Heredity and Evolution

The governing distinction is between phenotype and genotype. If T is the dominant tall allele and t is the recessive dwarf allele, both TT and Tt plants appear tall. Only TT is pure or homozygous, whereas Tt is hybrid or heterozygous and can pass the recessive t allele to offspring. Since appearance alone cannot distinguish TT from Tt, a tall plant is not necessarily pure. Therefore option A is correct.

A dihybrid cross follows two pairs of contrasting traits at the same time. In the classic cross RrYy × RrYy, independent assortment produces four equally likely gamete types, and their combinations give the F2 phenotypic ratio 9:3:3:1. This pattern indicates that the allele pair for one trait can separate independently of the allele pair for the other, under the conditions of Mendel’s model. Thus option A is correct.

The governing concept is that inherited information must be present in the genetic material of reproductive cells, such as sperm or eggs. During reproduction, these cells transmit genes to the offspring. Therefore, a change confined to body cells cannot normally enter the next generation. Option A is correct; B is false because body cells do not form offspring, C ignores the difference between acquired and inherited changes, and D is false because reproductive cells do contain chromosomes.

The governing principle is the distinction between acquired traits and inherited traits. Exercise can enlarge or strengthen muscles during an individual’s lifetime, but this bodily change does not normally alter the DNA of sperm or eggs. Since offspring receive hereditary information through reproductive cells, the exercise-developed muscle is not directly transmitted. Option A is correct; B confuses tissue with chromosomes, C is scientifically false, and D is false because offspring inherit genes.

The governing concept is that an acquired physical change is different from a heritable genetic change. Cutting the mouse’s tail affects its body after birth, but it does not normally change the genes in its sperm or eggs. The offspring therefore receive the usual genetic instructions for tail development and can grow tails. Option A is correct; B reverses the genetic effect, C is false because offspring have chromosomes, and D incorrectly treats development as purely environmental.

The governing concept is genetic recombination in sexual reproduction. A child receives hereditary material from two parents, and the combination of their alleles is not an exact copy of either parent. Independent assortment and the random union of gametes can produce new combinations, increasing variation among offspring. Option A is correct; B describes copying rather than sexual reproduction, while C and D are biologically incorrect.

The governing concept is that asexual reproduction usually produces genetically similar offspring from one parent, but copying DNA is not perfectly error-free. Occasional small mutations or copying errors can create differences in the genetic material. Environmental effects may also influence appearance, but the biological source of inherited variation here is copying error. Option A is correct; B describes sexual reproduction, C is false because chromosomes are present, and D is not a genetic explanation.

The governing concept is chromosome-number maintenance through haploid gametes and fertilisation. In normal sexual reproduction, each sperm or egg has half the species-specific chromosome number. If both reproductive cells carried the full number, their fusion would add two complete sets and double the chromosome number in the offspring. Repeating this in each generation would cause continual increase. Option A is correct; the other choices do not follow from fertilisation.

The governing concept is the inheritance of sex chromosomes in the XX–XY system. The egg from the mother contributes X, because human eggs normally carry X. A sperm from the father contributes either X or Y. An XY child must therefore receive X from the mother and Y from the father. Option A is correct; the mother does not normally provide Y, and neither parent can provide both chromosomes through one gamete.

The governing concept is that the mother normally contributes X through every egg, while the father’s sperm contributes either X or Y. An XX combination results when an X-bearing sperm fertilises the egg, and an XY combination results when a Y-bearing sperm does so. Therefore, repeatedly having daughters is not scientifically the mother’s fault. Option A is correct; the other options refer to biological factors that should be understood, not unfairly blamed.

The governing concept is natural selection acting on heritable variation. If a variation improves survival or reproductive success in a particular environment, individuals carrying it may leave more offspring. Because the variation can be inherited, its frequency may increase over several generations. Option A is correct, but this is a tendency rather than a guarantee. B is unjustified, C denies heredity, and D confuses an inherited trait with an acquired skill.

Natural selection acts on heritable variation when some individuals survive and reproduce more successfully than others. A colour that provides camouflage helps an organism escape predators, so its carriers may leave more offspring. If the colour is inherited, its frequency can increase over generations. Therefore option A directly shows selection, whereas identical organisms, no reproduction, or no environmental pressure provide no effective basis for selection.

Genetic drift is a random change in the frequency of alleles in a population. In a small group, each individual represents a larger fraction of the gene pool, so the accidental loss or reproduction of only a few individuals can change allele frequencies substantially in one or a few generations. Isolation prevents much gene flow from restoring the earlier frequencies. Thus A is correct; the other statements are biologically false.

Natural selection is non-random with respect to fitness: individuals with inherited traits that improve survival or reproduction tend to contribute more offspring. Genetic drift changes allele frequencies through random sampling, especially in small populations, and the change need not improve fitness. Both can alter populations over generations, but their causes differ. Therefore A states the essential distinction; C reverses it, while B and D are incorrect.

Long-term reproductive isolation stops or greatly reduces gene flow between populations. Once separated, mutation, recombination, genetic drift and natural selection can act differently in the two environments. Differences may accumulate until the populations can no longer produce fertile offspring with one another, a condition associated with separate species. Thus A is a possible result. The other options incorrectly claim instant identity, chromosome loss or a complete absence of change.

Geographical isolation separates populations and reduces interbreeding and gene flow. Evolutionary differences usually arise through many small changes, including mutation, recombination, drift and selection, rather than appearing in one generation. Over many generations these differences can accumulate and produce reproductive isolation, making speciation more likely. Therefore A is correct. B and C describe impossible instant events, while D contradicts the role of geographical isolation.

A biological species is commonly identified by the ability of its members to interbreed and produce viable, fertile offspring. External appearance alone may be misleading because unrelated groups can look alike, while closely related populations may differ internally. If two groups cannot breed successfully, reproductive isolation may indicate that they are different species. Hence A is the best answer; B and C ignore genetic or reproductive evidence, and D does not explain the failure to interbreed.

In undisturbed sedimentary rock, deeper layers are generally older than layers above them. Fossils from these successive layers provide a relative sequence of past life. Comparing their structures and types can reveal that organisms appeared, changed or disappeared through geological time, providing evidence for evolution. It cannot tell only today's temperature, food taste or a common age for all organisms. Therefore A is correct, while B, C and D are unrelated or false.

Fossils are preserved remains, impressions or traces of organisms from the past, so they reveal earlier forms and the sequence of biological change. However, fossilisation requires special conditions, such as protection from rapid decay, and many organisms leave no preserved record. The fossil record is therefore incomplete and must be interpreted with comparative anatomy, embryology, genetics and other evidence. Option A captures both its value and limitation; B, C and D are false.

Homologous organs share a basic structural plan because they were inherited from a common ancestor, but they may perform different functions in different organisms. For example, the forelimbs of humans, whales and bats have corresponding bones adapted for distinct uses. This pattern is called divergent evolution: one ancestral structure becomes modified in different directions. Therefore A is correct; B describes analogous organs more closely, and C and D do not explain the structural relationship.