate from the visible form it can take. Several words in the list could appear familiar in pea descriptions, but they are not interchangeable across characters. In particular, green and yellow may also be encountered when discussing seed colour, so the column label "pod colour" is essential context. A reliable mapping uses the exact character-trait relation rather than recognising a colour word alone. The completed set preserves four distinct Mendelian character pairs without mixing position, shape, and colour categories.

Explanation: Mendel's observations placed tall over dwarf for stem height, violet over white for flower colour, and axial over terminal for flower position. The fourth statement reverses the dominance relation for pod colour: green is the dominant trait and yellow is recessive. Correct use of these pairs requires more than recognising two words that belong to the same character; their dominance relation must also be retained. That distinction becomes important when predicting the phenotype of a heterozygote, since the dominant trait is expressed under complete dominance while the recessive trait can remain unexpressed. Accurate character-trait and dominance mapping provides the reference needed for interpreting later monohybrid and dihybrid crosses, where phenotype predictions depend on knowing which member of each pair is expressed in the heterozygous state.

Explanation: The table must be completed by matching each character with its proper contrasting trait, not by distributing familiar pea-trait words arbitrarily. Flower colour pairs violet with white, flower position pairs axial with terminal, and seed colour pairs yellow with green. Together, these pairings preserve the identity of three separate characters rather than mixing traits from different character pairs. The structure also illustrates why colour words need contextual labels: green is a valid trait in more than one pea character, so "seed colour" specifies the intended relation. Completing the record as white, terminal, and green preserves the distinction among the stated characters and keeps every row internally consistent. The table also tests a common source of error: the same descriptive word can belong to different characters, so a trait must always be interpreted with its character label. Here, green is valid for seed colour, while terminal belongs only to flower position and white to flower colour. Each missing entry is fixed by the established contrasting trait of its own character, so the three pairings must remain internally consistent.

Explanation: Violet and white are the two contrasting traits for the single character flower colour. The other pairs combine traits drawn from different characters: tall belongs to stem height, axial to flower position, inflated to pod shape, green can describe pod or seed colour depending on context, round to seed shape, and yellow to seed or pod colour. A contrasting pair must represent alternative expressions of one defined character. This is more restrictive than choosing any two observable pea features. Keeping that relationship clear prevents a common notation error in which symbols for unrelated characters are treated as if they were alternative alleles of one gene. The same character-to-trait discipline underlies every later Mendelian cross.

Explanation: For the listed characters, dwarf is recessive to tall for stem height, white is recessive to violet for flower colour, wrinkled is recessive to round for seed shape, and green is recessive to yellow for seed colour. The mapping requires two kinds of accuracy at once: each trait must belong to the correct character, and its dominance status must also be correct. A student who remembers only that "green" occurs in Mendel's pea list could misassign it unless the character is checked carefully. These dominant-recessive relations provide the vocabulary for interpreting the first hybrid generation, where the dominant phenotype is expressed while the recessive allele remains present but unexpressed under complete dominance.

Explanation: A true-breeding tall parent contributes only \(T\) gametes for this gene, while a true-breeding dwarf parent contributes only \(t\) gametes. Every \(F_1\) zygote receives one allele from each parent and has genotype \(Tt\). Under complete dominance, the presence of \(T\) is sufficient for the tall phenotype, so the heterozygous \(F_1\) plants appear tall. The cross does not produce parental homozygous genotypes in the first hybrid generation, since each offspring must combine one gamete from each pure parent. This symbolic relation introduces the genotype-phenotype distinction that later becomes essential when \(F_1\) plants form gametes and the recessive phenotype can reappear. The gametic step can be written as \(TT\rightarrow T\) and \(tt\rightarrow t\); fertilisation then gives \(T+t\rightarrow Tt\) for every zygote in this cross. The biological meaning is that parental homozygosity restricts each parent to one allele type, while the hybrid receives one member of the pair from each parent. Phenotype is then assigned only after the genotype is known, using complete dominance for the stem-height character.

Explanation: The Assertion is true. Crossing \(TT\) with \(tt\) produces heterozygous \(Tt\) offspring, so the allele associated with dwarfness remains in every \(F_1\) plant even though the plants are phenotypically tall. The Reason is also true and directly accounts for the observation. Complete dominance concerns expression in the heterozygote; it does not mean that the recessive allele is destroyed, diluted, or converted into the dominant allele. Retention of both alleles in \(Tt\) is what later allows separation during gamete formation and reappearance of the recessive phenotype in a subsequent generation. The distinction between absence of expression and absence of an allele is central to Mendelian inheritance. It also explains why observing only the \(F_1\) phenotype is insufficient to conclude that the recessive hereditary factor has disappeared.

Explanation: In the simple Mendelian treatment of pea stem height, the outcome depends on which alleles the two parents contribute, not on whether the tall parent supplies pollen or ovules. A true-breeding tall parent contributes \(T\), and a true-breeding dwarf parent contributes \(t\), so either reciprocal cross produces \(Tt\) offspring. With complete dominance, these heterozygotes are tall. Reversing parental sex does not change the allele combination in this autosomal Mendelian setup. The comparison is useful experimentally since it separates the inheritance relation from an imagined parent-of-origin effect. At this stage, the relevant conclusion is that the same two pure parental genotypes generate the same \(F_1\) genotype and phenotype when their sexes are reversed.