Heredity and Evolution

Let Y represent the dominant yellow allele and y the recessive green allele. Each heterozygous parent is Yy, so the cross is Yy × Yy. The four equally likely combinations are YY, Yy, Yy, and yy. Only yy produces green seeds because the recessive phenotype appears when both alleles are recessive. Thus 1 of 4 outcomes, or 25%, is green. The other three outcomes are yellow, so B, C, and D are incorrect.

Use T for the dominant tall allele and t for the recessive dwarf allele. A pure tall plant has genotype TT, while a dwarf plant must be tt. The cross TT × tt gives only Tt offspring because the first parent can provide only T and the second can provide only t. Every F1 plant is therefore heterozygous and phenotypically tall. A and C incorrectly predict dwarf offspring, while D confuses heterozygous Tt with homozygous TT.

For a single trait with complete dominance, the 3:1 phenotypic ratio is the expected result of crossing two heterozygotes. In Tt × Tt, the genotypes are TT, Tt, Tt, and tt in a 1:2:1 ratio. Three offspring show the dominant tall phenotype and one shows the recessive dwarf phenotype. TT × TT gives all tall, TT × tt gives all tall, and Tt × TT gives all tall phenotypically, so only option C fits.

A test cross is used to determine the genotype of an organism showing a dominant phenotype. The organism with the unknown genotype is crossed with a homozygous recessive organism. If all offspring show the dominant phenotype, the unknown parent is likely homozygous dominant; if recessive offspring appear, it is heterozygous. Therefore option B is correct. A involves two traits, C is reproduction within the same plant, and D is selection rather than a genotype-testing cross.

A dihybrid cross follows two pairs of contrasting traits at the same time. When the allele pairs assort independently, a cross such as AaBb × AaBb produces the F2 phenotypic ratio 9:3:3:1: nine with both dominant traits, three with the first dominant and second recessive, three with the first recessive and second dominant, and one with both recessive. Thus C is correct; the ratio does not prove acquired inheritance, linkage, or only sex determination.

For a trait controlled by a dominant and recessive allele, the dominant allele masks the recessive allele in a heterozygote. The recessive phenotype appears only when the individual receives a recessive allele from each parent, giving a homozygous recessive genotype such as aa. Therefore option B is correct. One recessive allele with a dominant allele produces the dominant phenotype, and the other alternatives do not describe a valid allele condition for expression.

The governing concept is variation, which is necessary for evolution. DNA copying is highly accurate but not perfect, so small changes may arise. If such a change occurs in reproductive cells and is passed to offspring, it becomes heritable variation. Natural selection may then favour or eliminate it. Therefore, option C is correct; the other choices incorrectly describe reproduction, acquired traits, or organism survival.

In the usual human XX–XY system, the mother’s egg carries an X chromosome, whereas a father produces two types of sperm: X-bearing and Y-bearing. Fertilisation by an X sperm generally produces XX, while fertilisation by a Y sperm generally produces XY. Thus the paternal sperm determines whether the zygote receives X or Y from the father. Option B is correct; the other statements misrepresent the chromosomes in eggs or sperm.

The governing concept is independent probability in sex determination. In the simplified human XX–XY model, each pregnancy involves a new fertilisation event: an X-bearing or Y-bearing sperm may fertilise the X-bearing egg. Therefore, earlier daughters do not alter the next probability, which is approximately 1/2 for a boy. Option B is correct; options A, C and D incorrectly treat births as predetermined or dependent.

An acquired trait develops during an individual’s lifetime because of activity, environment or experience and is not ordinarily encoded for transmission in reproductive cells. Exercise can enlarge muscles through training, so option B is the acquired example. Blood group, natural eye colour and pea seed colour are primarily determined by inherited genetic information. They are therefore distractors representing inherited characteristics rather than acquired changes.

Weismann repeatedly cut the tails of mice across generations, yet their offspring continued to develop normal tails. The governing principle is that a bodily change is not automatically inherited unless it alters genetic information in reproductive cells. Thus option A is correct. The experiment did not show that injuries are inherited, that tails lack hereditary control, or that reproductive cells are unrelated to traits.

Natural selection requires variation among individuals, and its evolutionary effect is strongest when the variation is heritable and affects survival or reproduction. Individuals with a favourable inherited trait are more likely to reproduce and pass that trait onward, so its frequency may increase. Option A contains both requirements. Learned-only, non-genetic and identical traits cannot be efficiently selected across generations in the stated sense.

The governing concept is natural selection acting on variation. A bacterial population may contain a few cells with inherited resistance before the drug is applied. The drug kills or inhibits susceptible cells, while resistant cells survive and reproduce, causing their proportion to increase. Therefore, option B is correct. This is not definite inheritance of an acquired trait, formation of homologous organs, or sex determination.

This is the classic industrial melanism example. The moth population already showed colour variation, and soot-darkened surroundings changed which colour provided better camouflage. Dark moths were less visible to predators, survived more often and reproduced more successfully. Their frequency therefore increased through natural selection. Option B is correct; artificial selection requires deliberate human choice, while test crosses and self-pollination are breeding concepts unrelated to this survival pattern.

Genetic drift is a random change in the frequency of alleles, rather than a change caused by a trait being advantageous. Random survival or reproduction has a much larger effect when the population is small, and isolation prevents outside individuals from restoring the original frequencies. Therefore, option B is correct. In a very large population, chance effects are usually weaker; no variation leaves little frequency difference to change, and equal selection does not describe drift.

A flood can kill individuals randomly, so the few deer that survive may carry some alleles in proportions different from those in the original herd. When these survivors reproduce, their accidental genetic sample becomes the starting point of the new population. This bottleneck effect is a form of genetic drift, making option A correct. The flood does not make every gene useful, deer do possess genes, and reproduction need not stop.

Gene flow normally mixes alleles between populations. If it remains absent, each population changes more independently through mutation, recombination, genetic drift, and natural selection. Over many generations, these differences may produce reproductive isolation, so the groups can become separate species. Option B is therefore correct. Isolation does not make populations identical, erase chromosomes, or automatically remove all variation; variation may instead accumulate differently in each group.

Reproductive isolation means that individuals from separated groups cannot mate successfully or cannot produce fertile offspring with one another. The immediate population-level consequence is reduced exchange of genes, or reduced gene flow. This makes option A correct and can later support divergence and speciation. It does not equalize lifespan, convert acquired characteristics into inherited traits, or affect fossil formation as a direct consequence.

Analogous organs perform similar functions but arise from different basic structures and evolutionary origins. Insect wings and bird wings both enable flight, yet their anatomy and developmental history are different. They illustrate adaptation to a similar function through separate evolutionary pathways, often called convergent evolution. Therefore option B is correct. Same origin with different functions describes homologous organs, not analogous organs.

The human hand and horse forelimb contain corresponding bones arranged according to a similar basic structural plan, inherited from a common vertebrate ancestry. Their functions are different: the hand is suited for grasping and manipulation, whereas the horse forelimb supports weight and movement. Similar structure with divergent function identifies homologous organs, so option C is correct. They are not analogous, vestigial, or artificial organs.