Heredity and Evolution

The governing concept is chromosome-number maintenance during sexual reproduction. Gametes are formed with half the chromosome number, or a haploid set. During fertilisation, the sperm and egg fuse and their chromosome sets combine, restoring the diploid number. If both gametes carried the full number, the chromosome number would double in every generation. Thus A is correct; the other options are unrelated biological claims.

The governing concept is the XX–XY pattern of human sex determination. A human egg normally contributes an X chromosome, whereas a sperm contributes either X or Y. When an X-bearing egg fuses with a Y-bearing sperm, the zygote has XY chromosomes, the usual male chromosomal combination. Therefore B is correct. The mother does not normally supply Y, so C and D are impossible, while A gives XX.

The governing concept is chromosomal sex determination in humans. The mother’s egg contributes an X chromosome, and the father’s sperm contributes either X or Y. If an X-bearing sperm fertilises the X-bearing egg, the resulting zygote has the XX combination, which is the usual chromosomal pattern associated with female development. Thus A is correct. XY is the usual male combination, and YY or a single Y cannot produce the required zygote.

The governing concept is the inheritance of sex chromosomes and the equal genetic contribution of the parents. A human egg normally carries X, while sperm cells are of two types: X-bearing or Y-bearing. The sperm that fertilises the egg therefore determines whether the zygote is XX or XY. Option A is correct. The mother does not provide Y, the father contributes a chromosome, and food does not determine chromosomal sex.

The governing concept is that acquired somatic changes are different from changes in hereditary material. Removing a mouse’s tail changes its body after birth, but it does not normally alter the DNA in its sperm or eggs. The offspring receive hereditary information through those reproductive cells and therefore retain the genetic instructions for tail development. A is correct; B, C and D contradict basic genetics and development.

The governing concept is germ-line inheritance. During reproduction, hereditary information is transmitted through the reproductive cells, such as sperm and egg, and these cells contribute genetic material to the offspring. A change restricted to ordinary body cells is not normally copied into that reproductive-cell DNA, so it is not inherited. Option A states this principle; B, C and D reverse or deny the role of reproductive cells.

The governing concept is variation as the raw material for natural selection. Sexual reproduction combines genetic information from two parents through meiosis and fertilisation, producing new combinations of traits. If environmental conditions change, some variants may survive or reproduce more successfully than others, allowing populations to evolve over generations. Therefore A is correct. The other options describe outcomes opposite to the role of sexual reproduction.

The governing concept is the evolutionary importance of variation. Asexual reproduction usually produces genetically very similar offspring, so many members of a population may share the same weakness. If a disease, temperature shift or other environmental change challenges that weakness, most organisms may be affected at the same time. Thus A is correct. Low variation does not guarantee strength, immortality or instant formation of new species.

The governing concept is natural selection acting on variation already present in a population. Individuals may differ because of inherited genetic variation; if one difference improves survival or reproduction, that variant can become more common over generations. Nature does not produce a trait because an organism suddenly needs it. Therefore option A is correct. Options B and C deny the need for variation, while D confuses acquired characteristics with inherited traits.

Natural selection requires three linked conditions: variation among individuals, a difference in survival or reproduction, and inheritance of the advantageous feature. If better-coloured insects escape predators more often and leave more offspring, their colour may become more frequent in later generations. Thus option A shows selection clearly. B removes variation, C removes reproduction, and D provides no selective difference.

Evolution is defined as a heritable change in the characteristics or gene frequencies of a population across generations. A useful feature that is not passed to offspring can help one individual, but it cannot accumulate in the population. Therefore option A is correct. B is false because useful traits are not always harmful; C incorrectly says traits automatically become genes; D is unrelated because populations can reproduce.

Both genetic drift and natural selection can change allele frequencies, but their causes differ. Genetic drift is a random change produced by chance, especially in small populations, and it may increase or eliminate a trait without regard to usefulness. Natural selection is non-random with respect to advantageous inherited traits because those traits improve reproductive success. Hence A is correct; B, C and D make incorrect generalisations.

Genetic drift is the chance-driven change in allele frequencies, and its effect is especially strong in a small, isolated population. A trait can become common simply because of random reproduction or the accidental survival of particular individuals, even when the trait gives no special advantage. Therefore option A is correct. Natural selection requires a relationship with differential fitness, while acquired traits and analogous organs describe different concepts.

The founder effect is a special case of genetic drift. When a new population is established by only a few individuals, the allele frequencies of those founders may differ from those of the original population. Their inherited traits can therefore become unusually common in the new group, purely because of its small starting sample. Option A is correct; the other choices concern nutrition, comparative anatomy, or iodine deficiency rather than population genetics.

Geographical isolation separates populations and reduces gene flow between them. Once exchange of genes is limited, mutation, recombination, genetic drift and natural selection can act differently in the separate environments. Over many generations, inherited differences may accumulate and reproductive isolation may develop, producing speciation. Thus A is correct. B, C and D contradict the effects of isolation or describe impossible immediate changes.

A biological species is commonly understood as a group whose members can interbreed and produce viable, fertile offspring. External appearance alone may be misleading, because closely related groups can look alike while being reproductively isolated. Therefore option A is the strongest conclusion, expressed cautiously as “may be.” B ignores reproductive isolation, C is unsupported, and D does not define a species.

Isolation reduces gene flow, but it does not by itself guarantee that separated populations will become different species. Speciation generally requires inherited genetic divergence to accumulate through mutation, recombination, drift and selection, eventually leading to reproductive isolation. Hence option A is correct. B is the opposite of divergence, while C and D make absolute biological claims that are false; isolated populations can still reproduce within their own groups.

In an undisturbed sequence of sedimentary rock, lower layers were generally deposited earlier than layers above them. Fossils found in successive layers therefore provide evidence about organisms living at different relative times. Comparing their structures can reveal a sequence of biological changes and support evolutionary explanations. Option A is correct; B reverses the usual order, while C and D ignore the historical information preserved by fossils.

The governing concept is that fossils provide historical evidence, but the fossil record is incomplete. A fossil forms only when suitable conditions preserve the remains or traces of an organism; many organisms decay, are eaten, or leave no preserved remains. Therefore option A is correct. Options B, C, and D are incorrect because fossils can be very old, are directly related to past organisms, and may reveal structure and habitat as well as feeding clues.

Homologous organs are structures inherited from a common ancestral body plan. Their internal arrangement is similar, although natural selection and different environments may modify their functions; for example, a forelimb can become an arm, wing, or flipper. Thus option A is correct. Option B confuses homology with identical function, while C states the opposite and D incorrectly describes inherited structural features as acquired traits.