Heredity and Evolution

For a gene, the two alternative forms are called alleles. When the alleles at a particular locus are identical, such as TT or tt, the individual is homozygous. Homozygous describes the genetic combination, not whether the trait is dominant or recessive: TT is homozygous dominant and tt is homozygous recessive. Therefore A is correct, whereas heterozygous means two different alleles.

Use T for the dominant tall allele and t for the recessive dwarf allele. The heterozygous tall parent is Tt, while the dwarf parent is tt. The first parent produces T or t gametes, and the second produces only t gametes. The offspring are Tt and tt in equal expected numbers, so the phenotype ratio is one tall to one dwarf. Hence A is correct; 3:1 applies to a heterozygote × heterozygote cross.

Mendel selected pea plants because they possessed several easily recognisable contrasting traits, such as tall versus dwarf stems and round versus wrinkled seeds. Pea flowers normally self-pollinate, allowing pure lines to be maintained, but Mendel could remove anthers and transfer pollen to control cross-pollination. Their relatively short generation time also made repeated crosses practical. Therefore A is correct; the other statements are false.

In a heterozygous individual, the dominant allele determines the visible phenotype and masks the expression of the recessive allele. Masking does not remove or transform the recessive allele; it remains part of the genotype and can be transmitted to offspring. If two recessive alleles later come together, the recessive phenotype can reappear, often in the F2 generation. Thus A is correct, while B, C and D misunderstand inheritance.

Genetic drift is a change in allele or gene frequencies caused by chance rather than by a consistent selective advantage. Its effect is especially strong in small populations because random sampling can greatly alter which alleles are passed to the next generation. A sudden reduction may produce a bottleneck effect, while a small founding group may produce a founder effect. Therefore A is correct; natural selection is non-random differential success, and the other choices do not describe this population process.

The governing concept is Lamarck’s theory of use and disuse and inheritance of acquired characters. Lamarck proposed that giraffes repeatedly stretched their necks to reach leaves on tall trees; increased use was believed to lengthen the neck, and this acquired change was then thought to pass to offspring. Option A states this idea. The other options do not describe Lamarck’s explanation.

The governing concept is the distinction between inherited genetic information and changes acquired by body parts. Weismann removed the tails of mice over several generations, yet their offspring continued to develop normal tails. This observation weakened the claim that an acquired bodily change is automatically inherited. Therefore, option A is correct. The experiment did not disprove chromosomes, dominance, or variation in sexual reproduction.

Darwin’s natural selection is based on heritable variation, competition, and differential survival and reproduction. Individuals in a population differ, and a variation that improves survival or reproduction can give its bearer an advantage in a particular environment. Such individuals tend to leave more offspring, so the useful inherited trait becomes more common over generations. Thus, option A is correct; the other statements contradict natural selection.

The governing concept is homology. Homologous organs have a similar underlying structural plan because they were inherited from a common ancestor, although their functions may differ after adaptation to different environments. For example, the forelimbs of humans, whales, and bats share a basic skeletal pattern but serve different purposes. Option A is therefore correct. Analogous organs share function but not basic origin, while vestigial organs are reduced remnants.

When the underlying forelimb bones are compared, bird and bat wings show the same basic skeletal arrangement, even though the wings have adapted for flight in different evolutionary lineages. A shared structural plan is evidence of common ancestry, so they are homologous in this comparison. Option A is correct. Analogous organs would have a similar function but different basic structural origin; vestigial and recessive are not suitable terms here.

The governing concept is analogy: organs are analogous when they perform a similar function but have different structural plans and evolutionary origins. Insect wings and bird wings both enable flight, yet an insect wing is an outgrowth of the exoskeleton while a bird wing is a modified vertebrate forelimb. Therefore, option A is correct. They are not homologous because their basic origins and structures differ.

The relevant principle is relative dating of sedimentary rock layers. When undisturbed layers accumulate, lower layers were deposited earlier and upper layers later. Consequently, fossils preserved in deeper layers are generally older than fossils in layers above them. Option A expresses this conclusion. The word “generally” is important because geological disturbance can overturn or mix layers, so “always” in option B is too absolute.

Fossils are preserved remains, impressions, or traces of organisms that lived in the past. By arranging fossils according to the relative age of their rock layers and comparing their structures, scientists can identify gradual changes and transitional features across time. This sequence supports the idea that organisms have changed through generations. Therefore, option A is correct; the other options make claims fossils cannot establish.

The governing concept is speciation through reproductive isolation. If separated groups no longer interbreed successfully, gene flow between them is reduced or stopped. Mutations, natural selection, and genetic drift can then cause the groups to accumulate different inherited characteristics. Over many generations, the divergence may become large enough for them to function as separate species. Thus, option A is correct; isolation does not instantly remove variation or end sex determination.

An acquired trait develops during an organism’s lifetime because of use, exercise, nutrition, injury, or environmental conditions. Evolution depends mainly on heritable variations, which arise from changes or recombination in genetic material passed through reproductive cells. Since an acquired change usually does not alter those genes, it is not normally transmitted to offspring. Therefore, option A is correct; the other options make absolute or factually incorrect claims.

Variation means differences in characteristics among individuals of the same species. During sexual reproduction, one set of genetic material comes from each parent, and meiosis and fertilisation produce new combinations of alleles. These combinations can make offspring different from both parents and from one another. Therefore, option A identifies a major source of variation. Food color, injury, and aging do not by themselves create inherited genetic combinations.

Asexual reproduction generally involves a single parent and does not involve the fusion of two different gametes. The offspring therefore receive genetic material that is almost identical to the parent and to one another. Small differences can still arise through occasional DNA-copying errors or mutations, so “less variation” does not mean “no variation.” Hence option A is correct; genes are present and offspring are definitely formed.

An allele is one form of a gene, and a diploid organism usually has two alleles for a trait. If the pair is represented by one capital letter and one small letter, such as Tt, the two alleles are different. A genotype containing two unlike alleles is called heterozygous. Homozygous means that both alleles are alike, while asexual describes reproduction rather than a genotype. Therefore option A is correct.

Phenotype refers to the observable characteristics of an organism, such as seed shape, height, or flower color. Genotype refers to the allele combination underlying a trait, such as TT, Tt, or tt. Phenotype results from the interaction of genotype with environmental influences, so the two terms are related but not identical. Therefore option A is correct; the other choices misuse or falsely equate the terms.

Let round seed be represented by R, the dominant allele, and wrinkled seed by r, the recessive allele. Two hybrid round-seeded plants have genotype Rr. In the cross Rr × Rr, the possible genotypes are RR, Rr, Rr, and rr. Only rr expresses the recessive wrinkled phenotype, so the seeds appear wrinkled when both parents contribute r; this is about one-fourth of the offspring.