Heredity and Evolution

Heredity is the transmission of genetic information and traits from parents to offspring, so it creates continuity and resemblance between generations. Variation means differences that arise among offspring because of genetic recombination, mutation, or environmental influence. Thus, heredity explains similarity, whereas variation explains differences. Options B, C, and D incorrectly deny these biological meanings.

The chapter combines two connected ideas. First, heredity explains how genetic traits pass from parents to offspring. Second, variation explains why offspring are not completely identical. When inherited variations accumulate over many generations, populations may change and diversify; this long-term biological change is evolution. The other options belong to unrelated areas of science.

Heredity is the transmission of genetic information and traits from parents to offspring, so it produces resemblance between generations. Variation means differences in inherited features among offspring, arising from recombination, mutation, and other biological processes. These differences provide the basis for natural selection and evolution. Hence option A correctly states the relationship; the other choices deny the meaning of heredity or variation.

An offspring resembles its parents because it inherits chromosomes and genes from them. However, sexual reproduction combines one set of genetic information from each parent in a new arrangement, and recombination can produce differences. Therefore resemblance and variation occur together, making option A the best answer. Chromosomes and gametes are essential, so options B and D are incorrect; food alone does not determine all traits.

A gene is a specific segment of DNA located on a chromosome. It carries hereditary information that contributes to the expression of a particular trait, often by directing the formation of a functional protein. This is why it is called a basic unit of heredity and traits. Option A is correct; genes do not independently make blood or digest food, and they are located within cells rather than outside them.

In humans, ordinary body cells are diploid and contain 46 chromosomes, arranged in 23 pairs. Gametes are produced by meiosis and are haploid, containing 23 chromosomes. During fertilisation, the egg and sperm nuclei fuse, restoring the diploid number of 46 in the zygote. Thus option A is correct; chromosomes are neither absent nor doubled in gametes, and they are not destroyed during fertilisation.

Let the allele for tallness be T and the allele for dwarfness be t. A pure tall plant has TT and a pure dwarf plant has tt. Their cross produces F1 offspring with genotype Tt. Since T is dominant, each heterozygous plant expresses tallness even though the recessive t allele is present. Thus option A is correct; the dwarf allele is masked, not destroyed.

In the F1 generation from a pure tall TT × pure dwarf tt cross, all plants are Tt. The dominant T allele masks the recessive t allele, so dwarfness is not expressed, but the t allele remains present. When F1 plants self-pollinate, the F2 combinations include TT, Tt, Tt, and tt. The tt plants express dwarfness, so option A is correct.

The governing concept is Mendel’s law of segregation. In a monohybrid cross such as Tt × Tt, the alleles separate during gamete formation, producing TT, Tt, Tt and tt in a 1:2:1 genotypic ratio. Because T is dominant, three offspring show the dominant phenotype and one shows the recessive phenotype. Thus option A is correct; the other choices incorrectly describe blending, absence of variation or chromosome formation.

The governing concept is dominance in a Mendelian genotype. A recessive phenotype appears only when the individual has two recessive alleles, commonly written aa. If one dominant allele, A, were present in Aa, it would usually mask the recessive allele and produce the dominant phenotype. Therefore option A is correct. Options B and C would normally produce a dominant appearance, while D is biologically impossible because a trait requires genetic information.

The governing concept is complete dominance. A plant with one dominant allele and one recessive allele is heterozygous, for example Tt. The dominant allele determines the phenotype, so the plant generally appears tall if T represents tallness, even though the recessive allele is still present and can be passed to offspring. Hence option A is correct. The recessive phenotype requires two recessive alleles, and heterozygosity does not prevent reproduction.

The governing concept is independent assortment during gamete formation. In a dihybrid cross, each parent can produce different gamete types because the allele pair for one trait separates independently from the allele pair for the other. When these gametes fuse, alleles may recombine into combinations not seen in either parent, such as round-green or wrinkled-yellow. Thus A is correct; genes are not destroyed, parents do contribute genes, and food alone does not determine inherited traits.

The governing concept is the F2 phenotypic ratio of a standard Mendelian dihybrid cross with independent assortment and complete dominance. Each parent in AaBb × AaBb can form AB, Ab, aB and ab gametes. Combining these gives four phenotype classes in the ratio 9 showing both dominant traits, 3 showing the first dominant and second recessive, 3 showing the first recessive and second dominant, and 1 showing both recessive traits. Therefore B is correct.

The governing concept is heredity: acquired traits arise during an organism’s lifetime because of practice, use, injury, or environmental conditions, whereas inherited traits are associated with genetic information transmitted through gametes. Thus, a learned skill or developed muscle is not normally passed directly to offspring. Option A is correct; B ignores the distinction, C wrongly says all acquired traits are inherited, and D confuses nutrition with heredity.

The governing idea is the difference between acquired characteristics and inherited genetic information. Training enlarges an athlete’s muscles by changing body tissues and performance during life; it usually does not alter the DNA of sperm or eggs in a way that encodes those muscles. Therefore the developed muscles are not directly transmitted. Option A is correct, while B and C are biologically false and D describes no normal effect of exercise.

The governing concept is that a body modification is not automatically a hereditary change. Cutting a mouse’s tail alters a somatic body part, but it does not normally alter the genetic information carried by its reproductive cells. If tailless mice are bred, their offspring are generally born with tails, showing that the acquired loss was not inherited. Option A is correct; B and C contradict heredity, and D is false.

Evolution requires more than the appearance of differences. A variation must be heritable, and if it improves survival or reproduction in a particular environment, it may become more common over successive generations. Neutral or harmful variations may disappear, while a non-heritable variation cannot be reliably passed on. Option A captures this requirement. B and C are absolute and false, and D misunderstands evolution as an immediate event.

Natural selection is governed by differential survival and reproduction, not simply by size. A useful trait gives an organism an advantage under particular environmental conditions, such as better camouflage, disease resistance, or efficient feeding. The organism may then survive longer and leave more offspring, increasing the trait’s frequency in the population. Option A is correct; B is an unjustified absolute, while C and D are biologically incorrect.

Natural selection changes trait frequencies because environmental conditions give some heritable traits an advantage in survival or reproduction. Genetic drift changes allele frequencies through random events, such as the accidental survival or reproduction of particular individuals, and it is especially influential in small populations. Thus option A correctly contrasts selection with chance. B is irrelevant, C is false, and D denies the environmental role in selection.

Genetic drift is random change in allele frequency. In a small population, each individual represents a relatively large fraction of the gene pool, so the accidental loss, survival, or reproduction of only a few individuals can greatly change the proportion of alleles. Repeated chance events may even eliminate an allele. Option A is correct; B, C, and D are biologically false explanations.