Heredity and Evolution

Natural selection is a non-random process in which heritable variations that improve survival or reproduction tend to become more frequent. Genetic drift, in contrast, changes trait or allele frequencies through chance events, particularly in small populations, whether or not a trait is advantageous. Therefore, option A correctly identifies the difference. The other options confuse an evolutionary mechanism with a life process or deny the role of variation.

In sexual reproduction, the offspring receives genetic information from both parents. For many traits, one allele or form is contributed by the mother and another by the father, producing a pair of alleles in the offspring. Thus, option A is correct. The other options wrongly exclude one parent or deny the presence of genetic information. Dominant and recessive relationships may determine which form is expressed.

Dominance describes expression, not destruction of an allele. When a dominant allele is paired with a recessive allele, the dominant phenotype is usually visible, while the recessive allele remains present in the genotype but is masked. Therefore, option A is correct. The recessive form is not automatically destroyed or converted into a dominant form, and it cannot destroy a chromosome.

A useful heritable variation can increase an organism’s survival or reproductive success. If individuals carrying it leave more offspring, the variation may become more frequent in successive generations. Over a long period, this change in frequency can alter the overall characteristics of the population and contribute to evolution. Thus, option A is correct; the other choices contradict inheritance and population change.

The governing concept is that heredity transmits genetic information from parents to offspring, while evolution describes changes in populations over many generations. Heritable variations can be passed on, and advantageous variations may become more common through natural selection. Therefore, option A correctly connects heredity with evolution. Acquired skills are generally not inherited genetically, evolution is not completed in one generation, and the two concepts are clearly related.

The chapter focuses on how traits are transmitted, how differences arise, and how populations change over long periods. Heredity explains the transfer of genetic information, variation describes differences among individuals, and evolution is the gradual change that can result when inherited variations are selected across generations. Thus option A presents the three connected ideas. The other choices belong to life processes, mechanics, or chemistry rather than this chapter.

The governing concept is the relationship between heredity and variation. Offspring inherit genetic information from both parents, so many characteristics resemble those of the parents. During reproduction, chromosomes and genes are combined in new arrangements, and small changes may also arise, producing differences. Therefore option A is correct. The other choices deny inheritance or incorrectly claim that food alone determines every trait.

The governing concept is that a gene is a functional unit of heredity, not simply a label attached to an observable feature. A gene contains or represents information that contributes to the production or expression of a trait, often through proteins and cellular processes. Thus option A is correct. A gene is not a digestive gland, is not restricted to skin, and is normally transmitted rather than destroyed in every generation.

The governing concept is reduction division and maintenance of chromosome number. In humans, body cells normally contain 46 chromosomes, whereas each gamete contains 23. During fertilisation, one sperm and one egg unite, so 23 + 23 restores 46 in the zygote. If gametes carried the full number, chromosome number would double in each generation. Therefore option A is correct.

The governing concept is chromosome-number continuity through sexual reproduction. A human body cell has 46 chromosomes. If both sperm and egg also had 46, fertilisation would produce a zygote with 92 chromosomes. If that organism later produced full-number gametes, the next generation could have 184, so the number would continue rising rather than remaining stable. Hence option A is correct.

The governing concept is experimental suitability in Mendelian genetics. Pea plants offered several clear, contrasting traits, such as tall versus dwarf and round versus wrinkled seeds, making inheritance patterns easy to observe. They also self-pollinate naturally but can be cross-pollinated by hand, allowing Mendel to control the parent plants. Therefore option A is correct; the other choices contradict the useful features of peas.

The governing concept is dominance in a monohybrid cross. Represent pure tallness as TT and pure dwarfness as tt. Every first-generation offspring receives T from one parent and t from the other, producing Tt. Because T is dominant, all offspring appear tall even though the recessive t allele is present and can reappear in a later generation. Thus option A is correct.

The governing concept is the particulate, or discrete-unit, view of inheritance. If tall and dwarf traits blended completely into a permanent intermediate substance, the dwarf trait could not return unchanged in the second generation. Its reappearance shows that hereditary factors remain distinct, with the recessive factor hidden in the first generation. Therefore option A is the idea disproved by the observation.

The governing concept is a heterozygous genotype and phenotypic dominance. A plant with one dominant allele and one recessive allele has both forms in its genetic makeup, but the dominant allele controls the observable height under the usual Mendelian model. The recessive allele remains present but is not expressed in the phenotype. Therefore option A is correct; it does not mean the recessive allele has been destroyed.

The governing concept is dominance and recessiveness. A dominant allele can mask the expression of a recessive allele in a heterozygous condition, such as Aa. A recessive phenotype appears only when both alleles are recessive, represented by aa, because no dominant allele is present to mask it. Therefore option A is correct; option B gives a dominant phenotype, while C also expresses dominance and D is biologically impossible.

The 3:1 phenotypic ratio in a typical Mendelian monohybrid F2 generation results from segregation of allele pairs. For example, crossing Aa with Aa gives AA, Aa, Aa and aa in a 1:2:1 genotypic ratio; dominance converts this into three dominant-phenotype offspring and one recessive-phenotype offspring. Thus option A expresses the principle of segregation, whereas B, C and D contradict heredity.

The 9:3:3:1 ratio is the typical phenotypic outcome of a dihybrid cross between two individuals heterozygous for two independently assorting traits, such as AaBb × AaBb. The four phenotype classes occur in the proportions 9, 3, 3 and 1 because allele pairs separate and recombine independently. Thus option A is correct; the other choices deny the presence of two inherited traits or invoke an unrelated idea.

The governing distinction is between somatic changes and heritable changes. Exercise can enlarge or strengthen muscle cells in the individual’s body, but this acquired modification usually does not alter the DNA in the sperm or ova that form the next generation. Since it is not encoded in gametes, it is not directly transmitted to offspring. Therefore option A is correct; B is false, and C and D make unsupported or impossible claims.

The governing concept is that acquired changes in body cells are generally not inherited. Cutting a mouse’s tail removes or changes a body part, but it does not alter the DNA in its sperm or egg cells. Therefore, the genetic instructions for tail development can still pass to offspring, which may be born with tails. Option A is correct; B, C, and D contradict basic ideas about gametes, development, and heredity.

Natural selection acts across generations. A variation may help an organism survive or reproduce, but it can influence the population’s future only if the associated genetic information is transmitted to offspring. Inherited variation can therefore become more common when it improves reproductive success. Option A states this requirement. B is false because variation is not always acquired, while C and D misrepresent heredity and selection.