Mendel’s laws

The blending theory predicted that two parental traits would permanently mix, so a hidden trait should not reappear unchanged. Mendel crossed pea plants with contrasting traits and observed that the recessive dwarf trait disappeared in the F1 generation but reappeared in the F2 generation. This showed that hereditary factors remain discrete and are passed on separately. Therefore, option A is correct; B, C, and D contradict the actual observations.

Mendel selected pea plants because they have several easily observable pairs of contrasting traits, such as tall versus dwarf stems and round versus wrinkled seeds. They also have a short generation time, produce many seeds, and can self-pollinate or be cross-pollinated under controlled conditions. Therefore option A gives the main reason. Pea plants reproduce and produce seeds, so options B and C are false.

The governing concept is Mendel’s law of independent assortment. During gamete formation, allele pairs for different traits can separate and arrange independently, provided the genes are not linked in a way that prevents this pattern. In a dihybrid cross, this produces combinations such as round-yellow and wrinkled-green as well as new combinations. Therefore A is correct; B states the opposite, while C and D confuse inheritance with reproduction.

The blending theory suggested that parental traits would permanently mix, like two colours of paint, so a recessive trait should not reappear unchanged. In Mendel’s pea experiments, a trait absent in the first hybrid generation reappeared in the second generation, such as dwarfness after crossing tall plants. This showed that hereditary factors remain distinct and can separate into later offspring. Hence option A is correct.

In a Mendelian cross, a dominant allele expresses its trait in a heterozygous offspring, whereas a recessive allele can remain masked. If a tall plant is crossed with a dwarf plant and every F₁ offspring is tall, the observed phenotype indicates that tallness is dominant under the usual pure-parent assumption. Dwarfness is therefore recessive, not destroyed. Options C and D contradict the observed F₁ result.

In a typical monohybrid cross, the parental alleles combine in F₁ so that the dominant phenotype is visible, while the recessive allele remains present but masked in a heterozygous genotype. During gamete formation and fertilisation, two recessive alleles may unite in an F₂ offspring, producing the recessive phenotype again, commonly in a 3:1 phenotypic ratio. Thus option A is correct.

The governing concept is the design of Mendelian inheritance experiments. Pea plants were useful because they have several clear, easily recognisable pairs of contrasting traits, such as tall/dwarf plants and yellow/green seeds. They also produce many offspring and can self-pollinate, while controlled cross-pollination is possible. These features allowed Mendel to compare parental traits and count their appearance in successive generations. Therefore option A gives the key reason.

In a Mendelian cross, contrasting forms of a trait are represented by different alleles. In a heterozygous first-generation offspring, one allele may mask the expression of the other. The expressed form is called dominant, while the hidden form is recessive. Therefore A, dominance, is correct. Evaporation, excretion and physical balance do not describe allele expression or Mendelian inheritance.

A dihybrid cross follows two pairs of contrasting traits at the same time. Mendel’s results showed that, when the genes are not linked, the inheritance of one pair can occur independently of the inheritance of the other pair. This is the principle of independent assortment, so A is correct. Traits do not always travel together, and Mendel’s work supports inheritance of genetic factors rather than acquired traits.

Mendel’s principle of dominance explains why a dominant phenotype can mask a recessive one in the first filial generation. For example, an F1 plant with genotype Tt shows the dominant tall form, while the recessive allele t is still present in its genetic makeup. When alleles separate during gamete formation and combine again, the recessive phenotype can reappear in F2. Thus option A is correct; invisibility does not mean destruction or absence.

A pure-breeding line produces offspring with the same form of a trait generation after generation because it is genetically uniform for that characteristic. Mendel could therefore begin a cross with known parental forms and attribute differences in the offspring to the cross rather than to unknown variation in the parents. This made dominant and recessive inheritance patterns easier to detect. Option A is correct; the other choices contradict the purpose of controlled breeding.

In a monohybrid cross, the heterozygous F1 generation produces two types of gametes in equal proportions because the two allelic forms separate during gamete formation. Self-pollination then gives genotypes in a 1:2:1 ratio and, with complete dominance, phenotypes in a 3:1 ratio. The reappearance of the recessive phenotype demonstrates segregation. Thus option A is correct; blending would not restore the recessive form.

The governing principle is Mendel’s law of segregation. In a cross such as Tt × Tt, the two alleles separate during gamete formation. Random fusion produces TT, Tt, Tt, and tt in a 1:2:1 genotype ratio; because T is dominant, the visible phenotypes occur in a 3 tall:1 dwarf ratio. Therefore option B is correct. Independent assortment concerns the inheritance of two different traits, not the basic one-trait 3:1 result.

The governing concept is Mendel’s law of independent assortment. During gamete formation, the allele pair for one trait can separate and combine independently of the allele pair for another trait, when the genes are considered independently. This allows offspring to receive new combinations of characteristics. Thus option B is correct. The law of dominance explains why one allele can mask another, conservation of energy belongs to physics, and natural selection is an evolutionary process rather than the specific inheritance rule described here.

The governing concept is the paired-allele model of inheritance. In sexual reproduction, each parent forms gametes carrying one allele for a trait. At fertilisation, the offspring receives one allele from the mother and one from the father, restoring a pair. For example, Tt may result from T supplied by one parent and t by the other, allowing dominant and recessive relationships to be observed. Therefore option A is correct; the other choices deny biparental inheritance or confuse inherited traits with acquired characteristics.